Topband: KD9SV-OK1RR relays ???

Tom W8JI w8ji at w8ji.com
Sun Aug 30 00:27:45 EDT 2015


> It seems to me that a very fast operating preamp protection circuit could 
> be
> constructed employing a good fast saturating NPN switching transistor 
> across
> the antenna path. In receive mode the collector-base junction would have
> substantial reverse bias and the transistor can be chosen for low
> collector-base capacitance. With a fast switch like a 2N708 or something
> similar the switching time will, of course, FAR outperform a relay closure
> time.


This thread might have splintered. I was responding to this:

<<Your FT-747 only has one antenna input. It does not have a second
<<receiver. It doesn't even have a receive-only antenna input. You cannot
<<transmit and receive at same time. Why do you need a "front end saver"?


If it is a transceiver without an RX antenna point, the problem is adding a 
receive antenna to a transceiver that does not have a receive port.

If it is a transceiver with an RX port, the requirement for an external 
"front end saver" and what will work depends on the antennas, the power, the 
transceiver, and the antenna spacing.

A front end saver can be very simple with some radios, more complicated, or 
not needed at all.

An external switch is never easy to do correctly, unless the radio has good 
TX RX switching time sequencing.

I'm unclear what the application is, but a 10 mS relay is really too slow 
for either application. The sequencing issues I pointed out apply to both 
systems.

73 Tom 



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