SV-XPNDR-261 Mode C (ALT), and ADS-B

[cbretana]

I had ATC report to me inflight today that they were not receiving my Mode C (altitude data). Intermittently, they also reported they were not seeing any other transponder information.

My transponder is a Dynon SV-XPNDR-261 Transponder which includes ADS-B out, and after I landed I went to Flight Radar 24 and they had my complete flight data profile including position, altitude, and airspeed.... The Dynon SV-XPNDR-261 unit includes the ADS-B Out functionality, and it is connected to only the one antenna (a separate ADS-B In unit has it's own antenna), so I know the antenna is not the issue. DO these symptoms imply that the transponder unit itself has an issue?

 

[CanardMulti]

A couple of items: First and foremost, I'd check your ADSB out performance report for the flight in question using the free FAA tool:

Public ADS-B Performance Report

I don't know where Flight Radar 24 gets its data, but other private sites like Flight Aware get their data from individual users on the ground w/ receivers hooked up to personal computers that report what they see on line. There's a lot more of them than there are 'official' reporting sites, and it's what the FAA sees that counts.

Older antenna cables can lose efficiency, B&C connectors can loosen or develop a bit of corrosion either in their connection to the cable itself or to the antenna/ transponder. Lots of ways a signal can degrade between a good antenna and a good transponder.

A fair number of folks tend to let their 24 month transponder check slide. If you're getting close to due anyway, take it to a shop for the check. You'll know for sure then if you're putting out an acceptable signal.

Ken

 

[cbretana]

Well, I was assuming that because the Flight Radar 24 system had all the flight data, the only place it could be getting it from is the FAA database that collects ADS-B Data, so my ADS-B system, and antenna, must be functioning properly.

The fact that ATC was not receiving Mode C altitude or Mode S position information, made me suspect that something else other than the cable or antenna was causing the issue, since the ADS-B system uses the same cable and antenna. Is this not valid assumption? I checked the cable B&C connectors after we landed, and they seemed to be secure, but I did disconnect and reconnect them anyway. I am flying again tomorrow, and will not know whether this worked until then.

Funny you should mention a transponder check. I just (a week ago) had that performed by a local avionics shop, and it passed with no issues. I have flown four times since then, using ATC flight following on every flight,, and have had no complaints from ATC about mode C altitude data until today's flight.

 

[CanardMulti]

If your transponder was just given a clean bill of health, it would make it even more useful to check the ADSB performance tool for the SAME time period that ATC said your signal was missing. If you have ever seen one of those reports, there are a number of parameters. If the report does show problems, the place to start trouble shooting might be narrowed down.

Perhaps the shop that passed your transponder just a week ago might care enough or be curious enough to look at the failed transponder report from just a few days after they passed it and want to figure out why? Maybe it's just me, but I would think a paper print out actually documenting a failure would be more convincing than just a verbal squawk re ATC was unhappy. Certainly worth a try before sending your transponder back to Dynon for service.

Ken

 

[swatson999]

Could it be something as simple as the geometry of the XPDR antenna line of sight to the receiving station being blocked by a gear leg? Every once in a great while I have to switch COM radios so as to use the other antenna because of interference by a gear leg; could be something similar going on with your XPDR?

 

[cbretana]

Going to talk to Avionics shop tomorrow morning about this.

 

[soarhead]

This might be too simple but hasn't been brought up yet. Which GPS antenna are you using? The GPS-250 will produce ADSB-Out which can indeed be seen by apps like Flightaware but will not have enough power to satisfy FAA standards.requirements. Only the GPS-2020 will meet the FAA requirements.

Also, you can use the Skyview itself to run a basic diagnostic. In the setup menu it will say whether or not your system is meeting the FAA requirement and will also display any sentence errors that may be occurring.

 

[cbretana]

I have the GPS 2020, but I was under the impression that the GPS is not generating the RF output - It is the transponder that does that, no? The antenna is connected to the transponder, not the GPS, and the only connection the GPS has to feed any data to the rest of the system is through the Serial port to the main HDX DB37 connector harness. And the only connection that the transponder has to the rest of the system is through the transponder DB25, and it gets all of it's data from the rest of the system through the serial port input and output (pins 5&7). So it must be the transponder that is generating the RF output for Mode C and for ADS-B data.

But to answer your question, I am using a monopole antenna, (the Advanced Aircraft Electronics High Gain Transponder antenna, designed to be used without a ground plane).

I have it mounted on the inside of the fuselage side wall, which is fiberglass. I wonder, could this be caused by interference from other parts of the aircraft structure?

 

[soarhead]

You are correct, the GPS is not generating RF but your position data that gets sent via the XPDR is coming from it. Basically if the information being sent is not within the required parameters, you fail the test. I had the exact issue you are having when my aircraft was equipped with the GPS-250. The SV was even able to tell me ( either on the XPDR setup menu or GPS setup, I can't remember ) that the system was not meeting the FAA requirement. Meanwhile, I was able to see my entire flight on Flightaware. I flew the aircraft and requested a profile from the FAA. It failed. Once I installed the GPS 2020, the system was able to tell me that it was now meeting the requirement and sure enough, it passed. However, all that is meaningless seeing as you have the GPS 2020. Not sure if your antenna configuration would be the issue or not. I'm not the right guy to say anything about that. Just figured you might have had the same issue as me, in which case my experience might have helped. Interested to hear what the solution is. Cheers.

