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<title>dB, dBi, and dBd</title>
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<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10">No. 1: dB, dBi, and dBd<img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>A most convenient way to compare power is as a ratio between two powers. For this purpose, we have the decibel or dB. One dB = 10 log (P1/P2), where P1 and P2 are any two powers it is relevant to compare. A power ratio of 2 is 3 dB; a ratio of 4 is 6 dB; a ratio of 10 is 10 dB. These marker points will guide you to intermediate values. Your transceiver output is 100 watts. Your linear output is 800 watts. Hence, your power gain is 9 dB to the antenna.</p>
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<p>Antennas measure their power gain in the favored direction in dB. Since gain requires 2 powers to make the ratio, where does the second power come from? It comes from a standard reference. For most theoretical studies, the most common reference is dBi, decibels over an isotropic radiator. An isotropic radiator is a geometric point in free space with no material losses that radiates equally well in all directions. My rotatable aluminum backyard dipole at 35 feet over soils with average losses has a gain of over 7.5 dBi.</p>
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<p>A second standard is dBd, decibels over a dipole in free space made of lossless infinitely thin wire. This theoretic dipole has a gain of 2.15 dBi. Hence, my backyard real dipole has a gain of over 5.35 dBd.</p>
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<p>For many, if not most of the decisions you will make about antennas, neither dBi nor dBd are the real references. Suppose you wish to buy a 3- element monoband Yagi for 10. What improvement can you really expect over your dipole? Now your own antenna--an aluminum tube (with losses) antenna at a certain height over soil (with certain losses)--becomes the standard. Unfortunately, manufacturers do not rate their antennas by testing them on your mast in your yard. How can you use your knowledge of dBi and dBd to estimate the improvement the prospective antenna will make?</p>
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<p>One way is to make antenna comparisons theoretically and extrapolate to your yard. Table 1 shows the gain figures for 3 antennas: a pretty good 3-element Yagi, a pretty good 2-element Yagi, and a dipole, all in free space. The 2-element Yagi has a forward gain of about 4.3 dB over the dipole, while the 3-element Yagi adds another 1.8 dB, for a total of 6.1 dB over the dipole. In the favored direction and in front-to-back ratio for nulling out QRM, you can expect similar performance from equivalent antennas mounted at the same height over the same terrain.</p>
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<pre>Table 1: Relative Antenna Gain using dBi and dBd and Antennas in Free
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Space
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Antenna Gain Gain over Gain over
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in dBi dipole 2-element
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Yagi
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Dipole 2.1 ---- ----
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2-element Yagi 6.4 4.3 dB ----
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3-element Yagi 8.2 6.1 dB 1.8 dB
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Antenna Gain Gain over Gain over
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in dBd dipole 2-element
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Yagi
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Real Dipole 0.0 ---- ----
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2-element Yagi 4.3 4.3 dB ----
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3-element Yagi 6.1 6.1 dB 1.8 dB
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</pre>
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<p>Do not expect much more improvement than this, even if a manufacturer cites much higher numbers. The beams in this example can be tweaked for a little more gain or a little more front-to-back ratio, but not much and not both at once. And the ability of the antenna to hold its gain and front-to-back ratio over the entire 10-meter band will suffer.</p>
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<p>As you might expect, it does not matter whether you make the gain comparison starting with figures in dBi or figures in dBd. You will get the same result. The only requirement is that you use the same reference for all your comparison calculations.</p>
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<p>Since we started with a dipole, the charts seem to prefer dBd. That is an illusion, since we might have as easily started with a quarter-wave vertical, a 2-element Yagi (as column 3 shows), or ZL-Special. Moreover, had we placed the models over real ground, the gain numbers would have been intrinsically higher, beginning with a dipole gain of over 7.5 dBi or 5.4 dBd, but the differences would have been very similar.</p>
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<p>Fig. 1 shows the free space azimuth patterns for the three antennas. Their relative proportions are identical no matter whether they start with dBi or dBd.</p>
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<img src="ant1a1.gif" width="640" height="480" border="2" vspace="5" hspace="10">
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<p>The relative gain of the 2-element Yagi over the dipole is evident, as is the relative gain of the 3-element Yagi over the dipole and the 2-element Yagi. That is what is crucial to know.<br>
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<p><i>Updated 3-17-97. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i><br>
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<a href="anta.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to An-Ten-Ten-nas Page</a><br>
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<a href="../radio.html"><img src="../images/rtnhome.gif" align="bottom" width="40" height="40"> Return to Amateur Radio Page</a><br>
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<title>Notes on 10-Meter Antenna Tuners</title>
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<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10">No. 10 Notes on 10-Meter Antenna Tuners<img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>There are many excellent antenna tuners (ATUs) on the market that will match at 80 or 160 meters. On 10, the match may be more of an illusion than an effective reality with some designs. If you are a dyed-in-the- wool 10-meter operator and use an antenna system that calls for an antenna tuner, you may be better off building your own.</p>
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<p>So what is wrong with the commercial tuners? (Remember, not all of them are wrong. Compare the points made here with the features you see when you think about buying one.) A number of things go wrong in designs aimed at covering 160-10 meters.</p>
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<p><b>1. The basic design:</b> Most ATUs use a T-network with series capacitors and a shunt inductor, as schematically shown in Figure 1.</p>
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<img src="ant10a1.gif" width="373" height="273" border="2" vspace="5" hspace="9">
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</center>Figure 1. Typical Series-C T circuit.
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<p>This design is a high-pass filter and provides little, if any, suppression of harmonic energy. One can argue that we should not rely on the ATU to do the job our rig's output circuitry should be doing. That argument is sound, but if there is a design that will also provide some harmonic energy suppression, it may be preferable. Two designs fill the bill.</p>
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<img src="ant10a2.gif" width="406" height="232" border="2" vspace="5" hspace="9">
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</center>Figure 2. Typical PI network.
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<p>Figure 2 shows a PI network with shunt capacitors and a series inductor. It is inherently a low-pass filter.</p>
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<img src="ant10a3.gif" width="428" height="213" border="2" vspace="5" hspace="9">
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</center>Figure 3. Typical Series-L T circuit.
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<p>The circuit in Figure 3 is also a high-pass filter: it is a T-circuit with series inductors and a shunt capacitor.</p>
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<p>The reasons for using a series-C T are cost and size. The components for it are smaller for a given power level, since we can fit a 20+ microH coil (rotary or tapped) alongside a pair of 250 pF variable capacitors in a compact case. By contrast, the inductors for the other circuits may require up to and over 30 microH, and the capacitor for the series-L T may need values up to 1000 pF. That is, if you want to tune 80 or 160.</p>
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<p>If you only want 10 meters, then the values drop. A 5 microH inductor (or 2) and a 250 pF air variable (or 2) give all the range anyone should need for 10.</p>
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<p><b>2. Tapped inductors:</b> A tapped inductor may do the job you require, but for some antenna situations, you may not be able to find the 1:1 SWR point. That is not fatal if you can find a point under 2:1. However, a good rotary inductor (or 2 in the series-L T circuit) is superior, since there is no required setting that is unavailable to you. You can find some good near bargains in rotary inductors at hamfests. The same applies to capacitors.</p>
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<p><b>3. High-value components:</b> The higher the highest value of a capacitor or coil, the higher the lowest value. The large frames of high-value rotary inductors may limit the lowest value to well over 1 microH. Capacitors are even worse, because the high maximum value does not tell the whole story. Usually, capacitors follow the rule, but you also have to watch out for construction. I have an old military capacitor with a maximum value of 35 pF. Unfortunately, its lowest value is 17 pF. It is built within a set of frame plates that surround the stators and limit how low it can go. By contrast, a 100 pF old E. F. Johnson capacitor in one of my units has a minimum value of about 10 pF. Its supporting metal work consists of two small plates on either end of the unit. If you roll your own, look for the capacitor's minimum value as well as its maximum value.</p>
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<p><b>4. Closed tight cases:</b> On ten meters, some commercial ATUs achieve a match more with stray capacitance and inductance than with the higher Q variable components that are supposed to do the work. Steel cases that cut the inductive fields of coils and provide several pF of shunt capacitance to the other components complicate and usually hinder the basic work of the matching network. A spacious case, however much against the grain of today's stylistic fetish of ultacompactness, is a plus for an ATU.</p>
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<p><b>5. Poor ground paths:</b> Ideally, there should be one good ground point for the ATU network. With large components, this condition is often impossible to attain. However, grounding should be as direct and compact as possible--and as near to the ground lugs of the coax sockets as possible. It should not rely on the bite of a lock washer through a painted surface. A rivet used as a ground connector is a prelude to malfunction. There are no shortcuts to good, short, wide ground paths and good mechanical and electrical connections. They are not expensive, but they do take care.</p>
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<p>This list of difficulties is not an indictment of all ATUs. Many units have taken the trouble to be the best they can be as 80 or 160 through 10 meter units. However, you can probably do better yourself. For 10 (or realistically, 10-20 meters), a PI or series-L T circuit is achievable with parts you can obtain from hamfests. A PI might use a 5 to 10 microH inductor with a pair of 250 pF air variable capacitors of good power rating. A series-L T circuit might use a pair of 5 or 10 microH rotary inductors and a couple of 250 or 300 pF variable capacitors, with a switch to bring them into play either one or both at a time. A surplus or home brew case with plenty of room helps ensure that strays are minimized, as do well-planned ground paths. Consider a plexiglass case for the ultimate in avoidance of strays.</p>
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<p>Remember, multiband is not always the most efficient, even if it is the most compact. The bands that take the beating are the ones at the high and low ends of the multiband range. Ten meters, unfortunately, is one of those ends. Hence, if you use a 10-meter antenna system calling for an ATU, consider building yourself a customized unit with all the advantages and none of the difficulties.<br>
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<br></p>
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<br>
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<p><i>Updated 3-17-97. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i><br>
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<a href="anta.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to An-Ten-Ten-nas Page</a><br>
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<a href="../radio.html"><img src="../images/rtnhome.gif" align="bottom" width="40" height="40"> Return to Amateur Radio Page</a><br>
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<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10">No. 11 Mobile Antennas for 10 Meters<img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>I was asked to do a column on mobiles. While I claim no serious expertise on mobile antennas, there are some ideas worth considering, some information worth getting, and some safety thoughts worth taking very seriously.</p>
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<h4>Mobile in motion</h4>
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</center>Inherently, a mobile-in-motion antenna system, consisting typically of a shortened, loaded vertical antenna, is a very lossy system. Most of your power goes nowhere. However, do not let that discourage you, since some truly amazing hamming has been done mobile. Let the problem be your challenge. How can you minimize the losses?
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<p>The first step is to ensure that a mobile rig in your car will not do bad things to your car's computer and equally bad things to your car's warranty. ARRL has published some contacts with automakers and distributors where you can get some (but probably not all) of the answers.</p>
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<p>The second step is to prepare your car for a mobile antenna system. This involves close attention to power cabling, ground cabling, and decoupling RF from the battery and other auto electrical components. Use coax that can handle the automobile's hot and cold and greasy and dirty environment. Avoid foam insulated coax cables, as well as those like RG-8 and RG-58 with older types of jackets. RG-213 generally meets the needs of the mobile environment. The very best book I know of on the subject of mobiling is Don Johnson's (W6AAQ) <i><b>40+5 Years of HF Mobiling</b></i>, which is from World Radio. Please do not just slap a rig in the car, mount an antenna on the roof, trunk lip, or plastic bumper, and try operating. Read Don's advise and carefully prepare your vehicle for a mobile set-up.</p>
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<p>The third step is choosing an antenna. According to various sources, the bugcatchers (either Texas or California style) and the DKs (antennas that follow Don Johnson's design) appear to do the most consistent job across the ham bands. However, for 10, you can consider the Hamstick types or even an old fashioned quarter-wave whip with no loading.</p>
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<p>Mounting brackets come in every conceivable size and shape. Brackets used by truckers for CB antennas will attach the antenna almost anywhere on the vehicle. The big question is this: where?</p>
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<p>Since the vehicle serves as a ground plane for most mobile antennas, the best place is in the middle of the roof, which is often a bad place physically. The antenna hits every overpass (until it breaks, of course), and you have to put a hole in your new $15,000 to $30,000 car, an act to which warranty servers may not take kindly, especially if you lease your car.</p>
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<p>Since trunks are making a comeback after a decade of hatchbacks, the trunk lip is a possibility. However, do not rely on the set screws of the trunk lip mount for a ground to vehicle metal. Add a short, wide strap from the ground of the base to a good bolt in the car metal.</p>
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<p>Increasingly, you can find a metal bumper only on trucks and vans. However secure, the antenna base, where the highest current resides and creates the most intense part of the radiation field, is very close to ground without much of a ground plane below it.</p>
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<p>Trucks and vans are becoming more popular as wheels for mobiles, and folks seem less fearful about making holes in them. Side-mounting mobile antennas well up the van wall or cab side is considered an acceptable alternative to roof mounting, and it lowers the tip of the antenna by a foot or so. Bumpers are strong on many of these vehicles: consider a sturdy extension to the antenna base to raise the entire assembly.</p>
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<p>Once everything is in place, including the rig, do not just hit the road chatting away. If you have not watched a weaving car with a cellular phone in the driver's hands, you have not been paying attention. While some ham-drivers can learn to drive and talk and twiddle knobs all at once, many others are high-speed unguided missiles. Unless you are certain that you are safe, let your passenger do the operating. I learned that the semi-hard way--from a close call rather than from an accident. It was inches from being the other way around.</p>
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<h4>Mobile not in motion</h4>
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</center>The safest way for the driver to operate mobile is to stop the vehicle and operate a while. Here, the driver can select a potentially good site to maximize radiation in useful directions, as well as operate safely. Parking lots, open fields, and hilltops are all likely operating points.
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<p>Once you stop your car, you should instantly realize that your antenna options increase dramatically. You can take a modified mag mount CB antenna, resonated for 10, and slap it on top of the vehicle so you can monitor the band while you assemble and raise some kind of antenna less dependent on the car for its ground plane.</p>
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<p>With careful planning, you can create a dipole or a 2-element beam that breaks down into sections that store within the trunk or a similar storage space. A little tool kit with pliers, screwdrivers, and nut drivers can have a permanent place in the trunk. Here are a few tips for such antennas:</p>
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<ul>
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<li>1. Avoid sheet metal screws as element length connectors. Their holes will wear out just when the band opens to Asia. Use hose clamps with a nut driver (usually faster and more sure-footed than a screwdriver).</li>
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<li>2. Protect the elements and the center connectors when not in use. A cheap golf-club bag (are there any such things any more?) or something similar is a good protective carrier.</li>
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<li>3. Store any loose hardware in a bag with a seal inside a plastic box with a clasp. Finding all but one absolutely necessary screw is Murphy's favorite joke on you. Better yet, have no loose hardware--keep it attached to one or the other of the pieces separated for storage. Then, keep a few EXTRA pieces in that bag in a box.</li>
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<li>4. Keep the antenna sturdy enough to store well, but as light as possible for easy assembly and raising. Remember, the antenna does not have to withstand gale-force winds in this application. Smaller diameter, thinner wall tubing will do a good job and prevent back pain.</li>
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</ul>
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<p>The mast can be as simple as 4 5' sections of TV mast. There are also a number of extendable aluminum poles, most never intended as masts, that will support a light antenna in very small breezes. The search for the perfect stationary-mobile mast is endless.</p>
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<p>Guying the mast, even for a brief stop is a good safety measure. One workable system requires a little 3/16 or 1/4 inch rope and a single concrete block. The mast slips into one of the block's holes, which generally is enough to prevent the end from skidding. A side bracket attached both to the door frame (of a car and perhaps the side rail of a pickup) holds the mast a few feet up. The guy ropes go from near the top of the mast to the other side of the car and are tied to the fore and aft bumper frames with a slight tension that holds the mast in place, but still lets you turn it.</p>
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<p>You can also make up a tubular bracket a few feet long and attach it firmly to a pickup's rear bumper. The mast slides into it and is self-supporting.</p>
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<p>Most studies show that over flat land, low horizontal antennas do best at a 5/8 wavelength height, which is just about 20' on 10 meters. Although the take-off angle is high (above 20 degrees ordinarily), short skip operation is quite good, and dx is often only about 2 S-units down from elevations above 1 wavelength (35'). Even though the take-off angle decreases with each increment in antenna height, try to avoid heights between 3/4 and 7/8 wavelength, since ground reflections tend to cancel some of the radiation in the lowest lobe, which is the most useful lobe for skip. If the land is not flat, diffraction effects can wash out some of these phenomena.</p>
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<p>However, a 20' mast is the most many hams can handle mechanically in the field. Those who can get above 30' up need to ensure both personal safety and the safety of passengers. Not to mention saving that big wheeled investment from damage by a $10 homebrew antenna.</p>
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<p>Of course, nothing says you cannot use a well-installed regular mobile antenna. When the band is open, antenna efficiency only makes a difference in pileups (station pileups, not vehicle pileups).</p>
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<p>This style of mobile operation is more leisurely than mobile on the fly. It is also safer for everyone.</p>
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<p><b>Drive safely. The life you save may be mine.</b><br>
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<br></p>
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<p><i>Updated 3-17-97. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i><br>
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<br></p>
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<br>
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<a href="anta.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to An-Ten-Ten-nas Page</a><br>
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<a href="../radio.html"><img src="../images/rtnhome.gif" align="bottom" width="40" height="40"> Return to Amateur Radio Page</a><br>
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<title>Multiband vs. 10-Meter Dipoles</title>
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<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10">No. 12 Multiband vs. 10-Meter Dipoles<img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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<center>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>If we had unlimited money and space, we could ask the following question sensibly: What is the best 10-meter antenna? The answer might be about 4 stacked long-boom, many-element Yagis from about 70' on up on a rotatable tower on a one-hill island surrounded by ocean. Even that answer might get an argument. However, for most of us, the simple question of what the best 10-meter antenna might be is an exercise in irrelevancy.</p>
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||||
<p>A more sensible question is usually this one: which of two alternatives is the better antenna? Each alternative might fit our yard and bank account. However, no general answer is possible. Even if one antenna does outperform another, there are always a number of other factors that affect the final decision. Can I maintain this antenna? What will my neighbors say when they see it? Is it compatible with the rose garden?</p>
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||||
<p>With those qualifications in mind, let's compare two antennas on performance alone just because people have asked a certain question: which is better: a multiband wire antenna like a G5RV or a center-fed Zepp on the one hand, or a dipole cut for 10 meters on the other hand? And let's presume you have the room and supports for both antennas to make this a real comparison.</p>
|
||||
<p>If you like to work all of the HF ham bands and are limited to a single antenna, then the multiband wire is certainly an antenna to consider. An "80-meter" 135' center-fed Zepp, a G5RV 102' dipole, and a "40-meter" 67' center-fed Zepp are all long doublets, center-fed with parallel transmission lines and differ only in length. All require an antenna tuner, although some antennas show a low impedance on some bands. The 135' Zepp obviously works better on 80, although the 102' center-fed antenna does quite well there, and all three antennas can show respectable performance from 40-10.</p>
|
||||
<p>In fact, some folks praise these longer antennas because they show some gain over a 10-meter dipole on 10-10's favorite band. Unfortunately, that kind of claim is like preferring $10 bills over $5 dollar bills. I'll take the $5s if you give me enough more of them than the $10s. The bill size is not relevant until you know how many of each are at stake.</p>
|
||||
<p>When comparing any of the longer multiband wires with a 10-meter dipole, the extra gain is not relevant until we answer the question of where it goes and, equally, where it does not go. A lot of variables go into the answer to the "where" question, but we can give a glimpse into the answer with Figures 1, 2, and 3. Each shows the azimuth pattern of 3 antennas (2 dipoles and one multiband antenna) modeled at a height of 35' (a typical amateur backyard installation) over real ground at a 14-degree angle of maximum radiation at 28.5 MHz.</p>
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||||
<p>Think of these patterns as looking down on the antenna from overhead. The dipoles are the simple figure-8 double loops. For the loops reaching out to 0 and 180 degrees, the antenna runs up and down the page through the center of the diagram. For the loops reaching maximum at the top and bottom of the diagram, the antenna runs left and right across the page. Each multiband antenna also runs left and right across the page.</p>
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<p>Figure 1 shows the 135' Zepp. It has four lobes that exceed the dipole limit by a good bit. However, notice how narrow they are. Notice also the nulls in the pattern. with some careful planning and some good luck in where your yard trees go, you might align the antenna so that one or more of the lobes points right where you find the stations you like to work most. Then again, you might end up aligning the antenna so that nulls point at your second and third favorite spots.</p>
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<p>Figure 2 shows the dipoles with a G5RV. Again, there are higher-gain narrow lobes, but aimed more at the 45-degree point on the pattern. The shorter antenna creates fewer lobes. Again, you might use this information to hit one or more of your targets, and possibly miss a few desirable targets.</p>
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<p>Figure 3 shows the same data for the 67' Zepp, about the length of a 40-meter dipole. There are only 4 lobes for this shorter antenna, but, of course, 4 nulls as well.</p>
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||||
<p>Each of these antennas has its place in some ham's yard, especially for working the lower HF bands (and each antenna will show a different pattern on each band). But is any really significantly better than a dipole on 10 meters? Gain is nice, but those nulls can drive you crazy.</p>
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||||
<p>Well, the dipole also has nulls off each side, so it too is limited. However, with an overall length of between 16' and 16.5' (depending on the element diameter), it is not too difficult to put up a rotatable dipole. You can hand-rotate the antenna mast or use a TV rotator. Then the nulls disappear. More correctly, they go where you put them, as you broadside the antenna to the desired signal. Hence, you only have to rotate a dipole less than 180 degrees to get full 360-degree coverage.</p>
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||||
<p>A stack of TV masting with a house clamp only roughly tight or guys in guy rings would permit you to hand turn the mast. You can attach a short level rod to the mast to make turning easier. Except in odd late afternoon shorter skip conditions, where signals seem to come from every direction, you will likely only have to change the antenna's orientation every few hours.</p>
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||||
<p>Am I "pushing" the rotatable dipole? Not really. Part of my point is that gain is not everything, especially if it does not point anywhere useful. Part of my point is that pointing is more simply done on 10 meters than many people believe, especially if they only look at monster 20-meter beams. Part of my point is that a multiband wire antenna is a very useful antenna for working all the bands. And part of my point is that, with a little ingenuity, a 10-meter dipole can do a lot of useful work for us without being unduly noticeable or expensive. Even if you already have that long wire, you might also consider adding a rotatable dipole to the antenna "farm." Now, if it is simple enough, you might even take it apart, toss it in the truck or trunk, and go portable with it--and put it back up when you get home.<br>
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<p><i>Updated 3-17-97. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i><br>
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<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10">No. 14 EDZs: Stacked and Unstacked<img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>After an article on EDZs (extended double Zepps) appeared in <b><i>Communications Quarterly</i></b> in Fall, 1995 (<a href="../mu/mu3a.html">link</a>), I received some mail and phone calls on a version of the antenna I had not covered. So I thought I would bring it to your attention. The EDZ is a good fixed wire antenna for those who do not wish to fool with towers and rotators, but do wish to work some DX when the band fully reopens. It helps to have high trees or other tall poles to support the ends, and you have to like ATUs (antenna tuning units or transmatches). Even if you do not like these things, read on--they can grow on you.</p>
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<h4>The Basic EDZ</h4>
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</center>The basic EDZ is a 1.25 wavelength wire antenna fed at the center. Unlike a dipole, precision wire cutting is not required. A 10-meter EDZ needs about 44 feet of copper wire. The feedpoint impedance will be about 130 ohms with about 680 ohms of capacitive reactance. I recommend you use 450-ohm parallel feedline to an ATU. Although you can match the EDZ to 50- ohm coax at the antenna feedpoint, you would lose the ability to use the antenna on other bands. For every band 20-meters and up, the antenna is longer than a dipole and thus shows a little gain above a dipole.
