There have been several posts about the need for a proper RX-only antenna
switching/grounding circuit. Most have dealt with lightning protection, a
few with interaction between the RX port and the radio/amplifier. Here's
some proof of the need for such a switch.
I had a Beverage ant here on the farm whose fed end came to within about 20
feet of one set of guys for my vertical. Whenever I selected that antenna,
and while using my FT990/Clipperton amp on topband, the amp would suddenly
break into oscillation and pull a lot of current. I eventually moved that
Beverage antenna to another location and that solved my problem. I thought
that was the end of it.
Well, I have started to use the 990 with an AL80 at our summer home, and
last weekend I went to tune up for the contest and WHAM!...the amp broke
into oscillation! Again, I was using a Beverage whose end is located fairly
close (20 feet) to guys on my tower.
I discovered that the switching cable to my Bev disconnect/TX switch somehow
got pulled out and the Bev was not being switched to ground, and the RX port
of the 990 was was not being grounded in TX (I do not switch the linear
through this circuit...I plan to now!).
So the problem is obviously the 990 switch board; while a small PCB relay is
used to switch the antenna connection, there is obviously considerable
bleed-through to the RX port, creating feedback which neither amp seems to
like! I don't know exactly what is happening here, but when both the RX
port and the Bev are grounded in TX, the problem disappears.
I suspect that this problem resides with several other radios besides the
990. I also wonder what kind of garbage is being radiated on TX when
running barefoot in this condition when everything seems to be OK!
So, if you don't have a RX port switching/grounding circuit on your
transceiver, it is time to get building! (By the way, KD9SV's excellent
circuit in QST a couple of years ago is now available commercially built and
tested. While Gary's circuit uses reed relays, I employ open-frame units
with wider spacing for a little more protection in case of a nearby touch of
Thor).
Bill, VE3NH
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