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<title>Antennas From the Ground Up Index (42 Articles)</title>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>The following series of articles was originally prepared for publication in <i>Low Down</i>, the journal of the Colorado QRP Club. The series is designed to present fundamental antenna information, concentrating most fully on lower HF band wire antennas and related topics, to QRP operators. Since <i>Low Down</i> has devolved into a small newsletter, I have terminated the series and will place the remaining pre-prepared items here.</p>
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<blockquote>
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<b>Note</b>: This series (with revisions and enhancements) are available in book form under the titles from MFJ:
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<ul>
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<li><i><a href="https://mfjenterprises.com/products/mfj-3306" target="_blank">Antennas From the Ground Up, Volume 1</a></i> (Numbers 1 - 21)</li>
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<li><i><a href="https://mfjenterprises.com/products/mfj-3307" target="_blank">Antennas From the Ground Up, Volume 2</a></i> (Numbers 22 - 42)</li>
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</ul>
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</blockquote><br>
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<ol style="line-height: 1.5;">
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<li>
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<a href="gup1.html">Where Do Antennas Fit in Your Communications System?</a>
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</li>
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<li>
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<a href="gup2.html">The Resonant Half-Wavelength Center-Fed Antenna</a>
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</li>
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<li>
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<a href="gup3.html">Azimuth, Elevation, and Antenna Modeling</a>
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</li>
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<li>
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<a href="gup4.html">Making a Dipole Fit the Space Available</a>
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</li>
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<li>
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<a href="gup5.html">A 135' Center-Fed Multi-Band Dipole Data Compendium</a>
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</li>
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<li>
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<a href="gup6.html">A 102' Center-Fed Multi-Band Dipole Data Compendium</a>
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</li>
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<li>
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<a href="gup7.html">How to Make Your Tuner Work on Every Band</a>
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</li>
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<li>
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<a href="gup8.html">Horizontal vs. Vertical Antennas on the Low HF Bands</a>
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</li>
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<li>
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<a href="gup9.html">Fundamentals of Off-Center-Fed Dipoles</a>
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</li>
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<li>
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<a href="gup10.html">Harmonic Operation of OCFs</a>
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</li>
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<li>
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<a href="gup11.html">A 135' Off-Center-Fed Multi-Band Dipole Data Compendium</a>
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</li>
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<li>
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<a href="gup12.html">A 135' End-Fed Multi-Band Dipole Data Compendium</a>
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</li>
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<li>
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<a href="gup13.html">What Happens Along a Length of Feedline</a>
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</li>
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<li>
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<a href="gup14.html">ATUs, Delta, and Tuner Losses</a>
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</li>
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<li>
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<a href="gup15.html">More on ATUs, Delta, and Losses</a>
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</li>
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<li>
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<a href="gup16.html">Noise, Antennas, and Receiving Systems</a>
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</li>
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<li>
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<a href="gup17.html">Vertically-Oriented, Horizontally Polarized 1 wl Loops</a>
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</li>
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<li>
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<a href="gup18.html">Multiband Use of VOHPLs</a>
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</li>
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<li>
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<a href="gup19.html">Vertically Oriented, Vertically Polarized 1 wl Loops</a>
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</li>
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<li>
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<a href="gup20.html">Some Facts and Fantasies About Standing Wave Ratios</a>
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</li>
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<li>
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<a href="gup21.html">A Short Look at Wire Beams</a>
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</li>
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<li>
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<a href="gup22.html">A Horizontal 80-Meter Multi-Band Loop Data Compendium</a>
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</li>
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<li>
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<a href="gup23.html">Where to Place Your Impedance Matching Efforts</a>
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</li>
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<li>
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<a href="gup24.html">The 75-Ohm 1/4 Wavelength Matching Section</a>
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</li>
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<li>
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<a href="gup25.html">A 1/2 Wavelength Inverted-L Multi-Band Antenna Data Compendium</a>
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</li>
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<li>
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<a href="gup26.html">A 3/8 Wavelength Inverted-L Multi-Band Antenna Data Compendium</a>
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</li>
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<li>
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<a href="gup27.html">Differentiating Among Many Types of Grounds</a>
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</li>
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<li>
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<a href="gup28.html">Why Parasitic Beams Work</a>
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</li>
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<li>
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<a href="gup29.html">Interesting Alternatives to the Yagi</a>
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</li>
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<li>
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<a href="gup30.html">A Collection of Inverted-Vee Patterns</a>
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</li>
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<li>
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<a href="gup31.html">Handling Parallel Feedlines</a>
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</li>
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<li>
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<a href="gup32.html">When and Which Parallel Feedline to Use</a>
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</li>
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<li>
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<a href="gup33.html">Unwanted Currents and Their Suppression</a>
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</li>
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<li>
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<a href="gup34.html">The 44' Doublet as a 40-10 Meter</a>
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</li>
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<li>
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<a href="gup35.html">Sorting Out Bi-Directional Phased Arrays</a>
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</li>
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<li>
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<a href="gup36.html">The Dual Expanded Lazy-H for 80-10 Meters</a>
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</li>
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<li>
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<a href="gup37.html">A Potpourri of Bent Dipoles</a>
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</li>
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<li>
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<a href="gup38.html">A Collection of Quadrant Antenna Patterns</a>
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</li>
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<li>
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<a href="gup39.html">A Nearly All-Band Vertical Doublet</a>
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</li>
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<li>
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<a href="gup40.html">The Terminated Wide-Band Folded Dipole Antenna</a>
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</li>
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<li>
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<a href="gup41.html">The Terminated (Very) Longwire Antenna</a>
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</li>
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<li>
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<a href="gup42.html">The Terminated Vee-Beam and Rhombic</a>
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</li>
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</ol>
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<br>
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<img src="../images/colorbar.gif" align="middle" width="540" height="4">
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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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<p class="topics-footer">Topics: <a href='../topics.html#magazine-columns'>Magazine Columns</a></p><br>
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<title>Where Do Antennas Fit in Your Communications System?</title>
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<h1><img src="../images/yagicad.gif" align="left" width="125" height="62" border="2" vspace="5" hspace="10">ANTENNAS FROM THE GROUND UP<img src="../images/w4rnlwwr.gif" align="right" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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<h3>1. Fantastic Feedlines,<br>
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or Where Do Antennas Fit in Your Communications System?</h3>
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<center>
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<h3>L. B. Cebik, W4RNL</h3>
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<br>
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We can all buy or build a box full of parts called a radio, or a transceiver, if we want to be precise. Then we add accessories: headphones, keyers, paddles, computers, antennas. . .
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<p>Antennas? Unfortunately, feedlines and antennas are too often looked at as accessories, as if the "rig" was the most important thing in the world in a communications system. And that has blocked our attention to and understanding of antennas. So let's start all over again by looking at a communications system. See Figure 1 for a sketch of a basic system.</p>
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<p>As you can see, a true communications systems consists of at least two stations: either a minimum of two or no communications will occur. Each station receives and transmits. Each needs devices to receive and to transmit. What transmitting and receiving have in common--usually, but not absolutely always--is a transmission line and antenna. For some special purposes, like working 160 meters, some folks use separate receiving and transmitting antennas, and separate feedlines.</p>
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<p>Although this may seem very basic, let's review the functions of these components of the communications system:</p>
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<p><b>Transmitter</b>: a collection of circuits (composed of components) that generates RF electrical energy and modifies it to include intelligence.</p>
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<p><b>Receiver</b>: a collection of circuits (composed of components) that separates the intelligence from the incoming RF electrical energy for station use.</p>
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<p><b>Transmission line</b>: transfers RF electrical energy from the transmitter to the antenna with minimal loss. Also transfers RF electrical energy from the antenna to the receive with minimal loss. Required only when the antenna is remote from the transmitter.</p>
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<p><b>Antenna</b>: a transducer that converts RF electrical energy into electro- magnetic radiation or fields and which converts intercepted electro- magnetic radiation into RF electrical energy.</p>
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<p>Interestingly, if we connect the transmitter and receiver directly to the antenna, as we virtually do with VHF handhelds (walkie-talkies?), we can eliminate the most interesting part of the communications system: the transmission line. Transmission lines are not interesting just because they seem mysterious. They are especially interesting because they are the key to everything else. Some traditional treatments of communications electronics begin with transmission line theory. From there, they derive lumped components (capacitors and inductors) and then they also derive antennas.</p>
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<p>Notice that I said "lumped components (capacitors and inductors)." I did not say "capacitance and inductance. One of the things we have to do to understand antennas and transmission lines is to separate in our minds the ideas of things--the capacitors and inductors--from the basic phenomena-- capacitance and inductance. Capacitors and inductors are just the things we use to implement controlled amounts of capacitance and inductance when and where we want them.</p>
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<p>Transmission lines also exhibit capacitance and inductance. Every length of wire exhibits inductance, and every pair of surfaces (like wire surfaces) exhibit capacitance. Although transmission lines come in many varieties ranging from one wire to many, the two wire line--coax or parallel--is most familiar to us. Two wires with a finite length equals capacitance and inductance.</p>
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<p>Transmission lines have something else of note: standing waves. A standing wave is nothing more than the variations in voltage and current along a transmission line. When measured and plotted on a graph, they replicate an electrical wave as we are accustomed to picturing it: something more or less like a sine wave with many possible variations.</p>
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<p>Standing waves are functions of basic electrical properties: If we pass a current through a wire, it generates an electro-magnetic field. If the electrical energy is alternating current (AC) and high enough in frequency (RF), then that field will radiate, that is, be detectable at a distance. It will be detectable unless we can control and confine it. Transmission lines confine the field by having equal but opposite voltages and currents on each wire in a two wire system in close enough proximity so that the fields cancel each other.</p>
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<p>For a full account of the parameters of coaxial transmission lines, see <i>QEX</i>, August, 1996, pp. 3-10.</p>
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<p>Now let's look at our lumped components: If they were large enough, they would have standing waves. They still have fields. If they did not, the grid-dip oscillator would not work. Of course, we try to control and confine component and interconnection-wire fields by a lot of methods. We confine inductor fields with iron or ferrite coil cores. We position components relative to each other for minimum coupling. We install shielding to keep inside fields from interacting with outside fields. Remember, however, that we can make use of component radiation. For example, we can make a large planar coil for the plate circuit of a tube and use it simultaneously as a lumped component in the circuit and as a loop antenna not especially less efficient than those used externally to the transmitter. (See, for example, <i>Communications Quarterly</i>, Winter, 1994, pp. 7-8.)</p>
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<p>Since every capacitor exhibits some inductance and every inductor exhibits some capacitance (again, controlled to a minimum for predictable circuit use), every lumped component is a sibling of a transmission line. Just ask VHF designers who use transmission line components instead of inductors.</p>
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<p>What about the other end of the transmission line: the antenna? Are they transmission lines? According to a traditional analysis, still very useful for low and very low frequencies, we can visually and mathematically picture an antenna as a transmission line spread apart so that the wires are collinear (end to end) rather than parallel. See Figure 2 for a very simplified sketch of the antenna and one of its several types of fields. A lot of stuff happens if you do open up the transmission line in this way.</p>
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<p>First, the fields around the wires are no longer confined by equal but opposite adjacent fields. Second, the intensity of the field and its phase may be just far enough away so that they add to the total field intensity in some directions and subtract from it in others.</p>
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<p>In a transmission line, only enough power is consumed to overcome the losses of the wire system relative to establishing the fields of the standing wave. The remainder of the power supplied by the source is available at the other end of the line for the load. Of course, if we manage to unbalance the transmission line, it will act like an opened transmission line for that part of the voltage and current not in balance. In other words, it will act like an antenna.</p>
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<p>The antenna is the transmission line's load in a transmitting situation because its fields are no longer confined. The current along the line yields fields that may spread without limit (with respect to the system). In short, the electrical energy is transformed into electro-magnetic radiation, and it must be resupplied from the source to maintain the ever-expanding fields.</p>
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<p>If you are ready for a little surprise, the energy of an antenna does not in fact travel in the antenna wire or tubing. It travels along the field lines in Figure 2 outside the wire or tube. In addition, the energy in a transmission line exhibits the same property. We shall look deeper into this mystery in another installment.</p>
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<p>On the receiving side, the antenna is always intercepting electro-magnetic fields, which set up currents in (or along) the antenna elements. The pattern of interception is the same as transmission, so that maximum current is generated from intercepted fields in the same direction as the maximum radiation of transmitted fields. The transmission line is in series with the currents, acting as the antenna's load and setting up equal but opposite polarity voltages and currents along its wires, voltages and currents that are amplified and processed by the receiver as radio frequency electrical energy..</p>
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<p>Or, to put it another way, the opened transmission line converts currents to fields and fields to currents. Antennas have capacitance and inductance, just as do transmission lines. More nearly correctly, antennas have capacitive and inductive reactance, from which we can calculate equivalent capacitances and inductances. Antennas of certain lengths show no reactance (we call them resonant), while antennas of other lengths show either an inductive or capacitive reactance at the feedpoint. We can cancel out the reactance with a lumped component having an equal amount of the opposite type of reactance. What is handy is that we can do the canceling right at the antenna terminals or remotely with an antenna tuning unit (ATU). Yes, we shall look at how ATUs work in a future column.</p>
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<p>So, what's your point? Any antenna theorist might well object to this account as too simplified (and hence distorting to some degree). In fact, for a more accurate account of how antennas fundamentally work, see Kenneth McCleish, W7TX, "Why an Antenna Radiates," <i>QST</i>, November, 1992, pp. 59-63. His account is traceable to Maxwell's Laws (whereas mine may be traceable only to Murphy's Laws).</p>
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<p>Actually, there are several points. First, the account does capture an essential interrelationship at basic levels among every part of the communications system. Second, it is important to every operator with any technical inclination to develop an interest in and appreciation of how the seemingly separate elements of a communications system interrelate.</p>
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<p>Third, learning about antennas and transmission lines requires exactly the same concepts we have to master to learn about circuits and components. We just use them in a different pattern of priority. For example, we match one circuit's output to another's input, and that is a matter of impedance matching and transformation--just like antenna work. We cancel out unwanted inductance with capacitance and vice versa both in circuits and in antennas. Understanding resistance, reactance, capacitance, and inductance will get you more than half way through both circuitry and antennas--and all the way through transmission lines.</p>
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<p>Fourth, the account will help us talk some of the same language as we explore antennas. Part of the problem of reading magazines articles about great new antennas is that each author tends to use antenna and transmission line words a little differently. Understanding an author requires that we catch on to how he is using the words. This little introductory picture of how antennas, transmission lines, and circuits interrelate will help you catch on to how I use various antenna concepts. And that will help you understand the next few installments when we really get down to business.</p>
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<p>Although I shall keep the math of these articles to a minimum, you should be willing to do some of it. Unless you just love hand calculation, I recommend that you get a copy of <a href="../radio.html#HAMCALC">HAMCALC</a>. This collection of ham calculation programs uses GW BASIC (supplied with the structured array) so that you can look at the equations behind the calculations just by listing any of the programs. And you can also run through a string of input values and get a feel for the pattern of the results. This will help build your intuitions about what is sensible and what is problematical about some antenna or feedline problem you are having. Any similar collection of calculation programs will do equally well, but I know of none that are both so comprehensive and so open to inspection.</p>
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<p>Get a copy of the latest HAMCALC from George Murphy, VE3ERP, 77 McKenzie Street, Orillia, Ontaria, Canada L3V 6A6. Include a $5 donation, which George uses to pay for disks and postage from Canada and then donates the excess to the Canadian National Institute for the Blind to support its ham radio work.</p>
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<p>Finally, to understand antennas and get a good feel for them, you have to change your way of looking at and thinking about them--unless you already think about them in something like the way we have started here. We shall not stick to the transmission line analogy for antennas, but we shall in every installment always think of antennas as integrated parts of our communications systems.<br>
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<br></p>
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<img src="../images/colorbar.gif" align="middle" width="540" height="4">
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<p><i>Updated 4-27-97. © 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></p>
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<center>
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<p><a href="groundup.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to series index 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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<p class="topics-footer">Topics: <a href='../topics.html#transmission-line'>Transmission Line</a></p><br>
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<h1><img src="../images/yagicad.gif" align="left" width="125" height="62" border="2" vspace="5" hspace="10">ANTENNAS FROM THE GROUND UP<img src="../images/w4rnlwwr.gif" align="right" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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<h3>10. Up, Up, and Away<br>
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||||
or Harmonic Operation of OCFs</h3>
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</center>
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<center>
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<h3>L. B. Cebik, W4RNL</h3>
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</center><br>
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<br>
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<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center><br>
|
||||
<br>
|
||||
In our last episode, we looked at the operation of the off-center-fed dipole (OCF) and discovered no magic--only something like a þregularþ center-fed dipole. However, many of the claims made for the OCF have to do with the convenience or the power of using the antenna on its harmonics. To be thorough, we must look in a little more detail at this aspect of the OCF. Perhaps there is magic left in the antenna yet.