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<p>At 10-meters, an EDZ up 40' in the air shows about 3.5 dB gain over a dipole. Of course, both antennas are bidirectional. The EDZ gets its gain by shrinking the side-to-side beamwidth of its two main lobes. The dipole has a half-power beamwidth of about 78° while the EDZ cuts that to about 30° with two small side lobes in each direction. See Figure 1</p>
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<p>Some folks view the sidelobes as QRM producers. If you agree, see the <b><i>ARRL Antenna Compendium</i></b>, Vol. 4 for an EDZ with capacitors in the elements to eliminate them. However, you may lose some performance of the antenna on other bands.</p>
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<p>Almost any wire antenna for 10 up in the air 40' has a take-off angle of 12° above the horizon. Take-off angle means the elevation of the lobe with the most power in it. The half-power points are ±6° around that angle, so there is plenty of power in the low angles needed for efficient DXing. But perhaps we can do a little better.</p>
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<h4>Stacked EDZs</h4>
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</center>Stacking any two antennas will do two important things for you: 1. It will increase the system gain a bit and 2. it will lower the take-off angle a bit. Both those advantages are DXer delights. So let's get crackin' and stackin'. The system I shall show you derives from an article in the November, 1968, <i>CQ</i> by John Schultz, W2EEY called "The Expanded Lazy-H Antenna." (My thanks to Henry Pollock, WB4HFL, for sending me a copy of the article.)
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<p>A vertical spacing of 5/8ths of a wavelength is about the most efficient gain producer when stacking antennas. This applies to Yagis and other arrays as well as EDZs. At 10 meters, this means a spacing of about 22' or so. Again, this number is not critical ±a foot. Figure 2 shows the general scheme. Assuming you have the side supports, there are only a few questions to ask about stacked EDZs.</p>
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<p>1. What do I get for my trouble? First, you get 3 to 4 dB additional gain in both directions, depending upon which wire you set at the 40' level, where we placed our one-wire EDZ. If you can put the second wire at about 60' or so, you will get the higher gain, plus a lower take-off angle of 9°. That puts the bulk of your power in the 10-meter far-DX zone. Figure 3 also shows a secondary lobe at about 28° for shorter paths. If you place the lower wire at about 20' up, you will get the lower gain and a basic take-off angle of 14°. That is still worth while.</p>
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<p>2. How do I aim single and stacked EDZs? With care, since they are fixed. In much of the US, the NE-SW corridor gets you both Europe and VK- ZL-land. Or you can broadside N-S for South America and over-the-pole and back scatter. Or, you can broadside to Asia. Finally, if you have lots of trees or poles, you can build 3 EDZ stacks and switch among them (a nice wiry dream).</p>
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<p>3. How do I feed the beast? The easiest feed system is to run 450- ohm parallel feeders from each feedpoint to a common center. At that point, you will see something like 75 ohms resistive and about 460 ohms inductive impedance. This situation gives us a choice.</p>
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<p>a. If you wish to use the antenna on 10 meters only, you can insert on each side of the line junction a 24 pF capacitor. This is a series connection on each side, with the ends then connected to each side of a coax connector. The capacitors cancel out the inductive reactance, leaving only about the same mismatch you would get with a dipole--and that's manageable.</p>
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<p>b. If you wish to use the antenna on all bands from 20 meters on up, just connect more 450-ohm parallel feeder and bring it to the shack and ATU. On 20 you will get dipole gain or better, with more gain on the other bands between 20 and 10.</p>
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<p>4. What does it cost? Well, #14 wire runs about 7 to 12 cents a foot, depending on its nature, and 88' of wire is in the $8.00 range. Add some insulators and rope for supports. Then add the 450-ohm parallel feeder, which is cheaper than coax. Even if you have to erect some supports (4 sections of 10' TV mast per side, plus rope guys and tie-down ring-screws in the ground), you might run the bill to $75 to $100. That will barely buy the TV rotator, let along the tower and very light beam that goes with it.</p>
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<p>Note that, as in all antenna work, we must compromise. The stacked EDZ requires supports and fixes your general directions to two, but saves you many dollars, pounds, yen, francs, marks, or lire. The long Yagi with the same gain gives you freedom of direction, but empties your pockets and requires constant maintenance. (Remember, Murphy's law says that when the rotator goes bad, it is always stuck in a direction with no hams in sight.) So the EDZ and the stacked EDZ array are suitable only for some people. But perhaps you are one of them.<br>
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<p><i>Updated 4-23-97. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i><br>
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<a href="anta.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to An-Ten-Ten-nas Page</a><br>
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<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10">No. 15 Coax: The Short Story<img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>One good way to think about antennas is as a system: the antenna system. The antenna system consists of all components that get power from the rig (or, on reception, to the rig) into the transducer (that is the antenna proper) that converts the electrical energy (the energy that shocks) into electro-magnetic radiation (the energy form that lets your signal bounce off the ionosphere to all that good DX). Some of those components are inside your rig: the filtering and impedance matching components that establish a 50-ohm impedance at the coax connector.</p>
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<p>The remainder of the components are external: the antenna proper, the transmission line, the transmatch or antenna tuning unit (2 names for the same item, and it may also be in your rig's case), and any matching components at the antenna terminals (such as a beta or gamma match for a Yagi). All of these have to be working well to ensure maximum efficiency of energy conversion into RF radiation.</p>
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||||
<p>The weak link in the system is often the transmission line, and just because it is a simple, no-adjustment component. It does not start off as the weak link: it just gets that way because we ignore it. Most 10-10ers use coax to feed their antennas, so let's confine our discussion to coaxial cables as transmission lines.</p>
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<center>
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<h4>Some Coax Basics</h4>
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||||
</center>Everyone knows what coax looks like. There is a center wire, which may be solid or stranded. Surrounding the center wire is some insulating material: it may be a solid, translucent plastic or a white softer foam. Around the insulation is a layer of conductors: these may be a mesh or braid of copper, or in recent cables, the conductor may consist of a foil with a mesh around it. Over these layers is a coating--usually a black jacket on ham wires, but sometimes gray or even white for special purpose cables.
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||||
<p>For the moment, forget the outer cover. The center wire, the insulating material, and the wire mesh that determine the characteristics of the cable as a transmission line. We can be a little more precise by remembering that alternating currents at HF frequencies travel at the surface of the wires. Hence, we are concerned with the outer surface of the center wire and the inner surface of the wire mesh: these two surfaces determine what the characteristic impedance of the cable will be. Of course, most of us use 50-ohm cables. Since getting a 50-ohm impedance depends on the ratio of the diameter of the center wire and the circle made by the wire mesh, we can make many different sizes of 50-ohm coax.</p>
|
||||
<p>Here is a table of the most common 50-ohm coaxial cables we hams use:<br></p>
|
||||
<pre>Cable # Outer 50 MHz Maximum Velocity
|
||||
Diameter* Loss** Voltage*** Factor****
|
||||
RG-174 0.1" 5.7 dB 1100 .66
|
||||
RG-58 0.195" 3.1 dB 1400 .66
|
||||
RG-58 Foam 0.195" 3.3 dB 1400 .79
|
||||
RG-8X Foam 0.242" 2.1 dB 2000 .75
|
||||
RG-213 0.405" 1.3 dB 3700 .66</pre>
|
||||
<p>* Outer diameter includes insulating jacket.<br>
|
||||
** Losses at 10 meters will be about 2/3rds this figure; losses are approximate and vary somewhat from one listing to another.<br>
|
||||
*** Maximum operating voltages (rms) also vary from one list to another.<br>
|
||||
**** Velocity factors on foam cables vary from one listing to another.</p>
|
||||
<p>What can we learn from the table? Actually, several things.</p>
|
||||
<p>1. The fatter the cable, the lower the losses in the cable. So, use the fattest cable you can afford and handle in your situation.</p>
|
||||
<p>2. The fatter the cable, the higher the power handling capability (as roughly indicated by the operating voltage). So if you use an amplifier, use RG-213 or better.</p>
|
||||
<p>3. The outer diameter of the cable determines what coax connectors to use. RG-213 requires regular PL-259 plugs. RG-8X, being the same size as RG-59 (a 70-ohm cable) requires UG-176 adapters. RG-58 requires a UG-175 adaptor. Do NOT forget the adapters for the smaller cables: they relieve stress on the wires for longer-lasting connections.</p>
|
||||
<p>4. The velocity factors tell us how long a cable wavelength is relative to that same wavelength in free space. The cables with solid insulation will be an electrical wavelength long at only 2/3rds the length of the wave in free space. Notice that the foam insulations have higher velocity factors than the solid insulation types. For all foam and for cables with new numbers, be sure to check the velocity factor just in case you have to make up a cable that is a specific fraction of a wavelength long.</p>
|
||||
<p>In addition to these basic cables, there are a number of new cables. Beldon and others make a very low-loss version of RG-213 under company stock numbers. It is for VHF or for very long (100' +) runs of coax. And it costs more. There are also marine cables with jackets that resist sun, salt water, acids in the air, and almost any other contaminant. They also cost more.</p>
|
||||
<center>
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||||
<h4>What Cable to Use</h4>
|
||||
</center>Determining the best cable for an application is a combination of cable characteristics and practical realities. While no absolute rules are possible, here are some scenarios and recommendations.
|
||||
<p>1. Mobile or portable: Since power with likely be no more than 100 watts and cable runs will be under 35' in most cases, RG-58 should do the job.</p>
|
||||
<p>2. High power contesting: Use RG-213 or better cable to handle the power and the longer cable runs to permanent antennas. One of the low loss cables may be in order if you use very high towers or antenna supports.</p>
|
||||
<p>3. Moderate power, but over 65 years old: Consider RG-8X. It is lighter and easier to handle than RG-213, but has more power handling ability and lower loss than RG-58. (For every tens years of age over 40, also consider lowering your tower by 10' to a height you can climb.)</p>
|
||||
<p>4. Coast-line or industrial area installations: consider some of the marine cables with their extra protection.</p>
|
||||
<p>5. Buried cable installation: RG-213 (an improved version of the old RG-8) should work fine, but for minimum maintenance, consider one of the cables with greater resistance to chemical and abrasive wear.</p>
|
||||
<p>Check at hamfests: sometimes a dealer will run a special on one of the marine or low-loss cables, bringing the price down to nearly the same as RG-213. I tend to avoid preassembled cables, since the crimped connectors are not as well-connected to the cables as carefully assembled regular connectors. However, I do use them for quick tests and for portable work. After a while, I end up replacing the connectors with soldered versions.</p>
|
||||
<p>The UG series of connectors was made to be taken on and off with ease, and to some degree, they depend on the abrasive effects of connecting and disconnecting to keep the center pin and the threads clean. If you use them at the antenna terminals, be prepared to disassemble and clean the connectors at least once a year, even if you use plenty of coax sealant to weather protect the connection.</p>
|
||||
<center>
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||||
<h4>Coax Problems</h4>
|
||||
</center>There are two types of coax problems: mechanical and electrical.
|
||||
<p>Mechanical problems and maintenance: Nothing lasts forever (except love). Inspect your coax and connectors at least once per year or whenever you have any signal strength problems. Clean the connections and reseal them, if outdoors. Wipe down the coax to remove dirt and chemical build- up. Try to install your coax as much in the shade as possible to reduce the rate of sun deterioration of the outer jacket. For buried installations, check for standing water around the cable and improve drainage if necessary. Be certain that you use plenty of support on vertical runs (for example, up the leg of a tower). Avoid long, unsupported horizontal runs: use a support rope to which the coax is taped at small intervals. Coax was not made even to support its own weight. Avoid tight corners: the fatter the coax, the larger the radius of a corner. (This also prevents deformation of the cable, which can change its electrical characteristics.)</p>
|
||||
<p>After five years of outdoor use, replace your coax. It may have some good life in it, but the odds are that your signal strength on both transmission and reception have dropped as the aging cable ate more of the signal power. If you want to use the old cable, cut away and discard a few feet from each end, along with any segments with visible jacket damage. Then use the remaining cable for those shorter noncritical indoor runs of a few feet each. Or remove the jacket and extract the braid as a grounding strap. But keep your outdoor coax fresh.</p>
|
||||
<p>Electrical problems and maintenance: Remember that the transmission line characteristics of the coax involved only the outer surface of the center wire and the inner surface of the wire mesh. The wire mesh also has an outer surface perfectly capable of conducting HF electrical energy. In fact, it can act just like an antenna in parallel with your "real" antenna. That phenomenon can ruin the performance of your system (unless it is designed into the system, as in some wire antenna schemes). Hence, you need something to isolate the outer surface of the coax from the antenna currents. Many beam manufacturers already provide a "balun" or "line isolator" to take care of that potential problem. If your system does not have one, consider purchasing a 1:1 current balun or W2DU-type choke balun to install between the antenna terminals and the coax run.</p>
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<p>I also take one other precaution. Instead of running one piece of coax from the antenna terminals to the rig, I use several. The first goes from the beam terminals (actually, the choke balun) to the mast, around the rotator with a large strain-relief loop, and down abut 8 feet along a tower leg. I have grounded my tower legs as best I can, and I have a plate on the leg with a bulkhead (double female) coax connector. The first length of coax ends here and the second begins. electrically, the tower and the outer coax braid surface are at the same potential. The second coax length goes to another plate installed where the coax enters my home. At this weather protected point, when weather threatens, I can move the coax from the house run to a plate with coax connectors and a long pipe into the soil. Keeping heavy charges outdoors is my goal, especially when I am on vacation. This measure protects my home. A third run goes from the home- entry plate to a plate at the edge of my operating table. Routinely, whenever I am not on the air, the coax, ground strap, computer connector, and AC cord to the station are all completely disconnected, so that the station is isolated from any electrical dangers. This measure protects my rig. Finally, short lengths from the plate to my antenna tuners and rigs complete the connections.</p>
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<p>Do I get some losses from having so many coax connectors? Probably a little. But so far, in 40+ years of operating, I have had no problem with static charge build-up on the antenna, power line surges, ground surges, or other weather- and thunderstorm-related problems.</p>
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<p>Coax makes a very effective transmission line, but its effectiveness depends on sensible selection, thoughtful installation, regular maintenance, and timely replacement. You might want to check out your transmission lines before the sunspots numbers take your mind off everything except operating.<br>
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<p><i>Updated 3-17-97. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i><br>
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<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10">No. 16 Antenna Noise<img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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<h3>L. B. Cebik, W4RNL</h3>
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We often hear reference to "noise" in antenna work, but often we are not sure what kind of noise is being talked about. So let's talk about noise and antennas. "Noise" comes in a wide variety of styles, but here is one way to divide the group into usefully smaller chunks:
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<p><b>1. Man-made noise:</b> This category includes the usual machinery sparking, faulty signs, auto engine sparking, etc. As you can see from thinking about the sources, it largely derives from spark generation and hence produces useless RF over a wide frequency range. Most human-made noise is vertically polarized and of ground wave propagation. Hence, ground-mounted verticals are most susceptible to this category of noise. A horizontal antenna generally shows an immediate 3 dB reduction. Additionally, antenna elevation also helps reduce the noise level. Finally, a narrow-band antenna also reduces the total amount of noise energy in this category from reaching the receiver. A parallel feedline-ATU arrangement sometimes shows improvement over the same antenna fed with coax by filtration action, i.e., narrowing the bandwidth of the energy allowed to reach the receiver.</p>
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<p>One technique that has been the subject of recent articles is the use of a short vertical noise sensing antenna (long enough to pick up local noise but too short for effective reception of propagated signals), inverting its signal, and combining the result with the regular antenna signal. With proper adjustment, local human-made noise can be cancelled quite effectively, with only slight reductions in received signal strength. The benefit lies in the large improvement in signal-to-noise ratio, the truer mark of effective reception.</p>
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<p>Except for very near by sources, such as an arcing pole pig, Man-made noises create the most problems on the lower HF bands.</p>
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<p><b>2. Atmospherics:</b> There are two sources of "atmospheric" noise and energy coupling to antennas:</p>
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<p><b>a. Sparks:</b> Nature also generates wide-band sparks in the form of lightning. There are other atmospheric noise sources, but especially on the lower HF bands, QRN is largely propagated lightning signals. As with all spark energy, the energy decreases as the frequency increases, hence, the quieter high bands. There is little difference in the reception of propagated spark energy between vertical and horizontal energy, since the polarization is lost in the skip refraction. Narrow-banding the pre-receiver reception system can reduce the total energy from such signals that reaches the receiver front end.</p>
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<p><b>b. Charges:</b> The more that air molecules strike each other, the more they lose electrons and become charged. The thinner the atmosphere, as at high altitudes, the longer molecules can stay charged before recombining with lost electrons. It is from phenomena such as these that we get the static charge build-up on antennas. For most home antenna systems, charge build- up was no real problem with tube grids, but a real problem with solid-state front ends. The longer the antenna wire, the windier the location, and the drier the air, the more likely that static charge can build to damaging proportions. At the very least, static charge collection on an antenna is an additional noise source and problem.</p>
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<p>For some antennas mounted very high, the energies involved could not be drained effectively before damage occurred to antenna elements. At the extreme, the development of the quad loop was to solve HCJB's end coupling problem with its Yagis: at the high altitude of Quito, Ecuador, the energy coupling was burning the ends off the antenna elements.</p>
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<p>Loop antennas have no ends: hence, for a portion of the incoming energy, there is a reduction in the amount of energy coupled to the antenna from wire-end capacitance. Where the high voltage region is distributed across a wire length, whether vertical or horizontal, capacitive coupling is minimized. For this reason, some operators find quads and other loop antennas quiter than Yagis and dipoles.</p>
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<p>Regardless of antenna type, static charge is simple to drain away. One technique is to have the antenna at DC ground. Some antenna designs are naturally at DC ground. Loops go from the coax center to coax braid, and if the braid is well grounded, the charge does not build up. Placing an RF choke across the antenna terminals or from the hot terminal to a ground line can continuously drain charge build-up. In some multi-band antenna systems, parallel feed lines can carelessly omit this protection, but a pair of RF chokes, one from each line to ground where the feedline enters the house, can protect equipment. However, remember that the impedance level at that point can be high, requiring a very high value of RF choke to ensure that significant signal energy does not go through the choke.</p>
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<p><b>3. Mixing products:</b> Two signals, neither of which is on the frequency to which we are tuned, can be mixed and produce a third signal (or a bunch of signals) that may fall on a frequency we want to use. The cure for mixing products begins by locating where the mixing occurs. If the mixing occurs in the receiver, then filtration of the unwanted frequency (or frequency range) is the best solution. If the mixing occurs externally to anything one's receiving and antenna system can control, then there is no cure immediately at hand. However, such problems often involve violations of technical standards by one or both of the signal generators involved as the sources of the mix, and patient bureaucratic pressure can sometimes alleviate the problem. If the mixing occurs within one's antenna system, then there is usually something wrong with the system--bad connections, unwanted couplings, less than optimal tuning set-ups: all of these are correctable and should be part of one's routine periodic maintenance on the antenna system.</p>
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<p>These are not all the noise sources. Power company equipment problems, such as arcing pole pigs, require a simple procedure: locate the problem transformer, keep on reporting the situation until you get action, and hope there is a ham on the technical staff that handles such complaints. RFI from light dimmers and other home products that use AC waveform chopping to control a voltage level has been noted in many articles and requires that we locate the source and cure it individually. Likewise with noise from computer timing circuits.</p>
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<p>Finally, some folks are condemned to live in areas where noise is beyond control and even beyond the ability of the best noise blanker to handle. The solution, short of illegally de-powering these sources, is to save money and move to a quiet location--or to concentrate on portable operation. However, antenna choice, feed system choice, filtration, noise cancelers, and noise blankers can go a long way toward reducing currently unlivable noise to a mere constant irritation.<br>
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<p><i>Updated 11-3-97. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i><br>
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<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10">No. 17 Some Misconceptions About SWR<img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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We have all read dozens of articles about SWR. So we all know that the Voltage Standing Wave Ratio is a complex function of the relationship between the feedpoint impedance of our antenna and the characteristic impedance of our transmission line. When the antenna feedpoint impedance is a pure resistance, the relationship is simple: SWR equals the larger of the two divided by the smaller of the two. If the antenna feedpoint exhibits reactance in addition to resistance, then the SWR is higher by a somewhat more complex calculation.
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<p>We also all know that generally, the better the match between the load, the transmission line, and the source (our transmitter outputs), the more power is consumed by the load. Hence, it is generally wise to strive for a well- matched antenna-feedline-transmitter system. So we place an SWR meter in the line in, at, or near the transmitter and monitor the SWR at that point.</p>
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<p>Despite all this knowledge, I still encounter some interesting misunderstandings about SWR. Of course, they come from outside the 10-10 ranks, so everyone can claim, "Well, I knew better than that." Even so, it may be useful to review a few of them.</p>
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<p><b>1. "My SWR is low, so my transmitter is safe."</b> In olden days when tube- type rigs had adjustable output circuits, folks worried about burning out tubes and other components "because" of SWR. Actually, the combination of resistance and reactance seen by the transmitter output circuit would sometimes permit only a small RF transferral. However, operators continued to load their finals to full DC plate input power. What is not RF in a final is heat, and that excess conversion of DC power to heat is what destroyed tubes and stuff around the tubes.</p>
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<p>Today's transistor rigs have feedback circuits that sample the reverse voltage at the output and automatically reduce drive to the finals in the event of a high SWR. Thus, it is pretty difficult to hurt a rig by connecting it to a high SWR output load. However, SWR is NOT the only thing that can hurt a rig. Overdrive, with or without SSB compression, is a source of major stresses on a rig's circuitry. However, the chief rig killer seems to be voltage surges coming from the antenna, the power line, or the ground. And that is a matter of safety that calls for measures outside the rig--like disconnecting the antenna, power cord, and system ground to totally isolate the rig when not in use.</p>
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<p><b>2. "My antenna system is fine, because the SWR is better today than when I put it up three years ago."</b> The fact of a lower SWR over time is often true. However, the conclusion drawn is false. If the SWR is lower than it used to be, the chief reason is an increase in losses in the system. Losses represent that portion of energy converted to heat along the line and at the antenna terminals, energy that is no longer available as energy to radiate. As systems age, cables become "lossier," terminals become corroded, and a variety of other things contribute to the problem.</p>
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<p>Yes, a lowering of SWR can indicate problems, not improvements. It is not impossible, but it is exceedingly rare for an antenna system to change its feedpoint impedance to match the transmission line. It is so rare that the lowering of SWR with time should always be taken as a sign that it is time for antenna system maintenance. Clean, deoxidize, tighten, and seal, as appropriate. If things do not improve, replace the outdoor coax with new stock (but save the old stuff for noncritical uses if it has any life left in it).</p>
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<p><b>3. "My antenna is operating very well because my SWR is a perfect 1:1 match."</b> Unfortunately, my dummy load gives a nearly perfect 1:1 match, and I cannot hear anyone when it is in the line. SWR is one measure of impedance match, but it is not an indicator of the quality of antenna performance as an antenna. Antennas convert radio frequency energy--a form of AC voltage and current--into electro-magnetic radiation (and also the reverse for reception); and they also manage to focus that radiation in various patterns. How well an antenna does this job is only indirectly connected with the impedance match to the transmission line carrying the energy to be converted and directed.</p>
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<p>The practical consequences of this fact are pretty basic. First, before committing to an antenna, try to determine what kind of operating you want to do and select an antenna that will enhance that operation--within the limits of what you can handle in terms of finances, maintenance, and home site restrictions. Second, maintain your antenna regularly--even more regularly than most folks change automobile oil. Preventive maintenance will keep your antenna operating to its maximum ability. Third, if you build your own antenna for a long-term installation, use sensible quality materials. Stainless steel hardware is a must. Tubing and wire made for antennas or equally strong and conductive are necessary. Applying No-Ox or similar antioxidation conductive materials at connections of dissimilar metals is always a good idea.</p>
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<p><b>4. "My antenna has a feedpoint impedance of 100 ohms. Surely 50-ohm coax will give me lower losses than the more highly mismatched 450-ohm parallel feedline."</b> This misconceptions stems from the belief that SWR is a direct measure of the ability of an antenna to "absorb" energy and convert it into radiation. SWR is only part of the story.</p>
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<p>Every transmission line displays two kinds of losses: first is a basic loss based on two significant factors: the ability of the wires to handle RF currents and the leakage between wires through the insulation. Because any coax we can afford compromises cost vs. effectiveness, all common coaxial cables have a higher basic loss per 100 feet than parallel feedline, whether 300-ohm or 450-ohm. In fact, for the HF bands, most parallel feedline has a minuscule loss compared to coax.</p>
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<p>The second loss source is a result of SWR--or rather the mismatch that SWR indicates. Since peak voltages climb, leakage increases. Since peak currents climb, heat conversion losses are higher. In effect, SWR puts a multiplier on the transmission line's basic loss. Since coax begins with significant basic losses, additional losses due to SWR are that much more significant. Parallel transmission lines begin with almost insignificant losses, and the same or higher multipliers usually mean that losses are still insignificant. Under some common conditions, a parallel transmission line with a 10:1 SWR may have lower power losses than a coax cable with a 3:1 SWR. Parallel transmission line is almost always the best bet for multiband wire antennas that require an antenna tuner.</p>
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<p>But remember that even at 3:1 SWR on the lower bands, like 80 meters, coax losses will still be too low to worry about. If your 75 meter dipole shows an SWR at the low end of 80 within the limits of your rig's built-in antenna tuner to handle and you would like to work a little CW, go for it.</p>
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<p><b>5. "My meter shows the reflected power to be 25 watts. I'm worried about losing that power at the antenna and what it must be doing to my rig."</b> Most folks who see these kinds of readings have never looked seriously at their forward power under the same conditions. Suppose you set your rig to exactly 100 watts output. Your reflect power reads 25 watts on a decent meter. Your forward power will read at least 125 watts--perhaps a couple of watts more to account for the cable losses just described (and your rig will be putting out about 102 watts). The difference is 100 watts. Where is it--and where did the extra forward power come from?</p>
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<p>The reflected power simply returns to the forward direction and adds to the rig's power along the line. No need to worry about the rig, since it is not affected by the reflected power (except as the reverse voltage may activate a power reduction circuit). The antenna is receiving and converting 100 watts of power (less only the very small amount changed to heat due to cable losses). A receiving station cannot tell the difference in signal strength between an exactly matched dipole and one running a 10:1 SWR to a parallel feedline and ATU system. The received signal strengths will be the same, assuming the antennas occupied the same transmitting positions with the same propagation conditions. Both antennas converted just about 100 watts of RF energy into radiation. It may take about a dozen cycles for the high SWR system to build to full power and an equal number to return to zero, but when you have millions of cycles per second to use, those few make no difference to the signal intelligence.</p>
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<p>I hope these notes help all those "other" folks approach SWR and antennas a little more intelligently. <i>WorldRadio</i>'s Kurt N. Sterba occasionally runs into SWR msconceptions, and I assure you that his treatment is far more entertaining than mine--except to the sources of those misconceptions, who are technical writers who ought to know better. He is a good incentive for writers to keep things right and sensible. The best extended treatment of SWR and for SWR misconceptions is still Walt Maxwell's book, <i>Reflections</i>. Unfortunately, it appears to be out of print. You may want to petition ARRL to reprint it. Hopefully your library has a copy. Mine is too dog- eared to be borrowed. Anything right in these notes belongs to Walt. Anything wrong is likely to be noted by Kurt.<br>
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<p><i>Updated 3-17-97. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i><br>
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<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10">No. 18: The Simplest 3-Element Yagi?<img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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In a past episode, I described one of the simplest 2-element Yagis I have encountered. For those who would like a little more gain and a better front-to-back ratio, I want to describe a simple 3-element Yagi. Like the other beam, this one comes from Bill Orr, W6SAI, from his column in <i>Ham Radio</i> for May, 1990.
