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.