Lets see if I got this correct. A micro power oscillator injects RF
pulses into the receiver which then thinks this is a radar
interrogation pulse and fires out a normal TX pulse out of the
transponder.
This sounds very much like what I said befo " I know my 20
year old Cessna ARC transponder transmits at a low rate when you push
the test button."

The ARC RT-359A service manual says that the self test generates 60
MHz 1us pulses that is fed to the first IF amp and that the rest of
the circuit behaves as if it was a normal radar reply. It also says
that you must use a dummy load or an antenna or damage may occur
because it is responding to the self test pulse by transmitting.

I am not sure how King and Narco do the same function since I do not
have a schematic to look at but most likely it is done the same way
which is what you said. It looks like we were both correct.

The end result is that it transmits when you push the self test
button.

On Wed, 10 Mar 2004 08:42:32 -0800, Jim Weir wrote:

You may have the only known transponder of that variety that has ever been
designed. In the overwhelming predominant world, the test button tests the
receiver only by injecting microwatts of oscillator into the front end.

Jim


(John)
shared these priceless pearls of wisdom:

-Humm, I forgot about that Jim. However most have a self test button
-that causes the transponder to transmit some pulses. I know my 20
-year old Cessna ARC transponder transmits at a low rate when you push
-the test button.
Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com