|Subject:||[TowerTalk] Modelling a basic stack|
|From:||Bill Tippett <firstname.lastname@example.org>|
|Date:||Mon, 10 May 2004 07:20:37 -0400|
Very well stated!
W5WVO wrote: >Since you're talking about stacking two A50-5S antennas, however, let me also submit to you that stacking two of a poor antenna is a twice-poor idea. And the A50-5S is a very poorly designed antenna. It is intended to be idiot-proof (which, I assume, is why all the elements are equidistant) rather than optimized. I used YO to redesign this 5-el antenna on a 17.5' boom, and the optimization process yielded a design that is very close to the M2 6M5X, which is a very good antenna for its size. You can modify the A50-5S in this fashion for the cost of a 6-ft piece of aluminum tubing (UPS shippable from Texas Towers) and two hose clamps. About ten bucks. For this lofty expenditure, you will get about a 1.5 dB increase in forward gain, an improvement in 3 dB beamwidth of 8 degrees (54 down to 46), and a front-to-rear that is awesomely better, typically better by 15 dB or more depending on height above real ground and other variables.
This is exactly why YO7 is such a value for the money. HRO lists the A50-5S at $180, so two would be $360. With little more than another $100 for YO7, you can turn the mediocre design of this antenna into one which is customized for your needs. Want to maximize performance for the 6-meter DX band only? Want maximum F/R in a particular area of the band? Want maximum gain only? YO7 can do any of these...literally in minutes.
In just ONE minute using YO 7.61 on my old 250 MHz PII, I got the following free-space results for two stacked A50-5S antennas assuming equal (33%) weighting for Gain, F/R and SWR:
Antenna 50.0 MHz 51.0 MHz
Stock: Gain 12.96 13.01 F/R 16.83 12.73 SWR 1.20 3.25
One-minute optimization: Gain 13.11 13.20 F/R 24.16 25.06 SWR 1.14 1.38
This was based on only 126k iterations but, if you really wanted to fully optimize, you could let the model run overnight. The beauty of YO7 is the ability to quickly customize your tradeoff preferences for Gain, F/R, SWR and frequency range and do so within a stack (which is different than for a single stand alone Yagi). Many models of monoband antennas (including KLM) are included in Brian's software which makes your job very easy. I had not seen what I felt was an accurate model of KLM (including one that W4RNL sent me) until I got YO7.
Many existing Yagi designs were made before modeling was practical (KLM and Cushcraft). Most newer antennas (Hy-Gain, M2, Force 12, etc) have been developed using modeling (Force 12 uses the professional version of YO7 and possibly others do as well), so the improvement potential may not be as great. However, even newer antennas could probably be tweaked using YO7 for individual tradeoff preferences.
If a person is going to the expense of stacking, I simply cannot understand why anyone would NOT spend another $100 to truly optimize their system performance to meet their exact needs.
73, Bill W4ZV
P.S. What is the difference between YO7 and earlier versions? Global optimization...described in the July 2000 issue of CQ.
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
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