[TowerTalk] Binomial feed for three stacked 40M Yagis

[Thomas Cathey]

Hi Steve,

Thanks so much for doing a double check for me.

As you said, maybe it's not worth the extra effort. Think I'll feed them as
50 ohms each and put it to bed.  It will be easier and less prone to error
in matching.

Appreciate the help, OM.

Tom, K1JJ



-----Original Message-----
From: towertalk-bounces at contesting.com
[mailto:towertalk-bounces at contesting.com] On Behalf Of Steve Hunt
Sent: Friday, June 19, 2009 9:59 AM
To: towertalk at contesting.com
Subject: Re: [TowerTalk] Binomial feed for three stacked 40M Yagis

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
>
>