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<p>This beam has two essential items for the casual home builder. First, the antenna has a 50-ohm feedpoint impedance with a wide SWR bandwidth. In fact, the SWR stays below 1.4:1 up and beyond 29 MHz when the antenna is modeled for 28.5 MHz. Second, the characteristics (gain, front-to-back ratio) are almost constant for the entire first MHz of 10 meters. The gain ranges from 12.3 to 12.5 dBi, while the front-to-back ratio remains better than 18 dB across the band. Figure 1 is an azimuth plot of the antenna taken every quarter MHz from 28 to 29 MHz at a 35' height. The differences from one line to another are too small to make any difference at all.</p>
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<p>For these benefits, there is a price to pay: lower gain. From a 3-element Yagi on a 12' boom, one can squeeze out another dB of forward gain and improve the front-to-back ratio by a couple more dB. However, these beams have two disadvantages for the home builder. First, they have a narrower bandwidth where these properties show up. Beyond certain frequency limits, the pattern begins to deteriorate. Second, the feedpoint impedance tends to be very much lower--in the 20-25 ohm range. The lower feedpoint impedance requires a matching network. Although adjusting matching networks can become routine, the process can take the pleasure out building one's first beam that works.</p>
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<p>Let's go ahead and build one of these wide-band specials. First, we need some dimensions. Figure 2 supplies the data.</p>
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<p>For 10-meters and simple installations, hardware store tubing is convenient and accessible. 1" aluminum tubing comes in 8' sections at most hardware depots. You will need 6 pieces, plus a shorter length of 7/8" tubing. You will also need a boom 12' long. One trick I have used is to take two pieces of 6' long 1.5" diameter aluminum tubing and lay them end to end. Now take two lengths of 1 3/8" tubing (or 1.25" if the larger tube is not available). Cut one smaller tube size length in half. Slide 3' of the 6' length of the smaller tubing inside one end of the larger tubing and fasten with stainless steel sheet metal screws. Slide the other piece of large tubing over the exposed 3' of smaller tubing, and fasten. Now slide the 3' lengths of the smaller tubing inside each end of the 12' boom, and fasten (but in such a way as not to interfere with the element mounting to come). We now have a 12' boom adequate for this simple beam.</p>
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<p>Now make up 3/8" plywood plates about 6" wide and 3' long. coat them well against the weather. You can taper the pieces outward from the center to save a few ounces of weight. Find stainless steel U-bolts that go around the boom and around the elements. Let the plates go below the boom, and the element go below the plates.</p>
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<p>Notice that two of the elements require aluminum pieces that are longer than the twin 8' sections of tubing we bought. Likewise, from the director, we shall cut off short section of tubing. Save them. For the reflector, set the tubes to the required extension. Take a 3' length of the 7/8" diameter tube and slide it inside one of the 1" diameter elements and fasten with sheet metal screws. Now slide either or both pieces of remnant director tubes over the 7/8" tube and position them so that they fit under the inner U-bolts. Again, fasten them to the 7/8" tube with screws. Now slide on the other 8' reflector section and fasten. The short segments of exposed 7/8" tubing will make no significant difference to antenna performance. (If you are a perfectionist, you can purchase one extra section of 1" tubing to make the reflector 1" in diameter for its entire length.)</p>
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<p>Since the director only requires trimming to specification and mounting with a pair of U bolts on each side of the plate, the last item of concern is the driven element. Mount the element leaving a 6" space at the center. Slide 18" sections of 7/8" into each 1" tube and bring the ends close together. (The perfectionists can cover the exposed 7/8" tubing with short lengths of 1" tubing for a uniform driven element.) Fasten loosely and begin thinking about a boom-to-mast mounting and a coax connection. A small plate with a coax connector and the shortest possible leads to the antenna elements takes care of the connection. I like to use a 1:1 choke balun of W2DU design (ferrite over a length of coax), and this can be taped to the boom.</p>
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<p>Mount the antenna about 10' off the ground pointing straight up. Adjust the driven element for resonance. Now get the beam up about 35' and begin waiting for sunspots.</p>
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<p>This beam has only one more element than our 2-element special, but it is twice as large and twice as ungainly with its 12' boom. However, its performance improvement over the 2- element Yagi will show up instantly--or as soon as there are some folks to work. You will notice the effects of the added gain and front-to-back ratio on your local net. The bill will come to about $75 or so, which is not bad, if you have priced commercial beams lately.</p>
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<p>There are beams with higher performance figures, but none any simpler to build than this wide-band special.<br>
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<p><i>Updated 2-21-98. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i><br>
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<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10">No. 19: A 3-element 10-Meter Beam on an 8' Boom<img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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<h3>L. B. Cebik, W4RNL</h3>
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As the sunspots return, the search for an inexpensive beam antenna continues. We have looked in past columns at 2-element Yagis and a wide-band 50-Ohm feed 3-element Yagi on a 12' boom. We can shrink the boom length down to 8' if we are willing to accept almost a half dB more gain, a good front-to-back ratio, and a matching network at the feedpoint to transform an impedance of around 25 ohms.
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<p>The design is adapted for hardware store aluminum tubing from a design by Brian Beezley (K6STI) in the YA collection that comes with the ARRL Antenna Book. The original design called for smaller tubing that is often hard to find. The revision calls for 7/8" and 3/4" diameter tubing, which may be found at outlets like Lowes or Home Depot.</p>
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<p><b>Figure 1</b> shows the dimensions of the antenna. The inner 8' of tubing for each element is 7/8" diameter, with the remaining length consisting of 3/4" diameter stock. Three 8' lengths of 7/8" tubing and six 6' lengths of 3/4" tubing are all you need. You can fasten the tubing lengths with sheet metal screws, stainless steel hose clamps (with stainless steel screws), or "pop" rivets. Do not waste weight by using too much overlap in the tubing sections: 2-4" will do nicely. The reflector and the director are continuous elements, but the driven element is split in the middle for the matching system. I use small well- varnished (spar varnish) plywood plates to hold the elements to the boom--you can see the details in almost any of the other Yagi columns.</p>
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<p>My favorite 2-element boom--Schedule 40 PVC--gets a bit heavy by the time the boom reaches 8' long. So you might want to use 1.25" or larger diameter aluminum. A light-weight section of TV masting (the kind so light that I would never recommend that it be used as a mast) would also work if you are sure that it will stand the weather and not rust out in the first season.</p>
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<p>The key to the antenna is the matching system. My favorite for small antennas is the beta match, but a gamma or Tee match would do as well. The driven element length is set for a beta match, with a resistance of about 25 ohms and a reactance between 20 and 25 ohms. The reactance is the virtual series reactance needed by an L-circuit to transform the 50-Ohm coax cable impedance down to the 25-Ohm resistive antenna impedance.</p>
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<p>However, an L-circuit also needs an inductive reactance across the line, as shown in <b>Figure 2</b>. The inductive reactance can be a coil, as shown in the lower view. For 28.5 MHz and the 2:1 transformation needed here, the coil needs to be about 0.28 µH. A coil about 0.5" in diameter and wound with #12 copper house wire will have 6 turns spread over about 3/4" of length with half-inch leads to meet this need.</p>
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<p>The inductive reactance can also be a shorted transmission line stub or "hairpin." The hairpin can have many dimensions, depending on the wire thickness and the spacing. With a spacing (S) of 2" between wires, a hairpin of 1/8" wire will be about 7.7" long (L), while a similarly spaced hairpin of #12 wire will be about 6.8" long.</p>
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||||
<p>In real life, not everything is exact, and the hairpin or coil may need adjusting to achieve a 1:1 SWR at the target frequency (28.5 MHz in this case). You can adjust the length and/or spacing of the hairpin. Likewise, you can squeeze or spread the coil turns. If neither trick works, adjust the length of the driven element and try again.</p>
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<p>How well does the antenna work? Across the first MHz of 10 meters, the gain ranges from about 7.3 dBi to 7.7 dBi (free space gain. In comparative terms, this is about halfway between the gain of the wide-band 50-Ohm beam and a highly optimized 3-element beam with a boom length of about 12' or so. The gain is high enough to compete favorably with most 3-element tri-banders on 10 meters. <b>Figure 3</b> shows the pattern at 3 point across the band.</p>
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<p>The front-to-back ratio is about 18 dB at the band edges, but for most of the span between it is well over 20 dB, peaking above 27 dB near center band. Not too bad for an 8' boom.</p>
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<p>The entire beam will weight between 8 and 10 pounds, including the boom, depending on the materials used. An old TV rotator will turn it, and a guyed push-up mast or roof-top "tower" will put it at a good height. Remember that the fairly high front-to-back ratio is not only a QRM killer, but also a closed back door. In a local net or similar operations, you may also want to have a simple vertical or dipole around to tell when there are signals off the rear waiting for you to turn the beam in their direction.<br>
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<p><i>Updated 10-2-98. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i><br>
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<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10">No. 2: Invisible and Hidden Antennas<img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>If you live in an apartment, condo, or development with antenna restrictions, you may need to hide or disguise your antenna. Here are some ideas to get you started. First, some general principles, then some basic antenna types.</p>
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<p><b>Principles:</b> The principles are few: 1. Safety: keep the antenna as far away from people as possible for their safety. 2. Propagation: put the antenna as high and in the clear as possible for maximum efficiency within the limits of needing to keep it out of sight. 3. Antenna tuner: with few exceptions, the hidden or invisible antenna will become a random length end-fed wire. Any of the T-matches (for example, MFJ's tuners) will do a good job matching the antenna to your rig. 4. Expectations: do not expect the performance of a long Yagi. Instead, expect to make a lot of great contacts.</p>
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<p><b>Antenna Types:</b> The basic hidden antenna types are four: 1. disguised verticals, 2. thin wires, 3. attic antennas, and 4. loading existing metal. There are dozens of variations on each theme. But, forget loading the bedsprings.</p>
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<p><b>1. Disguised verticals:</b> You can encase almost any type of vertical antenna in PVC, and call the outdoor structure a flag pole. For working dx from a development with restrictive covenants, this route is often useful, especially if you use a multiband vertical. Construction varies with the circumstances.</p>
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<p><b>2. Thin wires:</b> Wire as small as #28 enamel makes a good antenna for 100- watt transceiver use. It is often invisible enough to leave running from a second-story apartment window to a tree. Or, you can put it on a reel and wind it out and in as necessary. Alternatively, you can string it in the attic, turning corners wherever necessary. I have used it with success taped to the edges of a second story ceiling. How much wire? How much have you got? Just do not end up with tight coils; try for maximum length in each direction used.</p>
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<p>Thin wire antennas, indoors or out, are random length wires fed at the end and need a transmatch. If possible, locate the transmatch where the wire leaves the building. Otherwise, try a piece of RG-213 from the tuner to the antenna wire. Ignore the coax losses. Try a sleeve balun (such as those sold by Radio Works) where the coax leaves the building. Ground the coax shield where the antenna begins and the tuner and every piece of equipment in the shack well--very well! The object is to do everything possible to keep the RF outside or away from the rig. Some combination of sleeve balun, ferrite cores, and the like will likely solve any RF-in-the- shack problems. Keep experimenting. And be willing to spend a few dollars on wide braid to use as grounding strap; it works well, as does a good, long ground rod (replaced every few years).</p>
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<p><b>3. Attic antennas:</b> Way up and away from everything is best, but attics are not usually as bad as you might think. The frame and roof shingles do not absorb too much RF, and the braces can help support an antenna. In the attic, you can usually find room for dipoles for many bands, even if you have to bend the ends down or to the side. Mount the dipole as high in the structure as you can, and away from any metal duct work, metallic duct insulation, or house and phone wiring. Feed an attic dipole just as you would an outdoor antenna (coax and 1:1 choke balun). Trim it to resonance or minimum SWR.</p>
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<p>The attic may also hold a 2- or 3-element Yagi fixed on your favorite target. You can adjust element lengths for thinner wire elements and do away with the boom. If you use tubing, getting it into the attic may be a problem if the opening is small. First, build and test the antenna, then cut it into pieces that will fit the opening, developing couplings to reconstruct it on site. (Back before cable, I put a 12'-long TV log periodic in the attic just that way: I used wire on a wood frame, cut the wood into entry size pieces, and used linking pieces to reassemble the frame before restringing the wire. Great reception from Atlanta about 70 miles away.)</p>
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<p><b>4. Loading existing structures:</b> Perhaps the most obvious aluminum structure to load is the gutter and down spout. Treat the system like a thin wire antenna and use a tuner, driving a ground rod near the feed point. Be sure to securely bond the joints, since painted aluminum joints with pop rivets make a shaky electrical system. Expect changes in pattern and antenna tuner settings with water, snow, or leaves in the gutter.</p>
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<p>You can also load patio umbrellas, existing aluminum flag poles, and other metal structures. You can shunt feed a grounded flag pole. Avoid iron and steel structures if an alternative is possible, since their losses are fairly high, but use them in a pinch. Even the frame of a sliding patio door will radiate and receive.</p>
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<p><b>Anticipated results:</b> Compared to a high and free outdoor antenna, these substitutes will likely disappoint you for local area point-to-point work on 10 meters. However, do not sell them short with skip. Their low height will give high take-off angles, but still produce very solid contacts, even in 10-10 contests. The idea is NOT to try to compete with the 1.5 kW linears feeding 7-element quads at 105' altitudes. Instead, find modes of operation to which the antenna is better suited: rag chewing, nets, and even informal contesting. Most of these hidden or invisible antennas will usually outperform loops and other tiny antennas (but they will also work in making contacts).</p>
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<p>If your situation calls for these measures, develop an attitude: be on the lookout for better places (higher and bigger) for the next design. Most of these antennas are cheap, but do not skimp on quality coax, connectors, ground rods, baluns, and filtration. Use the same high quality components you would put into an outdoor tower and beam. Part of your attitude adjustment should be forgetting ultimate antenna efficiency, except when comparing two of your own models to see which one to keep.</p>
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<p>Last, be careful and considerate: keep RF out of the power lines and phone lines. It is often easier to try a different kind of hidden antenna than to filter or revise parts of the phone wiring.<br>
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<p><i>Updated 3-17-97. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i><br>
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<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10">No. 21: A Simple 2-Element 10-Meter Quad<img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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<h3>L. B. Cebik, W4RNL</h3>
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Quads are popular with many folks, and sometimes for good reasons. So let's see what a dedicated 10-meter 2-element quad might look like after some work in the home shop. We shall use only materials available locally: wire, PVC, and some hardware.
