On Feb 21, 7:02*am, Andy wrote:
On Feb 20, 10:10*am, kd6veb wrote:

Hi Gang
* This thread prompted me to think as to how the MXR unit ignores my
transponder in my glider which is being pinged regularly where I fly?

As Darryl has explained the MRX is designed to distinguish between
your own transponder and the transponders of threat aircraft.

However in my PA-28 the MRX seems to get confused at least once on
every flight, usually about 10 minutes after takeoff. *What happens is
that I see a target rapidly converging on me in both altitude and
distance then it stays about 0.6 miles from me at the same altitude.
If I cycle power on the MRX the target, which was my transponder, goes
away usually for the rest of the flight.

I adjusted the suppression level but still have not eliminated the
problem. *I know that my transponder is a bit low on power until it
has been responding for about 10 minutes and I assume this is
contributing to the problem. *It's a bit puzzling that, before the
spurious target converges on me, the MRX reports my own transponder
squawk and pressure altitude correctly. *This would seem to imply that
the spurious converging target is not the first acquisition of my
transponder signal.

I have not sent the MRX back to ZAON for testing since I thought the
problem may have been caused by a detuned aircraft transponder
antenna. *It is missing tip ball. *After a bit of investigation I was
able to obtain the manufacturer's drawing of the tip ball and found it
was made of acrylic, not brass as I had assumed, *so it would not have
been a factor.

Substitution of a different KT 76A would be the next thing to try but
I don't have a spare just lying around

Andy



Andy

I would suspect other things before the transponder itself, including
the antenna and encoder. Does the MRX see a local ghost in other
aircraft? If so obviously I'd suspect the MRX more and want to get it
checked, or that may point to the MRX not seeing enough transponder
signal in your PA-28 to reliably pick it as a local transponder.

One test is to hit the local mode button on the MRX when this happens
and see if the MRX is using the local transponder for altitude or not
(a job for a front seat PAX... don't crash into a real threat while
doing this!). If it is then one explanation (grasping at straws
here...) could be that you are seeing strong ground reflections with
enough time delay to look like a second transponder. But in that case
I would hope the relative altitude to be fairly consistently around
"0.0" (or possibly garbled unintelligibly/ignored ?) but there could
be complex reasons while the relative altitude does not exactly report
within +/-"0.1".

The problem is the MRX not seeing enough local transponder signal. You
might try moving the MRX to a different location on the glareshield
and see if that makes this happen less. Keeping the antenna clear of
metal structures.

If the transponder antenna was hit/bumped and that caused the ball to
break off be careful that there was no other damage internally that
could affect radiated power esp. if somebody had to bend the rod
straight. The ball being plastic is a bit strange. I'd find an all
metal one next time.

Another thing is just to try to confirm that the MRX internal altitude
and the encoder altitude are close. The KT 76A does not display your
encoder altitude and the MRX will show you its altitude not the
transponder altitude if it thinks the transponder is not local so
there is no simple way to really check this for sure. (You wanted to
replace the KT76A with a GTX 330 or TT-31 anyhow right?). But if you
see a threat that is not mostly at +/- "0.1" or so relative altitude
then maybe altimetery is more likely a suspect. When you say that the
altitude of the ghost threat gets closer how much relative altitude
error does it start with and how close does it get? How smoothly? Any
idea if at that time your transponder is being continuously
interrogated?

The MRX internal altimeter measures cockpit ambient pressure and your
transponders/encoder is measuring aircraft static. You might just be
getting unlucky on climb-out or other things (like opening a cockpit
vent, or effects from the encoder heater cycling) that pops the
encoder and MRX altitude just far enough apart, especially if there
was already a baseline error, that causes the MRX to think your
transponder is no longer local. There are several ways how I could
wave my hands to explain a ghost target that might appear at a
changing/converging altitude, but they would be just wild-ass guesses.
And you also might need to throw in real threat aircraft that you
never actually see into this picture.

Unless the aircraft is IFR equipped the encoder accuracy is not
usually tested during the biannual transponder checks. However the
encoder operation would have been checked when it and/or the
transponder was installed). The RF output power and signal is tested
in the biannual tests however. Obviously any other problems or
strangeness with the static system could be a culprit here--what
happens if you pull the alt static in flight? What altitude difference
do you see? (at different airspeed/vents open/closed etc. ?)

Anyhow a long-winded way of saying I have no idea what is really going
on :-)


Darryl