The measured impedance will change depending on the length of the coax. It
should match the antenna feedpoint impedance at 1/2 wavelength multiples.
The SWR should not change with coax length, except for the change due to
loss.
John KK9A
Subject: Re: [TowerTalk] Elevated vertical
From: "Mike Ryan" <mryan001@tampabay.rr.com>
Date: Sat, 24 Dec 2011 09:17:48 -0500
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
Gerald is correct here. I think one has to start with the feed line. IF
W7WHY wants his antenna resonant at 7.125 it might be in his best interest
to find out what the velocity factor is for his coax. Then, cut the cable to
have the swr the same at BOTH ends based on the freq and the velocity
factor. I am scratching my head but I think this works out to be even
multiples of 1/4 waves or something like that ( sorry, it's been too long
since I cared about this stuff). In that way, the swr will be the same at
the antenna end as it is at the transmitter end ( or should be), and if the
cable is cut to the correct length, it should be about flat. Trouble is
when he then changes freq this will go away as the coax is then not cut for
a resonant freq. any longer. How much difference it will make is anyone's
guess. But this thread began I THINK with him saying he had swr of 1.5 and
1.7 etc. I don't know what he is worrying about anyway. At hf freqs the
loss is pretty negligible. Merry Christmas all.
-Mike
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