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<p>Why a Quad? Quads beams consist of 2 1 wavelength (approximately) loops, ordinarily arranged so that one is the driven element and the other is the reflector. Since the elements are double the size of the half wavelength elements in a 2- element Yagi, we expect more gain-and we get it. In fact, a 2- element quad has almost as much gain as the 8'-boom Yagi or the wide-band Yagi we discussed in recent columns. However, the 2-element quad does not have as much gain as a fully optimized long-boom 3-element Yagi. Also, the front-to-back ratio of a quad fluctuates around the 20 dB figure, more like a 3-element Yagi than a 2-element Yagi.</p>
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<p>Physically, a quad beam--when compared to a Yagi--trades volume for horizontal area. Horizontally, even the 8'-boom Yagi requires a rectangle about 8' long by 16.5' wide. A quad of similar performance has a footprint only 5' long by 9.5' wide, or about 1/3 the area. However, the Yagi is flat, while the quad occupies a 450 cubic foot volume. (That just means that it is as high as it is wide.)</p>
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<p>If you decide that you want a quad for most of the HF bands, I recommend that you obtain one of the commercial models. These fairly complex structures are engineered for maximum strength and durability. However, if you need only a 10-meter model (and later want to tuck 6-meter or 2-meter quads within the same framework), then you can build one yourself with simple tools and materials.</p>
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<p><b>The Quad Beam Structure.</b> As shown in <b>Figure 1</b>, a good 10- meter quad can be built from two wire loops. If the wire you use is #14 bare copper (stranded or solid), the driven element loop is 105.3" per side or 421.2" overall. The reflector loop is placed 60" behind the driven element and is 110.3" per side or 441.2" overall. If you decrease the wire size of a loop antenna, the total length becomes smaller as well (and not longer, as with a linear wire antenna element). For #18 bare copper wire, the dimensions per side drop to 105" and 110" respectively--not a big drop. but a noticeable one.</p>
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<p>These dimensions are for bare wire. Insulated wire has a 2-5% velocity factor, depending on the type and thickness of the insulation. It will require different dimensions that I have not modeled and tested.</p>
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<p>If we could just starch the wires and toss them in the air, the quad would be a very simple antenna. However, we need a supporting structure, shown by the dotted boom and arms in <b>Figure 1</b>. The support structure should be nonconductive, although a metal boom usually does not affect antenna tuning. For 10-meters, the arms can be strong fiberglass, quality bamboo (with lighter wire elements), or thin-wall PVC.</p>
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<p>The sketch shows what one might do with an all-PVC structure. The boom can be about 4-5' 1" or 1.25" nominal Schedule 40 PVC. (1" nominal PVC is closer to 1.25" in diameter, while 1.25" nominal PVC is closer to 1.5" in diameter.) The arms can be thinner-wall SDR-135 PVC, 0.5" diameter nominal. Each of the 4 arms per element should be just about 6.5' long, but lets add another half foot to each.</p>
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<p>Since PVC comes in standard 10' lengths, we shall need 8 pieces of the half-inch diameter stock, along with 4 end-to-end couplers (plus a 5' piece of fatter Schedule 40 for the boom). You can either drill the ends of the arms for the wires or use half-inch Tee fittings--although the smoother fittings add weight to the assembly.</p>
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<p>Assuming a 5' boom, use a 1 1/8" drill-mounted hole cutter to cut two pair of aligned holes in each end of the boom. The outer pair at each end should be about 6-9" in from the end, and the inner set another 1.5" further in. The hole-pairs should be at right angles to each other at each end, and the hole sets at each end should be very closely aligned with each other.</p>
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<p>Now we can make the wire supports. First, place an end-to-end coupler in each hole and run a stainless steel nut and bolt through the boom and coupler to fasten the assembly. Using PVC cement, add 7' lengths of the half-inch thin-wall PVC to each coupler end. Add Tees to the ends, or drill out and deburr generous holes for the wire about 1/2" in from each arm end.</p>
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||||
<p>Use a length of cord marked the same length as the wire that will replace it--with some excess. Run the cord through the holes and stress the arms outward toward the boom end until the cord matches the element size. Tie it off. If you use fresh nylon cord, you can leave it in place even after you add the wire--it will not hurt anything. Since cord stretches, you may experience some flex reversals, but after a few of these, you will learn how to keep the arms flexed in the correct direction.</p>
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<p>Now add the wires. Make the reflector loop solid by twisting the ends of the wire together and soldering. The driven element requires a small insulated plate and a coax connector.</p>
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||||
<p>Assuming you have or can build a boom-to-mast mounting plate and have the U-bolts to mount the structure, you are almost ready to go. But let's not hurry.</p>
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<p><b>Performance.</b> The 2-element quad we have just built was designed for best front-to-back ratio from about 28.25 through 28.75 MHz. Below 28.25 MHz, the front-to-back ratio decreases rapidly to a little over 10 dB. Above the target frequency range, the front-to-back ratio decreases more slowly so that the 10 dB figure is not reached until about 29.4 MHz.</p>
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<p>Forward gain of this antenna is maximum at the low end of the band and decreases about 0.1 dB per 100 kHz. Gain is best in the first half MHz of 10 meters, rivaling the gain of the 8'-boom Yagi. It remains as good as any 2-element Yagi all the way to the top end of 10 meters.</p>
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<p>Since I am not trying to sell you anything, notice that I have told a story about what happens to the gain and front-to-back ratio rather than citing peak figures for each. <b>Figure 2</b> shows the azimuth pattern at the elevation angle of maximum radiation when the antenna is modeled at 1 wavelength up (about 35' high).</p>
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<p><b>Figure 2</b> shows not only the variation in front-to-back ratio and the rear pattern, but also the gradual reduction in gain with increasing frequency.</p>
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<p><b>Feeding the antenna.</b> The feedpoint impedance of the antenna alone varies from about 70 to 150 ohms across 10 meters. since we anticipate feeding the antenna with standard 50-ohm coaxial cable, we need to add one more component: a matching system.</p>
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<p>The simplest and most effective matching system for this antenna is a 1/4 wavelength section of 75-ohm coaxial cable, such as RG-11 (for higher power) or RG-59 (for lower power levels). Both cables have a velocity factor of 0.66, which means that a full wavelength of cable is 0.66 of the wavelength in free space. Let's use 28.5 MHz as our design frequency. A wavelength at this frequency is just about 34.5' long. A quarter wavelength is a little over 8.6' or 103.5" long. The quarter-wave matching section is 0.66 of this or 68.3" long.</p>
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<p>A perfect quarter wavelength will transform an impedance higher than 75 ohms to a lower impedance, which is just what we need. In fact, when fed with a 50-ohm cable to the shack, the quarter-wave section shows under 2:1 SWR all across the 10- meter band. <b>Figure 3</b> shows anticipated SWR curve.</p>
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<p>Do not operate this antenna without the quarter-wave 75-ohm matching section or an equivalent matching circuit.</p>
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<p>If a 2-element quad meets your space and operating needs, this one will do the job about as well as it can be done. Feel free to alter the support structure using materials with which you are comfortable in your shop. (For me, PVC is tinker toys for adults.) A TV rotator will easily turn this light antenna. For best results, be sure the bottom wire is at least 20' up, and a tower or mast height of 35' is a good minimum target height for excellent DXing.<br>
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<p><i>Updated 10-2-98. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i><br>
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<a href="anta.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to An-Ten-Ten-nas Page</a><br>
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<title>The Handy Quarter-Wave Matching Section</title>
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<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10">No. 22: The Handy Quarter-Wave Matching Section <img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>In the quad project we looked at last time, we specified the use of a 1/4 wavelength matching section of 70- to 75-ohm coax between the antenna feedpoint and the 50-ohm coax run to the shack. Let's find out why we needed it and how the matching section does its work. These types of matching sections are handy and easy to make, so you may find them useful in future antenna projects.</p>
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||||
<p><b>The Raw vs. the Matched Antenna</b>: Let's start by comparing the feedpoint impedance of our quad both with and without the matching section. The figures are based on a quad beam that is self-resonant just below 28.25 MHz. The 1/4 wavelength section of 75-ohm, 0.66 velocity factor coax was cut for 28.5 MHz and turned out to be 68.3" long. All SWR figures are relative to 50 ohms.</p>
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<pre>Frequency Without matching section With matching section
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||||
in MHz Feed Z SWR Feed Z SWR
|
||||
(R+/-jX Ohms) (R+/-jX Ohms)
|
||||
28.00 70.6 - j29.2 1.81 66.9 + j27.1 1.73
|
||||
28.25 96.8 + j 3.4 1.94 58.1 - j 2.7 1.17
|
||||
28.50 125.1 + j25.9 2.63 43.1 - j 9.1 1.28
|
||||
28.75 150.1 + j39.4 3.23 34.9 - j 8.5 1.51
|
||||
29.00 168.6 + j47.7 3.67 30.7 - j 7.1 1.68
|
||||
29.25 180.8 + j54.8 3.97 28.2 - j 6.0 1.81
|
||||
29.50 188.2 + j63.1 4.22 26.6 - j 5.4 1.91
|
||||
29.75 192.6 + j73.6 4.45 25.1 - j 5.2 2.02</pre>
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<p>Without the matching section, the SWR figures for the quad are high enough that the automatic shut down feature of most current solid state rigs would reduce rig output to almost nothing. With the matching section, the SWR figures within 10 meters permit normal operation.</p>
|
||||
<p>As the unmatched impedances go up, the matched impedances go down. This gives us a clue to how the matching section operates. Every length of coax of any characteristic impedance (Zo) is an impedance transformer. For odd lengths, the transformation is complex. However, when a length of coax is exactly 1/4 wavelength long at a given frequency, the transformation is simple, especially if the impedance to be transformed is wholly resistive. We can use a calculator to handle this equation:</p>
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||||
<b>Zmatched = Zo2 / Zantenna</b>
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||||
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<p>This little formula is only approximate in the real world. for example, with the quad, there is a reactive part of the antenna feedpoint impedance in every line of the table. Moreover, the matching section is only an exact 1/4 wavelength at 28.5 MHz and nowhere else. However, if the reactances are not too high and the frequency span is not to great, the simple equation makes a good approximation. As we look at the table, for a single ham band and for reactance values less than half the resistive values, the simple equation works well enough for antenna building.</p>
|
||||
<p>Remember that for purely resistive impedances, a 2:1 50-ohm SWR accommodates an impedance range of 25 to 100 ohms. This resistive range shrinks when we combine reactances with resistance. However, note the 4:1 range of impedance that these SWR limits can handle. (Also remember that the 2:1 ratio is somewhat arbitrary as a set of limits. It's chief effect is noted by automated power reduction circuits in transceivers. Apart from this, there would be little difference in radiated power between, say, SWRs of 1.8 and 2.5.)</p>
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||||
<p>With a 1/4 wavelength 75-ohm matching section, again in purely resistive terms, we can take antenna feedpoint impedances between just above 56 ohms up to 225 ohms and transform them to values that fit the 50-ohm 2:1 SWR limits--again, a 4:1 range. Notice that our quad does not reach 225 ohms when the matched SWR exceeds 2:1, but notice also that there is considerable reactance that accompanies the resistive value at 29.75 MHz.</p>
|
||||
<p>Likewise, at 28 MHz, we would expect the antenna impedance of 70.6 ohms to yield about an 80-ohm figure instead of the 66.9-ohm figure that actually emerges. However, not only do we have reactance at the antenna feedpoint, but as well the matching section is shorter than 1/4 wavelength at this frequency. Hence, the impedance does not undergo a full quarter wavelength transformation. (Likewise, above 28.5 MHz, the impedance undergoes more than a 1/4 wavelength transformation.)</p>
|
||||
<p>These are the finer points of using a 1/4 wavelength matching section that affect the matching range by just a little bit and throw the actual impedances somewhat off the calculated results from the simple formula. But the simple formula works well enough for most ham antennas. To be on the safe side, if you have a range of antenna feedpoint impedances from about 80 to 200 ohms, then a 1/4 wavelength section of 75-ohm coax will transform them to values appropriate to a 50-ohm feedline and transceiver system.</p>
|
||||
<p><b>Other Applications</b>: 50-ohm and 75-ohm coax cables are the ones most easily obtained by hams, even though other values are available from manufacturers. However, this fact does not limit us to matching only values above 50-ohms to our 50-ohm system. If you cut 2 lengths of 75-ohm cable to 1/4 wavelength and connect them in parallel (center conductor to center conductor and braid to braid at both ends), you have a 37.5- ohm cable. If we plug this value into the simple equation, we find that we can match impedances values below 50-ohms up to values within the 2:1 50-ohm SWR limits. This is useful for Yagis and other antennas that often have feedpoint impedances in the 20-35 ohm range. The double line can be a bit bulky, but that is about its only significant disadvantage over other matching methods.</p>
|
||||
<p>Consider another situation: At certain wire antenna heights below 1/2 wavelength, the feedpoint impedance of a dipole is not 70 ohms, but more like 80-95 ohms. The 75-ohm matching section would transform these values to a 70-60 ohm range. However, we can broaden the range over which these values apply by first running a section of 50-ohm cable that is 1/2 wavelength long or a multiple of 1/2 wavelength (allowing, of course, for the cables velocity factor). Cut the 50-ohm cable for a frequency at the band center, such as 7.15 MHz for a 40-meter dipole. Since the cable is short at the low end of the band, the impedance will be higher than at the antenna at the same frequency. Equally, since the cable is long at the high end of the band, the impedance will also be higher than at the antenna terminals. The result will be band edge values closer to 100-120 ohms.</p>
|
||||
<p>Now, if we plug in our 1/4 wavelength 75-ohm matching section, we have lower SWR values across the band than we would have by placing the 75-ohm matching section at the antenna terminals. In fact, such a system can, with some dipole heights on 80 meters, cover more than 4/5 of the band with under 2:1 SWR.</p>
|
||||
<p>75-ohm quarter wavelength matching sections (and derivatives) make up a quite flexible array of methods for adapting 50-ohm transmission line to antennas that do not present 50-ohms at their feedpoint terminals. However, they do have some major limitations. Because a length of coax is 1/4 wavelength long at only one frequency, this technique is for monoband antennas only. If you have a multiband antenna, you will have to use some other method of matching your 50-ohm coax/transceiver system to the antenna.</p>
|
||||
<p>Likewise, the transformations become far more complex the higher the reactance at the antenna feedpoint. Hence, the quarter wavelength matching system is also only for low- reactance matching situations, such as the one shown in the table for the quad. If you have higher reactances, you may need a different matching system.</p>
|
||||
<p>But where the 1/4 wavelength matching section is suited to the task, it is simple, inexpensive, low-loss, and effective. Those are pretty good credentials.</p>
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<p><i>Updated 1-24-99. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i></p>
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<p><a href="anta.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to An-Ten-Ten-nas Page</a></p>
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||||
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<title>Some Notes on Antenna Safety</title>
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<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10">No. 23: Some Notes on Antenna Safety <img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>What would you like to protect from harm? Yourself? Your family? Your home? Your station? Your neighbors and their property? I guess that is enough to start a column on safety. Obviously, we can never finish it, but perhaps a few ideas might help you fill some gaps in the safety net around all you wish to protect and preserve.</p>
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||||
<p><b>Static Build-Up</b>: As the wind and weather pass over the antenna elements, a charge builds up. On long wire antennas, it can generate enough of a charge to draw a visible spark across a gap to ground. On smaller antennas it can create unnecessary static. The best way to avoid problems here is to be sure that the antenna is at DC ground. Some antennas are designed with a DC ground; others are not. For low impedance antennas (less than 150 Ohms feedpoint impedance) fed with coax, the simplest way to insure a DC ground is to place an RF choke across the antenna terminals. This gives you a DC ground path but leaves the antenna RF-hot. At 10-meters, a 100 micro-H RF choke provides about 1800 ohms of reactive isolation, but a low resistance to charge build-up. Just be sure your coax braid is grounded somewhere along the line so that the charge has somewhere to go other than to your equipment.</p>
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||||
<p>Parallel feedline systems, such as all-band doublets require a different treatment, since the impedances they may see can range up to 5000 ohms and more. Here we can use high value resistors where the feedline enters the house. Run a 1 Mohm resistor from each line to a ground rod where the line meets the shack. It is also useful to add a knife switch or other disconnector (with the alternate contacts directly grounded) to remove the connection to the equipment when the antenna is not in use.</p>
|
||||
<p><b>Station Grounding</b>: Everyone is familiar with the need for a common ground bus for all station equipment with as short a lead as possible to a ground rod outdoors. If you have a tower, it should also be ground rodded. All ground rods (including the house ground at the electrical service entry) should be connected together by the largest copper wire you can obtain. Some folks use copper flashing cut into long strips. These days, braid is not recommended because its very large surface area open to weather deteriorates it quickly. Use solid mechanical connections, not solder (which melts in the presence of lightning like a teenager in the presence of a rock star: instantly).</p>
|
||||
<p>If ground rods are widely separated, add ground rods in a row from end to end, spaced about the length of each ground rod apart. This is not over-kill--just the opposite. It is over-safe by holding the resistance between everything in your house and shack at the absolute minimum. Even a slight resistance may give a lightning surge an alternative path to ground through your "stuff."</p>
|
||||
<p><b>Towers and Coax</b>: Some studies have shown that the bulk of lightning current travels in the shield of the coax. A considerable voltage difference can exist between a tower leg and the coax braid if the first connection between the two is at the bottom of the tower. You may want to consider breaking your coax near the top of the tower, having one section to the antenna with the flex loop to accommodate rotation and another section down the leg of the tower. Securely fasten a plat to a tower leg and use a double female connector with chassis nuts (often called a bulkhead connector) to join the two coax lengths. This gives you a connection between the coax braid and the tower leg. This is not the only ground you should use, but an extra one.</p>
|
||||
<p><b>Total Disconnection</b>: During violent electrical storms, surges can enter the station equipment from several sources: the antenna lines, the AC power lines, and ground. When dealing with surges, do not think in terms of complete electrical paths. A surge can charge components, cases, and chassis, and create high voltages across components that are designed for low voltages. Hence, during violent storms, do a complete disconnect.</p>
|
||||
<p>Outdoors, disconnect all antenna feed lines and reconnect them to a ground rod in the station ground system. Indoors, disconnect antenna feedlines from the station equipment. Unplug all equipment--or that power strip you use as a master switch. Most power switches are single pole, so one must unplug in order to break the neutral and ground line paths along which surges can travel. Also disconnect the station equipment from the master ground lead to the outdoor ground rod system. If you do some careful planning, you can make all these moves in under a minute. Just be sure that all connections are accessible and require no tools to connection and disconnection.</p>
|
||||
<p>While it may be true that a direct strike is rare, most damage occurs from nearby strikes that place heavy voltage surges on power lines, antennas, and the ground. You can do a lot to keep them outdoors, where they belong.</p>
|
||||
<p><b>Towers</b>: Towers, whether free standing or on the roof top, require some special thought. Too few hams actually study tower installation before putting up the first one. Here are some things to think about:</p>
|
||||
<p><i>1. What are the requirements for the tower base, including the concrete, rebar, excavation, etc.?</i> Never under-support a tower at its base.</p>
|
||||
<p><i>2. What, if any, are the guying requirements?</i> If a tower is a guyed model, install the guying system to at least the manufacturer's standards. Be certain that guys are correctly and adequately anchored--and that they do not present a hazard to those who use your yard. If the tower can use a building support, be sure the building is up to the job. Most roof-line facia boards on a house are not.</p>
|
||||
<p><i>3. Where does the tower go?</i> Place it where it cannot hurt neighboring property (or people) if the tower and antenna fall under the worst conditions imaginable. Also try to place it where it will not hurt your own home and family if it falls.</p>
|
||||
<p><i>4. Where do the feedlines go?</i> Be sure that feedlines do not create either an electrical or a physical hazard for family members or visitors.</p>
|
||||
<p><i>5. What are the codes?</i> The more urban your setting, the more you may be subject to codes and ordinances that require permits, special requirements (for example, conduit for control voltages over a certain value), inspections, and, in some cases, licensed installers. Do not under any conditions bypass these requirements. Investigate in advance to know what you have to do and what it will add to the cost of the installation.</p>
|
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<p><i>6. What about intruders?</i> Towers are in some places classified as attractive dangers, which is what they might be to a neighborhood child with an urge to climb. Hollering is a deterrent, but not protection. Consider fencing in or cladding the lower tower sections so that the tower cannot be climbed.</p>
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<p><i>7. Are any of the tower conveniences dangerous?</i> Crank-up and tilt-over towers are convenient, but usually rely on cables, pulleys, and gearing to go up, come down, and stay in position. Analyze the stresses on these auxiliary parts to ensure that they can safely handle the loads. Develop a maintenance schedule for inspecting and for replacing cables before their lifetime is ended. If they die on the job, disaster often results.</p>
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||||
<p><b>RF Safety</b>: RF hazards come in two varieties. One is radiation. Current FCC regulations provide standards for safe amounts. Use the worksheets and analyze your station, even if you think you may be exempt by virtue of power levels and spacings. Be sure.</p>
|
||||
<p>A second hazard comes from direct contact with antenna elements carrying RF currents. Ground mounted verticals and inverted Vee ends provide the greatest hazards, since both children and adults can come into contact with them--either directly or with some implement having a long metal handle.</p>
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<p>Isolate ground-mounted verticals with fencing or other barriers which are effective especially in preventing children from touching a potentially active antenna. A vertical in or adjacent to a children's play area may mean station silence during periods when children are present: if contact is not a danger, radiation may be.</p>
|
||||
<p>Elevate and insulate horizontal antenna ends above the level where anyone can touch them with a metal rod. Never underestimate the ingenuity of a child with a small mean streak or a large curiosity.</p>
|
||||
<p><b>Insurance</b>: Carry all you can afford that may be relevant to hazards that antennas may present. However, never let the existence of insurance be a substitute for the best possible practice in the installation and maintenance of antennas and their supports. The lives you preserve may include your own and those of your loved ones.</p>
|
||||
<p>This is not a complete look at all the facets of antenna safety. We have not even reminded you to keep your antennas and supports well away from power lines. But perhaps we have said enough to prompt you to do a periodic inspection of your own safety measures--and perhaps some reading into the handbooks and other literature. The three key words are these: protect, divert, and prevent. They are three ways of saying that you care enough to do the very best.</p>
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<br>
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<p><i>Updated 3-2-99. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i></p>
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<p><a href="anta.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to An-Ten-Ten-nas Page</a></p>
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<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10">No. 24: Three Ways to Skin a Quad Loop <img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>A single quad loop makes a compact and effective bi-directional array for 10 meters. It has somewhat more gain than a dipole, and most users note that it is quiet, that is, not as susceptible to local noise as an open-ended dipole. With some sort of supporting system, the loop is a good choice for many ham back yards.</p>
|
||||
<p>A quad loop does its work with the antenna in the vertical plane, like a giant fly swatter. Maximum radiation is off the two broad surfaces and is minimum off the edges.</p>
|
||||
<p>Now comes the hard part: deciding what kind of loop to use. There are at least three versions, each with advantages and disadvantages. Let's look at them in order of complexity.</p>
|
||||
<h3>The Standard Square Loop</h3>The first sketch (<b>Fig. 1</b>) shows a standard quad loop made from #12 AWG copper wire. (If you use a different size wire, you may have to change the dimensions just a bit for resonance.) This antenna is a proven performer, with about 3.3 dBi free space gain, which translates into about 8.3 dBi gain at 1 wavelength above ground for the bottom wire (about 35'). The elevation angle of maximum radiation is about 19 degrees, which provides access to low-angle incoming DX signals.
|
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<img src="ant24-qloop1.gif" width="356" height="378" border="2" vspace="5" hspace="9">
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<p>The feedpoint impedance is about 125 Ohms at the 28.5 MHz design resonance frequency. A quarter wavelength section of 75-Ohm coax (about 5.7' of standard RG-59 with a velocity factor of 0.66) will provide a very low-loss match for the 50-Ohm coax to the shack and provide less than 2:1 SWR over all of the first MHz of 10 meters, plus a little. With this set-up, the antenna has the broadest operating bandwidth of all of the loops we shall examine.</p>
|
||||
<p>The old standard way of making a quad loop is to use criss- cross spreaders of bamboo, fiberglass, or--more recently--PVC. However, there are no rules that forbid you from stretching the quad loop from its corners to trees or other vertical supports. You can also use tubular horizontal members and wires vertical sides, although you may have to adjust the dimensions--most likely to enlarge them a bit.</p>
|
||||
<h3>The Elongated Loop</h3>In July, 1996, K6STI wrote in QST of an old idea: by elongating the quad loop we can achieve a little more gain and, at the same time, bring the feedpoint impedance close to 50 Ohms for a convenient match with our standard 50-Ohm coaxial cables.
|
||||
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<img src="ant24-qloop2.gif" width="295" height="508" border="2" vspace="5" hspace="9">
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<p><b>Fig. 2</b> shows the dimensions of a #12 AWG copper wire loop meeting these goals. The feedpoint impedance is almost exactly 50 Ohms at 28.5 MHz. However, the 2:1 SWR operating bandwidth is only about 800 kHz, somewhat narrower than the standard loop with a matching section attached.</p>
|
||||
<p>The gain of the loop with the bottom wire at about 35' is 8.9 dBi (4.2 dBi in free space), and the taller assembly lowers the take- off angle to 17 degrees. Both the gain and the lower angle of maximum radiation contribute a little extra to our DXing efforts.</p>
|
||||
<p>Most likely, you would want to build a fixed version of this kind of loop by supporting the wire from its corners by ropes running to adjacent supports. As an alternative, you can build a rotatable version by using tubular horizontals and wire vertical sides (again, with dimensional adjustments that owe to the fat horizontal elements) attached to (but insulated from) a center mast. With a height about 3' taller than the normal quad loop, support requires a bit more work than the standard loop. Yet, if the top of this loop and the top of the standard loop are level with each other, this elongated loop loses some of its advantages in gain and lowered take-off angle.</p>
|
||||
<h3>The Hentenna</h3>The Hentenna is an invention of Japanese hams (and "hen" means "what is it?"--or so I am told). It consists of the full wavelength upper loop with a secondary lower loop that allows a close match to 50-Ohm coaxial cable. See <b>Fig. 3</b>.
|
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<img src="ant24-qloop3.gif" width="297" height="544" border="2" vspace="5" hspace="9">
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<p>The gain of this antenna, if the bottom wire is at the same level as the other two loops (1 wavelength or 35'), is about 9.8 dB (5.0 dBi in free space), with a take-off angle of 15 degrees, making it a good DX antenna among loops. However, its performance depends very much on the added height of the upper wire, which is nearly 19' from the bottom. The antenna is 60% as wide but more than twice as tall as the standard square loop. If we lower the top wire to parallel it with the top wires of the other two loop designs, the hentenna turns out to be only a little better than they are.</p>
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<p>The operating 2:1 SWR bandwidth is the narrowest of the three loops, about 600 kHz or a little over half of the first MHz on 10 meters with the design frequency of 28.5 MHz used here. Like the other loops, construction can be all wire, with the corners attached by thin (UV-resistant) rope to supports. Or, you can once more use larger diameter upper and lower horizontal members with wires sides and a wire feedpoint element for a rotatable antenna.</p>
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<h3>Pattern Comparison</h3>To give you a better idea of what to expect from each antenna, here are two elevation patterns, each of which contains patterns for all three loops.
