Rob Cherney
shared these priceless pearls of wisdom:

-
-I guess we're having a critical design review...

I guess so.


-So, what's the isolation at around 140 MHz? It's not 20 dB, it's more
-like 40 dB. In addition, since there are two internal switches, they
-can be wired in series. But, for the sake of argument, let's say that
-we're limited to 40 dB of isolation. That's 0.32 volts instead of the
-3.2 volts that you quoted. That's at or below the conduction voltage
-of the protection diodes, assuming that they're placed at the input of
-the receiver.

But that's not the common failure mode. The common failure mode is for the
PTT/relay circuit to fail to respond to a "put me in transmit" mode. Thus, the
only isolation you have is from the splitter.

If you're really worried about this failure mode, put two
-independent relays in series.

Isn't that flawed logic like carrying a bomb with you on an airliner? The odds
of TWO people having a bomb are much smaller.


Lastly, I would not select a Wilkinson divider for the receiver. I'd
-use a transformer hybrid.

Your call. I note that under the common failure mode that your transformer
hybrid will have to handle the whole 10 watts. That's going to be one hell of a
transformer. The Wilkinson splitter will handle 50 watts without a whimper.

Jim


Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com