Expert: Marc Ebelini - 11/10/2009

Question
Question re intermittent transponders:

Set-up:   There are two transponders installed in my plane, one is a KT-76 and the other is a KT-76A.  Each has its own encoder.  Each has a stub (quarter wave) antenna on the belly, about 10 inches apart, fed by RG-58 coax lines about 8 feet long.    There is a switch (SPDT) that selects which of the two transponders gets power from the AC bus.  They share a single 5 amp circuit breaker which goes to the radio master CB.    

The KT-76 has been installed for years.  The KT-76A is a recent addition.   When only the KT-76 was in place, I sometimes, not often, got reports of "intermittent mode C" but I was unable to trace that problem, and it seemed to be most commonly reported by the same sector in mid-state.   I figured their interrogator system was a bit off compared to most.  

However, since installing the second transponder the problem is worse.   I have been receiving reports from ATC of intermittent operation.   So, I switch to the second transponder and get the same reports. (!) Typically ATC will report either loss of mode C, or of all replies both Mode A and C.   The transponder reply lights appear to be blinking in normal patterns regardless of whether ATC can see anything from the transponder.   This is true with either transponder selected.   

In the last two weeks I've flown up and down CA twice, about 10 hours total, all IFR and so in contact with ATC all the time.  During that time, the transponders appear to have been "OK" about 3/4 of the time – it is hard to tell more accurately as ATC sometimes just made do with intermittent replies, and upon hand-off to the next sector all would be well, or not.

I had the two transponders checked at the local avionics shop, which has a ramp tester. All is OK on the ground - tested with aircraft battery only, aircraft engine off.   Also I pulled out the KT-76A and its encoder, and it bench tested perfectly in mode A and C, as it did before installation.    

The aircraft is Mooney M20C with a 14V electrical system.  It has a 50 amp generator and the stock voltage regulator.   In flight the ammeter reads about 5 - 10 amps (that measures the current output of the generator)-- hard to tell more accurately with the little meter in the old cluster.  If I turn on the landing light the reading increases  to around 25A, which sounds about right for the 250W lamp.    The reading jumps around 5 A or more.  I mention this as I thought poor voltage regulation might be a cause.

My question is, obviously, do you have any idea about what is going on here?

Could the two transponders be "talking" to each other via the rather close-spaced antennas?   One transponder is active at a time -- I am sure of that as the 14V power is selected, not the "inhibit" inputs (only the KT-76A has the inhibit input, anyway).  But, could the active transponder's replies be coupled into the other transponder's powered-down cavity and messing up the reply pulses?     

I know that each of these transponders has a cavity tube and a warm-up delay circuit.  Will the reply light blink even if the delay circuit is still inhibiting the HV to the tube?   -- My question comes from speculating that the voltage bus may be so noisy at times that the delay circuit gets triggered, which would stop replies for a minute or so.  

Are there any tests you could suggest that would help isolate the problem?


Answer
Sure a lot of info to digest. Since the problem started with the first transponder let's start with that. How close is the antenna to the engine? Generator noise could be a problem during low signal levels from the stations. To eliminate generator noise one must disengage the generator. The noise could be mechanical. Had that experience once. Disconnecting it electrically did not solve the problem. If the light blink consistently during the dropped interrogations then the transmissions become suspect. Ramp test boxes are not fool proof. The first step would be to ramp test and note the frequency then bench check withe box and bench gear. Note any change in the frequency measurement. If there is a difference change the antenna system. You mentioned RG-58. That is the wrong cable. It should be RG-58AU or CU. The later is recommended as it is oil resistant. Let me explain. On a newly installed KT76 installation my customer immediately had the same complaint. Swapped unit and made several ramp tests. This is when I noticed a shift in frequency during the test. Had my technician replaced the cable and antenna and the problem went away. One of my part time technicians worked for Grumman aircraft so he took the cable to their lab. The cable impedance had a sharp increase at the 1090 MHz frequency that lowered the output power. Because of the close proximity to the antenna the box never gave an indication of a reduced power. The antennas most the blade type are also suspect in such problems. That test requires a a variable section of of feed line sometimes referred to as the trombone that varies the line length to determine if the antenna is affected by cable length that changes the SWR. The antennas are within the wave length of each other but since they work quarter wave in should not cause a problem. I also like to see a greater distance when installing transponder and DME antennas. As you can see there can be many reasons for your problem. Like me know if you still have problems after trying some of my suggestions.