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<p><b>Fig. 4</b> compares the antennas using a common bottom horizontal wire height of 20'. This arrangement places the elongated loop top wire above that of the square loop, and the hentenna top wire above both the others. The advantage in gain and lowered angle of radiation for the larger kloops is clear.</p>
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<p><b>Fig. 5</b> reverses the procedure and places the top wires of all 3 antennas at 40' up. For many installations, top height is more absolute than bottom height. In this configuration, all three antennas have comparable TO angles, with only small gain advantages as the loops grow larger.</p>
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<img src="ant24-qloop5.gif" width="637" height="267" border="2" vspace="5" hspace="9">
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<p>Whichever loop you choose, assuming that a loop fits your operating needs, give your best ingenuity to construction. If at all possible, figure out how to make the antenna free standing so that you can rotate it by hand (if not by a TV rotator). You will need to turn at most less than a half turn, since the antenna is bidirectional. All of the loops have very deep side nulls on a plane with the wires, and just these nulls alone can get rid of more than half the QRM that might get in the way of your QSOs.</p>
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<p>Loops are also handy in contests, where you really do want to hear what is happening in most directions. You never know in advance from where your next contact will come. If you build the loop to be collapsible, you can set it up for Field Day and other hilltopping exercises.</p>
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<p>The basic quad loop is a versatile antenna that lends itself to many construction techniques. If you want a little more performance than a dipole can give and you think it is fun to have a fly swatter waving in the breeze above your QTH, then one of these three designs may be the next antenna to build.</p>
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<p><i>Updated 7-20-99. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i></p>
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<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10">No. 25: The 10-Meter L-Antenna <img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>A large number of 10-10ers are just getting started. They are unsure of whether to go with a vertically polarized antenna or a horizontally polarized antenna. They have heard that cross polarization reduces signal strength by some amount from 3 dB on upward. They do not yet know what the prevalent polarization is in the local area. They do not want to put a lot of money into their first antenna, but they would like to enjoy 10 meters for both skip and local contacts. They may also have some severe space restrictions. What's a person to do?</p>
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<p>There is a simple and cheap solution. But first, a little background. For skip paths, polarization makes no difference almost all of the time. The ionosphere skews polarization. Highly elevated beams have an advantage, but simple antennas perform well on 10 meters, whether vertical or horizontal.</p>
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<p>Polarization makes the greatest difference with local area contacts, where line of sight is the general rule. Some of the gang run verticals, some horizontals. How to get started before one discovers which types of operating are the most fun is the tough question.</p>
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<p>Why not make a simple L antenna out of hardware store supplies. Suppose you have a typical rooftop abut 25' up, and a chimney or similar solid mounting point. Less than 10' of aluminum tubing and less than 10' of copper or aluminum wire, along with some mounting hardware to keep the parts solidly mounted but insulated, will produce a nice compromise antenna.</p>
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<p>The vertical section consists of 2 pieces of tubing in the 3/4" diameter neighborhood. The tubing is self-supporting once you fix the base area solidly. The horizontal portion is wire, perhaps running along the roof ridge, elevated at least a few inches and more if you can manage it. Dimensions are not too critical. You can adjust the wire horizontal part by pruning the far end. And you can adjust the vertical part by sliding one section of tubing inside the other before clamping.</p>
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<p>Adjusting the antenna for minimum SWR will do just fine. The modeled feedpoint impedance is about 40-45 ohms, so it makes a good match for RG-58, RG-8, RG-8X, or RG-213 coax. I highly recommend a choke balun, such as the W2DU ferrite- over-coax designs, at the feedpoint to minimize RF on the outer braid of the feedline.</p>
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<p>What do you get for your $15 investment? A pretty good local antenna with both vertical and horizontal polarization. The patterns above assume the antenna is mounted atop the typical 25'-high rooftop crest. As the patterns show, the vertical part produces a vertically polarized signal (and receives the same) that is almost as strong (or sensitive) as a pure vertical. The horizontal wire produces a horizontally polarized signal (and receives the same) about half as good as a full size dipole. The result is a low-angle total pattern shaped like a kidney bean that will handle signals, whatever their polarization.</p>
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<p>By using light tubing for the horizontal piece, with perhaps a collapsible AM radio whip as the end section, you can make up a dandy portable or hill-topper antenna. A few nesting sections of PVC along with antenna pieces that also nest when not in use will make an antenna that takes almost no room in the car trunk. 5-10 minutes work, and you are on the air from your local scenic mountain or picnic area.</p>
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<p>To get a better idea of the antenna's operation, think of it as an inverted Vee that has been rotated until one leg points straight up and the other is parallel to the ground. Unlike the Vee, this antenna is a good candidate for a rooftop or an attic.</p>
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<p>In fact, if you must have an attic antenna, there is nothing wrong with using a Vee, either right-side-up or inverted, for getting a little polarization diversity to catch all the locals. Use 45-degree angles to maximize both types of polarization in the best compromise. If the L will not fit your attic vertically, then perhaps a Vee will fit the space.</p>
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<p>For outdoor use, the L is likely a better mechanical choice, since you need only one solid mount and a secondary mount to keep the horizontal wire horizontal. The antenna has a minimal profile, and you can even put a PVC tube over the vertical tubing and fly a flag from it.</p>
|
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<p>As with all elevated metal structures, be sure you have fat wire to a ground rod to bleed off charges built up by the weather. A 100 mH RF choke across the feedpoint terminals would not hurt either. Always build antennas with safety in mind.</p>
|
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<p>So, if you are undecided about vertical vs. horizontal polarization, if you do not want to put a lot of money into an antenna while finding out which you prefer, or if you just need a good, cheap local antenna for 10 (that will also do quite well with skip), then the L may be right for you. When you finally invest in your long term antenna, you may want to keep the L in place as a back-up or for emergencies.</p>
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<p><i>Updated 10-20-99. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i></p>
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<p><a href="anta.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to An-Ten-Ten-nas Page</a></p>
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<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10">No. 26: When Should I Use a Vertical on 10? <img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>Most of the antenna we have discussed in this column have been horizontally polarized. There are some good reasons for this fact. First, 10-meter horizontal antennas are fairly compact, with a half wavelength being about 16-17' long. Second, the shortness of a wavelength on 10 meters (35') generally simplifies the process of supporting a horizontal 10-meter antenna at a good height (at least 1/2 wavelength, with over 1 wavelength preferred for best performance). Third, even 3-element 10-meter Yagis are fairly light-weight, for easy support, even in field or hilltop operations.</p>
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<p>Nevertheless, there are some good reasons for using a vertically polarized antenna on 10. Although the gain of such antennas may not usually compete with a well-installed horizontal antenna of the same size, this factor is rarely a problem when the band is open. So let's look at the question of when to use a vertical.</p>
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<p>1. <b>Mobile in Motion</b>: The standard these days for mobile-in-motion operation is the short, center-loaded, magnetic mount vertical set on the car roof. Although the least efficient of almost any antenna used on 10, these antennas acquit themselves well. Full size 1/4 wavelength whips have gone out of vogue, especially with the increased use of plastics in autos. When auto bodies themselves become universally plastic or fiberglass, we may have to rethink the center-loaded mag-mount vertical for mobile operation.</p>
|
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<p>2. <b>Lunch-Time Operation</b>: With small rigs, short antennas, and an open band around noon, 10-meter lunchers are more numerous than we imagine. Since the lunch hour (or half-hour) is all too brief, operators want a system that wastes no time in set-up and take-down. The vertical--again, usually a mag-mount antenna in the parking lot--fills the bill.</p>
|
||||
<p>3. <b>Local Convention</b>: In some towns and cities, most of the locals may use vertical antennas. Sometimes, this represents a lot of mobile work; sometimes it represents former citizen's band operators who have joined the amateur ranks and cut down their old antennas to resonate on the higher frequencies. Since local work is mostly point-to-point, as in VHF operation, cross- polarized antennas result in major losses in signal strength. So if the local group is mostly vertical, then it will pay you to have a vertical at home (as well as on the car) to join the fun full strength.</p>
|
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<p>Since the path through the ionosphere generally skews signal polarization, distant stations will not suffer from being cross polarized relative to your antenna.</p>
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<p>4. <b>Lack of Space</b>: Many hams live in homes without large yard. So space for antennas must compete for space with play equipment, patio furniture, and flower gardens. A vertical may be the only antenna type the home owner can erect.</p>
|
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<p>The question here is not whether to use a vertical, but what kind of vertical to use. There are a number of multi-band verticals now on the market that will open many of the ham bands. They come in two major types.</p>
|
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<p>If the roof top is the mounting area of choice, then one of the 1/4 wavelength trap verticals may be best. The heaviest part of the antenna is mounted near the roof top or chimney mounting system for maximum support. The necessary radials, installed according to the antenna makers instructions, can run along the roof top. If the antenna is at the end of the house, radials in the open direction can be run to trees or fence post, well out of reach of children or adults.</p>
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<p>Where space is too restricted for an elevated radial system, one of the half wavelength verticals may be more fitting. Some demand an elevated mounting point and may rest well on top of a fence post, short flag pole, or even a mast attached to a deck post. Other models call for ground mounting and can be placed in the most clear usable place in the yard with buried coax.</p>
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<p>In all such installations, safety to children, family members, visitors, and neighbors is a top requirement. These antennas are rarely large enough to cause damage to neighboring property if they fall. Of course, they should be well clear of any utility lines crossing the yard. Finally, they should be isolated so that no one can get an RF burn by touching the antenna while in use. For some models, we achieve this last safety measure by elevating the antenna above reach, even by fence-climbers. Ground-mounted models require some extra thought. Setting up a flower bed and small fence around the antenna can keep most folks away. Sheathing the lower portion of the antenna in large-diameter black plastic down-spout drainage pipe for about 8' up is quite effective in preventing children from touching the antenna and has been found not to adversely affect performance. The protective sheathe can be attractively painted (with non-metallic paint) to call attention away from the antenna. Whatever the safety measures we take, we should also insure that they meet FCC requirements regarding RF exposure to other people.</p>
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<p><img src="ant26-1.gif" align="left" width="248" height="530" border="2" vspace="5" hspace="9"> Even hams with room for a host of horizontally polarized antennas may wish to consider installing one of these multiband verticals. They make good (even if not great) low-band antennas, provide back-up service in case the main beams collapse in high winds or ice, and allow the operator to match the polarization of locals using mobile whips or other vertical antennas. so even if you can afford the highest, the biggest, and the best, one of these simpler antennas makes good sense as part of the antenna farm.</p>
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<p>5. <b>Home Brewers</b>: Some of us like to build antennas. Some of us have to build antennas to save the cost of commercial versions. Whatever the reason, a vertical dipole for mounting at least 20 to 25 feet up at the center on a non-conducting mast is a good starter project. I suggest a vertical dipole, since it saves a lot of grief over where to run the radials for a quarter- wavelength ground-plane model. The vertical dipole also takes less space than a horizontal dipole and requires no turning for maximum signal.</p>
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<p>You can construct a vertical dipole from hardware store materials: aluminum rod or tubing (a little over 8'), PVC, and wood are the main ingredients. <b>Figure 1</b> shows in bare outline a vertical dipole I once used to capture Worked All Continents in about an hour at the height of a long-ago sunspot cycle. The 4x4 fencepost was the main support, with underground bracing from bagged concrete. The side rail 10' 2x4s supported a good quality 2x4 mast, with the 4" side running between the rails. Two long galvanized bolts braced the mast. Removing the lower bolt permitted tilt-over operation.</p>
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<p>The antenna itself began with an 8' length of aluminum tubing for the top extension. The lower part of the antenna consisted of insulated #12 house wire, purposely cut long. I tuned the antenna to frequency by trimming the lower wire for minimum SWR. Many local hams seemed initially horrified by the idea of a dipole made from unequal diameter elements and trimming only one end. They thought that terrible things would happen to performance, since the antenna was obviously as unbalanced as its builder.</p>
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||||
<p>Actually, virtually nothing happens except for a bit of building and adjusting convenience. Half wavelength antennas lose nothing in performance by being fed slightly (or even radically) off-center. The feedpoint impedance does not begin to change noticeably until the feedpoint is well off center. The only precaution was for safety: the dipole end is a high-voltage point on the antenna, so it had to be inaccessible to human touch when in operation.</p>
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<p>There you have it: some good reasons for using vertical antennas on 10 meters, whether they are commercial multi-band antennas or home brew specials. There are other reasons of a specialized nature that we could add. For example, if you live by the seaside, expect an exceptional increase in performance over the same antenna placed on a rocky hillside in the Smoky Mountains. Verticals have proven to be more than good enough in some island contesting locations. Some operators even prefer the wider beamwidth of a vertically oriented Yagi to one that is horizontal. Whatever the reasons, vertical have and will always have an important place in 10-meter operation, even if we never mention FM and repeaters at all (which I just did).</p>
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<p><i>Updated 1-12-2000. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i></p>
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<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10">No. 27: A Compact Aluminum Moxon Rectangle <img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>I often receive inquiries from folks who cannot quite support the width of a 10-meter Yagi (either 2 or 3 elements) because obstructions give them less than the 16.5' needed. Is there an antenna with decent performance that will fit in a space about 12-13' wide? If it can be home built to save money and require no fancy tuning or matching system, so much the better.</p>
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||||
<p>In fact, there is an antenna that fits this category almost perfectly. Imagine an antenna with the gain of a 2-element Yagi (6 dBi), the front-to-back ratio of a 3-element Yagi (>20 dB from 28.3 to 28.5 MHz), and an SWR of below 2:1 from one end of 10 to the other. In fact, imagine that the antenna has better than 15 dB front-to-back ratio all the way down to 28 MHz and still has about 12 dB front-to-back ratio at 29.7 MHz. (All figures are free space modeling estimates.) Imagine also that the antenna can be directly connected to 50-ohm coax with no matching system whatsoever (even though I always recommend a 1:1 choke balun). Imagine also that you can make it yourself from hardware store materials, that it will weigh about 10-15 pounds including the boom, and that you can make it in your garage with no special tools. Imagine also that when it is done, you will still have change from a $50 bill.</p>
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<p>The antenna is the Moxon rectangle. Past versions that I built using wire elements required lots of PVC to support them. However, if we translate the antenna into hardware store aluminum tubing, we can build a 1-boom version. Now peak at the sketch of the pieces in <b>Fig. 1</b>. Some 7/8" and 3/4" diameter aluminum tubing form the main elements, with 3/4" tubing for the side elements. The corners can use radius-bent tubing or be squared by making some corner supports from L-stock. The combination of 7/8" and 3/4" aluminum tubing lets you telescope the ends into the center for a precise fit or a center frequency adjustment.</p>
|
||||
<p>Since the end spacing and alignment is critical to make the antenna give its full performance, you can slide a piece of CPVC or similar lightweight, durable tubing either inside the ends or over the ends and lock them in place with sheet metal screws. The rigid spacer is also a good idea to limit the twisting force placed on the curved or right-angle corners. Sheet metal screws also connect the 3/4" and 7/8" tubing together. Be sure that all hardware screws are stainless steel.</p>
|
||||
<p>For element-to-boom plates, you can use any durable material. Spar varnished 3/8" plywood or 1/4" LE plastic make good plates. About 3" by 9" (or longer) plates give ample room to U-bolt the elements to the plate and have room for U-bolts that go over the mast. As with all good antenna structure, let the elements hang under the boom. What boom? Well, almost anything, from 1-1/4" nominal diameter PVC to a good grade of aluminum tubing (thicker-wall than the usual 0.55" hardware store variety) to a 5' length of spar varnished 1.25" diameter closet rod. Make up a boom-to-mast plate similar to the boom-to-element plates, only a bit more square, and you are in business.</p>
|
||||
<p>The dimensions of the antenna in the drawing are too fussy, being direct translations of the computer model used to generate the antenna. Just try to keep the dimensions within about 1/4" of the drawing, and no one will be able to tell any difference in performance. Squaring the corners or missing the dimensions by a half inch will shift the performance centers by about 100 kHz at most. In most cases, you will not be aware of any difference at all.</p>
|
||||
<p>Note that the antenna is just about 12.5' wide and under 5' front-to-back, for a turning radius of about 6'8" or so. Strapped up on the side of the house, the antenna is unlikely to overhang the neighbor's yard line. The antenna is light enough for hand rotating, but an old TV rotator might come in handy. Because of the antenna's characteristics, you may not need to rotate it much.</p>
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<p>The pattern figure shows the optimal front-to-back ratio in azimuth patterns 20' and 35' antenna heights. Note the very broad forward lobe that is almost a cardioid, giving reception and transmission as wide as your peripheral vision. Behind you is silence--or at least a large dose of silencing. Les Moxon, G6XN, uses a wire version of the antenna with both elements remotely tuned: that way he works the world just by electrically reversing front and rear elements with a fixed mounting. If you want to learn more about the Moxon rectangle, find the Spring, 1995, issue of <i>Communications Quarterly</i>, and look at pages 55-70 (<a href="../mu/mu2a.html">link</a>).</p>
|
||||
<p>It is unlikely that anyone will ever produce this beam commercially, since it is a monobander without the super gain that avid DXers and contesters crave. You can only get that kind of performance from many Yagi elements. However, you can build your own compact antenna with a pretty good chance of success on the first try. It will beat a fixed wire dipole or a vertical hands down. So if you need a compact 10-meter beam for your compact home site, then you might roll your own version of the aluminum Moxon rectangle.</p>
|
||||
<p>For further details on the construction of this antenna, see <i>The ARRL Antenna Compendium</i>, Vol. 6, pp. 10-13.</p>
|
||||
<p>See the <a href="../moxon/moxpage.html"><b>Moxon Rectangles and Online Calculator</b></a> page for more information.</p>
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<p><i>Updated 3-12-2000. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i></p>
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<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10">No. 28: Beta Coils and Hairpins <img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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||||
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<h3>L. B. Cebik, W4RNL</h3>
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<p>In a number of columns, I have mentioned the beta match. Let's take a brief look at what it is and what it does.</p>
|
||||
<p>The beta match appears to be simply a small coil or hairpin placed across the terminals of an antenna, most often a Yagi. Some folks mistake the coil for an RF choke, while others mistake the hairpin for a short circuit.</p>
|
||||
<p>Actually, the beta coil or hairpin is one part of an impedance matching circuit, where the remaining elements are invisible, if you do not know what to look for. Many Yagi antennas have feedpoint impedances in the 20 to 35 Ohm range, somewhat low for feeding directly with coaxial cable. We need to raise the impedance to 50 Ohms--and that is what the beta match system does.</p>
|
||||
<p>The coil is not the only element in the circuit. There is also a capacitor--or, more correctly, some capacitive reactance. We get that part of the circuit from the antenna element itself.</p>
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<img src="ant28-1.gif" width="530" height="371" border="2" vspace="5" hspace="9">
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<p><b>Fig. 28-1</b> shows how we move from a resonant driven element to a beta match. Let the resonant antenna impedance be low, say about 25 Ohms. If we shorten the element, the resistance does not change significantly, but the antenna becomes capacitively reactive, as the middle part of the figure shows.</p>
|
||||
<p>If we shorten the element by the right amount, we get the right capacitive reactance in series with the antenna resistance to go together with an inductance across the coil to make an L-circuit. An L-circuit is one of the fundamental impedance transformation circuits, and in this case, -Xa and XL together change the 25- Ohm antenna resistance to 50 Ohms.</p>
|
||||
<p>We can calculate the needed values if we know the antenna feedpoint resistance (Ra). (We know the coax has a characteristic impedance (Ro) of 50 Ohms.) First we calculate a value called "delta" by some and "working Q" by others. Delta = the square root of [(Ro/Ra)-1]. Now we can easily calculate the necessary values of capacitive reactance in the antenna (- Xa) and of inductive reactance to place across the terminals (XL). Xa = Delta times Ra. XL = Ro / Delta.</p>
|
||||
<p>Since these values are given as reactances, you need to convert the inductive reactance into a component value. The capacitive reactance will be developed by simply shortening the antenna element until the beta match gives us 50 Ohms.</p>
|
||||
<p>For reference, here is a small table of values we commonly encounter with beta matches with 50-Ohm coax for various values of antenna feedpoint resistance (Ra):</p>
|
||||
<pre>Ra 33 25 17
|
||||
Delta 0.7 1.0 1.4
|
||||
Xa 23.6 25.0 23.6
|
||||
XL 70.7 50 35.4</pre>
|
||||
<p>Notice that the capacitive reactance reaches a peak when delta = 1, while the inductive reactance gets smaller as the feedpoint resistance gets smaller.</p>
|
||||
<p>We have not yet converted these inductive reactances (XL) into a component value, because there are two distinct ways to achieve the required reactance across the coil. <b>Fig. 28-2</b> below shows them both:</p>
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<img src="ant28-2.gif" width="427" height="356" border="2" vspace="5" hspace="9">
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<p>The beta inductor is simply a coil with the value of inductance that provides the inductive reactance at the operating frequency. If you divide the required inductive reactance by the product of the operating frequency (in Hz) and twice pi, you get the right inductance.</p>
|
||||
<p>The hair pin version of the beta inductor is actually a small shorted stub of parallel transmission line. Rather than go through the calculation procedure, I shall simply once more recommend that you obtain a recent copy of <a href="../radio.html#HAMCALC">HAMCALC</a>, a suite of handy ham calculation programs in GW Basic. Among the selections on the disk is an excellent program that will calculate the dimensions of a hairpin for the match. It was written by Thomas Cefalo, Jr., WA1SPI. The program will also tell you the equivalent inductance in case you want to wind a coil. Other programs in HAMCALC will help you wind an accurate coil.</p>
|
||||
<p>Many antenna builders use the experimental technique of adjusting the driven element for a beta match. After calculating the beta coil or hair pin, they install it and then adjust the element length for a low SWR. Antenna modelers tend to determine the required element length in advance from their software and save some time fumbling for the right element length.</p>
|
||||
<p>Either way, the beta match results in a very low loss match. For inductor Qs over 100 (easy to obtain, but some maintenance is required to maintain the Q), losses will be well under 1%--and even less for the hairpin.</p>
|
||||
<p>If you like to build antennas, you should become familiar with the beta match. Some folks actually avoid the beta match because it is "too simple to really work." However, it does work, and very well indeed for antennas with moderately low feedpoint impedances. Since there are easy-to-use utility programs for calculating everything you need, there is no need to avoid either the beta match or antennas that require one.</p>
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<p><i>Updated 7-1-2000. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i></p>
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<p><a href="anta.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to An-Ten-Ten-nas Page</a></p>
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<title>Some Larger 10-Meter Yagi Designs</title>
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<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10">No. 29: Some Larger 10-Meter Yagi Designs <img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>Occasionally, I am asked to recommend some larger Yagi designs that one might build for 10 meters. I can do little better than recommend the designs by Dean Straw, N6BV, that appear in the K6STI, Brian Beezley, program, YA, which has been distributed with The ARRL Antenna Book. In past columns, we have noted other designs originating from YA/YO. We have also featured various 2-element and 3-element Yagis of interest.</p>
|
||||
<p>The present designs use 4, 5, and 6 elements. Each has been optimized for stable gain, front-to-back ratio, and impedance across the first MHz of the band. I have cross-checked each of them on other antenna modeling programs to confirm the numbers, and all appear to be very promising designs for the home builder.</p>
