Sounds like an issue with the center conductor contracting when it is cold
and expanding in length as it warms up. Have you tried checking with an ohm
meter both when it is OK and when the SWR is high?
Date: Sun, 01 Apr 2012 10:01:08 -0500
From: Richard Thorne <email@example.com>
Subject: [TowerTalk] Stange SWR/AttenuationpProblems, goes away when
the sun comes up
To: Towertalk <firstname.lastname@example.org>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
I finished my tower/antenna install Friday. 77' of 45g with 15' of mast out
the top. I have a F12 D240 mounted 10' up the mast and a F12 C31 at about
1' up the mast. The swr is where they are supposed to be on both antennas.
Now for the strange part.
Some time after dark my D240 becomes attenuated and the swr is very very
high. But after the sun comes up for about 2 to 3 hours, the attenuation
goes away and the swr is right where its suppose to be and signals are clear
Its been clear with no rain since the antenna has was put up Friday morning.
There must be some moisture in the air as dew does form on the grass and the
roof of my steel building/shop.
So I'm scratching my head on this one. I've switched out coax cables at the
shack, bypassed the polyphaser lightning suppressors, used the different
antenna inputs to my Elecraft K3, checked to be sure I didn't have a receive
antenna selected on the K3 etc.
I have a single 158' run of coax from my entrance panel to the top of the
tower and have it terminated with one of these
then another length of coax that forms the rotor loop, goes up the mast then
to the f12 balun (f12 B1) at the feed point of the D240. I have the same
setup via another coax cable going to the C31XR which doesn't exhibit this
I have two weep holes drilled in the bottom of each trap cover for moisture
All connection points are weatherized using 3m rubber splicing tape which is
then wrapped with Scotch Super 33. All coax, connectors etc are brand
The next step is to climb the tower and check the connections at the top of
the tower, if that's not the problem it will be time to bring out the crane
again and check the connections at the feed point.
Before I get to far into this I'd like to know if any else has run into this
symptom and what the final resolution was.
Rich - N5ZC
TowerTalk mailing list