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Watch Out For Loose TNC's....

Ironflight

Ironflight

VAF Moderator / Line Boy
Mentor
last week we were happily cruising across East Texas in the Valkyrie, watching the autopilot take us to RV Central to work on the RV-1. Out of the corner of my eye, I was surprised to see the 430W annunciate that the GPS signal had become unreliable - then it went into Ded Reckoning mode. I have seen this during a series of aerobatic maneuvers, but never straight an d level! I of course, immediately switched to GPS #2 (and could have gone to #3, or #4...or heaven forbid, looked at the countryside spread out below, and the city ahead...), and figured I'd troubleshoot it on the ground.

Sure enough, the 430W picked up no satellites at all for the meaning 20 minutes of the trip, so I suspected an antenna problem. Lo and behold, when I reached into the oil door to check the firewall-mounted antenna, the screw-on TNC connector had backed off almost all the way. I am sure it was tight when I did the Condition Inspection before Christmas, so I'll have to chalk it up to vibration.

Interestingly enough, when Garmin went from the 430 to the 430W, they switched the antenna connection from a BNC (which requires two motions to disconnect) to the more expensive (and clearly less positively secure) TNC. No way to safety wire it that I can see.

Are your TNC's tight?:rolleyes:

Paul
 
Ironflight said:
last week we were happily cruising across East Texas in the Valkyrie, watching the autopilot take us to RV Central to work on the RV-1. Out of the corner of my eye, I was surprised to see the 430W annunciate that the GPS signal had become unreliable - then it went into Ded Reckoning mode. I have seen this during a series of aerobatic maneuvers, but never straight an d level! I of course, immediately switched to GPS #2 (and could have gone to #3, or #4...or heaven forbid, looked at the countryside spread out below, and the city ahead...), and figured I'd troubleshoot it on the ground.

Sure enough, the 430W picked up no satellites at all for the meaning 20 minutes of the trip, so I suspected an antenna problem. Lo and behold, when I reached into the oil door to check the firewall-mounted antenna, the screw-on TNC connector had backed off almost all the way. I am sure it was tight when I did the Condition Inspection before Christmas, so I'll have to chalk it up to vibration.

Interestingly enough, when Garmin went from the 430 to the 430W, they switched the antenna connection from a BNC (which requires two motions to disconnect) to the more expensive (and clearly less positively secure) TNC. No way to safety wire it that I can see.

Are your TNC's tight?:rolleyes:

Paul
Click to expand...

Paul - was it really secure?

AMP says this about TNC connectors...

Threaded coupling interface ensures connector will not de-couple in vibration-intensive applications

http://www.amphenolrf.com/products/tnc.asp?N=0&sid=4F18AE805A88617F&

..and fron another source

TNC RF connectors are basically BNC connectors with threaded coupling and improved electrical performance at higher frequencies. TNC 50 ohm connectors provide repeatable electronic performance from DC to 11 GHz. TNC 75 ohm connectors are suitable for applications up to 1 GHz. All 50 and 75 ohm connectors are intermatable. TNC connectors are designed to accomodate cables from .150" to .250 OD. The TNC type connector is a weatherproof unit and with its threaded coupling ensures low noise and withstands shock and vibration. LTI TNC connectors are designed to conform to MIL-C-39012 specifications. Typical TNC series coaxial connector applications include airframe, missile, radar, cellular mobile phones, and precision electronic equipment industries.

http://www.rfconnector.com/tnc-connectors.php

Following on with another thread, perhaps your TNC connectors are also good locations for a Torque Seal marker...:)

They are the correct part for the job....:D
 
az_gila said:
Paul - was it really secure?


Following on with another thread, perhaps your TNC connectors are also good locations for a Torque Seal marker...:)

They are the correct part for the job....:D
Click to expand...

Well, yes, it was secure - I checked it in December, and the only way to check it is "hand tight". That's the point Gil - there is no "securing" mechanism.

Torque Seal doesn't, of course, secure anything - it simply shows you that the seal has been broken. I already knew that when the GPS signal went away!;)

I am sure that the RF engineers consider it the right part for the job - but from a mechanical, aeronautical standpoint, I'd say that there might be other considerations.

We're not going to change Garmin's design - I just want folks to know that these can loosen up on their own. Make sure that you don't hide them where you can't get access to check them.

Paul
 
Paul - I hope that was indeed your problem. My 430w has been intermittently doing what you describe for several years - satellite page shows no signals at all. It will do this for 1 minute or perhaps 45 minutes. It will come back and run for 3 months without issue. I'm going to check my connector again, but I doubt it will be my problem, hasn't been in the past.
 
AlexPeterson said:
Paul - I hope that was indeed your problem. My 430w has been intermittently doing what you describe for several years - satellite page shows no signals at all. It will do this for 1 minute or perhaps 45 minutes. It will come back and run for 3 months without issue. I'm going to check my connector again, but I doubt it will be my problem, hasn't been in the past.
Click to expand...

Yeah - your problem is a mystery Alex - I found mine almost completely spun off - I am sure the center conductor wasn't making contact.
 
I have a 70cm (440Mhz) ham radio antenna on the roof of my house. It has an N-type connector for the feedline. An N-type connector looks like a scaled-up TNC. This antenna was installed by me 15 years ago. I hadn't used it for a couple of years and last summer connected a radio to it and saw the VSWR was infinite. Climbing onto the roof I found the connector had unscrewed itself (it is inside a protected tube) and the cable was hanging loose. No or very little vibration to speak of, but of course it had experienced many temperature changes. I had assembled it hand tight yet somehow it loosened itself over time until it came completely apart. So I don't find it difficult to believe a TNC in a more vibration prone environment did it as well. Could be neither of us tightened it "enough" but since there is no torque spec it's sort of hard to tell when a connector like this is indeed tight enough.

