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Poor transponder performance - thinking of relocating the antenna

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,
 
Behind the aft baggage compartment bulkhead will work good. Like every other antenna, the little xpdr also requires a good ground plane. Try keeping the antennas located more toward the center of the fuselage rather than the sides. 3 foot spacing between antennas is good, or as far apart as practical.

Your statement:
"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."

The above check is performed with a calibrated test box, the exact distance from your antenna to test equipment antenna is measured and has a known loss. Based on this calculated loss, a power output from your transonder is measured and must meet the specs in 43 appendix F.
 
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R = interrogation rate (GTX-327)

Hi Everybody,

..., 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's not that the 327 never replies, it just does it infrequently. ...

David,

I had same concerns when I first flew with the '327. I called Garmin support which clarified on how the R symbol works. Unlike other transponders, the GTX-327 displays R if interrogated more than at a certain rate. See my VAF thread:
http://www.vansairforce.com/community/showthread.php?t=40224&highlight=transponder

If center does not see you, then you may have other issues. Ground plane for sure. The way it is mounted now will make the signal very directional.

I mounted ours just forward of the main spar, center. It gets blasted by exhaust and needs to be cleaned more often, however signal radiation has been great.
 
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While you may indeed have a problem, this is not one of those things where you correct a problem that you don't even know exists other than some "feeling". Remember in avionics the biggest cause of problems is solutions! :)

If you had a good transponder test from a good shop with a good box, then you're likely fine. The "R" indicator on the display tells you as the pilot almost nothing about how your box is acting. Moving things around is fine, but many times is just a waste of time unless you know exactly what/if the problem is and what the solution should be. Until you have something a little more concrete to work with, you're shooting at an unknown moving target of unknown size at unknown distance - in the dark.

Like I said, while it is possible you have an issue it's not probable given the unit in mention and the history with said unit and it's reliability in lots of installations. You'd be amazed how poorly a transponder can be installed and still work really well! If you do have an issue, it should easily be spotted by a shop with a good modern test box. Like Walt said, the modern digital boxes (like the IFR6000) are remarkable. They know the distance you are from the aircraft and take that into account. They look for more subtleties eith the transponder system and can detect problems that you'd never even notice from ATC.

In the end, at this point I'd do a little more work to find out some more solid details before I decided to move things. It may be beneficial to do so, but go in with a solid plan.

Just my 2 cents as usual.

Cheers,
Stein
 
While you may indeed have a problem, this is not one of those things where you correct a problem that you don't even know exists other than some "feeling". Remember in avionics the biggest cause of problems is solutions! :)

If you had a good transponder test from a good shop with a good box, then you're likely fine. The "R" indicator on the display tells you as the pilot almost nothing about how your box is acting. Moving things around is fine, but many times is just a waste of time unless you know exactly what/if the problem is and what the solution should be. Until you have something a little more concrete to work with, you're shooting at an unknown moving target of unknown size at unknown distance - in the dark.

Like I said, while it is possible you have an issue it's not probable given the unit in mention and the history with said unit and it's reliability in lots of installations. You'd be amazed how poorly a transponder can be installed and still work really well! If you do have an issue, it should easily be spotted by a shop with a good modern test box. Like Walt said, the modern digital boxes (like the IFR6000) are remarkable. They know the distance you are from the aircraft and take that into account. They look for more subtleties eith the transponder system and can detect problems that you'd never even notice from ATC.

In the end, at this point I'd do a little more work to find out some more solid details before I decided to move things. It may be beneficial to do so, but go in with a solid plan.

Just my 2 cents as usual.

Cheers,
Stein

Stein,

It could be a directional problem.

The test antenna is usually only used from one location, and a non-centered ground plane could easily cause a very directional response.

If you are buddies with the avionics tech, perhaps the test could be repeated at say 8 cardinal locations around your plane....
 
Stein,

It could be a directional problem.

The test antenna is usually only used from one location, and a non-centered ground plane could easily cause a very directional response.

If you are buddies with the avionics tech, perhaps the test could be repeated at say 8 cardinal locations around your plane....

