[TowerTalk] Modelling a basic stack

[Bill Tippett <btippett@alum.mit.edu>]

Hi Bill,

        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.

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