|
||||
<p>To review, our OCF looks like this:</p>
|
||||
<center>
|
||||
<img src="gup10-1.gif" width="497" height="338" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>I am taking this long on the OCF for a couple of reasons. First, the antenna has been the subject of considerable speculation, not to mention overzealous claims. These have obscured or ignored many of the basic facts about OCF operation. Second, antenna modeling is growing more popular. For newcomers to this higly educational activity, there is a temptation to accept what the machine says without checking the basic adequacy of the model fed to the machine. Hence, some added notes about modeling the OCF might prove useful to new owners of NEC and MININEC.</p>
|
||||
<p><b>How the Harmonic Stuff Got Started</b>: When the OCF first appeared on the ham scene, vacuum tube output stages were in vogue. Initially, these stages used parallel-tuned high-impedance output circuits with tuned link coupling to transform the tube's 1200 to 3000 ohm impedance to something closer to feedline impedances. Later, PI networks came into popularity for their ability to permit bandswitching, but wide ranging adjustment of the circuit remained possible.</p>
|
||||
<p>Either type of adjustable circuit would match the tube impedance to a wide range of output impedance requirements. A range of 20 to 200 ohms was not uncommon. Without an ATU, one might directly match lines presenting up to 4:1 SWR levels with good efficiency of power transfer. The OCF was popular as a directly matched antenna on its fundamental and most of its even harmonics.</p>
|
||||
<p>Even before the advent of fixed tuned solid state output stages, manufacturers began removing the "load" control from the new SSB transceivers. Thus was born the 2:1 SWR limit of ham transmitter output stages. Suddenly, OCF users began to have troubles effecting a direct match from the feedline to the transmitter. Since the dang system put RF into the shack anyway, many simply chose other types of antennas. From the 1960s to the 1980s, the OCF went into decline. Then came some US and Canadian hams with new antenna kits and new claims for the antenna. These resulted largely from the ease of making matching transformers using the more readily available iron and ferrite cores. We thought we could hit that magic 300-ohm or equally magic 100-ohm point and match our way to a mutliband antenna.</p>
|
||||
<p>Some purveyors of OCF kits said "Put them up at 35' in a field free and clear of ground clutter." When buyers complained that the antenna seemed not to work on one or another band, the sellers simply blamed the operator's site (and gave no refund). Some have suspected that other kit makers have intentionally or unwittingly used a fairly lossy matching device at the antenna-feedline junction to make a match on all advertised bands "at any cost."</p>
|
||||
<p>So we have two questions to answer: a. What happens to the impedance of an OCF on the even harmonics? And b. Are the patterns on these harmonics as good as they are claimed to be? Once more, we shall use our friendly old center-fed half wavelength dipole as the standard of comparison.</p>
|
||||
<p><b>Impedance</b>: First, the resonant lengths of an OCF are not directly harmonically related. The following four tables show the resonant frequencies of the fundamental and even harmonics for a 7.15 MHz OCF at 70' and at 35' for fundamental impedances in the 100-ohm and the 300-ohm vicinities. Resonance is defined for this exercise as a feedpoint impedance with less than 1 ohm reactance. Dimensions are listed as minus and plus either side of the feedpoint.</p>
|
||||
<pre>1. Length: -10.4'/+57.35'
|
||||
Height: 70' Material: #14 copper
|
||||
Frequency Impedance
|
||||
7.15 MHz 302 ohms
|
||||
14.34 MHz 138 ohms
|
||||
28.77 MHz 130 ohms
|
||||
|
||||
2. Length: -11.71'/+55.15'
|
||||
Height: 35' Material: #14 copper
|
||||
Frequency Impedance
|
||||
7.15 MHz 298 ohms
|
||||
14.56 MHz 117 ohms
|
||||
29.13 MHz 173 ohms
|
||||
|
||||
3. Length: -20.21'/+47.15'
|
||||
Height: 70' Material: #14 copper
|
||||
Frequency Impedance
|
||||
7.15 MHz 102 ohms
|
||||
14.38 MHz 107 ohms
|
||||
29.0 MHz 312 ohms
|
||||
|
||||
4. Length: -24.89'/+41.48'
|
||||
Height: 35' Material: #14 copper
|
||||
Frequency Impedance
|
||||
7.15 MHz 98 phms
|
||||
14.63 MHz 191 ohms
|
||||
29.37 MHz 115 ohms</pre>
|
||||
<p>The resonant frequencies of the harmonics of the fundamental frequency to which the antenna is originally cut depart more radically from true harmonics the closer the antenna is to the ground in terms of fractions of a wavelength. Note that since the model used in this exercise is for 40 meters, an 80-meter OCF at 35' and 70' would be only 1/8 and 1/4 of a wavelength in height, respectively.</p>
|
||||
<p>At true harmonics within the ham bands, the feedpoint impedance of the antenna on 20 and 10 meters would contain a large reactive component. Although this situation is not significant for a system consisting of parallel feedline and an antenna tuning unit, it presents major problems for systems using a balun and coaxial cable. Transmission-line transformer baluns of the type designed and investigated by W2FMI require resistive loads.</p>
|
||||
<p>The impedances at the feedpoint for ham bands which are not even harmonics of the antenna's fundamental frequency are not suited for balun operation. If the antenna is to be used on any of these amateur bands, parallel feedline and an ATU is a must.</p>
|
||||
<p><b>Patterns</b>: The azimuth patterns for a center-fed half wavelength dipole operated on harmonic frequencies are quite different from those of the harmonics of an OCF. Interestingly, with or without feedline radiation, the elevation angles of maximum radiation are the same for either antenna, making pattern comparison relatively uncomplicated. For comparison, here are the azimuth patterns for the center-fed antenna:</p>
|
||||
<p>1. 7.15 MHz; elevation angle 27 degrees</p>
|
||||
<center>
|
||||
<img src="gup10-2.gif" width="624" height="468" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>This pattern is not significantly different from that of an OCF when the antenna element alone is presumed to radiate. The side rejection of is about 11 dB.</p>
|
||||
<p>2. 14.15 MHz; elevation angle 14 degrees</p>
|
||||
<center>
|
||||
<img src="gup10-3.gif" width="624" height="468" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The second harmonic of a center-fed half wavelength dipole yields a full wavelength antenna with a high feedpoint impedance, along with higher maximum gain in a narrower beamwidth.</p>
|
||||
<p>3. 28.5 MHz; elevation angle 7 degrees</p>
|
||||
<center>
|
||||
<img src="gup10-4.gif" width="624" height="468" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>To get a feel for how the OCF differs in harmonic performance, let's look at azimuth patterns for the same frequencies and elevation angles for OCFs in which only the antenna element is presumed to radiate. These antennas are modeled for a 300-ohm feedpoint impedance.</p>
|
||||
<p>1. 7.15 MHz; elevation angle 27 degrees</p>
|
||||
<center>
|
||||
<img src="gup10-5.gif" width="624" height="468" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>This pattern is indistinguishable from the pattern of the center-fed dipole shown above. Wherever patterns are identical, antenna performance will also be identical. Harmonic patterns, however, will differ considerably.</p>
|
||||
<p>2. 14.15 MHz; elevation angle 14 degrees</p>
|
||||
<center>
|
||||
<img src="gup10-6.gif" width="624" height="468" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>This cloverleaf most resembles the 10-meter pattern of the center-fed dipole. The pattern may be more useful for general communications than the bi-directional pattern of the center-fed dipole, although the dipole pattern can reduce QRM from the sides more effectively.</p>
|
||||
<p>3. 28.5 MHz; elevation angle 7 degrees</p>
|
||||
<center>
|
||||
<img src="gup10-7.gif" width="624" height="468" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The 8-lobe pattern of the OCF operated on 10 meters is distinctly canted toward the longer end of the antenna. However, do not confuse the relatively high gain with that of a beam. Even a simple 2-element Yagi at a half wavelength height will have slightly more gain and at least 2 S- units of front-to-back ratio.</p>
|
||||
<p>Claims have been made to the effect that allowing the feedline to radiate provides benefits in the signal pattern. To test this claim, let's look at azimuth patterns for 7.15 MHz, 14.15 MHz, and 28.5 MHz for an OCF at 70' height and designed for the 300-ohm feedpoint. The radiating feedline is presumed to be restricted to 10' by use of a line isolator or W2DU choke.</p>
|
||||
<p>1. 7.15 MHz; elevation angle 27 degrees</p>
|
||||
<center>
|
||||
<img src="gup10-8.gif" width="624" height="468" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>2. 14.15 MHz; elevation angle 14 degrees</p>
|
||||
<center>
|
||||
<img src="gup10-9.gif" width="624" height="468" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>3. 28.5 MHz; elevation angle 7 degrees</p>
|
||||
<center>
|
||||
<img src="gup10-10.gif" width="624" height="468" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>In all cases, the antenna gain is down slightly. Only in 10-meter operation does the antenna show any marked effects from the vertical radiator, as four of its lobes disappear into one broad and modest lobe. For operational purposes, the likely effect of using the short section of feedline to supplement radiation from the antenna element is negligible.</p>
|
||||
<p>To be fair, let's also look at the case of using 35' of the feedline as a supplemental radiator. Compare each of the azimuth patterns (all of which have the same elevation angle of maximum radiation as the preceding patterns on each band) with its corresponding patterns above.</p>
|
||||
<p>1. 7.15 MHz; elevation angle 27 degrees</p>
|
||||
<center>
|
||||
<img src="gup10-11.gif" width="624" height="468" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>2. 14.15 MHz; elevation angle 14 degrees</p>
|
||||
<center>
|
||||
<img src="gup10-12.gif" width="624" height="468" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>3. 28.5 MHz; elevation angle 7 degrees</p>
|
||||
<center>
|
||||
<img src="gup10-13.gif" width="624" height="468" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>A comparison of the patterns using 10' of feedline radiator and 35' of feedline radiator demonstrates the variability of the effect of supplementing the antenna element's radiation. The 10-meter pattern has returned to its shape with no feedline radiation, while the 20-meter pattern shows more cant toward the long end of the antenna and the 40-meter pattern shows a measurable decrease in gain with only a slight widening of the pattern off the antenna element ends.</p>
|
||||
<p>In the final analysis, it is doubtful that feedline radiation will either significantly help or significantly harm the performance of an OCF antenna. Feedline radiation will affect the presumed impedance of the antenna, making it less predictable. Consequently, the use of an ATU is always a wise procedure.</p>
|
||||
<p>However, if one is going to use an ATU, then maximizing the use of parallel feedline is the surest way to reduce losses. As noted in the initial installment on this antenna, conversion to coaxial cable at the shack entrance may be necessary to prevent RF on equipment cases. This precaution is wise even at QRP, since RF in the shack is lost to propagation.</p>
|
||||
<p>We have one more question to ask: how does the OCF perform as an "all-band" antenna? But we shall save that question for next time.<br>
|
||||
<br></p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center>
|
||||
<p><i>Updated 1-25-98. © 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></p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center>
|
||||
<p><a href="groundup.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to series index 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>
|
||||
<p class="topics-footer">Topics: <a href='../topics.html#hf-multi-band'>HF Multi Band</a>, <a href='../topics.html#off-center-fed-dipole'>Off Center Fed Dipole</a></p><br>
|
||||
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|
||||
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|
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||||
<!DOCTYPE html>
|
||||
<html lang="en">
|
||||
<head>
|
||||
<meta charset="UTF-8">
|
||||
<meta name="keywords" content="Off Center Fed Dipole, HF Multi Band">
|
||||
<title>A 135' Off-Center-Fed Multi-Band Dipole Data Compendium</title>
|
||||
<link rel="stylesheet" href="../styles.css">
|
||||
</head>
|
||||
<body>
|
||||
<center>
|
||||
<h1><img src="../images/yagicad.gif" align="left" width="125" height="62" border="2" vspace="5" hspace="10">ANTENNAS FROM THE GROUND UP<img src="../images/w4rnlwwr.gif" align="right" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
|
||||
</center>
|
||||
<center>
|
||||
<h3>11. Unbalanced But Sane<br>
|
||||
or a 135' Off-Center-Fed Multi-Band Dipole Data Compendium</h3>
|
||||
</center>
|
||||
<center>
|
||||
<h3>L. B. Cebik, W4RNL</h3>
|
||||
</center><br>
|
||||
<br>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center><br>
|
||||
<br>
|
||||
The fundamental concepts and realities of Off-Center-fed (OCF) wire antennas developed in terms of 40-meter models apply to any other fundamental band with adjustments of height in terms of fractions of a wavelength. However, several folks have asked for more specifics for an 80-meter-based OCF in order to have reasonable expectations of performance.
