Meepus wrote:
On Tue, 26 Feb 2008 18:23:14 GMT, Joerg
wrote:

http://www.ssb.de/amateur/englisch/amsat/mvvfsp_e.shtml

Wow, cool link, that preamp controller would be just the ticket. I
know the Alinco amp has a connection which inhibits the amplifier, my
main question then becomes:

What cicuit would "see" the presence of a low- wattage (.5w) 440
carrier & bypass the receiving preamps


If you can't get a reasonable DC response via a diode you can amplify,
then detect. A nice stiff MMIC or something, they are cheap. But you'd
have to use one that won't croak at a few hundred mV of output level. As
I said I never did that myself since I don't trust RF-autoswitch relays.
Seen too much grief there.


Does cutting off the DC to a outboard preamp adequately protect the
preamps' FETs or is it necessary to =bypass= the receiving preamp?


On a well designed preamp with T/R relays, yes. Else you should provide
fast input protection diodes. I always pre-bias them a few hundred mV so
that they don't cause intermodulation during receive, when strong
signals are around. Take a look at the voltage levels at the worst case
SWR and see if that exceeds the transistor's datasheet limits. Hotrod RF
transistors often cannot take more than 3V, some even less.

The ARRL Handbook used to explain passive diode T/R switching for
VHF/UHF preamps. But I never really trusted that in terms of
intermodulation. Then again I didn't do much above 30MHz anyhow.

--
Regards, Joerg

http://www.analogconsultants.com/