|
||||
<p>The gain of a Yagi depends more on the boom length than on the raw number of elements. Therefore, each design will use a longer boom in conjunction with the increasing number of elements to achieve its objectives. Merely adding more elements within the same existing boom length will rarely produce any significant additional gain.</p>
|
||||
<p>The element listings use a combination of 5/8" (0.625") and 1/2" (0.5") diameter tubing. Only one side of the antenna, relative to the boom, is listed, with the other side being a mirror image. Both the outer segment length and the total half-element length is listed for convenience. The smaller diameter element sections should be at least 3" longer than the lengths listed for insertion into the larger diameter element sections. All dimensions will be in inches. The decimals in the tables correspond the 1/8" increments of length.</p>
|
||||
<p>The element assemblies are considered medium duty for maximum winds just over 90 miles per hour. For structural details of large Yagi construction, consult one or more of the many handbooks on antenna building. These beams are not casual projects, since they represent a considerable outlay for materials and result in large structures. Their weight may require a larger rotator and other improvements to your tower. Even the smallest of them should not be mounted on something so light as a telescoping mast.</p>
|
||||
<pre>4-Element, 14' Boom Yagi
|
||||
|
||||
Distance from Length of Length of Total (1/2)
|
||||
Reflector 0.625" 0.500" Element Length
|
||||
0.000 36.000 70.000 106.000
|
||||
36.000 36.000 63.875 99.875
|
||||
72.000 36.000 62.250 98.250
|
||||
162.000 36.000 53.125 89.125</pre>
|
||||
<p>The mid-band free-space gain of this antenna is about 8.4 dBi, with an excellent (greater than 20 dB) front-to-back ratio.</p>
|
||||
<pre>5-Element, 20' Boom Yagi
|
||||
|
||||
Distance from Length of Length of Total (1/2)
|
||||
Reflector 0.625" 0.500" Element Length
|
||||
0.000 36.000 71.375 107.375
|
||||
36.000 36.000 63.375 99.375
|
||||
72.000 36.000 62.750 98.750
|
||||
140.000 36.000 61.500 97.500
|
||||
234.000 36.000 56.375 92.375</pre>
|
||||
<p>The mid-band free-space gain of this antenna is about 9.7 dBi, with an excellent (greater than 20 dB) front-to-back ratio.</p>
|
||||
<pre>6-Element, 36' Boom Yagi
|
||||
|
||||
Distance from Length of Length of Total (1/2)
|
||||
Reflector 0.625" 0.500" Element Length
|
||||
0.000 36.000 70.875 106.875
|
||||
37.000 36.000 62.875 98.875
|
||||
80.000 36.000 62.375 98.375
|
||||
178.000 36.000 60.125 96.125
|
||||
305.000 36.000 59.250 95.250
|
||||
426.000 36.000 55.250 91.250</pre>
|
||||
<p>The mid-band free-space gain of this antenna is about 11.6 dBi, with an excellent (greater than 20 dB) front-to-back ratio.</p>
|
||||
<p>All of these antennas have feedpoint impedances that are designed for a beta or similar matching system. The resistive part of the impedance is in the low-20s, with a comparable amount of capacitive reactance. You may lengthen the driven elements to resonance without adversely affecting antenna performance. A resonant driven element of about 25 Ohms can be matched to a 50-Ohm coaxial cable with a quarter wavelength section of 35-Ohm coaxial cable. Such cable can be purchased for about $3.00 per foot. You may also fabricate a satisfactory line using parallel lengths of 75-Ohm cable with the braids and the center conductors each soldered together at each end.</p>
|
||||
<p>The smallest of these monoband 10-meter beams will generally outperform the 10-meter section of all but the very largest tri- banders. The longest of the beams is a serious DX-hunting machine. To obtain the maximum performance from any of these beams, place them at least 1 wavelength above ground and preferably closer to 2 wavelengths.</p>
|
||||
<p>Installing large beams at considerable heights above ground is not a casual process. First, there may be zoning and other legal restrictions or permissions to consider. Second, proper tower installation may cost many times more than the materials in the beam. Safety for yourself, your family, your property, and your neighbor's property are of paramount importance. So too is durability of the performance of the whole system, including the tower, coax, rotator, guys, grounding system, and the antenna.</p>
|
||||
<p>Before moving to large beams and complex tower installations, learn all that you can about every aspect of the task. If you have any doubts, consult professional tower installers for answers and for help. Do not violate or ignore applicable laws and ordinances. Do not settle for any installation that does not measure up to good engineering standards. And, for heaven's sake, do not rely on luck to keep your antenna in the air and lightning in someone else's yard.</p>
|
||||
<p>As you can see, a big antenna--even for 10 meters--is only the beginning and not the end of a much larger enterprise. If you have all the other pieces in place, then one of the N6BV designs may be the answer to the antenna part of the puzzle. Other designs, some with feedpoint impedances in the 35 to 50 Ohm range are also available from various ham magazines. Collect lots of designs and ideas before you start cutting aluminum. The more design articles you read, the more you will learn about the art of building big beams.</p>
|
||||
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||||
<br>
|
||||
<p><i>Updated 10-1-2000. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i></p>
|
||||
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||||
</center>
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||||
<p><a href="anta.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to An-Ten-Ten-nas Page</a></p>
|
||||
<p><a href="../radio.html"><img src="../images/rtnhome.gif" align="bottom" width="40" height="40"> Return to Amateur Radio Page</a></p>
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<title>Where Does the Gain Come From?</title>
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<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10">No. 3: Where Does the Gain Come From?<img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>Earlier, we looked at 3 antennas modeled in free space: a dipole, a 2- element Yagi, and a 3-element Yagi. All were made from aluminum tubing and all were monoband antennas with no traps or other lossy components. The patterns we showed made it clear that the 3-element Yagi had the most gain and the highest front-to-back ratio. But that does not make it very clear from where the gain comes, given a constant transmitter output.</p>
|
||||
<p>Fig. 1 repeats the azimuth patterns to refresh our memory. An azimuth pattern is like standing over the antenna and seeing its radiation. For free space patterns, we use 0ø elevation, since the pattern is strongest straight on.</p>
|
||||
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||||
<img src="ant1a1.gif" width="640" height="480" border="2" vspace="5" hspace="10">
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<p>The azimuth pattern does not give a whole picture of where the power is going. It does show that the beams project more power in the forward direction than the dipole, while "robbing" power from the rear. In reception terms, signals from straight ahead are strongest and weaker to the rear. However, they are weakest off the sides of the antennas.</p>
|
||||
<p>To accurately see the power pattern of an antenna, we must also look at it from the side in an elevation pattern. An elevation pattern is like standing beside the antenna and looking through the aluminum tubes, while seeing the radiation. Fig. 2 gives an elevation pattern in free space for the same 3 antennas (3-element, 2-element, and dipole). Notice the displacement of the beam antenna patterns to the right, which corresponds to the forward lobe of the beam antenna azimuth patterns.</p>
|
||||
<center>
|
||||
<img src="ant3a2.gif" width="640" height="480" border="2" vspace="5" hspace="10">
|
||||
</center>
|
||||
<p>Only an astronaut can have a pattern radiating below the antenna site. Antenna gain over real ground also makes use of radiation reflected from the ground. Fig. 3 shows elevation patterns for the three antennas over real ground at 35' feet, about 1 wavelength above ground. The azimuth patterns would resemble those of Fig. 1, but they would not be taken at horizon level. Instead, they would use the angle of maximum power, sometimes called the take-off angle. The elevation patterns display this angle clearly and show that horizontal antennas above a real ground have a number of higher angle lobes as well as the main lobe. The complex pattern results from adding and subtracting reflected radiation from the above- ground pattern, depending on its relative phase.</p>
|
||||
<center>
|
||||
<img src="ant3a3.gif" width="636" height="270" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Comparing Fig. 2 to Fig. 3 can lead to a misunderstanding, since the apparent radiation pattern is the same size; that is, the radiation just reaches the outer ring. However, that way of styling our drawings is just a convenience for making comparisons within each drawing. In actuality, the power in the lowest lobe of Fig. 3 for each type of antenna is much greater than the power in just the horizontal direction of Fig. 2. Expressed another way, all of the power represented by the spheres or near- spheres in Fig. 2 is flattened, thus putting greater power into the lobes that extend horizontally. The total power in the lobes is a result of adding reflections to the main or incident power going that way. The net result is gain over free space and gain of one antenna over another. And since the gain can be both calculated and controlled by good antenna design, we can--within limits--achieve a good bit more success than early radio pioneers in making and maintaining radio communications.<br>
|
||||
<br></p>
|
||||
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||||
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|
||||
<br>
|
||||
<p><i>Updated 3-17-97. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i><br>
|
||||
<br></p>
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||||
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||||
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|
||||
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|
||||
<br>
|
||||
<a href="anta.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to An-Ten-Ten-nas Page</a><br>
|
||||
<a href="../radio.html"><img src="../images/rtnhome.gif" align="bottom" width="40" height="40"> Return to Amateur Radio Page</a><br>
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<title>The V-Yagi</title>
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||||
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||||
<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10">No. 30: The V-Yagi <img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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||||
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||||
<center>
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||||
<h3>L. B. Cebik, W4RNL</h3>
|
||||
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||||
<p>In May, 1998, QST, NW3Z and WA3FET published an interesting 3-element beam for 40 meters, using one tubular element and two wire elements. The result was a light weight (for that band) beam with excellent SWR and F-B characteristics. It superficially resembled an old design by Dick bird, G4ZU, but had been optimized for performance resembling that of a Moxon rectangle: great F-B ratio, wide pattern forward, and direct coax feed. However, with an extra element, it had more gain. Why not adapt the design to 10 meter?</p>
|
||||
<center>
|
||||
<img src="ant30-1.gif" width="638" height="348" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>As Fig. 1 shows, the adapted antenna for 10 meters has decent gain and good front-to-back ratio across the first MHz of the band. The patterns are taken with the antenna modeled at 35' (1wl) up.</p>
|
||||
<center>
|
||||
<img src="ant30-2.gif" width="637" height="183" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The SWR curve strongly suggests that direct coax feed is certainly in order. As always, I recommend a choke balun at the feedpoint.</p>
|
||||
<p>Now that we have seen what the antenna can do, all we need to know is what the antenna looks like and how hard it might be to build. Like all Yagis, it will have significant side-to-side and front-to-back dimension. However, unlike the usual Yagis, only one of the elements will be made from aluminum tubing, while the other two will made from #14 AWG copper wire.</p>
|
||||
<center>
|
||||
<img src="ant30-3.gif" width="777" height="445" border="2" vspace="5" hspace="9">
|
||||
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|
||||
<p>As the sketch shows, the driven element is the longest part of the antenna. Overall, it is 197" long, with a insulated mounting plate to permit direct connection of the feedline to the element. The inner parts of the element are 48" lengths of 1" diameter hardware store aluminum, while the outer ends are made from 7/8" diameter tubing from the same source.</p>
|
||||
<p>The dashed lines crossing the driven element are booms to support the wire elements. They can be made from fiberglass or aluminum, with the center boom stronger, since it is about 12' long. The end support booms can be lighter, but need to support the wire ends. If the end booms are made from aluminum, they should be insulated from the driven element and set farther outward, so that the wire elements can terminate at an insulator. Simple UV resistant rope (3/16" diameter) connects the wire and end-support boom to the driven element ends, which helps prevent the wires from loading the end supports too much.</p>
|
||||
<p>The wires are each 94.4" long from the center boom to the end (at an insulator or at the support boom). At the ends, they should be between 30 and 31 inches from the driver. Since both wires are the same length, we need to load one of then to electrically lengthen it to become a reflector. A shorted transmission line stub about 65.55ø long will do the job, although you may want to adjust the exact length when tuning up the antenna. The length in degrees translates into 50.125" of RG- 58/RG-8 (velocity factor 0.66) line. You can use standard stub equations to calculate the length of 300-Ohm or 450-Ohm line as a substitute.</p>
|
||||
<p>The V-Yagi will not give all of the gain that a 12' boom is capable of giving if the elements were linear and fatter. In fact, this antenna is about 1 dB shy of maximum gain for the boom length. On the plus side of the ledger, the antenna is quite light, study, and has a smaller turn radius, since the corners are shortened by the slope of the director and reflector wires. A TV rotator should turn the beam with ease. However, you may not have to turn this beam as often as you might have to turn a standard Yagi, since the beam width is quite a bit wider.</p>
|
||||
<p>The V-wire for 10 meters is not for everyone, not even for every addict of home brew antennas. However, I have learned over the years that different folks have different needs, different skills, and access to different materials. So I never try to prejudge what mechanical designs are acceptable and which are not. That would limit folks to only my own level of construction ability. Instead, I pass along ideas for designs, and let those who can make good use of them have at it. Others can pass up this design, hoping for a more suitable one in the next column.</p>
|
||||
<p>Over the past 7 years or so, we have looked at a lot of antenna idea. And yet, we have only scratched the surface.</p>
|
||||
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||||
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||||
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|
||||
<br>
|
||||
<p><i>Updated 12-1-2000. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i></p>
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||||
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|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
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||||
</center>
|
||||
<p><a href="anta.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to An-Ten-Ten-nas Page</a></p>
|
||||
<p><a href="../radio.html"><img src="../images/rtnhome.gif" align="bottom" width="40" height="40"> Return to Amateur Radio Page</a></p>
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<title>A Triangular Vertical Array</title>
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<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10">No. 31: A Triangular Vertical Array <img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>Most beams we have discussed have been horizontal. I have not wanted to neglect fans of vertical antennas, but good designs that are not just horizontal antennas flipped 90 degrees are not easy to find.</p>
|
||||
<p>In some areas, the local gang use vertical whips and mobile antennas. Skip contacts do not care what polarization your antenna uses, since the ionosphere skews the polarization of the original signal. So the question is whether we can find a decent vertical antenna that will serve both local and distant needs.</p>
|
||||
<p>Well. . .how about a set of 3 vertical dipoles that require no rotator and only a simple weather sealed box with some relays inside. With a flip of a switch, you can cover the entire horizon in three broad lobes with little if any gap in coverage. The gain will not rival a long-boom Yagi, but the antenna array will give some gain and very decent a front-to-back ratio, with a direct coax feedline connection.</p>
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<img src="ant31-1.gif" width="554" height="322" border="2" vspace="5" hspace="9">
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<p>Let's start with three vertical dipoles, each 15.6' long made of 3/4" hardware store aluminum tubing. See <b>Fig. 1</b>. We shall set these up in an equilateral triangle 8.6' on a side (1/4 wavelength). If we run a mast up the middle of the array, we can use PVC or other means of supporting the vertical at just about their center points, leaving the feedpoint separation free for connections. The arms for such a system would have to be just about 5' long from the center mast.</p>
|
||||
<p>Next, we shall cut 4.97' (yes, 5.0' is OK) lengths of RG-8 foam or RG-58 foam coax. We must use the foam type to get the right velocity factor (0.78) so that the lengths are electrically correct. Each line goes from one feedpoint to a central waterproof junction box. The coax from the shack and a 12-volt DC line also enter the junction box.</p>
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||||
<img src="ant31-2.gif" width="627" height="456" border="2" vspace="5" hspace="9">
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||||
<p>The schematic diagram in <b>Fig. 2</b> shows the hook-up for 3 DPDT 12-volt relays in the box. (The switch is shown in the box, but it will be in the shack.) As we change positions on the switch, we activate one relay, moving the connectors so that the center of the coax from one antenna goes to the RF cable and the shield goes to the shack coax shield. The inactive relays let the coax from the other two antennas simply be shorted out.</p>
|
||||
<p>The shorted lengths of coax are stubs that electrically lengthen the other two antenna elements so that they work better as reflectors. Now we have 2 reflectors and one driven element in a vertical array.</p>
|
||||
<center>
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||||
<img src="ant31-3.gif" width="640" height="260" border="2" vspace="5" hspace="9">
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</center>
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<p>The elevation pattern created by this system and shown in <b>Fig. 3</b> gives us a low angle pattern, good for both local and dx contacts. The front-to-back ratio varies between 12.5 and 15 dB across the first MHz of 10 meters. The gain is very usable, but not world beating. However, remember that we saved the cost of a rotator with this array, and we can work the vertical locals with ease.</p>
|
||||
<p>In addition to all having gain and appreciable front-to- back ratio, we can cover the entire horizon just be changing the switch position. The switch simply converts one element from a reflector into a driven element, changing the overall heading of the array. The beamwidth of a vertical antenna is very large--well over 120 degrees between -3 dB points. Hence, three headings are enough to cover the world.</p>
|
||||
<center>
|
||||
<img src="ant31-4.gif" width="640" height="480" border="2" vspace="5" hspace="9">
|
||||
</center>The composite azimuth patterns in <b>Fig. 4</b> give you a good idea of how large the beam width is and how good the overall coverage is. If you construct the basic support system well enough, you will have an antenna with no moving parts other than the relays in the box. Servicing a relay at the top of the tower is easier than servicing a rotator.
|
||||
<p>The array can be fed directly with coaxial cable, as the SWR curve in <b>Fig. 5</b> shows.</p>
|
||||
<center>
|
||||
<img src="ant31-5.gif" width="640" height="181" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>All of the figures shown are modeled with the antenna at 35' at its center. The higher, the better, but the antenna will still work quite well with its lowest point at least 20' off the ground.</p>
|
||||
<p>Verticals acquit themselves very well on 10 meters, and this vertical array will add some directional gain and some QRM nulling. Relative to a beam, the cost will be low, and the maintenance should be easy. Possibly, it is time to think vertical.</p>
|
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<center>
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<img src="../images/colorbar.gif" align="middle" width="540" height="4">
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</center><br>
|
||||
<br>
|
||||
<p><i>Updated 05-01-2001. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i></p>
|
||||
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|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center>
|
||||
<p><a href="anta.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to An-Ten-Ten-nas Page</a></p>
|
||||
<p><a href="../radio.html"><img src="../images/rtnhome.gif" align="bottom" width="40" height="40"> Return to Amateur Radio Page</a></p>
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<title>What the Heck is a Stub?</title>
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<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10">No. 32: What the Heck is a Stub? <img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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||||
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||||
<h3>L. B. Cebik, W4RNL</h3>
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<p>Some of the antennas we have looked at in these columns have used transmission line stubs. I often receive this question: "Just what is a stub and what does it do?" So let's look at stubs. To keep things simple at the beginning, we shall confine ourselves to stubs shorter than a quarter wavelength.</p>
|
||||
<h3>The Basic Principle</h3>
|
||||
<center>
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||||
<img src="ant32-1.gif" width="553" height="223" border="2" vspace="5" hspace="9">
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<p>A shown in <b>Figure 1</b>, a stub is a length of transmission line. When the line is shorted at the far end, it acts as an inductive reactance and can actually replace a coil. When the far end is open, the stub acts as a capacitive reactance and can replace a capacitor. Stubs would be too large to use in HF circuits, but they are convenient in antenna applications, where space is usually no problem. They can handle high voltages and currents, often with greater ease and cost-effectiveness than lumped components.</p>
|
||||
<p>The amount of inductive or capacitive reactance is proportional to the length of the stub. However, the relationship is not linear. Let's look at how we calculate the reactance of a shorted stub to see why.</p>
|
||||
<center>
|
||||
<img src="ant32e1.gif" width="234" height="59">
|
||||
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|
||||
<p>where X<sub>L</sub> is the inductive reactance in Ohms, Z<sub>O</sub> is the characteristic impedance of the transmission line used for the stub, and L<sub>d</sub> is the length of the line in electrical degrees. since we are using lines shorter than 1/4 wavelength, L<sub>d</sub> will be between 0 and 90 degrees.</p>
|
||||
<p>For the same value of reactance, the higher the Z<sub>O</sub> of the transmission line, the shorter the line length. Hence, parallel transmission line is often used for inductive stubs to save space. Since the tangent of angles above 45° grows larger very fast, we usually restrict ourselves to modest value of inductive reactance so that we can prune the line length precisely without overshooting the mark.</p>
|
||||
<p>Shorted stubs for inductive reactance are more common than open stubs for capacitive reactance. The reason is easy to see from the formula.</p>
|
||||
<center>
|
||||
<img src="ant32e2.gif" width="186" height="76">
|
||||
</center>
|
||||
<p>where X<sub>C</sub> is the capacitive reactance in Ohms and the other terms have the same meaning as in the earlier equation. Smaller values of capacitive reactance (the most common need) require longer transmission line stubs. While some applications call for open stubs, shorted stubs for inductive reactance are far more common.</p>
|
||||
<h3>Application #1</h3>
|
||||
<center>
|
||||
<img src="ant32-2.gif" width="604" height="296" border="2" vspace="5" hspace="9">
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||||
</center>
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||||
<p><b>Figure 2</b> shows one common use of shorted (inductively reactive) stubs: to load an antenna element to make it electrically longer than its physical length. We know that a Yagi reflector is longer than the driven element, but in the figure, they are the same physical length. The load value of X<sub>L</sub> is 85 Ohms to make the reflector work like an element somewhat longer than the physical length would permit. Now we can place a coil in the load position. At 28.5 MHz, a coil of 0.47 microH would do the job, but its resistive losses might be of concern. Short stub losses are nearly negligible, so let's use a stub instead.</p>
|
||||
<p>At the given frequency of 28.5 MHz, a shorted stub might be made from either RG-58 (50 Ohms, 0.66 VF coaxial cable) or parallel transmission line (for example, 450-Ohms, VF 0.95). The first step is to take the ratio of X<sub>L</sub> and Z<sub>O</sub>. For 50-Ohm cable, this is 1.7, and for 450-Ohm line, it is 0.19. The second step is to figure the length in degrees. The "arctan" (backing out the degrees when you know the tangent of an angle) of 1.7 is 59.5°, while the arctan for the 450-Ohm line is 10.7°.</p>
|
||||
<p>Now let's figure the real line lengths needed. First, we know that a wavelength at 28.5 MHz is abut 34.5' long for a full 360°. 59.5° means a 5.71' length. 10.7° is 1.025' long at the same frequency.</p>
|
||||
<p>However, remember that the actual transmission lines had VF (velocity factor) values. The coax value means that its length needs to be 0.66 times the calculated amount, or 3.77' (45.22"). The parallel line had a VF of 0.95 and thus needs to be about 0.97' (11.7") long. Personally, I would use, if possible, the 450-Ohm line, since it is shorter, lighter, and easier to handle. But that is not the right decision for every situation.</p>
|
||||
<h3>Application #2</h3>
|
||||
<center>
|
||||
<img src="ant32-3.gif" width="648" height="335" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Some antennas use a beta match system, which requires an inductive reactance across the antenna terminals so that an antenna with a low impedance can match a 50-Ohm coax system, as sketched in <b>Figure 3</b>. Although coils are quite effective, most commercial beta matches use a "hairpin." The hairpin is nothing more than a U-shaped piece of wire, which is itself nothing more than a shorted parallel transmission line.</p>
|
||||
<p>Using standard equations for L-networks we can calculate the inductive reactance we need to place across the coil. If the antenna has an impedance of 25 Ohms, then the required reactance is 50 Ohms. We can make a hairpin from our 450-Ohm transmission line, using the same procedure. 50/450 = 0.11. The arctan of this number is 6.34°. This number of electrical degrees amounts to 0.61' (7.29") at 28.5 MHz. If the VF is 0.95, then the final length is 0.58' (6.93").</p>
|
||||
<p>For both applications, we would start with lines a little long and prune them to exact length. We can do this by having an adjustable set of contacts at the terminals or we can trim the shorted end and resolder the shorting bar.</p>
|
||||
<p>The math in this exercise is mostly to acquaint you with the general properties of stubs--which call for shorter lengths and which call for longer. Those inclined to do so can calculate a bunch of 10-meter stubs to see what the lengths look like for various values of X<sub>L</sub> and X<sub>C</sub> and different types of transmission line. Then when you encounter stubs in articles, presence and general dimensions will be familiar to you.</p>
|
||||
<p>The antenna array we discussed in the last column used stubs in the reflectors. I'll bet the next column also has a stub.</p>
|
||||
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||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center><br>
|
||||
<br>
|
||||
<p><i>Updated 07-01-2001. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i></p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center>
|
||||
<p><a href="anta.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to An-Ten-Ten-nas Page</a></p>
|
||||
<p><a href="../radio.html"><img src="../images/rtnhome.gif" align="bottom" width="40" height="40"> Return to Amateur Radio Page</a></p>
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<title>The Cheapest 2-Element Beam?</title>
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||||
<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10">No. 33: The Cheapest 2-Element Beam? <img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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||||