|
||||
<center>
|
||||
<img src="gup11-1.gif" width="620" height="429" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>For an 80-meter fundamental OCF , L will equal about 135' if the antenna is resonant. If the antenna is fed with parallel transmission line to an ATU, the exact length is not critical. However, for the sample patterns, it has been made resonant.</p>
|
||||
<p>We shall sample an 80-meter OCF at 35' height, a typical amateur installation. The frequency of basic resonance is 3.5 MHz to ensure that harmonics fall within the amateur bands. However, pattern samples will cover all 8 amateur bands between 80 and 10 meters.</p>
|
||||
<p>We shall also sample two points of antenna element-to-feeder junction: the 100-ohm and the 300-ohm points. The antenna dimensions for these two antennas are as follows:</p>
|
||||
<ul>
|
||||
<li>
|
||||
<p>100-ohm Connection: L = 135.25' D1 = 34.7' D2 = 100.55'</p>
|
||||
</li>
|
||||
<li>
|
||||
<p>300-ohm connection: L = 135.67' D1 = 18.2' D2 = 117.47'</p>
|
||||
</li>
|
||||
</ul>
|
||||
<p>Interestingly, this same antenna raised to a height of 70' shows the following resonant characteristics:</p>
|
||||
<ul>
|
||||
<li>
|
||||
<p>100-ohm Connection: L = 135.7' D1 = 50.5' D2 = 85.2'</p>
|
||||
</li>
|
||||
<li>
|
||||
<p>300-ohm Connection: L = 136.55' D1 = 24.1' D2 = 112.45'</p>
|
||||
</li>
|
||||
</ul>
|
||||
<p>35' and 70' are 1/8 and 1/4 wavelength heights, respectively, a region where we can expect large changes of antenna characteristics with small changes in height. The patterns and numbers associated with the antennas should thus be used with due caution. They are indicators, not absolutes.</p>
|
||||
<p>For each pattern, a feedpoint impedance figure, Zi, is given. Zi is the impedance assuming that the radiation is confined to the antenna element. Since this assumption will be violated in most installations using parallel feedline directly connected to the antenna, the impedance figures are best treated as indicators of moderate or high impedances, not as numbers to be expected in reality. The actual impedance presented to the ATU will be a complex combination of the degree to which the feedline contributes to radiation and the transformation of the antenna impedance along the line.</p>
|
||||
<h4>80 Meters: 3.6 MHz</h4>100-ohm Connection: Zi = 120 + j100 ohms -- Elevation angle = 45 degrees
|
||||
<center>
|
||||
<img src="gup11-2.gif" width="616" height="741" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Note: Since the actual elevation angle of maximum radiation is greater than 45 degrees for 80 and 40 meters, the azimuth patterns are taken at an elevation angle of 45 degrees.</p>
|
||||
<p>300-ohm Connection: Zi = 455 + j310 ohms -- Elevation angle = 45 degrees</p>
|
||||
<center>
|
||||
<img src="gup11-3.gif" width="618" height="741" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<h4>40 Meters: 7.15 MHz</h4>100-ohm Connection: Zi = 105 + j15 ohms -- Elevation angle = 45 degrees
|
||||
<center>
|
||||
<img src="gup11-4.gif" width="614" height="741" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Note: For a different slant on OCF antennas, see Chapter 3 of Bill Orr, W6SAI, <i>HF Antenna Handbook</i>. He presents some interesting variations on the versions studied here.</p>
|
||||
<p>300-ohm Connection: Zi = 185 + j20 ohms -- Elevation angle = 45 degrees</p>
|
||||
<center>
|
||||
<img src="gup11-5.gif" width="618" height="741" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<h4>30 Meters: 10.1 MHz</h4>100-ohm Connection: Zi = 690 - j890 ohms -- Elevation angle = 37 degrees
|
||||
<center>
|
||||
<img src="gup11-6.gif" width="620" height="741" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Note that differences between the 100-ohm and the 300-ohm patterns are beginnig to appear with respect to both azimuth and elevation.</p>
|
||||
<p>300-ohm Connection: Zi = 90 - j295 ohms -- Elevation angle = 36 degrees</p>
|
||||
<center>
|
||||
<img src="gup11-7.gif" width="618" height="741" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<h4>20 Meters: 14.15 MHz</h4>100-ohm Connection: Zi = 3145 - j1355 ohms -- Elevation angle = 28 degrees
|
||||
<center>
|
||||
<img src="gup11-8.gif" width="620" height="741" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>At 20 meters, there is a wide divergence between 100-ohm and 300-ohm patterns, with the 100-ohm pattern more closely resembling the pattern of a center-fed antenna.</p>
|
||||
<p>300-ohm Connection: Zi = 120 - j105 ohms -- Elevation angle = 25 degrees</p>
|
||||
<center>
|
||||
<img src="gup11-9.gif" width="620" height="741" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<h4>17 Meters: 18.1 MHz</h4>100-ohm Connection: Zi = 205 + j15 ohms -- Elevation angle = 20 degrees
|
||||
<center>
|
||||
<img src="gup11-10.gif" width="618" height="741" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Note that the 100-ohm and 300-ohm patterns are again similar, with difference confined to the minor lobes.</p>
|
||||
<p>300-ohm Connection: Zi = 170 + j65 ohms -- Elevation angle = 19 degrees</p>
|
||||
<center>
|
||||
<img src="gup11-11.gif" width="616" height="741" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<h4>15 Meters: 21.15 MHz</h4>100-ohm Connection: Zi = 135 + j240 ohms -- Elevation angle = 17 degrees
|
||||
<center>
|
||||
<img src="gup11-12.gif" width="622" height="741" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>On 15 meters, the 300-ohm version of the antenna shows considerably less gain to the short side of the antenna element.</p>
|
||||
<p>300-ohm Connection: Zi = 920 - j600 ohms -- Elevation angle = 17 degrees</p>
|
||||
<center>
|
||||
<img src="gup11-13.gif" width="618" height="741" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<h4>12 Meters: 24.95 MHz</h4>100-ohm Connection: Zi = 545 - j420 ohms -- Elevation angle = 15 degrees
|
||||
<center>
|
||||
<img src="gup11-14.gif" width="616" height="741" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Note that, although the number of lobes are the same, the 300-ohm version puts more energy into high lobes and less into lobes off the ends of the antenna.</p>
|
||||
<p>300-ohm Connection: Zi = 2930 + j350 ohms -- Elevation angle = 15 degrees</p>
|
||||
<center>
|
||||
<img src="gup11-15.gif" width="620" height="741" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<h4>10 Meters: 28.5 MHz</h4>100-ohm Connection: Zi = 1755 - j905 ohms -- Elevation angle = 14 degrees
|
||||
<center>
|
||||
<img src="gup11-16.gif" width="620" height="741" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Note the deep nulls broadside to the 100-ohm version of the antenna, compared to the smoother lobe structure of the 300-ohm version.</p>
|
||||
<p>300-ohm Connection: Zi = 875 - j925 ohms -- Elevation angle = 15 degrees</p>
|
||||
<center>
|
||||
<img src="gup11-17.gif" width="620" height="741" border="2" vspace="5" hspace="9">
|
||||
</center><br>
|
||||
<br>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center>
|
||||
<p><i>Updated 1-20-98. © 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></p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center>
|
||||
<p><a href="groundup.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to series index 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>
|
||||
<p class="topics-footer">Topics: <a href='../topics.html#hf-multi-band'>HF Multi Band</a>, <a href='../topics.html#off-center-fed-dipole'>Off Center Fed Dipole</a></p><br>
|
||||
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|
||||
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|
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<!DOCTYPE html>
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<html lang="en">
|
||||
<head>
|
||||
<meta charset="UTF-8">
|
||||
<meta name="keywords" content="EDZ / Zepp, HF Multi Band">
|
||||
<title>A 135' End-Fed Multi-band Dipole Data Compendium</title>
|
||||
<link rel="stylesheet" href="../styles.css">
|
||||
</head>
|
||||
<body>
|
||||
<center>
|
||||
<h1><img src="../images/yagicad.gif" align="left" width="125" height="62" border="2" vspace="5" hspace="10">ANTENNAS FROM THE GROUND UP<img src="../images/w4rnlwwr.gif" align="right" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
|
||||
</center>
|
||||
<center>
|
||||
<h3>12. A Tiger by the Tail<br>
|
||||
or a 135' End-Fed Multi-band Dipole Data Compendium</h3>
|
||||
</center>
|
||||
<center>
|
||||
<h3>L. B. Cebik, W4RNL</h3>
|
||||
</center><br>
|
||||
<br>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center><br>
|
||||
<br>
|
||||
A 1/2 wavelength wire antenna, fed at one end with parallel transmission line is also known as the end-fed Zepp. Figure 1 shows the general outlines of a Zepp cut for the 80-meter band.
|
||||
<center>
|
||||
<img src="gup12-1.gif" width="620" height="499" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The Zepp can be cut for other bands, but a 135' length of #14 wire at a height of about 35' is one of the most common ham installations. 35' is only about 1/8 wavelength at 80 meters and about 1 wavelength at 10 meters. Therefore, the Zepp will exhibit all the variability and sensitivity to terrain of any low horizontal antenna.</p>
|
||||
<p>The distinguishing feature of the Zepp is that it is fed at a high-voltage low-current point. Hence, its impedance is very high. More significantly, the impedance will usually have a large reactive component: capacitive if a tad longer than a multiple of a half wavelength, inductive if a tad short of a half wavelength. At the center point of an odd multiple of a half wavelength, the impedance of an antenna will be low with slow changes in reactance. At even multiples of a half wavelength, the impedance will be high. The reactance climbs to a very high value of inductive reactance and with only a tiny lengthening of the antenna (or increase in frequency) swings suddenly to a very high value of capacitive reactance that will gradually drop as the antenna is further lengthened (or the frequency climbs higher). 135' will prove a smidgeon long at all test frequencies.</p>
|
||||
<p>Because the antenna is fed at a high-voltage point, the current along the feedline is not nearly so unbalanced as some have believed, especially on the fundamental frequency. End-fed Zepps rarely create problems of RF in the shack. One side of the line goes to the very high antenna impedance, the other side to the almost indefinitely large impedance of the missing antenna connection. Hence, the lines are fairly well in balance. However, balance is not perfect, especially as the frequency is increased. If balance were perfect, the line would show very low values of impedance for most of each half wavelength, making a match to the ATU very difficult.</p>
|
||||
<p>Imbalance between the wires of the feedline does create some radiation from the feedlines. However, its consequences on the overall radiation pattern depend on the length of the feeder, the frequency of operation, and the angle at which the feeder leaves the antenna. We shall show patterns for the Zepp assuming a. that all radiation is in the antenna and b. also with a 34' 410-ohm feeder. The pattern you achieve with your Zepp will depend on your layout.</p>
|
||||
<h4>80 Meters: 3.6 MHz</h4>Without feedline: Z = 3000 - j60000 ohms -- Elevation angle = 45 degrees
|
||||
<center>
|
||||
<img src="gup12-2.gif" width="620" height="743" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Note: Since the actual elevation angle of maximum radiation is greater than 45 degrees, the azimuth patterns are taken at an elevation angle of 45 degrees.</p>
|
||||
<p>With feedline: Z = 15 - j420 ohms -- Elevation angle = 45 degrees</p>
|
||||
<center>
|
||||
<img src="gup12-3.gif" width="622" height="741" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<h4>40 Meters: 7.15 MHz</h4>Without feedline: Z = 2700 - j30000 ohms -- Elevation angle = 45 degrees
|
||||
<center>
|
||||
<img src="gup12-4.gif" width="620" height="741" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>All impedance figures are very rough approximations to indicate the range of values. They are in no case the actual figures individual layouts will achieve.</p>
|
||||
<p>With feedline: Z = 17 + j5 ohms -- Elevation angle = 45 degrees</p>
|
||||
<center>
|
||||
<img src="gup12-5.gif" width="618" height="741" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<h4>30 Meters: 10.125 MHz</h4>Without feedline: Z = 300 - j21000 ohms -- Elevation angle = 36 degrees
|
||||
<center>
|
||||
<img src="gup12-6.gif" width="620" height="741" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Note that at this frequency, the feedline radiation actually lowers the elevation angle of maximum radiation, a fairly rare occurrence with shorter lengths of feedline.</p>
|
||||
<p>With feedline: Z = 30 + j280 ohms -- Elevation angle = 32 degrees</p>
|
||||
<center>
|
||||
<img src="gup12-7.gif" width="622" height="741" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<h4>20 Meters: 14.15 MHz</h4>Without feedline: Z = 920 - j14000 ohms -- Elevation angle = 25 degrees
|
||||
<center>
|
||||
<img src="gup12-8.gif" width="622" height="741" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The feedline radiation below significantly rounds all the lobes of the azimuth pattern and reduces maximum gain while increasing overall broadside radiation.</p>
|
||||
<p>With feedline: Z = 7500 + j2000 ohms -- Elevation angle = 26 degrees</p>
|
||||
<center>
|
||||
<img src="gup12-9.gif" width="618" height="741" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<h4>17 Meters: 18.1 MHz</h4>Without feedline: Z = 2200 - j12000 ohms -- Elevation angle = 19 degrees
|
||||
<center>
|
||||
<img src="gup12-10.gif" width="622" height="741" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>On 17, the two patterns return to close coincidence. Whether patterns are similar or different will vary with feedline length and dress, as well as frequency.</p>
|
||||
<p>With feedline: Z = 55 - j400 ohms -- Elevation angle = 20 degrees</p>
|
||||
<center>
|
||||
<img src="gup12-11.gif" width="622" height="741" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<h4>15 Meters: 21.15 MHz</h4>Without feedline: Z = 410 - j10000 -- Elevation angle = 17 degrees
|
||||
<center>
|
||||
<img src="gup12-12.gif" width="618" height="741" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Note that even though the pattern differences appear very minor, they are enough to reduce maximum gain below by a full dB.</p>
|
||||
<p>With feedline: Z = 50 - j90 ohms -- Elevation angle = 17 degrees</p>
|
||||
<center>
|
||||
<img src="gup12-13.gif" width="620" height="741" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<h4>12 Meters: 24.95 MHz</h4>Without feedline: Z = 620 - j8000 ohms -- Elevation angle = 14 degrees
|
||||
<center>
|
||||
<img src="gup12-14.gif" width="620" height="741" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Note how slight thickening of the elevation lobes produces a significant decrease in the maximum gain of the main lobes of the pattern.</p>
|
||||
<p>With feedline: Z = 45 + j370 ohms -- Elevation angle = 14 degrees</p>
|
||||
<center>
|
||||
