[CQ-Contest] An Antenna Riddle

[Bruce Sawyer]

This being the day OF the contest, I suppose it's natural that "interesting"
things should happen.  So here's a little puzzle I hope somebody can explain
to me.  Not that I'm going to be able to do anything about it before the
contest starts, but I'm still curious.

I have one tower with a Force12 4 element 15 meter beam at the top (about
56') and a second KLM 5 element 10m beam about 7' under it.  (A 10' mast was
the longest I could get down to Little Cayman.  It's not like I'm within
driving distance of Texas Towers.)  That KLM 10m beam has always been a
fantastic performer.  When it works, I love it.  The problem is that it's
built as though it came from Starkville, MS.  When I first put it up this
year, the SWR curve was shifted up from where it should have been by about 1
MHz, and the performance was lousy.  Then about two months ago, the
reflector suddenly worked loose and fell to the ground.  Instantly, the
antenna was doing great.  I used it in several contests, and it was
wonderful.  Of course, there was no F/B, but at least the nulls on the side
were deep.  Then yesterday--naturally, the day before the contest--a tip on
one of the directors worked loose and came to the ground.  The antenna was
useless.

This morning I pulled the antenna down and put the director tip back in.  I
also put the reflector back on the beam.  When the reflector came down
before, I noticed that the shorting strap between the two element halves had
broken due to metal fatigue.  (This antenna always vibrates furiously in the
wind.)  I assumed that the open between those two halves was the cause of
the problem I had before.  So after getting the beam back up in the air,
it's behaving just like it was in the beginning--SWR curve shifted up 1 MHz
and lousy performance.

So what's going on here?  I checked everything out on that beam before
putting it up again.  Climbing towers in the noon sun on a windy day on
Little Cayman is NOT how I choose to amuse myself.   Is it likely that the
beam is fine, but simply coupling with the 15m beam?  That 15m beam is
working great, by the way.

73,
Bruce, ZF2NT


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