[TowerTalk] Antenna issue still puzzling me

John af5cc at fidmail.com
Sat Apr 20 19:22:51 EDT 2013

A couple of weeks ago I posted an email about the receive in my Icom 706
occasionally dropping down on some bands, and coming back after you click
the mic or key putting the rig into transmit. After some testing of things
in the antenna system-coax jumpers, low pass filter, tuner, I concluded
(with help from the list) that it must be in the rig. 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!

One test I made in the past 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. 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.

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. 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

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