Again it's possible, but not probable. Experience has shown that a transponder antenna placed almost anywhere on the underside of most RVs will perform quite well, even if not centered in the plane. I hear people use the word "directional" a lot, but the reaility is that it's not a common phenomena at all with transponder antennas on RV's. It does occasionally show up with comm/nav anennas in a wingtip or a few other select installations, but rarely a transponder.

Cheers,
Stein

Cheers,
Stein
 
Again it's possible, but not probable. Experience has shown that a transponder antenna placed almost anywhere on the underside of most RVs will perform quite well, even if not centered in the plane. I hear people use the word "directional" a lot, but the reaility is that it's not a common phenomena at all with transponder antennas on RV's. It does occasionally show up with comm/nav anennas in a wingtip or a few other select installations, but rarely a transponder.

Cheers,
Stein

Cheers,
Stein

Bad choice of words on my part...in this case "non-centered" means a ground plane that is totally missing on one side.

I agree on not centered on the fuselage being no big deal, but it seems like a chunk of ground plane simply not being there would make a difference...:)
 
Old wives tale?

I had read somewhere (the Internet?) that one should not install an antenna where exhaust and oil can form a film on it. The reason being that the FOD was carbon based and therefore conductive thus affecting the function of the attenna or shorting the main element to ground.

Old wives tale? Some truth? Very true?

Kinda makes me want to go out and stick a set of DVM probes in a quart of oil just to see what it reads. ;)
 
Good learning!

Hi All,

A lot of great info - thank you!

Alfio - really interesting read on the 327's behavior.

I've a couple of action items to follow up on and I'll see what road this leads down.
 
I had read somewhere (the Internet?) that one should not install an antenna where exhaust and oil can form a film on it. The reason being that the FOD was carbon based and therefore conductive thus affecting the function of the attenna or shorting the main element to ground.
Old wives tale? Some truth? Very true?
Kinda makes me want to go out and stick a set of DVM probes in a quart of oil just to see what it reads. ;)

Oily grime definitely degrades the efficiency of a transponder antenna. I doubt you would see any degradation at DC, but at transponder frequencies, it's a different story.
 
The transponder antenna on my -9 is in the same location as you describe. Only, my plane being a tail dragger there is a gear leg fairly close to it. I was afraid the gear leg would block the transponder?s transmissions.

During my Phase 1 I contacted a controller during some off hours and asked them to watch my transponder replies while I turned in circles. Flat circles, steep turns, runs in different directions without turns, etc. He told me he couldn?t tell any difference in the returns. Thus I have left the transponder antenna right where I put it during construction.
 
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.

That requirement for a 6" radius ground plane comes as a surprise to me because other "experts" have stated in the past that a TED transponder antenna requires a ground plane with a minimum radius the same size as the length of the antenna (specifically 2").

A friends of mine with a TED transponder antenna located in the same position as yours (front corner of cabin), but with a minimum 2" groundplane in all directions, reports entirely satisfactory GTX327 communications.

I'm guessing that if the TED transponder antenna really required a 6" radius groundplane in all directions there would be a whole lot of RVs out there with dodgy transponder communications. :)
 
The glider guys are having to install ground planes on their glass ships and need to make ground planes. I always thought the rule of thumb was 1/4 wavelength radius, but most sources say this is a minimum at transponder frequencies.

Interesting comment from RAMI engineering from a glider sales site -

I talked to Ben Ennenga at RAMI about transponder antenna ground planes. Some interesting notes from that conversation are below. Thank you Ben for the friendly and helpful support!

A ground plane is required (of course) with both rod style and blade style transponder antennas.
The smallest recommended ground plane would be 12 inches in diameter (6 inches radius).
A ground plane works about equally well regardless of whether it is made from a solid plate (such as aluminum) or instead made using 2 wires in a "+" arrangement. In either case, the radius should be at least 6 inches.
When working with carbon fiber glider fuselages, the antenna must be mounted on the outside of the fuselage, but it is OK to mount the ground plane on the inside.
The mounting screws for the RAMI AV-74 antenna are connected to the antenna ground. Therefore, you can connect the ground plane to either or both of the mounting screws.
It is OK for the ground plane wires ("+" configuration) to curve and follow the inside of the glider fuselage.
 