 

[GalinHdz]

But to answer your question, I am using a monopole antenna, (the Advanced Aircraft Electronics High Gain Transponder antenna, designed to be used without a ground plane).
View attachment 4899 I have it mounted on the inside of the fuselage side wall, which is fiberglass. I wonder, could this be caused by interference from other parts of the aircraft structure?

I have the exact same antenna in my airplane and it has worked flawless since 2012.

Is your entire airplane fiberglass? If so this is probably not the problem since fiberglass is "invisible" to RF radiation. If it is mostly metal with some fiberglass parts, then it can be an issue depending on where it is mounted.

Also, did this problem just start happening or has it been this way since you installed the equipment?

 

[maartenversteeg]

Also within a 'fiberglass plane', you will have to take care of the antenna gound-plane requirement , but since you mentioned that I assume you have created one of the appropriate size. For the metal airplanes this is very simple, but it takes a little thought for fiberglass constructions

 

[GalinHdz]

Also within a 'fiberglass plane', you will have to take care of the antenna gound-plane requirement , but since you mentioned that I assume you have created one of the appropriate size. For the metal airplanes this is very simple, but it takes a little thought for fiberglass constructions

Do NOT add a ground plane with the Advanced Aircraft Electronics High Gain Transponder antenna. It is designed with the ground plane built in and the instructions specifically state not to add a ground plane.

From the AAE Installation Instructions:

"ANTENNA USES & AIRCRAFT COMPATIBILITY
NO Ground Plane...
These antennas are designed to operate with the modern radios that are installed in today's composite airframes. Just bond them in and connect the coax cable, don't use a ground plane. Seal them into the structure, they last the life of the airframe. One antenna model works for communication, navigation, and for ELT (Three antennas in the aircraft, but all the same design), and this design is tolerant to installation errors. They will work in metal airframes when a plastic or fiberglass wing tip or similar plastic component has been available to provide the mounting structure."

It was made for non-metal installations where a ground plane does not exists. Hence my question "Is your entire airplane fiberglass?"

 

[Rhino]

Well, I was assuming that because the Flight Radar 24 system had all the flight data, the only place it could be getting it from is the FAA database that collects ADS-B Data...

Nope. They use independent ground receiving and reporting stations. I have one running on a Raspberry Pi on the desk next to me that reports to Flightaware. It generates around 20 to 30 thousand position reports every day. If ATC is reporting problems with your transponder data, and not your ADS-B signal, it seems very likely you have a transponder problem. They use the same frequency, so the antenna probably isn't your problem, though anything is possible. Have your transponder tested.

 

[cbretana]

GalinHdz,
. I have a Long EZ. So the antenna is mounted directly against the fiberglass side wall of the forward fuselage, ( next to where pilots left calf would be).

Of course my plane is not all fiberglass. The engine is steel, certain critical structural members are aluminum, etc., etc.

 

[Rhino]

The only place where fiberglass vs steel matters is with a ground plane for the antenna. Yours is designed to work without one, but it would still help if it had one. It also helps if the antenna is mounted vertically. Transponder antennas are vertically polarized, and work best in that position. Your antenna is also designed to be more tolerant of that than most antennas, but it would still work better mounted vertically. I still doubt this is antenna issue though, since issues were reported with your transponder data, but not with ADS-B. It's still possible of course, but I consider it unlikely.

 

[Dynon]

A couple of general comments: ATC may not comment at all on ADS-B, because even where mandated, they may not be actively using it in their procedures. So I wouldn't read too much into that. I'd also expect that if Mode-C was dropping out, that the extended squitter (ADS-B) data that piggy backs on the same signal might be unreadable as well.

The transponder does throw error codes in the alert area if it thinks it's not radiating enough energy, so if you're not seeing those, it's a bit less likely it's the transponder (but not impossible). I'd definitely check connectivity and see if it persists. If it does, get in touch with our support team (contact info in bio).

 

[GalinHdz]

The only place where fiberglass vs steel matters is with a ground plane for the antenna. Yours is designed to work without one, but it would still help if it had one.

No it will NOT help and actually degrades this particular antenna's performance. The AAE L2 is is not the typical transponder antenna you see on airplanes.

For the most part, with a Long EZ you do not have to worry about this antenna and the location. The antenna "could" be bad but highly unlikely. It sounds like you have a different issue and contacting DYNON is probably your best course of action right now.

 

[Rhino]

My mistake. The AAE L2 has a ground plane built in.

 

[cbretana]

Update... the issue has not reoccurred in the last six weeks.

 

[Rhino]

Darn gremlins.