> The solution...? An EXTERNAL antenna relay box, activated
when transmitting,
> that GROUNDS the receiving antenna at that time.
There really isn't much need to focus attention on whether a
switch grounds or floats a deselected antenna port. Optimum
isolation in a real-world system is virtually never when a
port is grounded. Sometimes grounding a port makes isolation
better, sometimes grounding makes it worse. The resulting
isolation really depends on feedline length, cable routing,
how things are grounded, and most of all the layout of the
box and the type of relay.
The main point is we break the path to the receiver input
port before the transmitter and amplifier switch on, and
that we not restore that path before the transmitter and amp
switch off. An open, a short, a combination of the
two.....it probably all works the same on average.
The problem with radios is the external receive antenna
ports in some radios are held off by a single series diode
with very low bias voltages and no shunt conduction clamp
anywhere on the path. Since the supply is 12 volts, they
can't have a hold-off voltages of more than several volts.
If that diode conducts because peak RF voltage exceeds diode
hold-off voltage, unwanted antenna voltage is sometimes
coupled right into the early pre-driver stages of the
transmitter.
All we need to do is limit the coupled peak voltage to a
safe margin below diode hold off and the problem goes away.
It isn't an issue of getting to millivolts or microvolts,
it's an issue of getting peak RF voltage below a few volts.
You can do that with an open, a short, or even a shunt diode
clamp. It isn't like we need to take it to microvolts or
even a few millivolts.
My 751A was corrected by placing a biased pair of silicon
diodes across the RX antenna line. They were set to conduct
at 1 volt, and clamp antenna voltage at that level.
<It behooves EVERY prudent Amateur running separate antennas
to take a moment
<to review the schematic of his rig, to ensure that ANY
receiving antenna is,
<indeed, grounded when sending. If not, an external
"mini-cased" relay is
<most assuredly NOT all that a difficult project to tackle
Not quite. Outside of being a very sharp technical person
who understands switching systems, I don't know how anyone
would be able to determine anything by looking at a
schematic. This is really a matter of monitoring your own
signal and taking reports given by others seriously.
73 Tom
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