The transponder antenna on my -9 is in the same location as you describe. Only, my plane being a tail dragger there is a gear leg fairly close to it. I was afraid the gear leg would block the transponder?s transmissions.

During my Phase 1 I contacted a controller during some off hours and asked them to watch my transponder replies while I turned in circles. Flat circles, steep turns, runs in different directions without turns, etc. He told me he couldn?t tell any difference in the returns. Thus I have left the transponder antenna right where I put it during construction.

Bill,

I don't doubt that the controllers were able to receive your signal, especially if you are within their control zone. Even an antenna that does not radiate well or evenly will work fine in close range. It seems to work well for you.

If you fly long distances and wish to avail yourself of flight following, or fly IFR, then an even radiation pattern will certainly help in maximizing tracking distance. In our case, flying in Canada means that control centers are far and few in between. An even transponder radiation pattern allows us to be tracked to a maxminized distance from the control center no matter what our horizontal orientation.
 
...If you fly long distances and wish to avail yourself of flight following, or fly IFR, then an even radiation pattern will certainly help in maximizing tracking distance. In our case, flying in Canada means that control centers are far and few in between. An even transponder radiation pattern allows us to be tracked to a maxminized distance from the control center no matter what our horizontal orientation.

Even on long trips, far from population centers, ATC has never questioned the ability to pick up my transponder while coming, going, or turning.

I see no reason for Dave to relocate the antenna.
 
Antenna Model Simulations

Several weeks ago, while re-assembling the TED after an experiment, I noticed two things: that there was a gasket on the exposed portion of the antenna where it mates up to the fuselage and that I had failed to properly clean any interior paint where the tag, washer, and nut are. I cleaned the area well, reassembled, and just had to go for a test flight. While about 50 miles east of Gray AFB (KGRK), I called up Gray Approach and requested a read-back of my Mode A and C while flying on the cardinal headings. All was spot on. I then followed this up with Austin approach (about 40 miles out) and all was good there, too. Knowing that Austin is a little busier than Gray, I only requested a single read rack while headed directly towards Austin. Do I think my installation is a good one: no, not really. Does it work: apparently so.

I also followed up w/ the local avionics shop on the field. They do indeed have a fancy xnpndr checker if I wish to pursue that avenue.

On the topic of harmful effects to xnpndr equipment due to oil film on your xpndr antenna, I don't think it matters much. I recently drove down the line at the airport looking a xpndr antenna installations - from Cessna trainers to a V-Tail. Some of those antennas may as well have been covered in grease they were so dirty! If this was truly causing equipment failures, I'm sure (and I'm assuming here) that folks would be doing a better job of keeping them clean and oil/grunge free.

While it's been raining like crazy around here lately, I got this idea about antenna modeling in software. It turns out that there is some really neat stuff out there - and it's free! If you are familiar with NEC-2 or happen to be an RF wizard, I'd appreciate your input on my work. I found this quite interesting and a very neat learning experience. There are still things I don't quite understand/grasp. Below are some far-field plots of the TED monopole in the corner of my fuselage. Know that I am not an RF engineer in any shape or form. Below is what I've been able to gather and learn. I make no guarantees that it is correct nor accurate.

ted_geometry.png
ted_nec_file.png


ted_horizontal_plane.png
ted_vertical_plane.png
 
I havent had the opportunity to question the most knowledgable guy yet. He was on vacation and OUR schedules don't exactly overlap anyway. I will post any findings/discoveries on the "controller" thread.

I have more things to discuss with him as well, ADS-B.
 
Switching to 978 ADS-B (out)

Allow me to revive this old thread with a new twist. I recently installed a Garmin GDL-82 UAT so that means my transponder is now broadcasting on 978 MHz rather than 1090 MHz. I am currently using an Archer SA-005 antenna cut for 1030 to 1090 MHz but I have reason to suspect that I'm going to need to change antennas. Apparently the only ADS-B ground station in the Asheville area is not picking up my 978 MHz UAT signal from only 12 NM away. That being the case, it won't send me TIS-B traffic in return. Asheville Approach has never had a problem receiving my UAT, but apparently that's not good enough for the ADS-B system - I am told the nearest ground station must receive my sqawk or it won't send me traffic data. I was in the pattern at AVL yesterday and two Marine Ospreys came buzzing in. They didn't show at all on my traffic screen - apparently because they don't have ADS-B (out). The ground station should have relayed that traffic to me, but didn't. I have read elsewhere that my Archer antenna doesn't perform well at 978 MHz so I'm buying a standard "spike" antenna cut to 978 MHz and will try it to see if things improve.

