Tom,
Modelling that in EZNEC over average ground I see little difference in
the main lobe, but some change in the small high-angle lobes. Best
result of the various current combinations I tried was with the current
in the middle Yagi 1.4 times the other two. But I doubt the difference
is worth the extra complexity in the feed system.
73,
Steve G3TXQ
Thomas Cathey wrote:
> Hi,
>
> I wonder if someone can help me out.
>
> I'm finishing up a homebrew project of three, full-size, 2 element 40M Yagis
> that will be stacked at 60', 125' and 190' on a free standing tower.
> Modeling shows the front-back and higher angle frontal lobes to be tighter
> and smaller when feeding the center Yagi at twice the current, or binomially
> fed.
>
> In AO, (Beezley's 'Antenna Optimizer') I have the sources set as:
>
> Wire 1 center 7 0
> Wire 3 center 10 0
> Wire 5 center 7 0
>
> Hope that's the correct source coding for binomial. Anyway, it produces a
> tremendous pattern compared to making all three sources as '10.'
>
>
> If done this way in the real world, I will match the center Yagi with a 25
> ohm input impedance and feed the outer and lower Yagis with 50 ohm
> impedances. Ie, the triple feed parallel connection will see a matched 25
> ohm load and two matched 50 ohm loads, all equal length.
>
> Binominal feeds are used with three verticals, etc all the time, but I've
> not seen information doing it with stacked Yagis. I understand it's done
> because the middle antenna is interacting with two outer antennas, so needs
> twice the current.
>
> Is this the right approach to feed a triple Yagi stack or should I feed them
> conventionally as three 50 ohm Yagis with equal feedlines, etc?
>
> Thanks.
>
> Tom Cathey, K1JJ
>
>
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