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Winter 2004 Issue |
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Using Antenna
Simulation Software |
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The antenna simulation software now available to amateurs certainly has moved antenna construction from “cut and try” to “calculate and build,” especially for Yagi-style antennas, obtaining professional-style results. In addition to the original design, I’ve used such software to analyze and reconfigure existing antennas.
In my situation, I had a number of Yagis I
wanted to cut down for use in my rover station.1 In your situation, at a
hamfest you may have obtained a Yagi without documentation and now you
would like to know its characteristics, or you might have an antenna to
which you want to add some elements, increasing the antenna’s gain. In
either case, you can use simulation software to guide you. I used YW, Yagi
for Windows®, obtained with The ARRL Antenna Handbook, for the examples
described herein, but other simulation programs should work equally well.
To characterize an existing antenna, use a
tape measure to make accurate measurements of element lengths and spacing
in inches or metric units, as required by the program you are using. Once
you have the described antenna as an input file to your software, you can
add or delete elements and see what effect it has on antenna gain and
pattern as well as the feed-point impedance. You can also change element
lengths if you are interested in altering the antenna’s resonance point or
bandwidth characteristics. My roving buddy, N3LJK, and I had obtained a FINCO 220-MHz 10-element Yagi without documentation at a hamfest (see photo). Simulations indicated the gain to be 12 dBi for the 9-ft. boom design. This antenna was too long for our rover station, but because its construction was a two-piece boom, it was easy to remove the front part of the antenna, leaving a 4.5-ft. long Yagi. Next I modeled it as a 5-element, 4.5-ft. long antenna, which produced a clean radiation pattern with a gain of 10 dBi and a front-to-back ratio of 11 dB at 222 MHz. The program also indicated that the existing gamma match would work in this new configuration, as measurements later confirmed.
Another hamfest special I have is a Cushcraft
432-MHz Yagi which consists of two 10-element Yagis on a 4-ft. boom and
which was originally used for satellite work with a phasing harness to
produce circular polarization. The size was feasible for roving, but it
looked like too many elements for the boom length. I was also concerned
about its resonance point. Modeling showed gain of 13 dBi and a
front-to-back ratio of 17 dB at 432.5 MHz, with a decent enough radiation
pattern. Consequently, I remounted one of the Yagis on a different 4-ft.
boom and sold the other Yagi on the original boom at a hamfest. Click here to return to this month's highlights
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Simulation Software to Reconfigure _________________ © Copyright 2004, CQ Communications, Inc. All rights reserved. This material may not be reproduced or republished, including posting to a website, in part or in whole, by any means, without the express written permission of the publisher, CQ Communications, Inc. Hyperlinks to this page are permitted. |
The author holding the 222-MHz antenna described in the text. |