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||||
<center>
|
||||
<h3>L. B. Cebik, W4RNL</h3>
|
||||
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||||
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||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
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||||
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||||
<p>"Can I build a small directional antenna for less than $20?" That was the challenge presented to me. The answer is yes. However, remember that this is a project designed for the dollar sign, so the gain and directivity will be modest. Here is what we need:</p>
|
||||
<ul>
|
||||
<li>32' #14 copper wire (Radio Shack and elsewhere)</li>
|
||||
<li>15' 2x2 (good quality)</li>
|
||||
<li>7' 1x2 or similar size Schedule 40 PVC</li>
|
||||
<li>1' 450-Ohm parallel feedline</li>
|
||||
<li>SO-239 coax connector</li>
|
||||
<li>Boom-to-mast bracket/hardware</li>
|
||||
</ul>
|
||||
<p>The antenna is a 2-element wire parasitic beam with the ends tapered back toward each other. The elements are the same physical length, but the reflector is stub loaded to increase its electrical length. (See the last column for information on stubs.) Gain is about half an S-unit over a dipole, and the front-to-back ratio is about 2 S-unit. Feed is a direct coax match, and coverage is about 900 kHz of 10 meters.</p>
|
||||
<center>
|
||||
<img src="ant33-1.gif" width="750" height="423" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p><b>Figure 1</b> shows both the wire and support frame dimensions. The 2x2 determines the side-to-side dimensions, while the cross piece determines the maximum front-to-back dimension. The end cross pieces keep the wire ends 9" apart.</p>
|
||||
<p>Use a good marine (spar) varnish (or modern poly equivalent) on all wood. Expect to re-varnish the wood annually. Be sure the 2x2 does not sag a lot. The cross pieces can be PVC or 1x2. If wood, give it the same spar varnish treatment.</p>
|
||||
<p>Details of coax connector mounting, wire termination, and mast support are subject to many variations, so I shall bypass them here. Check with any handbook for many different ways to achieve the same goals. Likewise, mast details are omitted here.</p>
|
||||
<p>The figure shows the stub dangling, but you can tape it to the center cross piece. Not shown is the 1:1 choke balun at the antenna feedpoint that I use on all antennas to reduce RF on the outside of the coax to the shack.</p>
|
||||
<center>
|
||||
<img src="ant33-2.gif" width="640" height="480" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p><b>Figure 2</b> shows the azimuth pattern with the antenna at 20' over average ground. 20' is the height of two sections of TV masting, which makes an adequate support system for hand rotating the beam. Whatever system you use to elevate the antenna (which I shall assume is as inexpensive as the antenna itself), be certain that the mast is well guyed or bracketed for safety.</p>
|
||||
<p>The antenna pattern is not a paragon of gain or rear nulling, but it does have some nice features. The forward beamwidth is wide enough so that a single twist of the mast will cover all of Europe or all of the VK-ZL area--or from each of these places, a single setting will cover all of the USA and Canada. The rear has only one lobe, so that there are no antenna positions susceptible to QRM from some quartering rear direction.</p>
|
||||
<center>
|
||||
<img src="ant33-3.gif" width="640" height="233" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p><b>Figure 3</b> is the modeled SWR plot across the first MHz of 10 meters. The antenna was designed for 28.35 MHz to provide a low SWR for the most popular segment (28.3 to 28.5 MHz). You can shift the SWR curve downward by increasing the length of both wires equally about 2" or so--or shorten them by the same amount to move a bit up the band. However, keep the wire end separation at the prescribed 9" with the end cross pieces.</p>
|
||||
<p>The antenna is related to the standard Yagi, although the elements are not exactly parallel. The element end coupling also relates the design to the Moxon rectangle, but the sloping wires prevent us from achieving the high front-to-back ratio of that design while still having any forward gain worth using.</p>
|
||||
<p>So while the performance is modest, the price is attractive. The sdloping wire array is a fun antenna to play with, since the cost is so low. However, the most important aspect of the design is that in an emergency--perhaps after some natural disaster has knocked down all of the regular antennas--this design can be fabricated from scraps from the rubble. You can make the electrical elements from house wiring that is no longer functional and lash together a frame with bailing wire.</p>
|
||||
<p>If you have no inclination to play with the design at this time, you may wish to file this column away in your notebook labeled "In the event of emergency or disaster. . .."</p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center><br>
|
||||
<br>
|
||||
<p><i>Updated 10-01-2001. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i></p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center>
|
||||
<p><a href="anta.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to An-Ten-Ten-nas Page</a></p>
|
||||
<p><a href="../radio.html"><img src="../images/rtnhome.gif" align="bottom" width="40" height="40"> Return to Amateur Radio Page</a></p>
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<title>The Turnstile Antenna on 10</title>
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<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10">No. 34: The Turnstile Antenna on 10 <img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>Net control stations often ask for an omni-directional horizontally polarized antenna. One of the simplest antennas to meet this need is the turnstile. The basic outline appears in <b>Fig. 1</b>.</p>
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|
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<img src="ant34-1.gif" width="492" height="372" border="2" vspace="5" hspace="9">
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||||
<p>Essentially, the turnstile consists of two dipoles at 90 degree angles to each other. One dipole is connected to the main feedline, in this case a 50-Ohm line (since the two dipoles together will give a 36-Ohm feedpoint impedance). Between the feedpoint of Dipole 1 and Dipole 2, we run a 90-degree or 1/4 wavelength section of 72-Ohm coax to effect the required phase shift between the two dipoles. It is this phase shift that gives the turnstile its nearly omni-directional pattern.</p>
|
||||
<p>For 1/2" to 5/8" tubing--or a combination of the two--the dipoles can each be 16.5' long--8' 3" on each side of center. This size will make them resonant at about 28.5 MHz. The dipoles must not touch each other. You can accomplish this by mounting them on opposite sides of a square plywood board or by keeping the feedpoint separations large enough so that nothing touches.</p>
|
||||
<p>Connect the main feedline across one dipole. Then connect the phasing line from the feedline connection points to the other dipole connection points. If you use a balun, connect it only to the main feedline. Let the phasing line droop. You can tape it together for control. But it is best to keep it spaced from any metal antenna mast you use to support the antenna.</p>
|
||||
<p>The phasing line length will depend on the velocity factor of the line you use. 72-Ohm coax comes with either solid or foam insulation. The solid insulation usually gives the line a 0.66 velocity factor, while foam lines have a velocity factor of about 0.78.</p>
|
||||
<p>Use the velocity factor as a multiplier on the basic 1/4 wavelength to determine the physical length of the line. 1/4 wavelength is about 8.63' at 28.5 MHz. A 0.66 VF line will be about 5.69' long, while a 0.78 VF line will be about 6.73' long. As with any antenna, be sure to weatherproof all connections to prevent rain from entering the coax line.</p>
|
||||
<p>Since the turnstile impedance is about 36 Ohms, a 50-Ohm feedline will show an SWR of between 1.3:1 and 1.4:1. Do not try to tune the antenna for a 1:1 SWR, since that will require shortening the elements below individual dipole resonance. The resultant pattern will no longer be omni-directional. On the other hand, once you have built the antenna close to the dimensions suggested here for both the elements and the phase line, it will be very broad-banded, covering all of 10-meters.</p>
|
||||
<p>What kind of performance can we expect from a turnstile. <b>Fig. 2</b> gives us an answer.</p>
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<img src="ant34-2.gif" width="422" height="413" border="2" vspace="5" hspace="9">
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</center>
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<p>The pattern is a blunted circle with only a 1 dB decrease from maximum gain along two of the flattened edges. This is as close to a perfect circle as you will come with a horizontally polarized antenna of this efficiency. The maximum gain is about 5 dBi when the antenna is 1 wavelength (about 35') above ground. At this height, the antenna has 2 elevation lobes, one at 14 degrees for long hauls and the other at 48 degrees for short skip and e-layer reflections. The local point-to-point abilities of this antenna are quite good.</p>
|
||||
<p>As always with horizontal antennas, the higher, the better--up to about 1.75 wavelengths or so--especially for local contacts. The turnstile gain is lower than the maximum gain of a dipole along, but remember that a single dipole has only two main lobes working at one time. The turnstile has 4 overlapping lobes. Since the power for any operation is constant, it must distribute itself over more territory--and hence will not be quite as strong as when we can limit its coverage to two lobes. (The uni- directional beam gets its gain from concentrating almost all of the power in one lobe.)</p>
|
||||
<p>The turnstile can be a useful antenna for net control stations. It may also be useful for folks who want a dipole, but do not wish to turn it with every new incoming signal. You can orient the antenna along any axis, so you have some options in fitting it into the space you have.</p>
|
||||
<p>However, the turnstile has limitations. For its area, gain is not high. In the same footprint, you can create a small 3-element beam with much higher gain and directivity. Also beware of the temptation of folding the ends around to reduce the space required by the crossed dipoles. The omni-directional pattern will become a bi-directional, weak set of dipole lobes. Finally, the turnstile is a monoband antenna, due to the requirement for the 90-degree phasing line. If you want a turnstile for another band, you will have to design the entire antenna--both dipoles and the phasing line--for the new design frequency.</p>
|
||||
<p>So the turnstile is a special-purpose antenna. If it has the pattern you need, use it. If you need a different pattern, then try one of the other antennas in this series.</p>
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<p><i>Updated 12-01-2001. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i></p>
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<p><a href="anta.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to An-Ten-Ten-nas Page</a></p>
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<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10">No. 35: A 10-Meter, 6-Element OWA Yagi <img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>There is a very interesting 20-meter Yagi design called the Optimized Wideband Antenna, or OWA. Although only one of several designs within this genre, developed by Nathan Miller, NW3Z and Jim Breakall, WA3FET, the 20-meter version is one of the most adaptable. It employs 6-elements in the space that many other designs use 5. <b>Fig. 1</b> shows the general proportions of the antenna.</p>
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<img src="ant35-1.gif" width="442" height="442" border="2" vspace="5" hspace="9">
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<p>The 20-meter antenna has several features that deserve special note. Director 1 is perhaps the most significant, since it represents the added element to previously standard 5-element designs. By the use of this parasitic element, the driver can be more closely spaced to the reflector and still show a feedpoint impedance very close to 50 Ohms resistive. Moreover, the antenna shows wideband VSWR characteristics, with values less than a 1.3:1 from 14.0 through 14.35 MHz.</p>
|
||||
<p>Not only is the feedpoint impedance quite stable, so too are the other main operating characteristics, including both gain and the front-to-back ratio. The antenna shows better than 10 dBi forward gain in free space models across the entire 20 meter band, with more than a 20 dB front-to-back ratio across the same span. Many 5-element designs show much larger variations in all three of the main Yagi parameters: gain, front-to-back ratio, and feedpoint impedance.</p>
|
||||
<p>The remaining elements of the OWA are also interesting. Directors 2 and 3 are either the same length or the forward director is slightly longer than the rearward member of the pair. Director 4 and the reflector are available for making small changes in the upper and lower frequency limits of the design to spread the operating characteristics across the desired bandwidth. The 20-meter band is about 2.5% of its center frequency. The OWA is capable if significantly greater operating bandwidths with little loss in any of its main characteristics.</p>
|
||||
<p>The reason for making extensive note of the OWA design is that it scales quite easily (but not without some readjustment) to create very usable Yagis for 10 meters. Although only a few hams have the wherewithal to construct a 48' boom Yagi for 20 meters, 24'-boom Yagis for 10 are more common--and more manageable. The resultant beam shows a free space gain above 10 dBi across the band, a front-to-back ratio always in excess of 20 dB, and very low 50-Ohm SWR values for direct coax feed systems.</p>
|
||||
<p>Scaling the initial OWA to 10 meters involved converting the 20-meter design into its equivalent uniform diameter element equivalent, scaling this antenna, and then creating a set of tapered diameter elements suitable for 10 meters, adjusting their lengths to be equivalent to the substitute model. The table lists the overall element length, the spacing from the reflector, and the exposed tubing lengths of each size tubing used on one side of the element. (Be sure to double the length of the largest size tubing and to have extra inches on the remaining sections for the tubes to nest.) All dimensions are in inches.</p>
|
||||
<pre>Element Overall Spacing 0.5" 0.375" 0.25"
|
||||
Length from Refl. Inner Middle Outer
|
||||
Reflector 216.8 ------ 35.75 35.75 36.9
|
||||
Driver 209.2 44.68 35.75 35.75 33.1
|
||||
Dir 1 199.36 69.26 35.75 35.75 28.18
|
||||
Dir 2 193.23 132.40 35.75 35.75 25.12
|
||||
Dir 3 193.24 192.83 35.75 35.75 25.12
|
||||
Dir 4 186.05 282.96 35.75 35.75 21.53</pre>
|
||||
<p>The antenna was scaled and reset to cover the span from 28 to 29 MHz. Some adjustment of the reflector and 4th director was required to achieve the added bandwidth. The first MHz of 10 meters represent a 3.5% operating bandwidth, about 40% greater than demanded of the 20-meter antenna. The following table shows representative modeled figures for 5 points across the band. All figures are based on free space models using NEC-4.1 with Leeson corrections invoked for greatest accuracy.</p>
|
||||
<pre>Parameter 28.0 28.25 28.5 28.75 29.0
|
||||
Gain dBi 10.00 10.10 10.19 10.26 10.27
|
||||
F-B dB 20.29 26.57 30.22 24.47 21.34
|
||||
Feed Z:
|
||||
R 38.4 41.9 44.4 44.6 36.5
|
||||
jX +5.0 -1.1 +1.3 +0.5 -2.6
|
||||
50-Ohm SWR 1.33 1.20 1.13 1.12 1.38</pre>
|
||||
<p>The greater bandwidth demanded of the 10-meter design shows most clearly in the rise in SWR and decrease in feedpoint impedance at the low and high ends of the bandpass. Nonetheless, the design meets all of the objectives. The gain changes only by about a quarter dB across the band. With further tweaking, the feedpoint impedance might be brought upward toward 50 Ohms a bit, but the reactance figures are extremely low for an antenna covering a full MHz of 10 meters. All of this suggests that the OWA design concept is capable of significant expansion beyond its original implementation.</p>
|
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<center>
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||||
<img src="ant35-2.gif" width="640" height="320" border="2" vspace="5" hspace="9">
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</center>
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<p>The antenna pattern itself is a model of good behavior, with no undesirable side or rear lobes. Note in <b>Fig. 2</b> the change in the shape of the rearward lobe across the band, which is a normal progression for well-behaved antennas of this type.</p>
|
||||
<p>Just for the drama, <b>Fig. 3</b> shows the 50-Ohm SWR sweep, taken at 0.1 MHz intervals across the band. There are no impedance spikes anywhere in the bandpass of the antenna.</p>
|
||||
<center>
|
||||
<img src="ant35-3.gif" width="466" height="187" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>As a high-performance 10-meter antenna covering all of the first MHz of the band, the 24' 6-element OWA is a worthy monoband competitor with other designs for the band.</p>
|
||||
<p>For those who prefer somewhat beefier construction, here are the dimensions of the NW3Z model on a 24' boom using 0.75" diameter inner tubing, 0.625" diameter middle tubing, and 0.5" diameter tip sections. Note that the element lengths and spacing are slightly different than the slim-element version above. When adapting Yagi designs, do not simply take the dimensions you find in an article and use whatever materials you have. Differences in element diameter and where the steps in the diameter occur will make a difference in the required element lengths and spacings. A hasty near-copy may not perform to full specifications.</p>
|
||||
<pre>Element Overall Spacing 0.75" 0.625" 0.50"
|
||||
Length from Refl. Inner Middle Outer
|
||||
Reflector 215.04 ------ 24.00 18.00 65.52
|
||||
Driver 207.18 43.86 24.00 18.00 61.59
|
||||
Dir 1 195.80 69.18 24.00 18.00 55.90
|
||||
Dir 2 191.76 131.30 24.00 18.00 53.88
|
||||
Dir 3 192.34 192.70 24.00 18.00 54.17
|
||||
Dir 4 183.14 282.00 24.00 18.00 49.57</pre>
|
||||
<p>All credit for the OWA design belongs to its originators. Further details can be found at the following website: http://www.contesting.com/nw3z/. This exercise has only shown that one of the implementations of the basic design can be advantageously adapted to other bands. None of the models presses any limit of the NEC and, therefore, they are quite reliable, both as analyses of the antennas and as guides to construction. Of course, using still another element diameter taper schedule than the ones shown will require resetting the element lengths to accommodate the materials used.</p>
|
||||
<p>In addition to being rather good Yagis of their size, the OWA designs may also serve as a standard against which to measure other designs that present themselves. Even if you never build one of these designs, the data provided here may be useful for comparative purposes.</p>
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<img src="../images/colorbar.gif" align="middle" width="540" height="4">
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<br>
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||||
<p><i>Updated 4-01-2002. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i></p>
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<img src="../images/colorbar.gif" align="middle" width="540" height="4">
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</center>
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||||
<p><a href="anta.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to An-Ten-Ten-nas Page</a></p>
|
||||
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<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10">No. 36: 4- and 5-Element OWA Yagis <img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>The OWA (Optimized Wide-band Antenna) design for Yagis means different things to different designers. For some, it means simply a low SWR across a passband (like 28-29 MHz), regardless of the impedance. To others, it means a low 50-Ohm SWR across the passband. Since many 10-meter users are more comfortable with a direct coax feed (with a choke balun for protection from common-mode currents down the line), let's take the latter approach.</p>
|
||||
<p>Last time, we looked at a 6-element 24' OWA using fairly thin elements. This time, let's look at some shorter designs using elements with larger diameters. In fact, in this episode and the next, we shall look at a total of 4 OWA design ranging from 13' to 36' long, all with the same element structure.</p>
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||||
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<img src="ant36-1.gif" width="250" height="461" border="2" vspace="5" hspace="9">
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||||
<p><b>Fig. 1</b> shows the element structure that all of the beam designs will use. The 4' center section is 3/5" tubing, with a short 1.5' (plus 3" for overlap) section of 5/8" tubing following. The "tips" are 1/2" tubing. All this can be obtained from sources that advertise in QST or CQ. In the element length tables, be sure to add 3" to the 1/2" tubing for overlap.</p>
|
||||
<p>For higher wind loads, you can let the 5/8" tubing go all the way through the 3/4" element. For lesser winds, the 3" overlap will do well.</p>
|
||||
<p>Of course, the driven element must be open at the center for the feedline connection. You can use a fiberglass tube or rod across the gap to keep the element aligned and to strengthen it.</p>
|
||||
<center>
|
||||
<h4>A 4-Element OWA</h4>
|
||||
</center>OWA design requires one additional element to achieve the 50-Ohm feedpoint impedance compared to beams with feedpoint impedances in the 25-Ohm range. Shorter OWAs must also be a bit longer than their 3-element low-Z counterparts for the same performance. While a 3-element Yagi with about 8 dBi free space gain needs a 12' boom, our OWA will need a 13.5' boom.
|
||||
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||||
<img src="ant36-2.gif" width="341" height="303" border="2" vspace="5" hspace="9">
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||||
</center>
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||||
<p><b>Fig. 2</b> provides the outline of the 4-element version, with the spacing between elements shown. The following table lists the element spacing from the reflector, the tip length, and the total element length from tip-to-tip for reference, all in feet.</p>
|
||||
<pre>Element Space from Tip Length Total
|
||||
Reflector Length Length
|
||||
Reflector 0 5.34' 17.68'
|
||||
Driver 5.12' 5.10' 17.20'
|
||||
Director 1 6.87' 4.57' 16.14'
|
||||
Director 2 13.00' 4.10' 15.20'</pre>
|
||||
<p>The elements are designed to be insulated from the boom. If you change materials or element structures, an entire redesign will be needed. Do not be casual in your construction of Yagi designs you find in handbooks. They simply will not perform up to the original design if you alter the element diameters or the tapering schedule.</p>
|
||||
<p>The 4-element OWA provides an average gain of over 8 dBi (free-space). The front-to-back ratio is above 20 dB across the 28-29 MHz span. The 50-Ohm SWR is exceptionally low. Hence, the beam is a fit candidate for reproduction in the home workshop.</p>
|
||||
<center>
|
||||
<h4>A 5-Element OWA</h4>
|
||||
</center>If we add about 6' to the boom and one more element (respacing all of the others), we can add about 1 dB to the overall gain of the OWA Yagi. A 19' boom will hold the elements, which use the same construction as the 4-element version.
|
||||
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|
||||
<img src="ant36-3.gif" width="342" height="412" border="2" vspace="5" hspace="9">
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||||
</center>
|
||||
<p><b>Fig. 3</b> gives us the outline of the 5-element OWA, as well as the inter-element spacing. The following table provides the structural details.</p>
|
||||
<pre>Element Space from Tip Length Total
|
||||
Reflector Length Length
|
||||
Reflector 0 5.52' 18.04'
|
||||
Driver 5.30' 5.16' 17.32'
|
||||
Director 1 6.70' 4.70' 16.40'
|
||||
Director 2 11.10' 4.65' 15.30'
|
||||
Director 3 18.70' 4.13' 15.26'</pre>
|
||||
<p>For the extra element and boom length, we get a gain that ranges from 9.1 to 9.4 dBi (free space) across the 28-29 MHz span. The front-to-back ratio is close to or above 20 dB over that same frequency range. The SWR is quite smooth across this range.</p>
|
||||
<p>Once more, the design is for elements insulated from the boom. There are many systems for mounting elements to the boom. I tend to prefer 6" by 12" plates of polycarbonate (a generic name for the material sold as Lexan), which has good sun (UV) resistance, good RF properties, and is very strong. 1/4" thick material works well at 10 meters. It is much more durable and maintenance-free than plywood (which needs periodic varnishing) or more common acrylics (which become brittle after a season or two in the weather). I also use stainless steel hardware throughout, including U-bolts with saddles for the elements and the boom. Other systems also work well, but this one satisfies my experimental needs, meaning that antennas are always being reconstructed into new configurations.</p>
|
||||
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|
||||
<img src="ant36-4.gif" width="480" height="400" border="2" vspace="5" hspace="9">
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||||
</center>
|
||||
<p>For reference, <b>Fig. 4</b> shows the SWR curves for the two antennas from 28 to 29 MHz. These are for direct connection, using only a 1:1 choke or balun to move from the balanced feed element to the unbalanced coax cable. You can purchase a balun, make your own choke from coiled coax, or obtain a choke that uses ferrite beads.</p>
|
||||
<p>Be sure to weather seal all connections. For coax connectors, a wrapping of 3-M #33 electrical tape starts the job, followed by one of the black butylate sealants. Some folks like to cover the butylate with one more #33 tape coating. Do not stint on weather protecting connections, since water leakage into the feedline can ruin the performance of an otherwise great antenna system.</p>
|
||||
<p>In a future column, we shall go over some of the good practices to follow in building your own antenna. However, we must first finish our designs for 3/4-5/8-1/2" elements. Next time, we shall look at 6 and 7 element versions of the OWA, both with a flat 50-Ohm Feedpoint impedance, and with an extra dB or so every time we add an elements.</p>
|
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||||
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|
||||
<br>
|
||||
<p><i>Updated 7-01-2002. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i></p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center>
|
||||
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|
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<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10">No. 37: 6- and 7-Element OWA Yagis <img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>Last time, we looked at some Yagi designs for fairly short-boom OWAs with direct 50-Ohm feedpoint impedances across the 28 to 29 MHz span. The 4-and 5-element designs provided about 8 dBi free-space gain and 9.2 dBi gain, respectively, which is quite good for monoband Yagis with their boom lengths.</p>
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<p>This time, we want to look at some longer versions of the OWA with more gain, but still keep the 50-Ohm feedpoint impedance. We shall look at 6- and 7-element Yagis on 24 and 36 foot booms.</p>
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<p><b>Fig. 1</b> reviews the element structure that we are using in this set of designs, a combination of 3/4, 5/8, and 1/2 inch diameter aluminum tubing. One reason that we shall review the 6-element OWA, when we already did an entire column on that antenna, is the new, larger tubing sizes. As a result of changing both the tubing sizes and the taper schedule (the places along the element length where we change tubing size), the element lengths will also change. However, the end result will be very similar performance between the two versions of the antenna.</p>
|
||||
<h4>A 6-Element OWA</h4>The original 6-element OWA is a very good antenna. So too is the new version. The only difference is in the tubing, which may be a matter of what an individual builder likes to use or has access to in his or her local area. Let's see how the new version turns out.
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<img src="ant37-2.gif" width="325" height="481" border="2" vspace="5" hspace="9">
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<p><b>Fig. 2</b> gives us the outline of the 6-element OWA, as well as the inter-element spacing. The following table provides the structural details.</p>
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<pre>Element Space from Tip Length Total
|
||||
Reflector Length Length
|
||||
Reflector 0 5.46' 17.92'
|
||||
Driver 3.66' 5.13' 17.26'
|
||||
Director 1 5.77' 4.66' 16.32'
|
||||
Director 2 10.94' 4.49' 15.98'
|
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Director 3 16.06' 4.51' 16.02'
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Director 4 23.50' 4.13' 15.26'</pre>
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<p>The free-space gain of this 6-element OWA varies from 10.1 to 10.2 dBi across the band, with a front-to-back ratio that never falls below 21 dB. Only at the upper limit of the frequency spread does the SWR get above 1.25:1 with a direct 50-Ohm feed system (including the recommended 1:1 choke/balun to suppress common mode currents).</p>
|
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<h4>A 7-Element OWA</h4>Adding one more element increases the boom length from 24' to 36' or so. Whether the added length is worth the effort depends on how important another dB of gain is to the operator.
