How should I test a coaxial cable?

Question

I'm a ham radio operator, but haven't been very active on HF bands for a while. I have an inverted-V antenna and a long run (approximately 25m) of coax that runs from my upstairs "shack" down the wall, through a conduit that I buried in a trench and up near a tall oak tree in which the antenna is mounted.

Other stations don't seem to be able to hear me very well, and so I am suspecting that the coaxial cable may be waterlogged. Other than the obvious tests with an ohm-meter, how can I test the coax to see if it is still OK?

I don't have an antenna analyzer, but do have an antenna tuner and a dummy load.

Answer 1

You can measure a lot of things (impedance, velocity factor, distance to short-circuit, distance to open-circuit, ...) with a TDR (Time Domain Reflectometer) as shown and explained in multiple tutorials.

For example:

Cheap and simple TDR using an oscilloscope and 74AC14 Schmitt Trigger Inverter
How to measure coax velocity factor VF and impedance Z
"TDR" or Time Domain Reflectometer, build and use this circuit
Determining Velocity Factor of coaxial cable
Understanding DTF or Distance To Fault, using a TDR
Determining Coax Impedance with a TDR

Answer 2

Have you thought that mounting your antenna in a tree may be 90% of your problem. Most hams do every thing in the realm of feasibility to get their antennas mounted on towers that are higher than surrounding buildings, vegetation and trees.

Consider this. Try seeing how bad a GPS receiver works in a woods or forest with a tree canopy overhead. First hand experience will show that it works pretty bad.

Answer 3

If you disconnected the antenna, and shorted the end of the coax (or left it open-circuit), you should be able to verify that from the other end. Using a transmitter, you shouldn't be able to transfer any power into it. Anything that looks like a load it actually loss (heating the coax).

There are a number of downsides to this test. The transmitter isn't necessarily going to like operating into this. Not all transmitters have an adjustable output network ("tune" and "load"--do any of them have this anymore?). And, of course, you shouldn't be radiating any of this while testing.

A better instrument would be a noise bridge. This lets you measure the impedance looking into the feedline. Again, it shouldn't look like a resistor. And again, I hope I haven't dated myself with something else you can't buy. But it's an ingenious little gadget.

Answer 4

Great way I have tested Coaxial Cable is by using a return loss tester. Plus it can test continuity. This product is very easy to use. Check out this video: https://www.youtube.com/watch?v=dEOxWTESY70

For a more advanced test you can use this graphical TDR: http://www.t3innovation.com/arcchaser