You've already got most of something equivalent built into your transponder.
The reply light in the AT-150 is triggered by the pulses in the reply pulse
train picked off just before they go to the transmit cavity. If the reply
light is blinking without pushing the ident or holding the switch in the test
position, then the transponder is receiving proper interrogations and
generating replies. If ATC doesn't see you but your reply light is blinking,
then either your cavity is shot, the high voltage power supply for the cavity
is broken, or your reply timing is so far off that atc doesn't recognize the
reply. All 3 cases warrant a visit to the avionics shop. On the other hand,
if the reply light is not blinking , then operating the transponder in the test
position may be helpful in diagnosing the problem. The test mode in the
AT-150 injects a stream of pulses into the receiver that mimics pulses
normally recieved. If the test pulses are properly recognized by the pulse
logic, then replies are generated and the reply lamp lights. If the reply
light lights for test mode, then the pulse decode and reply generation logic in
the transponder are probably OK. Using these facts plus being somewhere where
ATC interogations reach your antenna should be enough to determine the health
of the system. YOu can also use one of the transponder monitors or passive
traffic detectors to monitor the reply from the transponder.

Note also that you can't legally remove the transponder and put it back in its
tray without a sign-off. It is specifically called out as a not-allowed owner
maintenance action in the FARs.

That said, a unit that exercises the transponder and monitors it for a reply
could be put together with a small number of parts, however the design time
would not be economical because there would be no market for such a device
seeing that only an avionics shop can legally remove/reinstall the transponder
and those shops already have transponder test sets that do far more. The
transponder replies to a 2 pulse sequence. The third pulse (which is between
the other two), if present, is supposed to suppress the transponder's reply.
Each pulse is 1030MHz (or is it 1090?...I always get the transmit and receive
frequencies mixed up), lasts nominally for 0.8 usec. For a mode C
interrogation, the leading edges of the pulses are 21 usec apart, for mode 3/A,
they are 8 usec apart. A pulse 2 usec after the first pulse will cause the
reply to be suppressed. If you kept the transmit power to a few milliwatts,
while not strictly legal, it would probably not raise anyone's hair. As
stated before though, this unit would tell you little more than what you can
already determine using the reply light and being either at an airport or in
the air where you can recieve ATC interrogations.

A simple interrogator circuit could be built using a 1030MHz oscillator gated
by a 0.8 usec wide pulse that happens every 8 us (mode 3/A). That circuit
could take the place of the ATC interrogations for testing on the ground at an
airport where you don't normally recieve ATC interrogations on the ground. You
could use that in conjuction with the reply light to test the transponder the
way you are intending. A suitable oscillator circuit could be found in the
ARRL handbook. The gate circuit could be done with a 10 bit digital shift
register clocked by a clock with a 0.8 usec period and arranged in a ring
shifting a single '1' through the 10 bits.



John P wrote:

A friend had a problem with his transponder. It didn't seat all the way in
the rack and would not connect with the coax(AT150).

I started thinking.....(I know) Does anyone out there know of a way to make
a simple transponder field unit that will transmit
the necessary pulses( 3, I think) at theone gig and change and then a
simple receiver to see if it responded(one shot and led).
Don't need squawk code or altitude..Any ideas....Jim!

This is probably an illegal device but at very low power, hmm?

John Prince Pitts N3DR

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