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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