Coax testing, access to only one end, how?

[Charles Mampe]

Hi, I think I have a compromised cable for a FLARM antenna. It appears it was caught in the canopy at some point.

Using the range tester, it appears the antenna is not doing a great job (short range, others have issues seeing me).

Yes, I could replace it and redo the test. I want to check what I have though. I believe the antenna end is molded in, it's not really accessible. The only other thing I can think of is to do a continuity test to see if the "crush" shorted the shield to the center conductor.

Thoughts?

If shorted, can I open it up, shorten a bit and reuse, or just toss it and start new?

[Christopher Kimble]

If the dielectric between the shield and conductor gets crushed, it disturbs the continuous transmission-line impedance, and would likely cause signal reflections at that point, resulting in a noisy signal.  You may have good DC continuity and no DC short, but the signal is high frequency, not DC. 

[Ryan Bluestein]

I think hooking it up to a VNA and looking at the S11 of the cable might be a good place to start if you have access to that equipment. Another option would be an SWR meter. There seems to be enough radio guys in soaring that you could find something to borrow.

If the dielectric did get crushed I think you should be fine to just cut the cable and terminate it at the new end. Hopefully you've got some excess already in the cable.

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[Lynn Alley]

Nano VNAs cost like $90 on Amazon.  You might need some help using it.  If you know a ham radio hobbyist, that would be a great resource.  If not, there are online videos on how to use a VNA to test an antenna.

At the end of the day, a VNA is the much better way to test an antenna.  Multimeters just can't do the job.

[Charles Mampe]

Thanks for the reply's everyone. I still have time to decide what to do.

I didn't think just a DVM would be enough. It could say, "it's a shorted cable", but not "it's a good cable".

[Moshe Braner]

Replacing the antenna is easy and not expensive, and in the worst case you will then have a spare left over.  But, many of the antennas sold as "915 MHz" (assuming you are in the USA, 868 in EU) are not really tuned to the frequency, or in other ways not good.  Anybody who has access to a VNA, can you report which ones are good?  I've had decent results with a couple of dipole antennas sold by "Data Alliance".

[Charles Mampe]

Moshe, yes, I'm in the US (North NJ). A replacement would be sourced from one of the US glider bits vendors. I realize there might be a cheaper way, but FLARM, to me, is safety equipment and not something to cheap out on. I believe they are about $50US each.

[Charles Mampe]

OK, follow up.

Ship is in the shop for spring stuff ("B" mod to ASW-24E) and things.

Pulled the 2 FLARM antennas. The "A" has a dipole housing screwed together. It was opened, and TADA......it must have been yanked at some point and connections lost. Cable was shortened a bit (beyond where it was pinched) and soldered back to stubs. The "B" antenna has a glued together dipole housing. It was worked open but looked good inside. It was also shortened.

Next few weeks hopefully I can fly and do another test. If not, then I wait until Perry contest.

[Dan Daly]

One other thing to watch. On FLARM COREs, the GPS connector, I believe MCX, is more fragile than it looks. Someone 'helped' me by removing a club FLARM going out for a no-transmit repair.  I found the connector end stuck in the FLARM receptacle, and the remainder was in the glider at the end of the coax. Bottom line, they should be handled with care. Pic attached. I now stock a spare of all antennae.

[Chris Behm]

Hello Charles-

So you are saying that if I want to shorten my antenna lead for flarm, instead of cutting and crimping a new end with a proper tool, I may be able to do it from the antenna end as you descibed?

[Mark Mocho]

The standard dipole FLARM antenna just has two soldered connections inside the rectangular enclosure. I had reports from other pilots that I was not appearing on their Flarm displays and I was also not receiving well. I opened the enclosure and found broken connections in both antennae. Resoldered them and life was good again. The breaks were caused by rough handling and an immediate 90-degree flex where the coax exited the enclosure. I added a couple of layers of stiff heat shrink as a strain relief that keeps the coax straight for a couple of inches before trailing off to the Flarm Core. Range analysis afterward shows excellent coverage.

This is definitely the way to adjust cable length. And if you suspect decreased performance, remove the two screws from the enclosure and check the connections.

[Charles Mampe]

Yes Chris, you can open the plastic by the antenna poles and just do a cut/strip/resolder there. My "A" antenna had 2 screws holding the housing together, my "B" antenna was glued or ultrasonically welded together. The case was successfully opened up to do the work. I can only state what we found and did, no guarantee you will find the same. I am looking forward to trying it out soon.

[Chris Behm]

Thanks Mark and Charles-

I'm pretty handy with a soldering station and will keep this in mind for a future cockpit cleanup. I hate bundled wires for all the right reasons. A single service loop or "S" is plenty....

Kind Regards,

Chris

[Mark Mocho]

I agree about bundled wires. I wish there was an easy way to eliminate all the extra cabling associated with the LX 90xx system, but there are just too many pins in the various D-sub connectors to bother with. I'd rather fly and then drink beer to suppress my mild OCD tendencies.

[Moshe Braner]

Additionally, a longer-than-necessary cable reduces the signal strength.  Especially at a high frequency (FLARM is at around 900 MHz) and when the cable it thin (as it is for these sort of antennas).  OTOH a simple soldered-wires connection also reduces the signal.  Then again, if this is already the arrangement in the antenna, there's nothing to lose.  Never splice the cable in the middle, though, e.g., if it got pinched.

[Tom]

Here is a short video on how to use a VNA to test an antenna cable:

www.youtube.com/watch?v=91ZRTFZ40rw

It is not as hard as it sounds!

Tom 2G