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[TowerTalk] TH-2 Large SWR changes during day

Subject: [TowerTalk] TH-2 Large SWR changes during day
From: Brian Alsop <>
Date: Sat, 20 Oct 2012 02:39:56 +0000
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This has been a nightmare.

I got a new TH-2 a few months ago.  Mounted on the roof at 22'.
10/20M behaved as expected. Tuned for the CW segment. 15M had a high SWR at the low end of the CW band. I tried tuning it out early in the day by lengthening the intertrap dimension.. It worked until later in the day. Magically the high SWR problem was back. It has an odd characteristic where there is a steep rise in SWR (relative to manual curve) on the lower side of resonance and a slope approximately matching the manual on the higher side of resonance.

MFJ said it was probably "interaction" with the roof, wiring et al and to get the antenna up higher and in the clear.

I did so. It's at 40' on a tower clear of house, trees etc, with non-conductive guys and no other antennas nearby. All dimensions were rechecked, ring clamps retightened, screws/nuts checked for tightness.

Guess what.  Same problem.

Discussion with MFJ indicated they thought it was a bad 15M reflector trap. They would send me one under warranty. That was yesterday. When it shows, I can't say.

Since that discussion, I have more data.

I have a weather station I can monitor humidity. There is an interesting correlation between SWR and humidity. High humidity leads to low SWR's and dry conditions to higher on the low end of the band.
The opposite is so on the high end of the band.

Coax in first installation was RG8X. The new installation uses an unbroken length of LMR-400 except for an RG213 rotor loop. A "yagi balun" (really turns of coax on a core)is used.

Now for some data:

freq  % humidity  SWR
21000   100%    1.3
         49%    2.0
         32%    2.1

21040   100%    1.0
         49%    1.5
         32%    1.6

21200   100%    1.9
         49%    1.5
         32%    1.4

100% humidity is accompanied by heavy condensation.

My gut feeling is aluminum doesn't behave this way! It has to be in the traps. The real question is reflector or driven element trap?

The fact that the above problem is non-existent on the other two bands indicates to me the problem is not in the coax, balun or rotor loop.

Of course, this humidity vs SWR is only a correlation. Is there a credible cause-effect?

Any help is resolving this issue would be appreciated.

73 Brian/K3KO

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