[TowerTalk] CushCraft Query

[john at kk9a.com]

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.

John KK9A


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
changes.

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.


Jim