Both W4ZV and KD9SV have identified the cause of some of the garbage we are
hearing on the band: RX-only antennas which are not disconnected during
transmit. Most current transceivers employ a PCB-mount relay to switch the
main and AUX antennas, and these do not offer very much isolation between
antenna jacks. Furthermore, in some radios the unused jack is not grounded,
leading to the problems we are hearing.
I had this problem occur myself a few years ago with a transceiver and two
different linears. In both cases, the linear started pulling very heavy
current, as if it was going into oscillation. It did this only when one of
my Beverages, passing within about 75 feet of the TX antenna ,was connected
to the RX antenna port. I had established a nice feedback loop, with energy
from the TX vertical being coupled through the RX antenna and back into my
transceiver.
I installed what KD9SV calls a "front end saver", the design for which
appeared in his CQ Magazine article, February, 1997. My version is a bit
different, as it removes power to the remote relay switching system for my
Beverages, and this grounds everything, including the RX port on my
transceiver. For those not having an idiot-proof linear amplifier, this
system is also a "linear saver"!
I strongly urge those not using such a system to install one. If you are
not a builder then Gary's excellent design is available commercially ready
to plug 'n play.
Let's clear up the garbage on the band-if you use an external RX-only
antenna, install some means of disconnecting it and grounding the port
during TX.
Bill, VE3CSK
_______________________________________________
Topband mailing list
Topband@contesting.com
http://lists.contesting.com/mailman/listinfo/topband
|