[TowerTalk] Modelling a basic stack

[Bill VanAlstyne]

Well, since this has become at least in part a referendum on YO, I'm going to
toss my hat strongly into the YO ring. My experience with it has been very
positive.

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.

The modified antenna is easily matched to the line by re-tweaking the existing
gamma match and driven element length using an MFJ 259B. I was able to achieve
zero reactance and a flat 1:1 SWR at 50.150 MHz with a little patient fiddling.

So IMO, this is where you should start -- with a decent antenna. If you already
have an A50-5S, I strongly recommend this modification. (I wrote a paper on it
if anybody's interested. Or do the optimization yourself.) For ten dollars (or
less, if you you can get a 6' X 1.5" OD tube locally and not pay the shipping),
you'll probably get as much benefit as you would by stacking two stock A50-5S's,
with a lot less hassle, expense, and strain on your rotator.

If you don't already have an A50-5S, just one word: don't. The stainless steel
hardware used in this antenna is (IMO) of poor quality, and is prone to galling.
I had to replace all my U-bolts and nuts when I disassembled my A50-5S for the
first time. If you put yours together without some kind of anti-seize
lubrication, you probably will, too.

Bill / W5WVO


Tim Duffy K3LR wrote:
> Hello Bert:
>
> I have used YO and AO with tremendous success for over 10 years and
> 100s of different designs. I am amazed by the correlation from
> computer model to real world.
> Most every antenna designed works first time without any iteration.
>
>
>
> YO is a tremendous antenna modeling tool and a real value for the
> price.
>
> Good Luck and 73,
> Tim K3LR
>
>
> N4CW at aol.com wrote:
>
>> I've never modelled a stack before. I have successfully modelled
>> yagis, and
>> in this case, I want to model a stack of two Cushcraft A50-5S yagis,
>> spaced 1/2
>> wavelength apart, with the lower yagi being 40 feet over real ground.
>>
>> I'd appreciate being guided to some good references regarding
>> stacking before
>> I get in too deep :~)
>>
>> 73,
>> Bert, N4CW
>>
>> _______________________________________________
>>
>> 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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>
> _______________________________________________
>
> 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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