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
>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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