[amsat-bb] Re: 70cm antenna SWR problem solved?
domenico.i8cvs at tin.it
Thu Jun 5 17:46:04 PDT 2008
----- Original Message -----
From: "Michael Tondee" <mat_62 at netcommander.com>
To: "amsat-bb" <amsat-bb at amsat.org>
Sent: Friday, June 06, 2008 1:30 AM
Subject: [amsat-bb] 70cm antenna SWR problem solved?
> After several unsuccessful attempts at solving my 70cm antenna problem
while it was still on the mast, I resolved today to just go ahead and get it
down off there. Not that big a deal to do really and besides I've been
thinking of rebuilding the whole shootin match anyway.
> I was able to use my 2 meter SWR meter after a fashion once I got the
antenna inside. I could see fluctuations in the amount of power the antenna
was taking as I tried various things. The problem lies in the area of the
matching and feedpoint of the driven element. Again those of you familiar
with the WA5VJB " cheap yagi" concept will know that the driven element is
bent back upon itself forming a U shaped section on one side. Those less
familiar with the design might not understand this but there's something
going on right there. Either with the spacing between the legs of the U or
the braid and center conductor being too close together at that point where
they are soldered. Changing the spacing of the U legs didn't seem to help
but spreading the center conductor and shield out from each other right
where they are connected to the DE made a marked difference. I suspect it's
because at this point they are very close to the "matching area" of the
antenna. Hope that makes se!
> At any rate I have decided to rebuild both my 70cm and 2 meter antenna. I
will keep 11 elements on 70cm but I'm going from 4 to 6 elements on 2
meters. The other change will be to replace the boom material. I used PVC
pipe before and the antennas have developed a bit of a sag along their
booms. I will be using wooden booms coated with spar varnish this go around
and possibly add reinforcement if any sag appears evident. BTW, both of
these are linearly polarized as I don't have an SWR analyzer to figure out
velocity factor on cable while making phasing harnesses. Maybe someday!
> Thanks much for everyones help and 73,
> Michael W4HIJ
Hi Michael, W4HIJ
Before to rebuild your WA5VJB cheap yagi antenna look here:
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