<img src="gup12-15.gif" width="620" height="741" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<h4>10 Meters: 28.5 MHz</h4>Without feedline: Z = 470 - j7000 ohms -- Elevation angle = 13 degrees
|
||||
<center>
|
||||
<img src="gup12-16.gif" width="620" height="741" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Although your situation is unlikely to copy this one, studying the upper and lower patterns for each band may give you a better idea of how to compare the details of antenna patterns.</p>
|
||||
<p>With feedline: Z = 4750 + j720 ohms -- Elevation angle = 13 degrees</p>
|
||||
<center>
|
||||
<img src="gup12-17.gif" width="620" height="741" border="2" vspace="5" hspace="9">
|
||||
</center><br>
|
||||
<br>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center>
|
||||
<p><i>Updated 1-21-98. © 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></p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center>
|
||||
<p><a href="groundup.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to series index 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>
|
||||
<p class="topics-footer">Topics: <a href='../topics.html#edz-zepp'>EDZ / Zepp</a>, <a href='../topics.html#hf-multi-band'>HF Multi Band</a></p><br>
|
||||
</body>
|
||||
</html>
|
||||
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||||
<!DOCTYPE html>
|
||||
<html lang="en">
|
||||
<head>
|
||||
<meta charset="UTF-8">
|
||||
<meta name="keywords" content="Transmission Line">
|
||||
<title>What Happens Along a Length of Feedline</title>
|
||||
<link rel="stylesheet" href="../styles.css">
|
||||
</head>
|
||||
<body>
|
||||
<center>
|
||||
<h1><img src="../images/yagicad.gif" align="left" width="125" height="62" border="2" vspace="5" hspace="10">ANTENNAS FROM THE GROUND UP<img src="../images/w4rnlwwr.gif" align="right" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
|
||||
</center>
|
||||
<center>
|
||||
<h3>13. Great Expectations<br>
|
||||
or What Happens Along a Length of Feedline</h3>
|
||||
</center>
|
||||
<center>
|
||||
<h3>L. B. Cebik, W4RNL</h3>
|
||||
</center><br>
|
||||
<br>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center><br>
|
||||
<br>
|
||||
<p>We have looked (and will continue to look) at antennas that present complex feedpoint impedances, that is, impedances composed of both resistive and reactive components. In the shorthand that we use for calculations, Z = R ± jX.</p>
|
||||
<p>We have previously noted that a transmission line is (among other things) an impedance transformer. An impedance presented to the line as a load will appear as a different impedance at every point along the line. We can calculate precisely the value of impedance for any point along the line, as illustrated in <b>Figure 1</b>.</p>
|
||||
<center>
|
||||
<img src="gup13-1.gif" width="594" height="458" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Knowing the values of impedance along a transmission is very handy. For a monoband antenna, we can select the point along a line where R is 50 Ohms and insert a compensating reactance so that we can then run coax the rest of the way to the shack. For multiband antennas fed with parallel line, we can estimate how much line to add or subtract in order to let the values fall within the range of adjustment of our ATU.</p>
|
||||
<p>To calculate the value of impedance anywhere along a transmission line, we need only know the following: the load impedance, the characteristic impedance and velocity factor of our feedline, the line length in any units, and the frequency of operation. For ultraprecision, we can improve the result by knowing the line loss figures for the feedline type and the frequency of operation. However, for many purposes, assuming lossless lines will provide all the accuracy needed.</p>
|
||||
<p>Now some good news and some bad news. The bad news is that, even though the necessary equation looks straightforward, the use of complex numbers (a ± jb) results in a tedious job for hand calculation. However, we have good news: we can resort either to Smith charts or to computer programs to do the heavy work for us. <a href="../radio.html#HAMCALC">HAMCALC</a> has a program called "Transmission Line Performance" that will calculate both single values and tables of values of voltage, current, and impedance (and their phases), as well as resistance and reactance along a lossless transmission line.</p>
|
||||
<p>Actually, for impedance, we only need to look at ½ wavelength of line, since in lossless lines, the values repeat themselves at that interval. In real lines with some loss, the values will change slightly as the line length is increased in multiples of ½ wl.</p>
|
||||
<p>Note, however, that values of voltage and current only change once per full wavelength. Although this fact will not affect the work we shall do in this installment, failure to appreciate it has led to some interesting errors in analyzing antennas. For example, most accounts of the ZL Special, a phased 2- element array, have viewed it as a 135° phased impedance antenna. However, antenna phasing is a product of antenna currents, not impedance. When this fact is appreciated, the antenna is more properly analyzed as a -45° phased current array.</p>
|
||||
<p>For most low-HF wire antenna matching problems, we can focus on the impedance. The question that faces users of multiband antennas is whether the length of line in use will present to the ATU a set of values for resistance and reactance that fall within the tuning range of the ATU. The actual range of permissible values varies with the ATU network type and components. But let's arbitrarily adopt these limits: R should range from about 20-25 Ohms to several hundred Ohms while jX should run no higher than ± a few hundred Ohms. These limits would place almost all the common ATU types within their high-efficiency matching zones.</p>
|
||||
<p><b>Line Length and Degrees</b>: The first step in analyzing our situation is finding the line length we have. Actually, we need only know where along the last ½ wl the line ends. We can calculate this length by plugging the following equations into a hand calculator:</p>
|
||||
<p>1. Divide the total length of line by the length of ½ wl of line, adjusted for the velocity factor (VF) of the line in use. Common 450-Ohm line has a VF of 0.95, while common 300-Ohm line has a VF of 0.80. The tables below list line lengths for various increments and frequencies. Use the figure for 180° for the nearest frequency to the one in which you are interested.</p>
|
||||
<p>2. Throw away the integer (the part left of the decimal, if any) and save the decimal part. Multiply this number times 180 to get the number of degrees along a half wavelength of transmission line that represents the termination point of your line.</p>
|
||||
<p>Virtually all transmission line work is done in degrees relative to a full 360° circle. 180° represents the half-circle in which we are interested. The tables below list the lengths of transmission line for various degrees along the line for selected HF frequencies that are useful for estimating your line length. Line lengths are in feet for 450-Ohm and for 300-Ohm lines.</p>
|
||||
<pre>
|
||||
80-meters 3.6 MHz
|
||||
L(°) L(ft): 450-Ohm 300-Ohm
|
||||
30 21.63 18.21
|
||||
60 43.26 36.43
|
||||
90 64.89 57.68
|
||||
120 86.52 72.86
|
||||
150 108.15 85.00
|
||||
180 129.78 109.29
|
||||
|
||||
40-meters 7.15 MHz
|
||||
L(°) L(ft): 450-Ohm 300-Ohm
|
||||
30 10.89 9.17
|
||||
60 21.78 18.34
|
||||
90 32.67 27.51
|
||||
120 43.56 36.68
|
||||
150 56.27 45.85
|
||||
180 65.34 55.02
|
||||
|
||||
30-meters 10.125 MHz
|
||||
L(°) L(ft): 450-Ohm 300-Ohm
|
||||
30 7.69 6.48
|
||||
60 5.38 12.95
|
||||
90 20.51 19.43
|
||||
120 30.76 25.90
|
||||
150 38.45 32.38
|
||||
180 46.14 38.86
|
||||
|
||||
20-meters 14.15 MHz
|
||||
L(°) L(ft): 450-Ohm 300-Ohm
|
||||
30 5.50 4.63
|
||||
60 11.01 9.27
|
||||
90 16.51 13.90
|
||||
120 22.01 18.54
|
||||
150 27.51 23.17
|
||||
180 33.02 27.80
|
||||
|
||||
17-meters 18.1 MHz
|
||||
L(°) L(ft): 450-Ohm 300-Ohm
|
||||
30 4.30 3.62
|
||||
60 8.60 7.25
|
||||
90 12.91 10.87
|
||||
120 17.21 14.49
|
||||
150 21.51 18.11
|
||||
180 25.81 21.74
|
||||
|
||||
15-meters 21.15 MHz
|
||||
L(°) L(ft): 450-Ohm 300-Ohm
|
||||
30 3.68 3.10
|
||||
60 7.36 6.20
|
||||
90 11.04 9.30
|
||||
120 14.73 12.40
|
||||
150 18.41 15.50
|
||||
180 22.09 18.60
|
||||
|
||||
12-meters 24.95 MHz
|
||||
L(°) L(ft): 450-Ohm 300-Ohm
|
||||
30 3.12 2.63
|
||||
60 6.24 5.26
|
||||
90 9.36 7.88
|
||||
120 12.48 10.51
|
||||
150 15.60 13.14
|
||||
180 18.73 15.77
|
||||
|
||||
10-meters 28.5 MHz
|
||||
L(°) L(ft): 450-Ohm 300-Ohm
|
||||
30 2.73 2.30
|
||||
60 5.46 4.60
|
||||
90 8.20 6.90
|
||||
120 10.93 9.20
|
||||
150 13.66 11.50
|
||||
180 16.39 13.80</pre>
|
||||
<p><b>Sample Cases</b>: Since impedance transformations are a function of the number of degrees along a line we take a reading, regardless of frequency, we can look at some transformations that are interesting and list them in terms of degrees. You can then translate them into actual line lengths for your situation, depending on the line you are using and the frequency at which you encounter something similar to the example.</p>
|
||||
<p>1. Mismatch with no load reactance:</p>
|
||||
<pre>Load Z = 150 ± j0 Ohms
|
||||
Line Length R ± jX
|
||||
(degrees) 450-Ohms 300-Ohms
|
||||
30 193 + j223 185 + j120
|
||||
60 450 + j520 343 + j223
|
||||
90 1350 + j0 600 + j0
|
||||
120 450 - j520 343 - j223
|
||||
150 193 - j223 185 - j120
|
||||
180 150 + j0 150 + j0</pre>
|
||||
<p>Note the symmetry of the variations of values for this line with no reactance. Still, none of the values seems to exceed what a good ATU might handle. Line length is thus not at all critical.</p>
|
||||
<p>2. Mismatch with small inductive reactance.</p>
|
||||
<pre>Load Z = 150 + j30 Ohms
|
||||
Line Length R ± jX
|
||||
(degrees) 450-Ohms 300-Ohms
|
||||
30 208 + j260 206 + j153
|
||||
60 538 + j564 419 + j226
|
||||
90 1298 - j259 577 - j115
|
||||
120 380 - j475 282 - j209
|
||||
150 179 - j188 166 - j90
|
||||
180 150 + j30 150 + j30</pre>
|
||||
<p>Note the disruption to the symmetry due to the presence of reactance. Although the values appear reasonable, I would avoid a 90° length, if possible.</p>
|
||||
<p>3. Mismatch with small capacitive reactance.</p>
|
||||
<pre>Load Z = 150 - j30 Ohms
|
||||
Line Length R ± jX
|
||||
(degrees) 450-Ohms 300-Ohms
|
||||
30 179 + j188 166 + j90
|
||||
60 380 + j475 282 + j209
|
||||
90 1298 + j259 577 + j115
|
||||
120 538 - j564 419 - j226
|
||||
150 208 - j260 206 - j153
|
||||
180 150 - j30 150 - j30</pre>
|
||||
<p>Notice the pattern of values and signs in the comparison of this case and the preceding one, where the only change is the type of reactance in the load.</p>
|
||||
<p>4. Mismatch with larger inductive reactance.</p>
|
||||
<pre>Load Z = 150 + j150 Ohms
|
||||
Line Length R ± jX
|
||||
(degrees) 450-Ohms 300-Ohms
|
||||
30 290 + j438 339 + j316
|
||||
60 1172 + j598 781 - j52
|
||||
90 675 - j675 300 - j300
|
||||
120 213 - j321 142 - j132
|
||||
150 137 - j69 115 - j8
|
||||
180 150 + j150 150 + j150</pre>
|
||||
<p>This plot reveals that as the reactance goes up, part of the curve of values along the line grows quite steep. In this case, it is the first 90° of each half wavelength. The second 90° provides the best region for connection to an ATU.</p>
|
||||
<p>5. Mismatch with larger resistance and inductive reactance.</p>
|
||||
<pre>Load Z = 600 + j150 Ohms
|
||||
Line Length R ± jX
|
||||
(degrees) 450-Ohms 300-Ohms
|
||||
30 643 - j105 435 - j252
|
||||
60 435 -j180 200 - j165
|
||||
90 318 - j79 141 - j35
|
||||
120 307 + j50 155 + j90
|
||||
150 397 + j164 267 + j221
|
||||
180 600 + j150 600 + j150</pre>
|
||||
<p>As the resistive component of the antenna feedpoint impedance goes considerably higher than the characteristic impedance of the feedline, (also making the ratio of resistance to reactance greater), the best matching region becomes the mid-region of the half wavelength line section, where the rate of change of resistance and reactance values is also the lowest. However, none of the values shown should present most ATUs with any problems.</p>
|
||||
<p>6. Mismatch with the larger resistance and a high capacitive reactance.</p>
|
||||
<pre>Load Z = 600 - j1000 Ohms
|
||||
Line Length R ± jX
|
||||
(degrees) 450-Ohms 300-Ohms
|
||||
30 138 - j371 81 - j314
|
||||
60 83 -j85 41 - j92
|
||||
90 89 + j149 40 + j66
|
||||
120 89 + j149 69 + j268
|
||||
150 1189 +j1216 365 + j812
|
||||
180 600 - j1000 600 - j1000</pre>
|
||||
<p>As the reactance grows larger than a resistance which is already larger than the characteristic impedance of the line, the region of easiest match to an ATU shifts. For this capacitive reactance, 30° to 120° becomes the best region; for an equivalent inductive reactance, 60° to 150° would be the best region.</p>
|
||||
<p>7. Mismatch with very large resistance and inductive reactance.</p>
|
||||
<pre>Load Z = 2000 + j2000 Ohms
|
||||
Line Length R ± jX
|
||||
(degrees) 450-Ohms 300-Ohms
|
||||
30 295 - j960 116 - j606
|
||||
60 77 - j327 33 - j203
|
||||
90 51 - j51 23 - j23
|
||||
120 59 + j192 28 + j143
|
||||
150 138 + j587 70 + j432
|
||||
180 2000 +j2000 2000 + j2000</pre>
|
||||
<p>As the resistance and inductive reactance reach the 2K level, as is common when 80-meter and 40-meter antennas are used on WARC bands, line lengths near 90° become more favorable for easy matches. Note that 300-Ohm line becomes less favorable for use, since at 90° it present a very low impedance to the ATU.</p>
|
||||
<p>8. Mismatch with very large resistance and capacitive reactance.</p>
|
||||
<pre>Load Z = 2000 - j2000 Ohms
|
||||
Line Length R ± jX
|
||||
(degrees) 450-Ohms 300-Ohms
|
||||
30 138 - j587 70 - j432
|
||||
60 59 - j193 28 - j143
|
||||
90 51 + j51 23 + j23
|
||||
120 77 + j327 33 + j203
|
||||
150 295 + j960 116 + j606
|
||||
180 2000 - 2000 2000 - j2000</pre>
|
||||
<p>Notice the reversal of the progression of values, but not signs attached to the reactance, compared to the previous case. Also compare cases 7. and 8. to cases 2. and 3.</p>
|
||||
<p>Cases 7. and 8. also show why the use of the 4:1 balun built into many ATUs should be avoided. With higher feedpoint impedances, the region of best match along the line encounters low impedance values. Assuming a perfect 4:1 ratio and no loss (actually, a rather poor assumption when there is significant reactance), the values seen by the ATU would be very low, either beyond the range of the ATU or subject to further losses in the network.</p>
|
||||
<p>9. Mismatch with very large resistance and extremely large inductive reactance.</p>
|
||||
<pre>Load Z = 2000 + j10000 Ohms
|
||||
Line Length R ± jX
|
||||
(degrees) 450-Ohms 300-Ohms
|
||||
30 18 - j863 8 - j556
|
||||
60 5 - j286 2 - j185
|
||||
90 4 - j19 2 - j8
|
||||
120 5 + j234 2 + j162
|
||||
150 13 + j707 6 + j487
|
||||
180 2000 +j 10K 2000 + j10K</pre>
|
||||
<p>Note that the transformer effect of the feedlines, as their characteristic impedances become very small compared to the magnitude of the feedpoint impedance (especially the reactance), confines the high impedance values to a small region near 0 and 180°. The remainder of the line section shows very low values of resistance.</p>