I have, as I suspect many of us do here, a TNC connection for a Garmin WAAS antenna in both of my airplanes. I am going to make a point of checking the torque of these frequently from now on. I'd hate to lose the GPS at an inopportune time due to this connection coming apart.
 
Ironflight said:
Yeah - your problem is a mystery Alex - I found mine almost completely spun off - I am sure the center conductor wasn't making contact.
Click to expand...

"The TNC type connector is a weatherproof unit and with its threaded coupling ensures low noise and withstands shock and vibration. "

It is designed for shock and vibration.

I would also look at the source of the actual connector you used on the cable end, and whether it came from a reputable manufacturer.

Mil-spec parts are relatively easy to obtain in the connector line and certainly would be worth it for this application.

One other possibility is that the connector you used was rated for RG-58 cable and not RG-400 - I have seen connectors advertised by VAF vendors for RG-400 that were not spec'd for that size cable by the manufacturer.

Tyco's products are here, with full part numbers -

http://www.te.com/commerce/Document...nglishENG_CS_1307191_TNC_Connectors_0307.pdf

Do you know the part number of the connector you used?
 
Ironflight said:
Torque Seal doesn't, of course, secure anything - it simply shows you that the seal has been broken. I already knew that when the GPS signal went away!;)
...
Paul
Click to expand...

Torque Seal does confirm that indeed you did check it - needed as our older memory cells start wearing out...:) You only apply it AFTER you have tightened it....:D

It would also give you an idea if it was slowly loosening - I don't have a TNC in front of me, but it is MANY turns from tight to falling off.
 
Adel Clamp

Ironflight said:
Interestingly enough, when Garmin went from the 430 to the 430W, they switched the antenna connection from a BNC (which requires two motions to disconnect) to the more expensive (and clearly less positively secure) TNC. No way to safety wire it that I can see.
Click to expand...

Paul,
You might consider putting an appropriately sized adel clamp on it and then safety wiring the clamp. I know it is less than elegant, but I had a pesky tach cable connector on my Bonanza that was prone to vibrating loose no matter how much it was tightened and that method solved the problem.

Don
 
cfiidon said:
Paul,
You might consider putting an appropriately sized adel clamp on it and then safety wiring the clamp. I know it is less than elegant, but I had a pesky tach cable connector on my Bonanza that was prone to vibrating loose no matter how much it was tightened and that method solved the problem.

Don
Click to expand...

That's not a bad idea if you are going to hide one in a non-accessible area!
 
AlexPeterson said:
Paul - I hope that was indeed your problem. My 430w has been intermittently doing what you describe for several years - satellite page shows no signals at all. It will do this for 1 minute or perhaps 45 minutes. It will come back and run for 3 months without issue. I'm going to check my connector again, but I doubt it will be my problem, hasn't been in the past.
Click to expand...

Hi Alex,

The 430w has a known issue with losing satellites. We have three of them and we can replicate the problem by transmitting at low radio frequencies. You can see the strength bars drop when you transmit. We have a YouTube video we sent to Garmin demonstrating this. Garmin is aware of the problem and they suggested rerouting wiring and moving the antenna location which we have done. It didn't seem to change the problem.

You might give it a try to see if that is what is going on with yours.

David
 
David, please see this thread for much more detail on my problem... It is not related to any com transmissions, it has happened at high altitude, no one around. A youtube video shows how, in a particular failure mode, the 430 will wipe out all other gps's on board. Not a real good failure mode. Any further comments on my problem are best put on the above thread please.
 
az_gila said:
One other possibility is that the connector you used was rated for RG-58 cable and not RG-400
Click to expand...

Can't really see where that would cause the connector to unscrew itself. The threads have no way of knowing what cable is crimped inside :D

Possible some lube was left on the threads making it easier to come undone?
(I really like the Adel camp idea....)
 
Firewall mounted GPS antenna

Paul, an explanation of your firewall mounted GPS antenna, or photo, would be appreciated. I would like to mount one there on my -6. Bob.
 
rjaflys said:
Paul, an explanation of your firewall mounted GPS antenna, or photo, would be appreciated. I would like to mount one there on my -6. Bob.
Click to expand...

Just a very simple shelf Bob, with a "shelf" area big enough for the base of the antenna, and two little triangular braces with flanges extending down the firewall for support. I don't have any pics handy - someone else might (it is a VERy common thing to do).
 
Ironflight said:
Well, yes, it was secure - I checked it in December, and the only way to check it is "hand tight". That's the point Gil - there is no "securing" mechanism.

Torque Seal doesn't, of course, secure anything - it simply shows you that the seal has been broken. I already knew that when the GPS signal went away!;)

I am sure that the RF engineers consider it the right part for the job - but from a mechanical, aeronautical standpoint, I'd say that there might be other considerations.

We're not going to change Garmin's design - I just want folks to know that these can loosen up on their own. Make sure that you don't hide them where you can't get access to check them.

Paul
Click to expand...

What about a bit of self-fusing tape wrapped around the connector? Or some other sealing tape I have for antenna/coax connections (have to find the name of it...can't recall at the moment...but it's designed to seal coax connectors for antennae)?

ETA: This stuff...got some when I picked up some conductive gaskets for my comm antennae...

http://www.avdec.com/products/tapes/stretchseal
 
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