Intermittent Transponder Mode C

[cbretana]

I have a SV-XPNDR-261 transponder, hooked up to an ADVANCED AIRCRAFT ELECTRONICS HIGH GAIN TRANSPONDER ANTENNA.

Recently, over the past few months, I have had recurring intermittent Transponder and Mode C Failures (as reported by various ATC controllers). Sometimes the controller reports that they have lost radar contact completely, and at other times they report that they have just lost Altitude data...

Am I correct that these symptoms (that at times only the Mode C altitude signal is missing), indicate that the coax and antenna are probably okay, and that the issue must be in the SV-XPNDR-261 itself?

 

[Rhino]

Altitude info is sent via a 12-bit binary code called a Gillham code. There is no difference I'm aware of in the RF signal sent to the antenna, and RF is all the antenna cares about. It wouldn't know a binary code from horse crap. So if the altitude encoding is all you're missing, I don't see how that could be caused by the antenna.

 

[GKC Aviation]

If ATC can still see your primary return (Mode A), but are missing the Mode C (altitude) data, then it's likely either your transponder or the altitude encoder are at fault. More likely the encoder from past experience.
The encoder transmits data to the transponder either over the Gillham code wires, or via RS-232 in a more modern installation.
Find someone with transponder test equipment to diagnose it, otherwise it's all guesswork.

 

[andresmith76]

I have a SV-XPNDR-261 transponder, hooked up to an ADVANCED AIRCRAFT ELECTRONICS HIGH GAIN TRANSPONDER ANTENNA. View attachment 5085Recently, over the past few months, I have had recurring intermittent Transponder and Mode C Failures (as reported by various ATC controllers). Sometimes the controller reports that they have lost radar contact completely, and at other times they report that they have just lost Altitude data...

Am I correct that these symptoms (that at times only the Mode C altitude signal is missing), indicate that the coax and antenna are probably okay, and that the issue must be in the SV-XPNDR-261 itself?

Your deduction regarding the coax & antenna being okay are correct. If either of these components were bad/inoperative, ATC would receive nothing from your transponder. If ATC has reported a partial loss of signal (altitude data), then the problem is from the originating device and not the coax/antenna/RF transmission system.

 

[jakej]

I have a SV-XPNDR-261 transponder, hooked up to an ADVANCED AIRCRAFT ELECTRONICS HIGH GAIN TRANSPONDER ANTENNA. View attachment 5085Recently, over the past few months, I have had recurring intermittent Transponder and Mode C Failures (as reported by various ATC controllers). Sometimes the controller reports that they have lost radar contact completely, and at other times they report that they have just lost Altitude data...

Am I correct that these symptoms (that at times only the Mode C altitude signal is missing), indicate that the coax and antenna are probably okay, and that the issue must be in the SV-XPNDR-261 itself?

I take it that you have checked the Txpdr & serial port settings & the pins & sockets? Did the gentle pull test for those too to confirm they are properly seated/ locked in place ? “ lost radar contact completely” is what prompted my input.

 

[cbretana]

As to the "gentle pull test ", Not yet, will do that next.

 

[Rhino]

I'm looking at what jakej is, and am a bit curious about the report from ATC. An antenna can't cause the loss of just altitude data, but the loss of radar contact comment is confusing. Nothing in your aircraft has anything to do with radar contact. They get radar contact if you have no electrical signal at all. It's just a reflection of the signal from their radar bouncing off your aircraft. I suspect they meant a loss of all transponder signal instead, but I learned long ago not to presume something without sufficient evidence. In any case, your antenna system could indeed cause a loss of all transponder data, but not altitude data alone. Not knowing exactly what ATC means makes this a bit more complicated. Like others, I would suspect the encoder first and the transponder second. But without more information, an antenna fault still seems very unlikely.

 

[cbretana]

Rhino,

The possibility exists that on those occasions where the controller reported "Loss of Transponder", he was not being completely accurate, and in fact all that he was noticing was loss of Mode C. In one of the cases when this happened, I did not query him immediately about what parts of the data were missing, but shortly after he reported the "Transponder Failure", he said that he saw the basic position signal again, but the Mode C was still out...

Charly

 

[jakej]

Charly - here are a couple of ‘out there’ situations I’ve seen with Transponders. I’m not a ‘licensed‘ tech however i find that helps as I wasn’t trained to think in one direction .
1st - composite aircraft, Txpdr would drop out sometimes & the range was reduced ( on a route that the customer travelled frequently).
A shop pulled the unit & bench tested (all good), then replaced the RG58 coax to antenna with RG 400 (good idea) however drop out symptoms were the same. He contacted me ( Glasair owner as well) & I offered to look at it. After accessing antenna I found the ‘problem’ within 10 seconds ( he had spent approx $1500 up until this point).
This was the answer - looking directly into the antenna BNC (socket) fitting there is a brass like tube, that is a smaller diameter than the pin on the cable, has splits in it making 4 segments so that when the cable is attached the pin from the BNC fitting spreads them slightly & are tight against the pin. 2 of those segments were missing, go figure. Replaced antenna, problem solved.
2nd - similar issue with another aircraft in CTA - Txpdr was dropping out, Licensed Techs ( I was the Service Manager) supposedly checked everything including KT76 A, replaced antenna cable & bench tested - all good. Aircraft went flying & returned due Txpdr ‘failure’. On further investigation I found they’d only replaced the main cable from the 1ft pigtail at the rack end ( a lot of the early Kings, Narco etc,etc had a short cable ( pigtail) at the Txpdr end to assist in troubleshooting. When that old RG58 pigtail was removed the ‘fault’ was obvious - the shield from that cable is soldered to the 90* fitting & with age etc everyone of the shield strands were broken resulting in the reported Txpdr fault. What was happening was sometimes a single or more wire strands would & then would not make contact due cable movement due temperature or vibration. ‘Pigtail’ cable replaced = problem solved.
HTH, sorry to be so long winded.

 

[cbretana]

I am already using RG 400 coax. But I also have ADSB In, using separate, but same model antenna. Think I'll switch them and see if that fixes the Mode-C, and causes the ADS-B In system to act up..

 

[jakej]

I figured you would have the RG400 - my point was to say that there can be other less obvious reasons why we have problems with avionics/ electrics. Mechanical problems usually can easily be seen & then fixed - electronics are a different animal, you can’t see where those pesky electron critters are going .
The solution to your situation may be helpful to others too - please report when you know, thanks.