|
||||
<p>10. Mismatch with very large resistance and extremely large capacitive reactance.</p>
|
||||
<pre>Load Z = 2000 - j10000 Ohms
|
||||
Line Length R ± jX
|
||||
(degrees) 450-Ohms 300-Ohms
|
||||
30 13 - j707 6 - j486
|
||||
60 5 - j234 2 - j162
|
||||
90 4 + j19 2 + j9
|
||||
120 5 + j286 2 + j185
|
||||
150 18 + j863 8 + j556
|
||||
180 2000 - 10K 2000 - j10K</pre>
|
||||
<p>These final two cases demonstrate why half wavelength loops and similar antennas are unusable with standard lines and ATUs. Indeed, with the end- fed Zepp, if the feedlines were not at least slightly out of balance, effecting a match with common forms of ATUs would not be possible, since the impedances presented to the feedline would be even higher.</p>
|
||||
<p>Exploring the behavior of impedances along a feedline can be very instructive in estimating the most favorable line lengths for presenting the ATU with not only a load it can match, but one which it can match efficiently. The samples presented here only scratch the surface of the subject. However, they do show some of the emergent patterns of how the values change with different ranges of antenna feedpoint impedances. I heartily recommend that you get a copy of HAMCALC and run the values presented with some of the antennas we have already explored and will yet explore in future episodes. What you learn just might increase your antenna system efficiency a notch or two.<br>
|
||||
<br></p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center>
|
||||
<p><i>Updated 11-11-98. © 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></p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center>
|
||||
<p><a href="groundup.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to series index 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>
|
||||
<p class="topics-footer">Topics: <a href='../topics.html#transmission-line'>Transmission Line</a></p><br>
|
||||
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|
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<head>
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<meta charset="UTF-8">
|
||||
<meta name="keywords" content="Matching">
|
||||
<title>ATUs, Delta, and Tuner Losses</title>
|
||||
<link rel="stylesheet" href="../styles.css">
|
||||
</head>
|
||||
<body>
|
||||
<center>
|
||||
<h1><img src="../images/yagicad.gif" align="left" width="125" height="62" border="2" vspace="5" hspace="10">ANTENNAS FROM THE GROUND UP<img src="../images/w4rnlwwr.gif" align="right" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
|
||||
</center>
|
||||
<center>
|
||||
<h3>14. A Good Match<br>
|
||||
or ATUs, Delta, and Tuner Losses</h3>
|
||||
</center>
|
||||
<center>
|
||||
<h3>L. B. Cebik, W4RNL</h3>
|
||||
</center>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center>
|
||||
<p>Losses from the insertion of antenna tuning units or transmatches between transmitters and transmission lines have bothered numerous folks for some time. There are two questions: how much loss can I expect? and how do I minimize the loss? Both are good questions with good answers. Unfortunately, they are not super simple answers. But some old material combined with some new material can make the answers easier to come by. Those who like the math involved can focus on the equations, while those interested in operational matters can concentrate on the tables and the resulting rules of thumb to minimize losses.</p>
|
||||
<p>Most of this installment and the next appeared originally in <i>QRP Quarterly</i> for January, 1996. I have added some material, especially about 2-element L-C ATUs, and rearranged some of the ideas. This review of the estimate of losses in ATUs is also a good review of the most common types of network ATUs in use in amateur circles these days.</p>
|
||||
<p>Most ATUs used today are L-C networks. The most common configuration is the C-L-C Tee network. However, L-C-L Tees and C-L-C PIs are in use, as are SPC and Ultimate Transmatch designs. Loss figures can be calculated for all such networks.</p>
|
||||
<p>The old material that contributes to determining ATU losses appears in Terman's <i>Radio Engineers' Handbook</i> (McGraw-Hill, 1943), pages 210-215. Based predominantly on work done in the 1930s by W.I. Everitt, Terman's analysis of classic impedance-matching networks is still referenced by current handbooks. The relevant part for considering ATU losses is the term, delta, which is a justifiably simplified measure of power dissipation in networks. For any impedance matching network, the primary power dissipation culprit is the inductance, and inductor losses can be calculated.</p>
|
||||
<p>The efficiency of L-C networks is dependent upon the impedance transformation ratio and the phase shift. PI networks and L-C-L Tees generally exhibit a large angle of phase retardation, while the C-L-C Tee circuit shows a large angle of phase advance. In contrast, simple L-networks show small angles of phase advance or retardation. Losses increase with increasing transformation ratios and tend to be larger when the phase shift is either very large or very small. The delta figure takes both into account. (Note, in some explanations of networks, but not in all, Terman's delta goes under the name of "working Q," "circuit Q," or "network Q.")</p>
|
||||
<p>The concept of delta has certain limitations. First, it ignores capacitor losses. This is justified for well- designed capacitors at HF and below, but capacitor losses may become significant at the boundary between HF and VHF (about 30 MHz). Second, the loss figure ignores the effects of strays, both capacitive and inductive, within a real ATU, especially one cramped inside a metal case. At 10 meters, the principal L and C in a matching circuit may be composed more of strays than of component values. Third, the technique will assume we know the coil Q in figuring ultimate efficiency and losses. On this score, we mostly guess, but a figure of 100 for good air wound coils is a conservative guess. Good powdered iron core toroids may yield more. Of course, the "suck out" effect of shorted coil turns is also ignored. Moreover, only networks, and not inductively coupled ATUs, are covered.</p>
|
||||
<p>The developer of the mathematical analysis used here, ZL1LE, recently ran a comparison between delta calculations and a more complex analysis published in <i>QEX</i>. The results showed an exceptionally high degree of correlation above load impedances in the 20-25 Ohms range, with the delta calculations being the more pessimistic below that level.</p>
|
||||
<p>Within these limits, the Terman-Everitt delta yields a fair estimate of loss and efficiency. As Terman notes,</p>
|
||||
<center>
|
||||
<img src="gup14e1.gif" width="368" height="74">
|
||||
</center>
|
||||
<p>In other, equally approximate terms, network efficiency = 1 - delta/Q. (Multiply this figure times 100 for a percentage value of efficiency).</p>
|
||||
<p>Now if we only knew how to calculate delta, the rest would be simple. Terman provides graphs for estimating delta. Each type of network requires its own calculation, and it can be a bit tedious to calculate delta for several combinations of component settings. However, Brian Egan, ZL1LE, has derived equations for delta-calculations and added them to his program TUNER.BAS. This versatile program is now included in the collection of programs called <a href="../radio.html#HAMCALC">HAMCALC</a>, made available by George Murphy.</p>
|
||||
<p>Among the networks for which ZL1LE includes calculations are PIs, C-L-C Tees, SPCs, Ultimates, and L-C-L Tees. By running the program through a selection of values, one can learn the trends of tuning that will minimize losses. Operationally, that is the best we can hope for with a fixed set of impedance conditions presented by the transmission line from the antenna. From the general relationship of power lost to power delivered, it is clear that we always want to tune for minimum delta.</p>
|
||||
<p>For almost any loed presented to the tuner, there is usually more than one set of values for the ATU components that will effect a match so that the SWR meter shows a 1:1 SWR ratio. Not every combination is of equal quality in terms of losses within the ATU. Any limitations we encounter on the range of component variability-- such as switched coil taps or switched fixed capacitors--can also limit our ability to achieve the lowest possible loss setting.</p>
|
||||
<p>I ran TUNER.BAS through a number of exercises, and here are some results for some configurations. I chose 10.1 MHz, because I was working on a network problem for that band. To standardize the tests, I chose the following load conditions for each network: 150 +/- j0 Ohms, 150 + 100 Ohms, and 150 - j100 Ohms. Looking at a straight 3:1 SWR purely resistive load is a good beginning point, while exploring the network with both capacitive and inductive reactances indicates limitations of the network with certain types of loads. This test is limited, since a higher delta at a 3:1 impedance transformation ratio is not indicative of the network's overall capability. Some networks are more efficient with lower transformation ratios, while others grow more efficient as the transformation ratio increases.</p>
|
||||
<p><b>PIs</b>: This familiar low-pass network configuration appears in Figure 1.</p>
|
||||
<center>
|
||||
<img src="gup14-1.gif" width="445" height="222" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>For those interested, the Egan equation for delta for a PI network (where omega = 2piF wherever it occurs) is the following:</p>
|
||||
<center>
|
||||
<img src="gup14e2.gif" width="300" height="78">
|
||||
</center>
|
||||
<p>Here is a small table of results for 10.1 MHz:</p>
|
||||
<pre>Input = 50 Ohms; Output = 150 =/- j0 Ohms;
|
||||
Maximum permitted L = 1.365 µH
|
||||
L 1.364 C1 172 C2 179 delta 2.2
|
||||
or 1.364 192 185 2.5
|
||||
L 1.2 C1 36 C2 150 delta 1.5
|
||||
L 1.15 C1 15 C2 149 delta 1.5
|
||||
|
||||
Input = 50 Ohms; Output = 150 + j100 Ohms
|
||||
Maximum permitted L = 1.640 µH
|
||||
L 1.639 C1 140 C2 197 delta 2.5
|
||||
L 1.6 C1 84 C2 187 delta 2.2
|
||||
L 1.5 C1 26 C2 182 delta 1.9
|
||||
|
||||
Input = 50 Ohms; Output = 150 - j100 Ohms
|
||||
Maximum permitted L = 1.640 µH
|
||||
L 1.639 C1 140 C2 100 delta 2.5
|
||||
L 1.6 C1 84 C2 90 delta 2.2
|
||||
L 1.5 C1 26 C2 85 delta 1.9</pre>
|
||||
<p>In all three cases, the lists stop where one of the values approaches the minimum usually obtainable with ordinary components. In this case, a value for C1 lower than about 20 pF is not normal for most ATUs. In many instances, as illustrated by the very first entry, two solutions to the matching problem are possible. After the first entry, only the more promising set of values is shown. PI networks are, within limits, equally capable of handling capacitive and inductive reactances in the load, with the limits of both being related to the range of C2. Notice that efficiencies of 98% are possible within the limits of the method. As Terman notes, increasing the ratio of the impedances to be matched increases the value of delta and decreases efficiency. For example, when the resistive component of the output impedance is 300 Ohms, with or without a reactive load, the lowest value of delta is about 2.3. To achieve the highest efficiency possible for any given matching situation, we must use the following rule of thumb:</p>
|
||||
<p><b><i>PI networks</i></b>: Use the lowest value of L that still permits C1 to be effective.</p>
|
||||
<p><b>C-L-C Tees</b>: This most common ATU network configuration appears in Figure 2.</p>
|
||||
<center>
|
||||
<img src="gup14-2.gif" width="446" height="323" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Whereas the PI network is a natural low-pass filter, the C-L-C network is a natural high-pass filter. However, this fact does not hamper its ability to match a wide range of complex load impedances.</p>
|
||||
<p>To determine delta, use the Egan equation</p>
|
||||
<center>
|
||||
<img src="gup14e3.gif" width="246" height="83">
|
||||
</center>
|
||||
<pre>Input = 50 Ohms; Output = 150 +/- j0 Ohms
|
||||
Maximum permitted C1 = 222.9 pF
|
||||
C1 222.8 C2 6027 L 1.65 delta 1.4
|
||||
C1 222 C2 1468 L 1.59 delta 1.5
|
||||
C1 220 C2 992 L 1.56 delta 1.5
|
||||
C1 210 C2 362 L 1.43 delta 1.8
|
||||
C1 200 C2 262 L 1.39 delta 2.0</pre>
|
||||
<p>Although a very low value of delta is theoretically possible, a realistic value of 2.0 or higher is to be expected with common components used in ATUs. As with a PI network, the C-L-C Tee yields higher values of delta as the output impedance is increased. At 6:1 ratio of output-to-input impedance, delta does not go below 2.1 with normal ATU components.</p>
|
||||
<pre>Input = 50 Ohms; Output = 150 + j100 Ohms
|
||||
Maximum permitted C1 = 222.9 pF
|
||||
C1 222.8 C2 154 L 1.65 delta 1.4
|
||||
C1 220 C2 132 L 1.54 delta 1.6
|
||||
or 220 196 1.85 1.3
|
||||
C1 210 C2 110 L 1.43 delta 1.8
|
||||
or 210 279 2.12 1.2
|
||||
C1 200 C2 98 L 1.39 delta 2.0
|
||||
or 200 396 2.34 1.2</pre>
|
||||
<p>In this table, two sets of matching values are possible, even within the range of normal ATU components. One set drives toward maximal efficiency as C1 is decreased, while the other set moves away from the highest efficiency under the same condition. The primary setting to watch, therefore, is that of C2.</p>
|
||||
<pre>Input = 50 Ohms; Output = 150 - j100 Ohms
|
||||
Maximum permitted C1 = 172.6 pF
|
||||
C1 172.5 C2 91080 L 1.37 delta 2.5
|
||||
C1 150 C2 456 L 1.42 delta 3.0
|
||||
C1 130 C2 223 L 1.52 delta 3.6</pre>
|
||||
<p>With respect to efficiency, the C-L-C Tee is less effective with capacitively reactive loads than with inductively reactive loads. With some load conditions, here a capacitive reactance of 100 Ohms combined with our standard 150-Ohm resistive component, a very low delta will not be possible with normal components. The best we can do is to keep delta as low as possible by following the common advice for C-L-C Tee tuners:</p>
|
||||
<p><b><i>C-L-C Tees</i></b>: Use the maximum capacitance possible, especially for C2, for a given matching situation.</p>
|
||||
<p>However, even with a delta of 3.6 to 4, efficiency with a coil Q of 100 will be in the neighborhood of 96%.</p>
|
||||
<p><b>SPC</b>: The SPC (series-parallel capacitance) network is the more general case of which the C-L-C Tee is a specific instance. Note that the C-L-C results from the SPC if we lower the value of the capacitor in parallel with L to zero.</p>
|
||||
<center>
|
||||
<img src="gup14-3.gif" width="446" height="321" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Ordinarily, the parallel capacitor is ganged as a dual unit with either C1 or C2. The requirement for a high-voltage dual-section capacitor has limited commercial production of SPC tuners. Figure 3 shows the more common configuration, with Cp and C2 ganged as equal-value units.</p>
|
||||