I have to say that I'm somewhat miffed that Garmin didn't address the antenna issue at all in their installation manual. I'm also miffed that nobody on the Internet seems to know how the ADS-B system actually works (or doesn't work) or, if they do, they're not talking. I'm having to stumble through all of this. :mad:
 
John,
I think I do understand adsb pretty well, and yes it?s complicated (?A camel is a horse that was designed by a committee?). Feel free to send me a PM with specific questions if you?d like to.
Your transponder uses 1090 MHz, not 978. No true transponders are on 978, just ADSB UAT systems.
Your particular Garmin system multiplexes the transponder on 1090 and the UAT on 978 into one antenna. Garmin was counting on the typical transponder antenna made for 1090 also working passibly well at 978 MHz. You have a rather special, non-usual antenna. I do not know its ?Q? (how sharp its resonance is) but if it in fact performs poorly at 978 that may be your problem.
 
Allow me to revive this old thread with a new twist. I recently installed a Garmin GDL-82 UAT so that means my transponder is now broadcasting on 978 MHz rather than 1090 MHz. I am currently using an Archer SA-005 antenna cut for 1030 to 1090 MHz but I have reason to suspect that I'm going to need to change antennas. Apparently the only ADS-B ground station in the Asheville area is not picking up my 978 MHz UAT signal from only 12 NM away. That being the case, it won't send me TIS-B traffic in return. Asheville Approach has never had a problem receiving my UAT, but apparently that's not good enough for the ADS-B system - I am told the nearest ground station must receive my sqawk or it won't send me traffic data. I was in the pattern at AVL yesterday and two Marine Ospreys came buzzing in. They didn't show at all on my traffic screen - apparently because they don't have ADS-B (out). The ground station should have relayed that traffic to me, but didn't. I have read elsewhere that my Archer antenna doesn't perform well at 978 MHz so I'm buying a standard "spike" antenna cut to 978 MHz and will try it to see if things improve.

I have to say that I'm somewhat miffed that Garmin didn't address the antenna issue at all in their installation manual. I'm also miffed that nobody on the Internet seems to know how the ADS-B system actually works (or doesn't work) or, if they do, they're not talking. I'm having to stumble through all of this. :mad:

Maybe you missed it but the GDL 82 Install manual has pages and pages of antenna info and a list of approved L band antennas, ground plane requirements for those antennas and cable specs. Not sure how much more Gamin could do for you. You chose to not follow the manual and used a "homemade" non-approved antenna and now you're pissed at them because it doesn't work?

I'm also curious if you did the transponder tests as called out in the manual when you installed the 82?

PS: I've done a number of xpdr tests with those antennas on glass aircraft and they generally are sub standard even for the xpdr freq.
 
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Maybe you missed it...
There's no need to get snippy, Walt. Yes, I read the manual numerous times and mostly followed it. Garmin specifies "Standard 50 ohm vertically polarized antenna with a VSWR < 1.7:1 at 978 MHz and < 1.5:1 at 1090 MHz.". My SA-005 is advertised as having an SWR of <1.2 from 1030 to 1090 MHz. I've flown across the country with it twice and never had complaints of poor transponder performance. That list of "acceptable" antennas Garmin printed were all TSO'd, expensive, and designed for metal airplanes. I'm not going to pay $200-$400 for a TSO'd Comant which wouldn't work in my fiberglass airplane anyway.

No, I didn't run the Pre-Installation Transponder Check because that test is intended to weed out sub-standard transponders. Mine is a new Garmin GTX-327 with 9' of RG-142B feedline so I know it's OK. If I had an avionics shop in my hangar like you do, I would have run the test anyway, but there aren't any avionics shops within 100 miles of my home airport.

