On 2011-06-23, at 1:22 PM, Radio K0HB wrote:
> My TH7 recently (and suddenly) shows 2:1 SWR on 15 & 20m. OK on 10m. My
> reaction is a bad 15m trap. Probably in a parasitic element, not a driven
> one? I’d like to only need to make one trip up the tower with the right
> parts, so any other possibilities? No obvious damage from the ground.
I had the very same thing happen here with my old Hygain TH3-Mark III three
element tri-bander, i.e. suddenly the SWR was A-OK (i.e. unchanged) on 10, but
sky high on 15 and 20 meters...
Well, maybe I shouldn't say sky high: the 1:1 SWR points on those latter two
bands had shifted UPWARD by some 120 KHz, or so, and the antenna had a very
narrow bandwidth on them (all three bands were originally optimized for the CW
ends of the bands).
Curiously enough, just as quickly as the malaise infected the antenna, in a few
short days it completely disappeared! I suspected that maybe I had rain
intrusion in one of the driven element traps, so I monitored the SWR both on
wet rainy days, & sunny dry ones---no change.
Maybe I had a nest of hornets, or spiders, that were attempting to take refuge
inside one of the traps, & the RF "zapped" them...? I don't know. I'm just glad
the thing is working well...well, for NOW, anyway! Hi Hi.
~73~ de Eddy VE3CUI - VE3XZ
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