[OKDXA] Antenna issue really puzzling me

[John]

This is DX related in that it involves my antenna, which allows me to work
DX,
and you can reply to me off the list if you like also to keep the reflector
from
getting too far off topic. I am hoping I can rely on the collective wisdom
of
list members who know far more than I do about antennas and electronics.

Here is my antenna system-105 feet or so of copper wire, attached to Radio
Shack
300 ohm twinlead, about 20 feet or so of that. The twinlead runs to a 4:1
voltage balun, which transitions to RG-11 75ohm coax, which then runs to the
shack. I then use a MFJ949D manual antenna tuner.

About a year ago I noticed the receive level in my Icom 706 was starting to
dropping down, first on 12m, and later on on some other bands, and coming
back
after you click the mic or key putting the rig into transmit. That first
suggested to me that it might be a relay problem, specifically on the low
pass
filter board since it wasn't happening on all bands. I did go through and
try
to test things in the antenna system after a friend say he had had a similar
problem once with a connector going bad in the feedline. I tested the coax
jumpers, low pass filter, tuner, and they all looked ok.

One test I also made was to switch over to the 6m yagi and see keying the
rig
would bring the receive back, since it is on a different feedline from the
dipole I am running. It did, which is one reason I concluded it must be a
rig
problem. I now see that there probably was a little energy making it to the
dipoles feedline anyways, since both antennas were on an antenna switch in
the
tuner, and I know that isolation isn't perfect. I sold the rig as a fixer
upper
and got a good deal on a replacement. Well it has the same problem! Same
symptoms, same bands (different brand and model of rig) everything! Talk
about
frustrating!

Tried hooking the 6m antenna straight up to the rig and didn't witness the
receive level dropping at all. That tells me it must be something in the
antenna system. Given this system, where is the problem likely to be-and
more
importantly, why would it occur on some bands and not others. It happened
mainly on 12m and 15m, and was starting to happen more on 17m and 30m. I
never
saw it occur on 10m or 20m, and only very rarely on 40m.

Anyone have an idea as to what might be going on?

73 John AF5CC