<p>The SPC departs from the C-L-C Tee in two significant operational characteristics. First, the actual values required for a given impedance transformation will differ. Second, the SPC tends to tune much more sharply. I recommend the use of verniers on the capacitors for ease of tuning. However, the equation for determining delta for the SPC is the same as that given for the C-L-C Tee.</p>
|
||||
<pre>Input = 50 Ohms; Output = 150 +/- j0 Ohms
|
||||
Maximum permitted C1 = 222.9 pF
|
||||
C1 222.8 C2 6027 L 0.04 delta 58.8
|
||||
C1 210 C2 362 L 0.64 delta 4.7
|
||||
C1 180 C2 176 L 0.69 delta 4.6
|
||||
C1 170 C2 152 L 0.75 delta 4.7
|
||||
|
||||
Input = 50 Ohms; Output = 150 + j100 Ohms
|
||||
Maximum permitted C1 = 222.9 pF
|
||||
C1 222.8 C2 154 L 0.82 delta 2.9
|
||||
C1 220 C2 137 L 0.85 delta 2.8
|
||||
C1 215 C2 118 L 0.87 delta 2.9
|
||||
|
||||
Input = 50 Ohms; Output = 150 - j100 Ohms
|
||||
Maximum permitted C1 = 172.6 pF
|
||||
C1 172.5 C2 91080 L <0.01 delta 1256
|
||||
C1 140 C2 305 l 0.52 delta 9.1
|
||||
C1 120 C2 172 l 0.76 delta 8.2
|
||||
C1 110 C2 136 l 0.88 delta 8.3</pre>
|
||||
<p>The SPC tuner improves the harmonic rejection characteristics of the C-L-C Tee (which is inherently a high-pass filter), but apparently at the cost of lower efficiency. All three tables show the values surrounding the lowest value for delta obtainable under the given load conditions. The SPC appears to do better with inductive loads than with purely resistive transformations or capacitive loads. The value of delta increases for resistive and inductive loads as the ratio of output-to-input impedance increases, but shows a slight decrease for capacitive loads at higher ratios. Because tuning points are quite specific, no general rule of thumb is possible for the SPC.</p>
|
||||
<p>The C-L-C PI, the C-L-C Tee, and the SPC are perhaps the most common networks in use at amateur stations these days. Of these, the C-L-C is the most common because it is the cheapest to produce commercially. PIs are less common at QRO levels because at least one of the capacitors must ordinarily have very high values of capacitance, which makes the component expensive.</p>
|
||||
<p>At QRP levels, receiving capacitors from older BC receivers have more than enough power-handling capability to make any of these or other networks feasible for the station ATU. Indeed, the entire concept of using one ATU for all possible bands can give way at QRP levels to ATUs that are optimized for a small collection of bands. For example, large value capacitors needed for the lower HF bands often have minimum capacitance values that are too high for maximum ATU efficiency at the upper HF bands. For these bands, it is possible to use lower value capacitors having very low minimum capacitance values. Likewise, the smaller coils needed for these bands may avoid the so-called suckout effect by reducing the number of turns shorted out for a given band. Monoband QRP transceivers are common practice, especially with kits. It is feasible to optimize ATUs for each band and to include them in the same case (thus also eliminating connector losses as well). <a href="gup15.html">More next time</a>.</p>
|
||||
<center>
|
||||
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|
||||
<p><i>Updated 1-31-99. © 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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<title>More on ATUs, Delta, and Losses</title>
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<h1><img src="../images/yagicad.gif" align="left" width="125" height="62" border="2" vspace="5" hspace="10">ANTENNAS FROM THE GROUND UP<img src="../images/w4rnlwwr.gif" align="right" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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</center>
|
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<center>
|
||||
<h3>15. One Good Match Deserves Another<br>
|
||||
or More on ATUs, Delta, and Losses</h3>
|
||||
</center>
|
||||
<center>
|
||||
<h3>L. B. Cebik, W4RNL</h3>
|
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</center>
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<center>
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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>In our <a href="gup14.html">last episode</a>, we explored the nature and losses of some common ATU networks, namely, the C-L-C PI, the C-L-C Tee, and the SPC. This time, we shall look at L-C-L Tees, McCoyþs Ultimate Transmatch, and simple 2-element L-C networks. The tables showing the trends in ë continue to be for 10.1 MHz. Values of capacitance and inductance will vary for the frequency in use, but the values of delta will remain the same for the same input and output values of resistance and reactance at any HF frequency.</p>
|
||||
<p>We are restricting ourselves here to networks, and we have set aside inductively coupled matching circuits. In some future installment, we shall examine these circuits, which are capable of excellent efficiency but suffer in many cases from some mechanical inconveniences for multiband use. Among commonly used wide-band inductively coupled ATUs on todayþs market, perhaps only the Z- match overcomes, at least for QRP operation, most of the mechanical problems. But let's, for the moment. return to our network programming.</p>
|
||||
<p><b>L-C-L Tees</b>: The L-C-L Tee, shown in Figure 1, has suffered from neglect until recent years. It is a natural low- pass filter, like the C-L-C PI shown earlier. However, whereas the intermediate impedance of the PI is lower than either the input or output impedances, the intermediate impedance of the L-C-L Tee is higher than either the input or output impedances. This results in higher voltages and lower currents at the junction of the three components.</p>
|
||||
<center>
|
||||
<img src="gup15-1.gif" width="446" height="222" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Determining delta for the L-C-L Tee requires attention to both inductors, which for simplicity are assumed to have the same Q in the following Egan equation:</p>
|
||||
<center>
|
||||
<img src="gup15e1.gif" width="443" height="113">
|
||||
</center>
|
||||
<pre>Input = 50 Ohms; Output = 150 +/- j0 Ohms
|
||||
Minimum permitted L1 = 1.114 µH
|
||||
L1 1.115 L2 0.07 C 152 Delta 1.4
|
||||
L1 1.12 L2 0.20 C 157 Delta 1.5
|
||||
L1 1.20 L2 0.77 C 176 Delta 1.8
|
||||
|
||||
Input = 50 Ohms; Output = 150 + j100 Ohms
|
||||
Minimum permitted L1 = 1.438 µH
|
||||
L1 1.439 L2 0.01 C 181 Delta 1.8
|
||||
L1 1.50 L2 0.16 C 180 Delta 2.0
|
||||
L1 2.00 L2 1.30 C 159 Delta 3.1
|
||||
|
||||
Input = 50 Ohms; Output = 150 - j100 Ohms
|
||||
Minimum permitted L1 = 1.114 µH
|
||||
L1 1.115 L2 1.51 C 145 Delta 2.1
|
||||
L1 1.12 L2 1.38 C 140 Delta 2.0
|
||||
L1 1.14 L2 1.16 C 129 Delta 1.9
|
||||
L1 1.43 L2 0.02 C 85 Delta 1.8</pre>
|
||||
<p>Despite the use of two theoretically lossy inductances in the L-C-L Tee, it maintains low values of ë over a wide range of load conditions and transformation settings. Delta climbs more rapidly with inductive loads than with capacitive loads away from optimal settings. The operative rule of thumb for maximum efficiency is this:</p>
|
||||
<p><b><i>L-C-L Tees</i></b>: Choose the lowest value of L2 that permits a match.</p>
|
||||
<p>This operative rule holds true at higher ratios of output-to-input impedance, even though values of delta increase at those ratios. L-C-L Tees are subject to losses from circulating currents in the shorted turns of either switched or rotary solenoid or toroid coils, losses that the value of ë cannot take into account. Therefore, construction of this network requires special care.</p>
|
||||
<p><b>Ultimate Transmatch</b>: Lew McCoy's Ultimate Transmatch (UT) is an interesting variation on the C-L-C Tee. It places a dual capacitor at the input of the network, one section in series with the signal path, the other section to ground. Figure 2 shows the general outline of the network.</p>
|
||||
<center>
|
||||
<img src="gup15-2.gif" width="443" height="321" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Calculating delta for the UT design requires the following Egan equation:</p>
|
||||
<center>
|
||||
<img src="gup15e2.gif" width="280" height="107">
|
||||
</center>
|
||||
<pre>Input = 50 Ohms; Output = 150 +/- j0 Ohms
|
||||
Maximum permitted C1 = 750 pF per section
|
||||
C1 749 C2 168 L 0.57 Delta 5.8
|
||||
C1 600 C2 161 L 0.68 Delta 5.0
|
||||
C1 400 C2 143 L 0.90 Delta 4.1
|
||||
C1 250 C2 113 L 1.17 Delta 3.8
|
||||
|
||||
Input = 50 Ohms; Output = 150 + j100 Ohms
|
||||
Maximum permitted C1 = 750 pF per section
|
||||
C1 749 C2 81 L 0.57 Delta 5.8
|
||||
C1 400 C2 75 L 0.90 Delta 4.1
|
||||
C1 250 C2 66 L 1.17 Delta 3.8
|
||||
|
||||
Input = 50 Ohms; Output = 150 - j100 Ohms
|
||||
Maximum permitted C1 = 546 pF per section
|
||||
C1 545 C2 >350K L 0.73 Delta 4.7
|
||||
C1 300 C2 615 L 1.06 Delta 3.8
|
||||
C1 250 C2 400 L 1.17 Delta 3.8
|
||||
C1 200 C2 256 L 1.30 Delta 3.9</pre>
|
||||
<p>Like other series-capacitance designs, the UT appears most effective with purely resistive and inductive loads: Required values for a match fall easily within the ranges of normal components just as the network approaches maximum efficiency. (If C1 is lowered further in these cases, delta increases.) However, with capacitively reactive loads, maximum efficiency has passed by the time C2 enters the range most normal for ATUs.</p>
|
||||
<p>Although the values for delta are higher than with some other designs, efficiency remains in the 96% ballpark. In fact, the efficiency of the Ultimate Transmatch appears to increase (at least to some limit) with increases in the ratio of output-to-input impedance. Using a 300-Ohm output resistance with either no reactance or 100 Ohms of inductive reactance, the UT achieved values of delta in the neighborhood of 2.8, while with 100 Ohms of capacitive reactance, delta was about 3.5. The rule of thumb for the UT design is not as simple as with some other designs:</p>
|
||||
<p><b><i>UT</i></b>: With normal ATU components (Cmax less than 400 pF), choose the highest value of C2 that permits a match within the range of C1.</p>
|
||||
<p>Remember that, for any of the network designs, as the frequency decreases, some of the efficiencies achieved in these 10.1 MHz tables may not be reached, since proportionately greater values of C and L may be needed for the same load conditions. Moreover, switched inductors may not provide the values necessary for highest efficiencies. This limitation is rarely a problem, since the value of delta in most cases "bottoms out" over a fairly wide range. Nonetheless, the rules of thumb continue to apply.</p>
|
||||
<p>Moreover, remember that these figures are based solely on losses in the inductor, one assumed to have a Q in the range of 100. Additional losses due to poor components, bad wiring and switching, stray inductances and capacitances, and coil "suck-out" are not accounted for by the calculations. If components get warm or arc under matched conditions at 100 watts, even though the calculations indicate less than 5% losses, believe your senses. Then analyze the components and construction of your ATU. Something other than network choice is very wrong.</p>
|
||||
<p><b>2-Element L-Circuits</b>: Although less versatile than 3-element networks, 2- element networks (L-circuits) are more fundamental. Each circuit consists of a series reactance and a parallel (shunt) reactance. The shunt reactance goes at either the input or the output side of the circuit depending upon whether one is transforming an impedance downward or upward, respectively. The reactances are normally of opposite types.</p>
|
||||
<p>Thought of in terms of reactance, rather than components, the reactances can be swapped with respect to type for the same impedance transformation. This will, of course, change the component values.</p>
|
||||
<center>
|
||||
<img src="gup15-3.gif" width="600" height="346" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Figure 3 shows two L-circuits, one for transformation upward, the other for transformation downward. As given, L is the series reactance and C is the shunt reactance. The result is a low pass filter. Had we placed L in the shunt position and C in the series position, we would have a high pass filter.</p>
|
||||
<center>
|
||||
<img src="gup15-4.gif" width="600" height="347" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Figure 4 redraws the L-circuits in their high-pass configurations. The top circuit, transforming downward, is the basis for the Beta-match, where the capacitance is actually a function of shortening the antenna element rather than being a separate lumped component.</p>
|
||||
<p>It is possible to treat the input and output impedances as purely resistive, as done in Figure 3, and then to add on any capacitive or inductive reactance necessary to cancel out load reactances. If we follow this procedure, the delta figure for the L-circuit simplifies to the follow expression:</p>
|
||||
<center>
|
||||
<img src="gup15e3.gif" width="145" height="86">
|
||||
</center>
|
||||
<p>Then, the required series and shunt reactances (Xs and Xp) can be calulated from simple equations:</p>
|
||||
<center>
|
||||
<img src="gup15e4.gif" width="256" height="75">
|
||||
</center>
|
||||
<p>Convert these figures to a capacitance and an inductance by use of the usual equations involving 2piF.</p>
|
||||
<p>When the load presented to an L-circuit is complex, that is, when it contains both resistance and reactance, the calculation of delta becomes somewhat more complex. However, we can show the trends by looking at our test case at 10.1 MHz. With L-circuits, with some exceptions in small regions where designs overlap under certain reactive loads, only one set of values provides a desired impedance transformation.</p>
|
||||
<p>In each test case, we shall be transforming an impedance of 50 Ohms upward, so the Rlo-to-Rhi versions of the L-circuits will apply. The interesting data will concern what happens to delta as we add either inductive or capacitive reactance. (LP = low-pass configuration; HP = high-pass configuration.)</p>
|
||||
<pre>Input = 50 Ohms; Output = 150 Ohms
|
||||
LP: Ls 1.114 µH Cp 148.6 pF Delta 1.41
|
||||
HP: Cs 222.9 pF Lp 1.671 µH Delta 1.41
|
||||
|
||||
Input = 50 Ohms; Output = 150 + 100 Ohms
|
||||
LP: Ls 1.438 µH Cp 181.3 pF Delta 1.83
|
||||
HP: Cs 172.6 pF Lp 2.946 µH Delta 1.16
|
||||
|
||||
Input = 50 Ohms; Output = 150 - 100 Ohms
|
||||
LP: Ls 1.438 µH Cp 84.3 pF Delta 1.83
|
||||
HP: Cs 172.6 pf Lp 1.370 µH Delta 2.49</pre>
|
||||
<p>Note that when the load is purely resistive, the high-pass and the low- pass configurations are equally efficient, that is, they have the same value of delta. When the load is inductively reactive, the high-pass configuration has the lower value of delta and thus has lower losses. When the load is capacitively reactive, the low- pass configuration has the advantage.</p>
|
||||
<p>Traditionally, we have built L-circuit tuners in the low-pass configuration for mechanical ease, not for the added filtering of transmitted signals. The attenuation of harmonics in a single L- circuit is somewhat minimal at best. However, most variable capacitors with any power-handling capability have ground rotator plates, and traditional construction has called for a metal chassis or case.</p>
|
||||
