[TowerTalk] CushCraft Query
john at kk9a.com
john at kk9a.com
Thu Jun 22 16:14:23 EDT 2006
Why would the impedance change with a different driven element. The
reflector and director lengths and spacing are not being changed and I
thought that is what sets the impedance. If I remember correctly the A3 is
a 50 ohm antenna and swapping the dipole for the DE would probably only
involve changing the lengths for the best SWR.
From: Jim Lux
Date: Thu, 22 Jun 2006 10:03:49 -0700
List-post: <mailto:towertalk at contesting.com>
At 07:06 AM 6/22/2006, Clay Curtiss W7CE wrote:
> > Could I replace the DE on the A3S
> > with the D4 and have a 40 - 10M yagi? (with no gain on 40, I know).
> > So I sent that query to CC and received a "I see no reason why that
> > would not work" sort of reply.than the one I got from CC.
>Chances are the impedance match will be poor if you try to use the D4
>as the DE on your A3S. The D4 is designed to give an impedance of
>~50 ohms with no parasitic elements. Add a reflector and director on
>20-10M and the impedance will drop to about 20-35 ohms depending
>on the design. Functionally it will work, but you will most likely have
>an SWR of about 2:1.
Not only will the impedance be different, but I'll bet the pattern would be
substantially different. The whole design of a Yagi depends on the mutual
coupling among the elements to get the current phases and amplitudes
correct. Change one element's characteristics and the whole shebang
Not to say that with a bit of work with a modeling program you could
probably figure it all out so that it would work acceptably. A 3 element
yagi isn't likely to be a design that is really, really picky.
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