> Folks:
>
> Wolf's idea sounds like it would make a good product (Are you
> listening, Gary KD9SV?) Merely switch the RX-only antennas during
> receive by using the rig's SEND line, or equivalent. Disable the
> relay power if you wish to receive on the TX antenna.
You have to be *very careful* when adding relays in the TX path. It
is a major headache that almost anyone who has sold products in
aftermarket accessory manufacturing knows all too well.
The RX antennas and RX amplifier system could be subject to TX signal
while the relay is switching unless the radio external TX control
line pulls down before the RF output appears by some speed faster
than the additional relay totally transfers and quits bouncing.
The TX control line also must release after the RF output totally
vanishes. That's less of a problem, but still can be a problem.
You'd be amazed at the timing design flaws in some rigs. Some
Kenwoods I know of continue to transmit AFTER the relay control line
drops by almost 50mS in the worse mode (FM)!
It is always better to use the external RX antenna port, if one is
available...and add protection there if needed.
Back-to-back diode clamps to limit RX antenna voltage are OK, but can
still allow the TX system to oscillate in some cases. This was the
case with my IC751A's, even with diode clamps. I had to open the
antenna lead (still used diode clamps for safety) when transmitting.
By the way, the best thing to do is add a small incandescent light
bulb in series with the antenna lead, before the preamps and diode
clamps. You can select a lamp that, when cold, has a filament
resistance of only a few ohms. While hot, the resistance will
increase greatly (often several times the cold value). Additionally,
it will act as a very fast acting fuse. Generally a 100mA 10-20 volt
lamp will work very well.
73, Tom W8JI
W8JI@contesting.com
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