I made a flight test today and the only GBT in the area is only picking up the signal from my GDL-82 when I get within 7 miles at 1,500' AGL. That leads me to suspect the antenna. Next week I'll let everybody know if the new antenna helps the situation.
 
Allow me to revive this old thread with a new twist. I recently installed a Garmin GDL-82 UAT so that means my transponder is now broadcasting on 978 MHz rather than 1090 MHz. I am currently using an Archer SA-005 antenna cut for 1030 to 1090 MHz but I have reason to suspect that I'm going to need to change antennas. Apparently the only ADS-B ground station in the Asheville area is not picking up my 978 MHz UAT signal from only 12 NM away. That being the case, it won't send me TIS-B traffic in return. Asheville Approach has never had a problem receiving my UAT, but apparently that's not good enough for the ADS-B system - I am told the nearest ground station must receive my sqawk or it won't send me traffic data. I was in the pattern at AVL yesterday and two Marine Ospreys came buzzing in. They didn't show at all on my traffic screen - apparently because they don't have ADS-B (out). The ground station should have relayed that traffic to me, but didn't. I have read elsewhere that my Archer antenna doesn't perform well at 978 MHz so I'm buying a standard "spike" antenna cut to 978 MHz and will try it to see if things improve.

I have to say that I'm somewhat miffed that Garmin didn't address the antenna issue at all in their installation manual. I'm also miffed that nobody on the Internet seems to know how the ADS-B system actually works (or doesn't work) or, if they do, they're not talking. I'm having to stumble through all of this. :mad:

A transponder can only transmit on 1090 so your transponder is NOT transmitting on 978, only your UAT does.

If your transponder is not transmitting (Off or disconnected) then your UAT does not receive the transponder code nor the reported altitude which it needs to send a valid ADS-B OUT signal which activate traffic. So something is not complete in either your settings or how you explained the situation.

Request an FAA ADS-B Performance Report for your airplane and see what it shows. That will help narrow down your particular situation.

:cool:
 
JYour transponder uses 1090 MHz, not 978. No true transponders are on 978, just ADSB UAT systems. Your particular Garmin system multiplexes the transponder on 1090 and the UAT on 978 into one antenna.
I have a BSEE and am familiar with multiplexing, but I don't understand that statement. Are you saying my antenna is transmitting a burst of 1090 and then a burst of 978 (time division multiplexing), or something more sophisticated?

JGarmin was counting on the typical transponder antenna made for 1090 also working passibly well at 978 MHz. You have a rather special, non-usual antenna. I do not know its ?Q? (how sharp its resonance is) but if it in fact performs poorly at 978 that may be your problem.
That makes sense. I don't have the equipment to sweep my SA-005 antenna for SWR. I wish I did. I'm pretty sure my SA-005 is broadband, though, due to those extremely wide driven elements. Not having any RF test equipment, I'll just try another antenna.

The only GBT is our area is "low power" so that may be the problem rather than my antenna. I can't find anything regarding it's "service volume". I may PM you when I have any intelligent questions to ask. Thanks for the offer.

All I know for sure right now is that I'm not getting any TIS-B service in the center part of the Asheville Class C. Maybe the FAA figured I didn't need it there.
 
I have a BSEE and am familiar with multiplexing, but I don't understand that statement. Are you saying my antenna is transmitting a burst of 1090 and then a burst of 978 (time division multiplexing), or something more sophisticated?
.

That?s it. In between transmissions the antenna is in receive mode.

BTW, it sounds like you have not had your transponder formally checked. Strictly speaking you shouldn?t be using it in flight if that?s the case.
 
All I know for sure right now is that I'm not getting any TIS-B service in the center part of the Asheville Class C. Maybe the FAA figured I didn't need it there.

That?s exactly where the FAA is most interested. At my home airport inside the mode C veil the FAA could track me right down to the ground on my performance report/rebate run, despite the fact that at low altitudes I am below radar coverage and not within range of a ground station - at least, none that talks to me! I suggest you follow the suggestion to request an adsb performance report, see what it says.
 