<p>Now add to this situation the fact that few hams have bothered to calculate the network efficiency of even the simple L-circuit (let alone other networks). Hence, they knew only that they either did or did not have a match. At power levels from 50 watts upward, the additional tuner losses seem to make little difference to the average operator. As a consequence, the high- pass L-circuit configuration has been neglected in all but Beta matches to Yagis and other low-impedance antennas.</p>
|
||||
<p>At QRP, where every avoidable loss is to be eliminated, the high-pass configuration for inductively reactive loads should be considered, especially in cases where the inductive reactance is high. Switches and jumpers can adapt the same coil and capacitor to use in high-pass, low-pass, high-to-low impedance, and low-to-high impedance combinations. Since the ATU network, whatever its configuration, is composed of passive components, no shielding is necessary. Non-metallic cases are not only satisfactory, they in fact reduce many stray capacitances that lower the overall component Q within the network.</p>
|
||||
<p><b>Single-Ended Networks</b>: All of the networks we have examined for efficiency (or losses, to look at the other side of the coin) are single-ended or unbalanced. Many of the uses to which they are put involve parallel or balanced transmission line. From early days, we have been taught not to use a single-ended network on balanced lines. ATU manufacturers who use unbalanced networks often install a 4:1 balun or transmission-line transformer between the network and a pair of so- called balanced output terminals.</p>
|
||||
<p>As we saw in a past installment, high impedance loads with large reactive components on balanced transmission lines do not present high impedances to the ATU terminals for most of each half wavelength of transmission line. Just the opposite prevails: the load the ATU sees is a very low impedance that needs no division by 4. Moreover, transmission-line transformers are efficient only with nearly pure resistive loads, and the best expert recommendations consistently tell us to compensate for reactance at the antenna terminals. We can effect a match that achieves a low SWR for the transmitter-to-line junction, but at indeterminate loss levels.</p>
|
||||
<p>So what can we do? Two answers have emerged from amateur experience.</p>
|
||||
<p>1. Some amateurs have found that, at least for their situations, the seemingly unbalanced network introduces no serious imbalance in currents and voltages along the transmission line. By floating the case, that is, leaving it ungrounded, the network performs normally and yields a perfectly or nearly perfectly balanced output. If a 1:1 balun or a line isolator (a W2DU- type balun choke or a series of split ferrite cores) is placed on the transmitter side of the ATU, any currents that remain on the outside of the coax are effectively blocked from the transmitter and other equipment cases.</p>
|
||||
<p>2. Other amateurs have had good success by transitioning to unbalanced line either at the ATU-to-transmission line junction or within a few feet of the ATU--perhaps at a window sill. Again, a line isolator prevents RF currents from flowing on the outside of the ATU case or the short section of coaxial line from the ATU to the window sill. The series section of coxial line introduces an altered impedance transformation and does impart some loss. Even the line isolator is not completely lossless, as W2FMI found some heating of the core material nearest the load in testing the W2DU balun under high power. However, the losses may be less than those accompanying alternative techniques of feeding power to the antenna. In such cases, this method may prove attractive.</p>
|
||||
<p>In the end, decisions fall into the domain of the practical. What is the most efficient and least lossy method of feeding my antenna among the practical alternatives I have? However, the more we know about networks, the greater the number of our alternatives.</p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center>
|
||||
<p><i>Updated 3-11-99. © 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>
|
||||
<center>
|
||||
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|
||||
</center>
|
||||
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|
||||
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|
||||
<p class="topics-footer">Topics: <a href='../topics.html#matching'>Matching</a></p><br>
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||||
<meta name="keywords" content="Vertical, Yagi, Quad, HF Lower, Noise / Receive">
|
||||
<title>Noise, Antennas, and Receiving Systems</title>
|
||||
<link rel="stylesheet" href="../styles.css">
|
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</head>
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||||
<center>
|
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<h1><img src="../images/yagicad.gif" align="left" width="125" height="62" border="2" vspace="5" hspace="10">ANTENNAS FROM THE GROUND UP<img src="../images/w4rnlwwr.gif" align="right" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
|
||||
</center>
|
||||
<center>
|
||||
<h3>16. Quiet, Please!<br>
|
||||
or Noise, Antennas, and Receiving Systems</h3>
|
||||
</center>
|
||||
<center>
|
||||
<h3>L. B. Cebik, W4RNL</h3>
|
||||
</center>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center><br>
|
||||
<br>
|
||||
<p>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, and here is one way to divide the group into usefully smaller chunks.</p>
|
||||
<p>Noise generally refers to any signal components at the point of demodulation except the intelligence of the desired signal. Noise may come from outside the receiver or be generated within it. In fact, all active devices in a receiver generate some noise, creating a so-called noise floor that determines the absolute minimum noise level for the receiver. Receiver designers constantly seek to improve the noise floor, but at the lower HF bands, other noise sources generally make the noise floor a hypothetical idea. Only at the upper HF regions can the receiverþs noise floor be perceived with an antenna connected.</p>
|
||||
<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>
|
||||
<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 canceled 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>
|
||||
<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. Near by sources include any number of household devices, such as touch control lamps, dimmer switches, and similar devices. The noise from these devices often enters the receiver not only though the antenna terminals, but as well through the power lines and the grounded case and chassis. Remember that the receiver does not care if a signal varies around some preset bias level on the main signal line or if it alters the level of the common bus while the bias remains constant. The cure for these noise sources lies at the sources themselves.</p>
|
||||
<p><b>2. Atmospherics</b>: Atmospheric noise refers to radio frequency energy generated by natural phenomena and carrying no man-made communications intelligence. There are two main sources of "atmospheric" noise and energy coupling to antennas:</p>
|
||||
<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. Lightning energy is generally AM modulated, and the difference between lightning modulation and some forms of rock music played by AM stations has been disputed. 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.</p>
|
||||
<p>Narrow-banding the pre-receiver reception system can reduce the total energy from such signals that reaches the receiver front end. One way to achieve a narrower bandwidth for signals entering the receiver is to place a narrow band, tunable filter at the antenna terminals. Except for pure receiving systems, this technique is not in wide-spread use by amateurs because it tends to attenuate outgoing signal power as well. The alternative is an equally undesired complex switching system to remove the filter during transmit cycles.</p>
|
||||
<p>An alternative is to use high impedance antennas that require sharp adjustment, usually of an ATU, as the frequency is changed. Even though the bandwidth of filters and sharply tuned antennas is wider than the IF passband of the receiver, the effects of noise reception are reduced. Receivers respond to signals and noise over a wide range, and this response can activate the AGC system, create thumps and other unwanted outputs, and otherwise disrupt reception of the desired signal. The narrower the bandwidth of the receiving system at the antenna terminals, the less noise is a problem.</p>
|
||||
<p>Since the perceived level of spark energy is the sum of energy received from all angles accepted by the antenna, narrowing the overall beamwidth of the antenna is one way to reduce noise levels. One might think that Yagis and other unidirectional antennas might achieve this, since they reduce radiation to and reception from the rear. However, their higher gains often offset this quieting for an overall tie with a dipole at the same height.</p>
|
||||
<center>
|
||||
<img src="gup16-1.gif" width="633" height="279" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>On the lower HF bands, one can feed certain vertically oriented loop antennas so that the horizontal radiation pattern is largely self-canceled. The horizontal portion of the antennas pattern is largely responsible for higher angle radiation in a broad vertical and horizontal beamwidth. By eliminating this portion of the pattern, as shown in <b>Figure 1</b>, the overall gain and receptivity to noise and signals is lowered. However, the remaining portion of the pattern is at low elevation angles that favor dx. Although the signals are usually weaker, their signal-to-noise ratio is much higher, resulting in higher quality reception. This technique, championed by ON4UN and other low-band DXers, proves that antenna gain is not everything, especially if that gain increases noise levels faster than it increases desired signal strengths.</p>
|
||||
<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>
|
||||
<p>For some antennas mounted very high, the energies involved could not be drained effectively before damage occurred to antenna elements. At the extreme, antennas can display St. Elmoþs fire. 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 did not produce a mere glow; it was burning the ends off the antenna elements.</p>
|
||||
<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 quieter than Yagis and dipoles.</p>
|
||||
<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>
|
||||
<p>Modern receivers usually employ noise blankers to eliminate as much noise as possible. Older receivers employed noise limiters. A noise limiter simply clips a signal as it exceeds a preset level. This level was set somewhat above the level of the strongest anticipated intelligent signal so that what was clipped had to be noise energy. Unfortunately, clipping often introduced distortion and mixing products (see next entry) into the remaining stages of the receiver chain.</p>
|
||||
<p>Noise blankers involve sending part of the signal energy to a separate amplifier. This amplifier is designed to detect energies of durations shorter than most forms of intelligence. Each energy pulse so detected is used to trigger a circuit that turns off a designated amplifier, usually in the IF chain. Hence, the amplifier is þblankedþ for a period too short for the human ear to tell. Gone is the noise pulse. Unfortunately, noise blanker circuits are imperfect devices: the sudden on-off square wave cycling can create distortion of the desired signal, as well as some mixing products.</p>
|
||||
<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.</p>
|
||||
<p>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. Some mixing problems occur from þfront-end overload;þ that is, signals are so strong that, regardless of frequency, they enter into the mixing and amplifying chain of the receiver. Very often for low-HF operators, these signals are AM broadcast band signals. The cure usually involves a trap or narrow bandpass filter at the antenna terminals.</p>
|
||||
<p>Along the way, we have mentioned ATUs, bandpass or trap filters, and other circuits external to a receiver's antenna terminals as possible aids in noise reduction. It is possible to end up with a string of small boxes, each containing a certain kind of filter, and each connected in series with cables and connectors. Although this situation is typical ham, it is better suited to the test bench than to a communication system. Cables and connectors are invitations to new ways of noise pickup. A better system would be to build all of the necessary external noise reduction tuning and filter devices into a single case with direct (or switched) connections between them.</p>
|
||||
<p>We have not covered 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. ARRL maintains a bibliography of articles on RFI and related matters for use by interested amateurs.</p>
|
||||
<p>A series of articles on antennas is hardly the place for political comment, but it is a sad though hardly surprising fact of life that commercial interests constantly pressure legislatures and regulatory agencies for relaxation of standards for interference and noise production. Counter- pressure from amateurs and others adversely affected by eliminable noise and interference is necessary to at least hold the line on the ever-noisier RF environment. Although there are no guarantees of success, especially since amateurs can rarely marshal the monetary resources for high-pressure lobbying, constant goading and enlightenment of regulators can at least minimize the rate of noise increase.</p>
|
||||
<p>Although every amateur operator has a vested interest in identifying, controlling, and eliminating man-made sources of noise, we should all remember that noise is a reciprocal issue. We should also go to great lengths to ensure that our stations and test benches are not noise sources relative to any other devices able to pick up RF energy. This includes such non-RF devices as modern telephones, along with their portable RF cousins. The less we tolerate noise and interference that might emerge from our own equipment, the more justified we are in insisting that other interests do likewise.</p>
|
||||
<p>The final defense against noise is ultimately the same as the final defense against both willful and unwitting QRM. That defense is to perfect our abilities as operators. When our equipment is state-of-the-art relative to noise elimination and reduction, we have nothing left to use in the war on noise but our well-honed skills as operators.</p>
|
||||
<p>Operators skills involve three areas of effort: equipment adjustment, reception, and transmission. We need to perfect our abilities to get the most out of the equipment at hand. This includes experiential knowledge of where to point pointable antennas for the best signal-to-noise ratio; how to integrate bandpass, bandwidth, and blanker adjustments for maximum effectiveness; and a myriad of other subtle receiver settings. In addition, we need to constantly practice receiving signals in order to tune our ears more finely than any electronic decoding device can match. Lastly, we need to transmit not only with the precision that makes code or voice reception Q5, but as well with the intuition that alters code or voice speed and patterns to fit the operator at the other end of the line.</p>
