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Re: [TowerTalk] Mixing a 20m yagi with a 40m yagi on the sameboom.....

To: Robert Thain <>,
Subject: Re: [TowerTalk] Mixing a 20m yagi with a 40m yagi on the sameboom.....
From: Jim Lux <>
Date: Wed, 30 Aug 2006 06:19:37 -0700
List-post: <>
At 04:46 AM 8/30/2006, Robert Thain wrote:
>I have a question from my radio club,
>We have a Hygain 4 element 20m monoband yagi (204) and a Cushcraft 40-2cd.
>We would like to put them on the same boom.
>Something like a  EF-420/240 from force12.
>Ideally we want to keep as much of the existing hardware as possible (Ie 
>keep the hygain boom)
>Have you done this (or similar)? Can it be done ? If so what changes need 
>to made ?
>Where should the 40m beam be placed on the '20m' boom. ie how should it be 
>interlaced ?
>Your input is greatly appreciated.
>We do not have any body who is familiar with antenna software, I "think" 
>it should be easier with that.

You need to spend half an hour getting familiar with antenna modeling 
software, and then spend half a day trying the models.

While there's not a "lot" of interaction in such a design, there's enough 
that you need to figure it out, so you don't inadvertently do it "really wrong"

Unless you have a proven way to do the interlace from someone who's done it 
before, you have a fair to middling chance of spending all the time putting 
it together, hauling it to the top of the tower, and then finding out it 
doesn't work like you expected.

By the way, the forward gain is pretty insensitive to these things, what 
you'll screw up will be the F/B or the bandwidth.

personally, I use the (free) 4nec2 engine, but others are fans of 
EZNEC.  Both of them would work fine for this kind of thing.

You're not necessarily looking for gnat's eyelash accuracy modeling here, 
but more putting the model together and making a "sensitivity analysis"... 
if I move the elements an inch forward or back, is there a huge difference 
in pattern.  If there isn't, then you can slap it together and be 
confident.  OTOH, if you put a quick model (no tapers, just straight 
elements) together, and moving the 40m elements relative to the 20m 
elements by 1/2 inch changes the F/B by 10dB, then you KNOW you've got some 
work on your hands.

>Please reply direct and I'll summarise.


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