There's no need to get snippy, Walt. Yes, I read the manual numerous times and mostly followed it. Garmin specifies "Standard 50 ohm vertically polarized antenna with a VSWR < 1.7:1 at 978 MHz and < 1.5:1 at 1090 MHz.". My SA-005 is advertised as having an SWR of <1.2 from 1030 to 1090 MHz. I've flown across the country with it twice and never had complaints of poor transponder performance. That list of "acceptable" antennas Garmin printed were all TSO'd, expensive, and designed for metal airplanes. I'm not going to pay $200-$400 for a TSO'd Comant which wouldn't work in my fiberglass airplane anyway.

No, I didn't run the Pre-Installation Transponder Check because that test is intended to weed out sub-standard transponders. Mine is a new Garmin GTX-327 with 9' of RG-142B feedline so I know it's OK. If I had an avionics shop in my hangar like you do, I would have run the test anyway, but there aren't any avionics shops within 100 miles of my home airport.

I made a flight test today and the only GBT in the area is only picking up the signal from my GDL-82 when I get within 7 miles at 1,500' AGL. That leads me to suspect the antenna. Next week I'll let everybody know if the new antenna helps the situation.

Maybe I was a bit snippy, but it bugs me when folks bash vendors because they fail to read and follow the installation guidelines they provide and then blame their problems on them.

I know everyone hates all that nasty drag that comes from an external antenna, but if you expect good performance from your radio equipment that's what it takes.
And yes any "normal" externally mounted antennas will work on your aircraft, you just have to be willing to install it.

The Comant CI-105 is included on the approved antenna list and meets all the specs, $166
http://www.aircraftspruce.com/pages/av/antenna_transponder/comant105.php

PS: the GTX327 install manual specifies no longer than 6' of RG400 (the 142 is almost identical). The reason we have the req'd 24 mth xpdr checks is to insure the "installed" system meets the minimum requirements.

Check out the antennas on the SR22 or Corvalis, all external.
 
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Actually, if you will check section 2.3.4 of the Installation manual, you will see that the max coax length is 8.8'. Cheers!

Hmmm, not sure what manual you have but this is from Table 2-3 of the GTX327 IM, Rev. R, which is the latest version:

Max. Length (feet – [m]) Insertion loss (dB/100ft) Carlisle IT Type MIL-C-17 type RG Type
6' 1.3" [1.86m]...................18...................0M17/128-RG400...........RG-400

PS: I was curious where you may have found the 8.8 number, after some quick checking I found it in the old GTX320 install manual dated 2001, sorry!
 
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Nah, it's a GTX 327 Manual Rev J from 2003, so I'm behind the times. Luckily, I can remove several feet of RG-142B from the transponder to the GDL-82. I'll do that and see if it helps. Any idea where to find the latest installation manual? Garmin doesn't seem to offer it on their site - only the Pilot's Guide.
 
I suggest you follow the suggestion to request an adsb performance report, see what it says.
I've done that after every flight. All the "failure" percentages are zero, but the "TIS-B Client" percentages suck. IOW The FAA can track me right down to the ground so, as far as they're concerned, my system meets all the 2020 requirements. What I'm unhappy with is that, for some reason, my connection with the ground station is weak so I don't get TIS-B traffic information on my Stratus/Foreflight/iPad unless I'm really close to the GBT. the problem could be my transponder antenna, or it could just be that the ground station signal is weak and I'm outside it's service volume. I wish the FAA would publish a list of ground stations and service volumes, but they refuse, for some reason. When I fly over the only ground station in my area, Foreflight tells me that it's "Low Power", but I have no idea what that means. If it were a VOR, the service volume would be publicly available, but not the GBTs.
 
Nah, it's a GTX 327 Manual Rev J from 2003, so I'm behind the times. Luckily, I can remove several feet of RG-142B from the transponder to the GDL-82. I'll do that and see if it helps. Any idea where to find the latest installation manual? Garmin doesn't seem to offer it on their site - only the Pilot's Guide.

Send me an email and I'd be glad to send you a copy of the latest manual.
Remove the extra cable and install an external antenna (with an internal ground plane) and I'll bet your problems will go away.
 