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<img src="ant37-3.gif" width="299" height="472" border="2" vspace="5" hspace="9">
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<p><b>Fig. 3</b> gives us the outline of the 7-element OWA, as well as the inter-element spacing. The following table provides the structural details.</p>
|
||||
<pre>Element Space from Tip Length Total
|
||||
Reflector Length Length
|
||||
Reflector 0 5.47' 17.94'
|
||||
Driver 3.66' 5.17' 17.34'
|
||||
Director 1 5.76' 4.64' 16.28'
|
||||
Director 2 10.89' 4.50' 16.00'
|
||||
Director 3 17.50' 4.51' 16.02'
|
||||
Director 4 26.40' 4.47' 15.94'
|
||||
Director 5 36.00' 4.11' 15.22'</pre>
|
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<p>The gain ranges from 11.2 to 11.6 dB across the first MHz of 10 meters, with a peak near mid-band. The front-to-back ratio is a solid 20 dB. Although the SWR curve reaches 1.6:1 at 29 MHz, it is very tame across the remainder of the band. In fact, <b>Fig. 4</b> gives us a sampling of both antenna SWR curves.</p>
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<img src="ant37-4.gif" width="480" height="400" border="2" vspace="5" hspace="9">
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<p>Remember that these designs are specific to the tubing sizes and transition points from one size to the next. Any deviations from the prescribed tubing schedule would require refiguring the designs.</p>
|
||||
<p>As with all OWA designs, we must have 1 extra element for the given boom length and gain in order to secure the relatively smooth 50-Ohm SWR curve across the band. The element marked as Director 1 in all of the designs does not so much contribute to the antenna gain as it functions with the reflector to establish the antenna bandwidth and feedpoint impedance.</p>
|
||||
<p>How big do you want to go? I have seen an 8-element OWA design for 10 meters with something over 12 dB gain. But it was 48' long. The design can be found at the NW3Z web site via your search engine (search for "NW3Z"). As well, these designs might be tweaked for even better performance with an optimizing program. The versions shown here were hand-developed. If you prefer a direct 50-Ohm feedpoint impedance in a good Yagi antenna, then the OWA designs are the way to go.</p>
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<p><i>Updated 10-10-2002. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i></p>
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<p><a href="anta.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to An-Ten-Ten-nas Page</a></p>
|
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<p><a href="../radio.html"><img src="../images/rtnhome.gif" align="bottom" width="40" height="40"> Return to Amateur Radio Page</a></p>
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<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10">No. 38: Notes on Home-Brewing 10-Meter Yagis <img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>Over the last several columns, we have examined the electrical designs of a number of Yagis ranging from 3 to 7 elements. Along the way, we have noted some construction pointers and preferences. However, let's pause a bit to see if we can pull some of those ideas together in one place--here.</p>
|
||||
<p><b>1. Element and Boom Stock</b>: For casual and portable operations, where an antenna is not going to be in the wind for long periods of time, hardware depot aluminum tubing is usually satisfactory for elements. It comes--at least in my part of the country--in limited sizes, beginning at about 3/4" diameter. So designs using this span of tubing can rely on local purchase.</p>
|
||||
<p>6063-T832 and 6061-T6 are the more commonly used aluminum tubing for Yagi elements. Commercial houses can use thin wall versions of this tubing for lighter elements, since they have the machinery to resize the ends for perfect lap joints. However, 0.55" to 0.58" wall thicknesses are the ones used by most hams--and these are the versions carried by vendors like Texas Towers and others. Usually, for shipping ease, the tubing comes in 6' lengths, so designs need to take this into account. If you need to put two lengths together at the center of an element to get, for example, 12' of 5/8" tubing, use a short piece (about 1') of 1/2 inch tubing inside the inner ends of both 6' pieces to form a good electrical joint and stiffen the element on its mounting. For split driven elements, use a piece of fiberglass rod.</p>
|
||||
<p>For short boom, such as for a 2-element Yagi, 1 1/4" PVC works well, but longer than 5' or so, the material sags and is heavy. Aluminum tubing makes a good boom. For modest beams up to 12 to 14 feet long, you can nest 1.125 and 1.25 inch tubing to make a very durable boom. If you have only 6' lengths, you can stagger the sections, making sure that they meet tightly at their ends.</p>
|
||||
<p><b>2. Boom-to-Element Mounting</b>: Commercial antennas often use special assemblies to join elements to the boom. Plates are bent and curved as necessary by equipment to which the average home builder has no access. However, if you like to build antennas, consider accumulating old broken beams from hams in your local area, even offering a few dollars to remove the "junk" from their yards. You will acquire a lot of unusable bent tubing and a number of quite unharmed boom-to-element plates (and also some boom-to-mast plates). Most of the junction assemblies will be reusable, even if you have to replace the original hardware with stainless steel nut and bolts from the local hardware depot. For 10-meter beams, if you have to build your own boom-to- element assemblies, it is best to design for elements that are insulated from the boom. (Directly connected elements and insulated elements usually require slightly different lengths to do the same job, so do not convert one type of assembly to the other without some redesign work.) <b>Fig. 1</b> shows a method that I prefer--meaning that it is certainly not the only method available.</p>
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<img src="ant38-1.gif" width="546" height="413" border="2" vspace="5" hspace="9">
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<p>Having used a lot of materials for the insulating plate, I have gone to polycarbonate, the generic name for Lexan. The material is UV resistant, has excellent RF properties, is exceptionally strong, and can be cut and drilled with woodworking tools. 6 by 12 inch plates of 1/4" material handle any 10-meter elements. Trimming the corners to save a little weight is feasible. Two U-bolts hold the boom and either 2 or 4 U-bolts hold the elements. The inner U-bolts are needed only if you have a split fed elements with no alignment rod running from end-to-end of the plate inside the aluminum.</p>
|
||||
<p>You can replicate the plate using good outdoor plywood in 1/4 or 3/8 inch thicknesses. However, plywood requires special attention both in the beginning and every year. Initially, seal the edges with a good exterior-grade fill. Then apply several coats of good spar varnish. I normally have done this to the wood before drilling U-bolt holes and then revarnished the word, getting varnish into the holes. An annual sanding and recoating is necessary, just as it would be for a wooden boat.</p>
|
||||
<p>Use stainless steel U-bolts, preferably with saddles. The saddles help prevent the U-bolts from collapsing the tubing when you tighten everything down. These will likely be a mail-order item. Drill the holes in the plate with precision to exactly fit the U-bolts with minimum amounts of free play (but no stress on the plate). This practice will keep your elements aligned as the wind tries to push them around.</p>
|
||||
<p>Note also that the elements are mounted beneath the plates, with the boom on top. This arrangement let's gravity help keep the elements level.</p>
|
||||
<p>There is a simpler method of mounting insulated elements. It involves a single U-bolt per elements that goes around the boom and through the element. It is best to use a curved plate that matches the element curve as a keeper. Between the U-bolt and the boom is a short section of split gray electrical PVC that acts as an insulator. (The gray PVC used for electrical conduit has the greatest UV resistance and hence will last longer in the sun before going brittle.) The tubing used in this system should be fairly large and should have an interior piece of tubing for reinforcement. Otherwise, the element will break off right at the U-bolt. This system requires even more careful machine work for good alignment and does weaken the element. So use the scheme, outlined in <b>Fig. 2</b>, with caution.</p>
|
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<<img src="ant38-2.gif" width="438" height="323" border="2" vspace="5" hspace="9">>
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<p><b>3. Boom-to-Mast Plates</b>: For very light antennas, you can use a non-metallic plate for the boom-to-mast junction. Again, plywood--with the proper treatment--can also do the job. However, I prefer metal plates either 1/4 or 3/8 inch thick, depending on the weight of the antenna and the diameters of both the boom and the mast. Predrilled plates are available from mail order sources, as is stock that lets you drill your own custom pattern of holes. The object of using a metal plate is to ensure a good electrical bond between the boom and the mast (and downward) so that the entire assembly (except for the elements) is a ground potential for bleeding charges off the antenna support system.</p>
|
||||
<p>For the larger U-bolts in this assembly, you can use saddle- types or you might wish to use muffler-type clamps. These U-bolt assemblies have a saddle with edges that some folks believe grips the boom and the mast better. However, if you do use these auto store components, be certain that all of the pieces are stainless steel. It will take a pair of U-bolts for the mast and another pair for the boom.</p>
|
||||
<p>You have a choice of shapes for the plate: a square (or rectangle) of a diamond. In principle, the diamond conserves the most weight and material. However, be certain that there is enough material beyond the U-bolt holes in a diamond plate to ensure that the material will not break under stress. <b>Fig. 3</b> will reveal the areas of potential weakness.</p>
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<<img src="ant38-3.gif" width="390" height="237" border="2" vspace="5" hspace="9">>
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<p>Alternatively, you might use for this assembly one of those commercially made fixtures with its complex bends and shapings to provide a maximum grip between the boom and the mast. Also, be sure the mount this fixture at the center of weight of the antenna boom.</p>
|
||||
<p>4. <b>A Note About Stainless Steel Hardware</b>: For antenna work, use stainless steel hardware everywhere. Make no exceptions. Although once hard to find, stainless steel nuts, bolts, U-bolts, and other fixtures are easy to locate at hardware depots. Do not use aluminum hardware, as it is very weak. All other hardware will create a bi-metallic junction that will corrode one or both of the pieces joined. Stainless steel has proven to be the most successful hardware for antenna jobs ranging from joining element sections of tubing to boom-to-mast plate mountings.</p>
|
||||
<p>The construction methods that we have noted will run the cost of a home brew Yagi only to about twice the cost of using junk-box components. However, the final cost will still be less than 1/3 the cost of a commercially made antenna--and it will last as long or longer than the commercial assembly. Even if the Yagi is home brew, if it is worth making, it is worth making well. Nothing is more frustrating than breaking a beam in the wind during a world-wide band opening.</p>
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<br>
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<p><i>Updated 10-10-2003. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i></p>
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<img src="../images/colorbar.gif" align="middle" width="540" height="4">
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</center>
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||||
<p><a href="anta.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to An-Ten-Ten-nas Page</a></p>
|
||||
<p><a href="../radio.html"><img src="../images/rtnhome.gif" align="bottom" width="40" height="40"> Return to Amateur Radio Page</a></p>
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<title>Frequently Asked Questions</title>
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<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10">No. 39: Frequently Asked Questions <img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>In our column for the last issue, we covered some fundamentals of the material side of beam construction. That exercise brought to mind a number of more general questions that folks have posed from time to time about antennas. So, let's try to give some succinct answer to these questions.</p>
|
||||
<p>1. <i>With a wire antenna, does insulation make a difference in performance?</i></p>
|
||||
<p>This question is actually two questions in one. First, once we get the antenna set correctly, insulation makes no difference in how well the antenna performs. The gain will be virtually the same, whether the wire is bare or covered with any of the standard insulating materials.</p>
|
||||
<p>Second, insulation will make a difference in how long a wire must be to be resonant. If a bare wire is resonant on 10 meters (actually some specific frequency on 10 meters) at a length of 200", then an insulated wire will be resonant at the same frequency at a length from about 190" to about 198". Insulation creates a velocity factor (VF) so that the required physical length is always shorter than the required electrical length. If we set the VF of bare wire to 1.0, then the VF of insulated wire will be from 0.95 to 0.99.</p>
|
||||
<p>The nature of the insulation and its thickness determine the VF of insulated wire. The higher the dielectric constant of the material and the thicker the insulation, the lower the VF value. Unfortunately, there is no handy chart that you can consult when you buy insulated wire to tell you what the precise VF is. For dipoles, you simply prune to resonance.</p>
|
||||
<p>If you are thinking about a wire beam, where the elements are not resonant on the design frequency, then the best policy is to use the exact wire specified in the article or handbook from which you draw the design. There are techniques for handling insulated wire in these cases, but they generally require efforts or instruments that the average backyard antenna builder does not have.</p>
|
||||
<p>2. <i>Does the feedpoint gap make a difference in the antenna element length?</i></p>
|
||||
<p>If a design that you are copying calls for an element length of 200", then this length remains constant, regardless of how large you make the feedpoint gap--within reason, of course. The gap can range (at 10 meters) from 1/4" to a couple of inches without disturbing the overall length of the antenna element. The leads from the feedline to the inner edges of the gap make up the missing element section.</p>
|
||||
<p>Remember that the feedline is in series with the antenna element. The feedline itself begins where the line is at its proper form to create currents of equal magnitude and opposite phase. When you turn the wires of a parallel feedline at right angles to the line to make the antenna connection, those wires are part of the antenna. When you separate the coax braid and center conductor to make the same kind of connection, those leads are parts of the antenna element. Even if not identical in diameter to the antenna element itself, the length is short enough at HF not to make any difference in the way the element performs.</p>
|
||||
<p>3. <i>I have a multi-band doublet (or loop, etc.) that I use for all or most of the HF region. Can I connect a 4:1 balun at the antenna terminals and use coax to the antenna tuner in the shack?</i></p>
|
||||
<p>The answer to this question is yes, but there are better ways to handle the situation. The impedance presented by the antenna varies from one band to the next, ranging from very high to very low. If your coax run is long--perhaps 100' or so, your losses will climb according to the SWR on the line and the frequency. Higher frequencies and SWRs multiply natural coax losses.</p>
|
||||
<p>Let's look at alternatives to this system. The most efficient system is to run parallel transmission line from the antenna to the antenna tuner. Parallel transmission line naturally has much lower loss than coaxial cables (except for the very low-loss hard lines used at UHF and microwave frequencies). Hence, the multiplier that comes with high SWR values on the line tends to increase the loss by only a very little. Hence, in most cases, the losses in a high SWR parallel line system will be less than the losses in a perfectly matched 10-meter coax system.</p>
|
||||
<p>The installation rules for parallel line--whether it is 300-Ohm TV line, windowed vinyl 450-Ohm line, or open wire 600-Ohm line--differ from the rules that apply to coax. Keep the line free and clear of everything by several times the width of the line. Conductive materials--even wet wooden posts or trees--can disrupt the balance in the line and reduce its effective operation as a transmission line.</p>
|
||||
<p>4. <i>If I run parallel into the house/shack, I get RF pick-up by the X (where X may equal the telephone, the TV, the rig, etc.). How can I have my parallel line and no interference?</i></p>
|
||||
<p>The losses of coax increase with line length. Hence, a short piece of coax--something less than about 20' at 10 meters--will not create significant losses. Even at a 10:1 SWR, 20' of RG-213 will have less than a 1 dB loss. Now lets assume that we can place the antenna tuner within 20' of where the coax would pass though the wall/window/etc. to reach the outside world.</p>
|
||||
<p>We can install our parallel line from the antenna to this same wall/window--keeping it free and clear of unbalancing forces in the outdoor run. At the entry point, we can install a 1:1 bead balun. From the coax end of the balun, we run our short length of coax to the antenna tuner.</p>
|
||||
<p>a. I specified a 1:1 balun, not a 4:1 balun. Sometimes the 4:1 balun will do the job--sometimes it will not. Since the antenna shows a different impedance on every band, it is rarely matched to the feedline headed toward the shack. At every point along the line, the impedance is transformed to a new value--and some of these values will be very low. Further transforming the impedance to a still lower value by a 4:1 ratio is likely to yield such a low impedance that the tuner may not be able to handle it. Hence, a 1:1 balun is the better choice.</p>
|
||||
<p>b. I tend to prefer bead baluns for this transition, although other 1:1 balun types may work. Bead baluns--originated by Walt Maxwell, W2DU--are compact and inexpensive (from such sources as the Wireman in South Carolina). No balun will be lossless in this application, but the losses will be modest at moderate power levels.</p>
|
||||
<p>c. At the coax end of the balun, run a short, heavy ground lead from the coax braid to a good ground rod. This measure helps prevent significant levels of RF from being on the outside of the coax braid and thus helps reduce coupling into conductors inside the shack.</p>
|
||||
<p>This system is not perfect, but it is effective for using an all-band antenna with parallel feedline while suppressing unwanted RF coupling to systems other than the amateur radio equipment.</p>
|
||||
<p>5. <i>I have followed all of the recommendations and my all-band antenna will still not load up properly on 1 or more bands. What should I do?</i></p>
|
||||
<p>Let's assume that your system is flawless--good connections, not opens or shorts, etc. In this case, the most likely cause of a failure of the antenna to load is the length of transmission line.</p>
|
||||
<p>Earlier, I noted that when the antenna feedpoint impedance does not match the impedance of the feedline, the impedance is continually changing along the line. It is possible to calculate the impedance at the shack end of the line, but only if we know the impedance at the antenna feedpoint. In most cases, we do not know the impedance at the shack end of the line.</p>
|
||||
<p>The impedance may be a complex combination of a resistive value and a reactive value. Your antenna tuner has limits to the maximum and minimum resistive value that it will accept and transform. The reactive part of the impedance is the more likely culprit, since most tuner designs will compensate for only limited reactance. Depending on tuner design, the compensation range will be better for one type of reactance (capacitive or inductive) than for the other type. In most cases of a failure to provide a good match to the 50-Ohm requirement of the rig, the culprit an impedance at the tuner terminals that is outside the matching range of the tuner.</p>
|
||||
<p>Wait--do not throw away the tuner. Instead, change the length of the line to obtain a different combination of resistance and reactance at the tuner terminals. You can splice in a section (4 to 10 feet) of parallel line, either permanently or with knife switches. Just keep these added sections from folding over each other or coiling up. In other words, they should be free and clear like the rest of the line.</p>
|
||||
<p>Sometimes, you can find a single length of line that allows you to tune all bands with your multi-band wire antenna. In that case, you can make the connections permanent. In other cases, you can find two line lengths that between them allow you to handle all bands. A simple knife-switching system makes quick work of changing the connections.</p>
|
||||
<p>6. <i>I have the ground line at the shack entry point. Is my system safe from lightning?</i></p>
|
||||
<p>Not especially. The safest system for an amateur station that does not need to operate during a thunderstorm is one that disconnects the antenna line from the shack lead and reconnects it directly to a ground rod. Then any charges that accumulate on the antenna go to ground and not to the equipment.</p>
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<p>In addition, set up your station so that you can do a complete disconnect in preparation for a thunderstorm. Disconnect--pulling the plug is best--the AC lines. Also disconnect the equipment from the outside ground line. Now the equipment is disconnected from every source of electrical surge.</p>
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<p>Sensitive solid state devices do not care if the supply line has a high surge or whether the ground side of the device has a high surge. In either case, the voltage across them is destructively high. Hence, isolating the equipment from both the supply voltage and the grounding lines is necessary to prevent surge damage. Surge protectors help, but when the storm is near, nothing succeeds like isolation.</p>
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<p>You can design your station to make all of these disconnects easy--easy enough to apply whenever you shut down after an operating session. The only step that requires a trip outdoors is changing the antenna lead from shack lead-in to ground rod. If you disconnect the antenna routinely along with the other indoor disconnects, you can save the trip outdoors for an impending storm and still be relatively safe. Of course, if you travel out-of-town, set up the station as if you will have a thunderstorm every day that you are gone.</p>
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<p>7. <i>I have a beam on a tower. The base of the tower is strapped to ground rods. My coax from the antenna enters the shack, and I can change it from the rig to a second connector that goes to a ground rod outdoors. Am I safe?</i></p>
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<p>Maybe--maybe not. Experts suggests that most of the current in a lightning strike is in the coax braid. This current is very high, capable of destructively melting the insulation and arcing to other conductors. It is best if this current never enters the house at all. Hence, having an outdoor grounding system so that the coax never gets indoors during a thunderstorm is best.</p>
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<p>There is a second move that you can make. Since your have well-grounded the tower, and since the bulk of destructive lightning-induced current is in the braid, consider this measure. Near the top of the tower, clamp a small plate to a tower leg. On the plate, install a coax bulkhead connector--a double female connector with mounting hardware. Now run the antenna lead from the antenna to the connector and a second length of coax from the connector to the outdoor disconnect near ground level. The coax braid is now at the same potential as the tower leg and connected to the ground system serving the tower. You can add a second plate and connector at the tower base for added safety.</p>
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<p>If you maintain this system so that the connectors make excellent contact with the plates and the plates make excellent contact with the tower legs, you shunt currents on the outside of the braid to ground. However, this measure does not remove the need to disconnect the coax from the shack lead-in before it enters the shack. It just adds to the overall protection.</p>
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<p>There are more questions on my FAQ list. But these are enough for one column.</p>
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<p><i>Updated 10-10-2003. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i></p>
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<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10"> No. 4: Antennas and Low Sun Spot Counts<img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>As much as we hate to admit it, 10 meter conditions have been rotten. Instead of sunrise to sunset contacts across the ponds east to west, we hear only some weak north-south dx and a few weaker U.S. stations (from the U.S. perspective, of course). Occasionally during the day, the band will open a little for east-west skip. So, what kind of antenna is best for these conditions?</p>
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<p>The answer depends on another big question: what kind of operating do you want to do? Let's look at three main answers: a. I want to work my local chapter and friends. b. I want to work those super weak signals, c. I'll take whatever I can get whenever I can get it. Each answer suggests some different antennas.</p>
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<p><b>Local:</b> Local chapters of 10-10 and other area communications are still wide open on 10 meters. That is because they are primarily point-to-point. (Note: we habitually call this ground wave, but usually it is not. Ground waves (or "surface" waves, as some call them) peter out on 10 very quickly. Just as on VHF, our elevated antennas look directly at each other: that is point-to-point.) Since most established hams have horizontal antennas, a vertical can be a disadvantage unless everyone is very close. Up to half the signal gets lost due to cross polarization.</p>
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<p>The basic local antenna is still the dipole. If possible, make it rotatable, even if by hand, and as high as you can get it. Aluminum rod or tubing about 16' long, fed with coax at the center on a single mast works quite well.</p>
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<p>If you live on the fringe of your local group, consider a simple 2-element beam, either fixed to point at the group or rotatable. The commercial HF5B is a typical compact multi-band design with a wide forward lobe (to catch everybody in the group) and modest front-to-back ratio. Since there is little trouble from QRM, the modest F-B ratio actually lets you hear someone off the back of the beam better than a station with a large beam designed to suppress virtually everything off the backside.</p>
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<p>You can build a 2-element Yagi from supply store parts. If you are into building antennas, you might want to consider other designs for the fun of it. The Moxon rectangle, a wire beam, has about the broadest forward lobe and excellent rear suppression, but a little less gain. The X-beam is a bit tricky to set up, but is quite compact (a square under 10' on a side). Linear loaded Yagis can cut the element length from 16 to 12 feet. And the ZL-special can be built from 300-ohm parallel line (TV ribbon) on a light frame or suspended at its ends and fed with coax. If any readers are interested in these designs, I'll present some details in future columns. Meanwhile, look at any of the antenna books to gather some basic ideas.</p>
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<p><b>Weak Signal Work:</b> Much of the weak signal work done on 10 during sun spot lows is via backscatter, that is, by bouncing signals off the edge of the ionized layer, as weak as it is. That same layer, weakly hovering over the tropics, is responsible for the north-south skip that appears while the east-west path is too weak to support communications. So many 10-10ers point their beams south (or, if in the southern hemisphere, north) and listen carefully.</p>
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<p>For the best results on backscatter and very weak skip, an antenna with the maximum forward gain is best. Again, the front-to-back ratio is unimportant, since there is no QRM. (However, few hams want the antenna just for sun spot lows, so they do pay attention to front-to-back ratio.) A long Yagi is perhaps best among the aluminum antennas. If designs are optimized, the gain depends on the boom length, so the longer the better (if you can support it).</p>
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<p>Some wire antennas are capable of excellent gain, even if bidirectional. The extended double Zepp is over twice as long as a dipole, but narrows the beam width and has gain over the dipole. Dipoles (that is, antennas fed in the center) of a full wavelength can be set about a quarter wavelength apart and fed 180 degrees out of phase for additional gain. So too can EDZ dipoles (1.25 wavelengths long). These antennas usually (but not always) require parallel feeders and an antenna tuner. But the materials are cheap. Run the wires east to west for north and south lobes. For sun spot low backscatter and transequatorial skip, a fixed wire beam is not a bad choice. Again, if there is any reader interest, I'll provide more details in future columns.</p>
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<p><b>Anything I Can Get:</b> For general operation, use what you have and be patient. First, learn the band's habits. When do the openings occur? In what directions? How long do they last? Then plan your operating accordingly. Second, be sure your receiver is quiet so you can hear what there is. Early synthesized rigs are noisier internally than the preceding generation of crystal-mixing rigs. Direct-digital synthesis in today's rigs still leaves some noise on 10. That's why Ten-Tec offers both synthesized and crystal-mixing rigs and why some hams hang onto their old Drakes, TS-520s, and so on. The antenna is part of a system, not a solution to everything.</p>
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<p>Third, plan your antenna for long term use, not just for the present lull in apparent activity. If you have room for antennas and like to build, have fun with experimental designs. But also plan the main system for the day when the sun spots return. (And, although we 10-10ers hate to admit it, that planning may involve operation on other bands, too.)</p>
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<p>Finally, use your antenna! Very often a band is only as dead as the operators sitting around complaining about it. Listen to 10 on contest weekends to discover its true potential. Contesters (inadvertently) QRM each other on a supposedly dead band. Vertical or horizontal, your antenna will do you no good if everyone thinks the band is dead without checking. Find net frequencies, especially the daily 10-10 nets, and listen. If you cannot hear the NCS, request a relay check-in to find out if anyone is in the right place to hear you. Try some "CQs" near frequencies that used to be active. Make your presence known. . .of course, within the boundaries of good operating practice. 10 meters is more open than you think.<br>
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<p><i>Updated 3-17-97. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i><br>
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