|
||||
<p>In the end, however, some folks are condemned to live in areas where noise is beyond control and even beyond the ability of the best noise blanker (in the receiver or between the ears) 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. Short of those drastic remedies, however, antenna choice, feed system choice, filtration, noise cancellers, noise blankers, and operator skills can go a long way toward reducing currently unlivable noise to a mere constant irritation.</p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
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</center>
|
||||
<p><i>Updated 7-11-99. © 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>
|
||||
<center>
|
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<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center>
|
||||
<p><a href="groundup.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to series index 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>
|
||||
<p class="topics-footer">Topics: <a href='../topics.html#hf-lower'>HF Lower</a>, <a href='../topics.html#noise-receive'>Noise / Receive</a>, <a href='../topics.html#quad'>Quad</a>, <a href='../topics.html#vertical'>Vertical</a>, <a href='../topics.html#yagi'>Yagi</a></p><br>
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<meta charset="UTF-8">
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<meta name="keywords" content="Dipole, Quad Loop, Delta Loop, Rectangle, Yagi, HF Lower">
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<title>Vertically-Oriented, Horizontally Polarized 1 wl Loops</title>
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<h1><img src="../images/yagicad.gif" align="left" width="125" height="62" border="2" vspace="5" hspace="10">ANTENNAS FROM THE GROUND UP<img src="../images/w4rnlwwr.gif" align="right" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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</center>
|
||||
<center>
|
||||
<h3>17. At Cross Purposes<br>
|
||||
or Vertically-Oriented, Horizontally Polarized 1 wl Loops</h3>
|
||||
</center>
|
||||
<center>
|
||||
<h3>L. B. Cebik, W4RNL</h3>
|
||||
</center>
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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>
|
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<p>There are many ways to make a 1 wl loop: a triangle, a square, a diamond, a hexagon, a circle. . .. What these loop antennas have in common is that they are resonant on a chosen fundamental frequency.</p>
|
||||
<p>Physically, we can set the antennas in three positions: horizontal (parallel to the ground), sloping (at some chosen angle), or vertical (at right angles to the earth. Horizontal and vertical orientation are the most fundamental. We have already presented some notes on horizontally oriented loops, so let's focus on vertically oriented loops.</p>
|
||||
<p>Before we dig into these loops, let's set up another distinction: horizontal vs. Vertical polarization. First, all low antennas (under 1/2 wl in height) have both horizontally and vertically polarized fields. So the question is not about exclusive polarization, as it might be if we were talking about a 2-meter Yagi at a height of 10 wl. Instead, the question is one of dominance.</p>
|
||||
<p>Second, the polarization of a loop's transmissions (and receptions) only indirectly affects the far end of the communications circuit. For the most part, the ionosphere randomizes the polarization of signals. Polarization of the antenna's pattern plays a major role in the shape of the radiation and reception pattern, and it is this shape that affects signal strength, signal-to-noise ratio, and whether the antenna favors high or low angle radiation.</p>
|
||||
<p>We shall look at vertically-oriented, vertically-polarized loops (VOVPLs) in a future installment, but for now, let's concentrate on vertically-oriented, horizontally-polarized loops (VOHPLs).</p>
|
||||
<p>Some folks like to think of VOHPLs as folded dipoles opened up into sundry geometric loops. That is fine, if you remember to stress the dipole aspect of the matter. We can expect virtually any VOHPL to act like a pair of dipoles, one above the other. The resulting patterns will usually be a compromise between the pattern of the higher dipole and the pattern of the lower dipole.</p>
|
||||
<p>The graph below (<b>Fig. 1</b>) charts the angles of maximum radiation for a 40-meter dipole, a 2-element wire Yagi, and three typical VOHPLs, the right-angle delta, a square loop, and a horizontal rectangle. The chart is based on practical considerations of lower HF-band wire antennas. Foremost is the fact that amateurs will install a wire antenna as high as possible, given the many constraints of backyard construction. Thus, the heights listed are for the top wire of each antenna. The premise is that if one can get a wire up to a given height, then whatever the wire antenna choice, its topmost wire will be at that height.</p>
|
||||
<center>
|
||||
<img src="gup17-1.gif" width="657" height="526" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>In all cases, the dipole and the wire Yagi have lower angles of maximum radiation than any loop. The explanation is simple: half of the loop's power is being distributed to a wire well below the peak of the antenna.</p>
|
||||
<p>One can thus expect a dipole to outperform a vertically-oriented loop with respect to low-angle gain. The natural question is this: why install a loop? There are three very significant positive reasons why one might choose a loop:</p>
|
||||
<p>1. The space between supports is too short for a dipole, but the supports are high enough to support the loop.</p>
|
||||
<p>2. There is only a single high support, thus favoring one of the Delta loops.</p>
|
||||
<p>3. The loop is inherently more immune to noise pick-up than an antenna with free ends, and the improvement in signal-to-noise ratio is worth the loss in other performance factors.</p>
|
||||
<p>The pattern of the VOHPL on its fundamental frequency is essentially the same, whatever the precise shape of the loop. <b>Figure 2</b> overlays azimuth patterns for a dipole, a rectangle, and a delta loop, all at top-wire height of 40' and with a 45° elevation angle. Patterns for increased height begin to show the peanut shape we associate with dipoles. Operationally, the rectangle most closely approaches the performance of the dipole, since its lower wire is highest among the loops.</p>
|
||||
<center>
|
||||
<img src="gup17-2.gif" width="640" height="480" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Let's tabulate some of the properties of the various VOHPLs we might choose as an antenna. All dimensions are for 40 meters (7.15 MHz), and are approximate, since actual resonant length will vary with height above ground. Likewise, feedpoint impedance figures are ballpark to guide initial feedline thoughts.</p>
|
||||
<p>All antennas were modeled on EZNEC-Pro over medium ground with #14 copper wire. All models have been stabilized by increasing the number of segments per half wavelength until further increases showed no significant changes. For new modelers, this technique is standard practice in the generation of reliable NEC models. Although dipoles may be stable with the minimum number of segments per half wavelength, antennas with more complex geometry, such as loops, may require more segments per half wavelength.</p>
|
||||
<p><b>Right-Angle Delta</b>: This antenna, as shown in <b>Figure 3</b>, uses a right triangle to hold down its height. Fed in the center, the impedance is 170-220 Ohms, depending upon height. The circumference is about 141', with a 58.4' base and a 29.2' height. Resonant dimensions will vary with antenna height.</p>
|
||||
<center>
|
||||
<img src="gup17-3.gif" width="474" height="309" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<pre>Height T-O Angle Gain
|
||||
Min. Max. (Degrees) (dBi)
|
||||
10.8 40 90 4.3
|
||||
20.8 50 64 5.3
|
||||
30.8 60 46 5.6
|
||||
40.8 70 37 6.1
|
||||
50.8 80 31 6.9</pre>
|
||||
<p><b>Equilateral Delta</b>: This antenna, as shown in <b>Figure 4</b>, is taller than the right-angle triangle. Fed in the center, the impedance is 105-135 Ohms, depending upon height. The circumference is about 144', with a 48' base and a 41.5' height. As with the right-angle delta, resonant dimensions will vary with antenna height.</p>
|
||||
<center>
|
||||
<img src="gup17-4.gif" width="501" height="411" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<pre>Height T-O Angle Gain
|
||||
Min. Max. (Degrees) (dBi)
|
||||
1 42.5 79 -1.6
|
||||
8.5 50 67 3.4
|
||||
18.5 60 53 5.0
|
||||
28.5 70 42 5.6
|
||||
38.5 80 35 6.2</pre>
|
||||
<p><b>Square Quad Loop</b>: The square loop, as shown in <b>Figure 5</b>, is often used as the standard for loops, although there is nothing electronically significant about its shape over and against other loop shapes. Fed in the center, the impedance is 115-145 Ohms, depending upon height. The circumference is about 145', with about 36.2' per side, with the exact resonant dimensions varying with antenna height.</p>
|
||||
<center>
|
||||
<img src="gup17-5.gif" width="392" height="384" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<pre>Height T-O Angle Gain
|
||||
Min. Max. (Degrees) (dBi)
|
||||
3.8 40 55 3.5
|
||||
13.8 50 46 5.3
|
||||
23.8 60 38 5.9
|
||||
33.8 70 33 6.6
|
||||
43.8 80 28 7.3</pre>
|
||||
<p><b>Diamond Quad Loop</b>: The diamond loop, as shown in <b>Figure 6</b>, is usually too large both vertically and horizontally for lower-HF use. However, the builder can shrink the diamond vertically and stretch it horizontally. Fed in the center, the impedance is 120-140 Ohms, depending upon height. The circumference is about 144', with about 36.1' per side, with variations in resonant length owing to antenna height.</p>
|
||||
<center>
|
||||
<img src="gup17-6.gif" width="392" height="381" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<pre>Height T-O Angle Gain
|
||||
Min. Max. (Degrees) (dBi)
|
||||
(A maximum height of 40' is not feasible
|
||||
with this configuration of the diamond
|
||||
loop. Note that the lowest maximum height
|
||||
is about 52'.)
|
||||
1 52 53 4.8
|
||||
9 60 45 5.5
|
||||
19 70 37 6.1
|
||||
29 80 32 6.8</pre>
|
||||
<p><b>Rectangular Loop</b>: The rectangular loop, as shown in <b>Figure 7</b>, is among the most practical of VOHPLs, since one can tailor its precise dimensions to the space available. The model used here, fed in the center, has an impedance of 200-270 Ohms, depending upon height. The circumference is about 138', with 49' horizontal wires and 20' vertical wires. However, exact resonant dimensions will vary with antenna height.</p>
|
||||
<center>
|
||||
<img src="gup17-7.gif" width="552" height="326" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<pre>Height T-O Angle Gain
|
||||
Min. Max. (Degrees) (dBi)
|
||||
20 40 63 5.5
|
||||
30 50 46 5.7
|
||||
40 60 37 6.2
|
||||
50 70 31 6.9
|
||||
60 80 27 7.7</pre>
|
||||
<p><b>Dipole</b>: As a standard for reference, here are the numbers on the half wavelength center-fed dipole, as shown in <b>Figure 8</b>. The model used here, fed in the center, has an impedance of 50-80 Ohms, depending upon height. The approximate resonant length is 67', but will also vary with height. A folded dipole will yield just about the same figures, with a higher feedpoint impedance.</p>
|
||||
<center>
|
||||
<img src="gup17-8.gif" width="555" height="262" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<pre>Height T-O Angle Gain
|
||||
Min. Max. (Degrees) (dBi)
|
||||
40 40 50 5.9
|
||||
50 50 39 6.2
|
||||
60 60 32 6.8
|
||||
70 70 27 7.6
|
||||
80 80 24 8.0</pre>
|
||||
<p><b>2-Element Wire Yagi</b>: For reference, here are the numbers on a 2-element wire Yagi, shown in <b>Fig. 9</b>. I note it here because it uses only about the same amount of wire as a full wavelength loop The driven element is 66' long, while the reflector is 70' long. The element spacing is 20'. The feedpoint impedance is between 50 and 60 Ohms, depending upon height.</p>
|
||||
<center>
|
||||
<img src="gup17-9.gif" width="454" height="360" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<pre>Height T-O Angle Gain
|
||||
Min. Max. (Degrees) (dBi)
|
||||
40 40 41 8.8
|
||||
50 50 35 9.5
|
||||
60 60 30 10.0
|
||||
70 70 26 10.5
|
||||
80 80 23 10.8</pre>
|
||||
<center>
|
||||
<img src="gup17-10.gif" width="640" height="750" border="2" vspace="5" hspace="9">
|
||||
</center><b>Figure 10</b> provides the azimuth and elevation patterns for the wire beam at a height of 50'. Although the antenna will not outperform a rotatable Quad or Yagi at 200', it will show significant gain and front-to-back ratio in its fixed position--hopefully aimed at DX.
|
||||
<p>Throwing the 2-element wire Yagi into the mix has a purpose: to force you to think in three dimensions about the space available for antennas and about all the options that may be available. There are other directional wire designs, such as the Moxon rectangle, which we shall review in another installment.</p>
|
||||
<p>Loop performance does not significantly change if fed at the center of the top rather than the center of the bottom. Inverting the delta designs so that the long horizontal wire is on top will improve performance, since a longer horizontal wire is now at maximum height. However, figures will still not outperform the basic dipole at the same maximum height. With the inverted delta, half the power still radiates from a lower conductor.</p>
|
||||
<p>Most of the loop designs will require matching at the antenna feedpoint, along the line, or at the equipment position (an ATU). Therefore, parallel feedline has been the transmission line of choice, especially if multiband operation is contemplated. For single-band operation, networks, X:1 baluns, and line sections have all been successfully used to match any of the loops to a 50-Ohm coaxial feedline.</p>
|
||||
<p>However, many antenna builders choose the VOHPL because it permits multiband operation with an ATU (or dual band operation with simpler matching means). So let's look <a href="gup18.html">next time</a> at multiband loops.</p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center>
|
||||
<p><i>Updated 9-8-99. © 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>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center>
|
||||
<p><a href="groundup.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to series index 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>
|
||||
<p class="topics-footer">Topics: <a href='../topics.html#delta-loop'>Delta Loop</a>, <a href='../topics.html#dipole'>Dipole</a>, <a href='../topics.html#hf-lower'>HF Lower</a>, <a href='../topics.html#quad-loop'>Quad Loop</a>, <a href='../topics.html#rectangle'>Rectangle</a>, <a href='../topics.html#yagi'>Yagi</a></p><br>
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