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  #1  
Old 09-12-2018, 08:50 PM
1flyingyogi 1flyingyogi is offline
 
Join Date: Dec 2015
Location: Long Beach, CA
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Default Intermittent Transponder problem

I replaced my KT76A with a GTX327 and it seems to work most of the time. In the past 20hrs of flying, 4 times I've had ATC tell me they can't see me. Once, recycling the transponder fixed the problem. One of the four times, they said they can't see my altitude. The other times, they can't see me at all (position or altitude). The interesting thing is that my ADSB can always see me, even when ATC can't. This leads me to think it's an antenna problem.

I was told that it's because of my post-style antenna. Like this one: https://www.aircraftspruce.com/catal...?clickkey=4083

I was told that the GTX 327 needs a CI-105 type of antenna. So my question is, will this one be ok? https://www.aircraftspruce.com/catal...?clickkey=4083

If someone can educate me about the different types of antennas and what's the difference between CI-101 vs. CI-105 and why there's such a big price difference within each category. For example, this one made by Comant is almost 3x as much as the Red Tail one. https://www.aircraftspruce.com/catal...?clickkey=4083

Which one should I get? Also, why did the post-style CI 101 work just fine with my KT76A, but not the Garmin GTX 327? I've never had a single problem with the KT76A.

Thanks.
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  #2  
Old 09-12-2018, 08:56 PM
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Carl Froehlich Carl Froehlich is offline
 
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The antenna makes no difference, assuming it is properly connected. Any of those three are resonate at the XPDR frequency.

Id first check the power leads on the XPDR breaker. A loose connection will have the symptoms you discribe.

Carl
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Old 09-12-2018, 09:59 PM
1flyingyogi 1flyingyogi is offline
 
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I also installed the uAvionix ADSB out/ in at the same time I did my transponder upgrade. I've read in another thread that the uAvionix could possibly be somehow interfering with the transponder and be causing this problem??
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Old 09-12-2018, 10:37 PM
wnplt wnplt is offline
 
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Location: Idaho
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Quote:
Originally Posted by 1flyingyogi View Post
I also installed the uAvionix ADSB out/ in at the same time I did my transponder upgrade. I've read in another thread that the uAvionix could possibly be somehow interfering with the transponder and be causing this problem??
I have a Uavionix and a 327 combo and have no problem.
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  #5  
Old 09-13-2018, 07:48 AM
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Raymo Raymo is offline
 
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Try pulling the unit out, cleaning the connectors and re-seat. You may have a pin in the try harness that is receding as well.
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  #6  
Old 09-13-2018, 08:44 AM
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Marc DeGirolamo Marc DeGirolamo is offline
 
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Location: Saskatoon,Saskatchewan,Canada
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Default Gear leg shielding

I have the same issue, often.....usually after takeoff. It is always the gear leg shielding the interrogation from the tower radar.
Try turning next time you get the question? See if this is your problem before digging into antennaes etc....
Ps: I am assuming your antenna is on the belly just back if the exhaust.
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Last edited by Marc DeGirolamo : 09-13-2018 at 08:46 AM.
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  #7  
Old 09-13-2018, 09:52 AM
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snopercod snopercod is offline
 
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Quote:
Originally Posted by 1flyingyogi View Post
...why did the post-style CI 101 work just fine with my KT76A, but not the Garmin GTX 327?
You mentioned ADS-B so may I assume that you installed a UAT like the GDL-82 in series with your GTX-327? Those spike antennas are tuned to just one frequency band, the 1030-1090 MHz that your transponder is using. Since the UAT broadcasts on 978 MHz, your spike antenna is no longer optimum at that frequency. What works best for a Transponder/UAT combination is a broadband antenna like the DeltaPop blade antenna you linked. All that said, I don't think your spike antenna is the source of your intermittent problem. I would look for a flaky coax connection. I had the shield braid pull partway out of the coax connector to my antenna once and it caused an intermittent problem similar to what you described.
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Old 09-13-2018, 01:34 PM
1flyingyogi 1flyingyogi is offline
 
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Location: Long Beach, CA
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Thanks for everyone's input. I'll look at my connections and coax again and see if everything looks good. My transponder antenna is just aft of firewall on the belly. The UAT antenna is on the belly below the baggage compartment. I don't think it's a problem of the gear leg shielding the signal b/c that has never been a problem before I changed transponder/ added ADS-B out.

Also to clarify, I'm not sure if my ADS-B "always" sees me, even when ATC doesn't. Just the few times I've looked down at my tablet, I've seen it. And since most of my flights are local, short flights, I'm not on with ATC that much, so my intermittent problem could be more frequent/ worse than I think.

When I did the install, I remember making a mental note that the connector from the coax cable to the back of the transponder looked really old and dingy. I was going to replace it but didn't because it's such a cramped space and the cable is so short that it would have been a PITA to work on. I'll take a look again this weekend.

Still looking for more insight and ideas on trouble-shooting so please keep them coming. Thanks!
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  #9  
Old 09-13-2018, 01:37 PM
1flyingyogi 1flyingyogi is offline
 
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I did NOT install my UAT in series with the transponder. It's using sniffer mode. I heard that it's difficult to make it work with the AK altitude encoder I have. I would need to buy some extra equipment and it's not worth the money/ hassle to put it in series.

Quote:
Originally Posted by snopercod View Post
You mentioned ADS-B so may I assume that you installed a UAT like the GDL-82 in series with your GTX-327? Those spike antennas are tuned to just one frequency band, the 1030-1090 MHz that your transponder is using. Since the UAT broadcasts on 978 MHz, your spike antenna is no longer optimum at that frequency. What works best for a Transponder/UAT combination is a broadband antenna like the DeltaPop blade antenna you linked. All that said, I don't think your spike antenna is the source of your intermittent problem. I would look for a flaky coax connection. I had the shield braid pull partway out of the coax connector to my antenna once and it caused an intermittent problem similar to what you described.
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  #10  
Old 09-15-2018, 01:03 PM
Moosetrax Moosetrax is offline
 
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Location: Cape Coral, FL
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Default GTX327 Intermittent

I just worked through the same issue. Start with the easy stuff, antenna, coax, and connections. Mine ended up being the unit itself. I was able to swap boxes with a local avionics shop and theirs worked fine. I opted to send my 327 back to Garmin for refurbishment. Flat fee $598. Good luck.
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