David_Nelson
Well Known Member
Hi Everybody,
This ended up being a lot longer than I had intended. For the folks in a rush, the questions are down below. What follows, is my journey, so far, on what I believe to be poor transponder performance.
For awhile now, I've been suspicious that my GTX-327 has not been performing as well as it should be in my -7A. By that I mean, I don't believe it has been replying to interrogations as often as I think it should. There have been some subtle hints. Namely, while on flight following I was asked if I was still with Houston Center while midway between San Antonio and Houston at 4500' . Or the time I was flying through Austin's Class C and they weren't getting Mode C; bear in mind that the 327's display was showing the proper pressure altitude so it was getting the proper data from the EFIS. Add to this, I've always noticed that the reply (the "R" that display's in the 327's LCD) symbol rarely lit up even when just outside Austin's airspace - or in it for that matter. I seem to recall that back in my training days, the "reply" light lit fairly often.
It wasn't until a couple weeks ago that a hanger mate asked, "How do you know you have a problem?". This got me thinking and a recent question over on the "Ask a controller..." thread somewhat confirmed that ASR (Airport Surveillance RADAR) has a period of around 3 seconds. In my mind, I should be replying every 3 seconds or so while within ASR range. It's not that the 327 never replies, it just does it infrequently. Because of this, I've also been hesitant to get close to Houston or Dallas airspace with a transponder that I don't have much faith in.
I'm determined to resolve this matter. Thus far, I've reseated the unit. I've ohm'd out the coax. Heck, I've remade the coax with quality AMPHENOL connectors. I've not exceeded the 8.8 foot limit of the RG-400. I've ohm'd out the TED monopole antenna. I've made sure that I have a good ground connection at the antenna. Add to this, the 327 passed its cert in Nov 2011; when this was done, a remote antenna was placed 10-15 feet away so I'd imagine a coat hanger would have worked Ok for a xpndr antenna.
It wasn't until the other day I was looking things over - again - and it dawned on me that this is a 1/4 wave monopole antenna and that a 1/4 wave antenna is dependent on a good ground plane to perform well. A little research later and I found a tidbit from Ben Ennenga at RAMI recommending that a minimum of a 6 inch radius ground plane be used for transponder antennas. And where did I install the antenna? About as far forward and to the right as I could get it - about 1.5 inches outboard of the right fuel vent. Ground plane to the front? Practically non-existent. To the right? Marginal. Essentially, I believe I have about a 25% ground plane and that it is affecting reception. Add to this, I've also got a fuel vent sticking out about 3/4" that probably isn't helping either. Hmmmmmm. I fashioned a temporary ground plane and took a test flight and things looked much better. Progress.
I know this general location has worked for others. What I don't know is where exactly others have put their antenna in this general location. For all I know, I may be just outside the "works ok" zone. What I do know, I want the next location to be "correct" as best as I can get it with what I have to work with.
With all that said/written, I'm thinking of moving the xnpdr antenna just aft of the baggage bulkhead on the right side. Garmin says that RG-400 with the GTX-327 is good for 8.8 feet and I think I can make that work. I've two comm antennas on the belly under the seat pans about 12 inches aft of the main spar. How far does the xpndr ant need to be from those? I plan to install an ADS-B antenna aft of the baggage bulkhead on the left side. Anybody know the distance requirements between it and the xpndr antenna? Any other good places I should consider given the above constraints?
I'm not opposed to having the unit bench tested. Without testing the entire system in the aircraft, I don't think that it will uncover anything given the above information. If you think otherwise, I'd like to know why.
Thank you,
This ended up being a lot longer than I had intended. For the folks in a rush, the questions are down below. What follows, is my journey, so far, on what I believe to be poor transponder performance.
For awhile now, I've been suspicious that my GTX-327 has not been performing as well as it should be in my -7A. By that I mean, I don't believe it has been replying to interrogations as often as I think it should. There have been some subtle hints. Namely, while on flight following I was asked if I was still with Houston Center while midway between San Antonio and Houston at 4500' . Or the time I was flying through Austin's Class C and they weren't getting Mode C; bear in mind that the 327's display was showing the proper pressure altitude so it was getting the proper data from the EFIS. Add to this, I've always noticed that the reply (the "R" that display's in the 327's LCD) symbol rarely lit up even when just outside Austin's airspace - or in it for that matter. I seem to recall that back in my training days, the "reply" light lit fairly often.
It wasn't until a couple weeks ago that a hanger mate asked, "How do you know you have a problem?". This got me thinking and a recent question over on the "Ask a controller..." thread somewhat confirmed that ASR (Airport Surveillance RADAR) has a period of around 3 seconds. In my mind, I should be replying every 3 seconds or so while within ASR range. It's not that the 327 never replies, it just does it infrequently. Because of this, I've also been hesitant to get close to Houston or Dallas airspace with a transponder that I don't have much faith in.
I'm determined to resolve this matter. Thus far, I've reseated the unit. I've ohm'd out the coax. Heck, I've remade the coax with quality AMPHENOL connectors. I've not exceeded the 8.8 foot limit of the RG-400. I've ohm'd out the TED monopole antenna. I've made sure that I have a good ground connection at the antenna. Add to this, the 327 passed its cert in Nov 2011; when this was done, a remote antenna was placed 10-15 feet away so I'd imagine a coat hanger would have worked Ok for a xpndr antenna.
It wasn't until the other day I was looking things over - again - and it dawned on me that this is a 1/4 wave monopole antenna and that a 1/4 wave antenna is dependent on a good ground plane to perform well. A little research later and I found a tidbit from Ben Ennenga at RAMI recommending that a minimum of a 6 inch radius ground plane be used for transponder antennas. And where did I install the antenna? About as far forward and to the right as I could get it - about 1.5 inches outboard of the right fuel vent. Ground plane to the front? Practically non-existent. To the right? Marginal. Essentially, I believe I have about a 25% ground plane and that it is affecting reception. Add to this, I've also got a fuel vent sticking out about 3/4" that probably isn't helping either. Hmmmmmm. I fashioned a temporary ground plane and took a test flight and things looked much better. Progress.
I know this general location has worked for others. What I don't know is where exactly others have put their antenna in this general location. For all I know, I may be just outside the "works ok" zone. What I do know, I want the next location to be "correct" as best as I can get it with what I have to work with.
With all that said/written, I'm thinking of moving the xnpdr antenna just aft of the baggage bulkhead on the right side. Garmin says that RG-400 with the GTX-327 is good for 8.8 feet and I think I can make that work. I've two comm antennas on the belly under the seat pans about 12 inches aft of the main spar. How far does the xpndr ant need to be from those? I plan to install an ADS-B antenna aft of the baggage bulkhead on the left side. Anybody know the distance requirements between it and the xpndr antenna? Any other good places I should consider given the above constraints?
I'm not opposed to having the unit bench tested. Without testing the entire system in the aircraft, I don't think that it will uncover anything given the above information. If you think otherwise, I'd like to know why.
Thank you,