Watch Out For Loose TNC's....

[Ironflight]

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?

Paul

[randyintejas]

Proseal that sucker

[az_gila]

Originally Posted by Ironflight

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?

Paul

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/t...8AE805A88617F&

..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....

[Ironflight]

Originally Posted by az_gila

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....

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

[DaAV8R]

Originally Posted by Ironflight

I of course, immediately switched to GPS #2 (and could have gone to #3, or #4)

Maybe a sign of the times. "We've lost number one" used to have an entirely different meaning

[FredMagare]

Originally Posted by Ironflight

Torque Seal doesn't, of course, secure anything - it simply shows you that the seal has been broken.
Paul

LocTite on the threads?

[AlexPeterson]

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.

[Ironflight]

Originally Posted by AlexPeterson

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.

Yeah - your problem is a mystery Alex - I found mine almost completely spun off - I am sure the center conductor wasn't making contact.

[molson309]

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.

[az_gila]

Originally Posted by Ironflight

Yeah - your problem is a mystery Alex - I found mine almost completely spun off - I am sure the center conductor wasn't making contact.

"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/DocumentD...ctors_0307.pdf

Do you know the part number of the connector you used?