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I've done that after every flight. All the "failure" percentages are zero, but the "TIS-B Client" percentages suck. IOW The FAA can track me right down to the ground so, as far as they're concerned, my system meets all the 2020 requirements. What I'm unhappy with is that, for some reason, my connection with the ground station is weak so I don't get TIS-B traffic information on my Stratus/Foreflight/iPad unless I'm really close to the GBT. the problem could be my transponder antenna, or it could just be that the ground station signal is weak and I'm outside it's service volume. I wish the FAA would publish a list of ground stations and service volumes, but they refuse, for some reason. When I fly over the only ground station in my area, Foreflight tells me that it's "Low Power", but I have no idea what that means. If it were a VOR, the service volume would be publicly available, but not the GBTs.

A Dynon tester found that the Tucson ground station was inop for many weeks and the FAA did not know about it. Apparently at that time the automated station did not have a transmitted data monitor function. IIRC the station was outputting a steady RF but no data.

Hopefully there is a better automated monitoring system now. :)

The station that ForeFlight says is "Low Power" might be worth a report to the FAA.
 
I emailed adsb.gov and reported that the reception from the ground station S of 0A7 seemed to be weak and asked them what the service volume was supposed to be. I flew down to SC yesterday and was picked up by another ground station which seems to be located on KGYH. My "Client Percentage" for the entire trip was only 41%, but at least my buddy - who was following me - could see my N-number on his traffic display. I'm still floundering around trying to figure all this out.
 
Have you looked to see if you?re getting in flight weather? Since that comes only from ground stations on 978 MHz, and they don?t need to be ?pinged? by you for wx, this info may help isolate the issues.
 
Still gotta look out the window

I flew through Newhall Pass the other day and saw a Gulfstream on the ILS to VNY who didn't show on my ADS-B. My EFIS showed "No Radar" which means no ground station reception. Apparently, the local ADS-B ground station wasn't transmitting. Lots of other traffic showing. Be careful - seeing traffic on your device doesn't necessarily mean you are seeing all the traffic.

Ed Holyoke
 
Have you looked to see if you’re getting in flight weather?
I have always gotten the weather just fine, even without the external antenna that I'm now using. I've flown across the Country twice and the FIS-B weather information never let me down. Logically speaking, the problem has to be that 1. My UAT isn't broadcasting a strong enough signal to "wake up" the GBT, or...2. The single GBT in my area isn't working right. Next time I'm in the Greenville SC area, I'll test my system against the GBT down there. Also, I'm getting my transponder re-certified very soon and will have the tech measure my broadcast signal strength.
 
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One would think that the Gulfstream would be ADS-B (out) equipped, but apparently not.

A Gulfstream will need to have a mode S-ES transponder, so maybe they?re waiting for a scheduled transponder check before upgrading?
Have you seen other ADSB traffic on 1090 MHz (e.g., any jets)?
 
Have you seen other ADSB traffic on 1090 MHz (e.g., any jets)?
Some, but many don't have it yet. This N975MD was a Citation. I went and spoke to the Captain after he landed to see what equipment he had. I was on the ground when I took this screen shot:

MvEZ8r.png


Edit: You can also see the GBT on this screen shot. It's the white tower 4 NM S of Hendersonville Airport.
 
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Okay. So it looks like your receiver works okay on both frequencies. Things still point to the transponder/UAT antenna.
 
Does this FAA Google Earth map help define/confirm your local coverage?
It might, if I knew what the colors meant. I pinned the GBT location down at the bottom, and the Hangar location up at the top. I don't know how to get rid of all that other junk.

UDhmT8.jpg
 
Here's the ground track of my recent flight to Pickens [LQK] overlaid on the coverage map. I was a "Client" only 41% of the time. I was at 4,500' most of the way. There is a second ground station in Greenville in the lower right of this screen shot. It looks like if I'm in a light green area, I'm SOL.

3QoaPn.jpg
 
Here's the ground track of my recent flight to Pickens [LQK] overlaid on the coverage map. I was a "Client" only 41% of the time. I was at 4,500' most of the way. There is a second ground station in Greenville in the lower right of this screen shot. It looks like if I'm in a light green area, I'm SOL.

3QoaPn.jpg

Google Earth has a legend section on the left to switch on/off coverage areas based on height.

The FAA also references their color coding to heights AGL, so your 4500 ft trip through the Smokey Mtns. may need calculating...:)
 
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