On Sep 29, 5:11 pm, Bill Horne wrote:
I have a run of RG-8 coax inside a conduit, and I'd like to keep it in
service if possible. How can I test it before I risk my finals trying to
put power into it?

The simplest way I know is to use a QRP wattmeter and a dummy load.

First, set up a rig to feed the dummy load directly, through the dummy
load. Use the highest practical band (meaning what you have gear for).
Set your rig to deliver a few watts and record the exact reading,
forward and reflected. (By running low power you don't risk your rig,
and you test that the dummy and wattmeter work).

Then connect the rig and wattmeter to one end of the cable to be
tested, and the dummy load to the other end. Recheck the forward and
reflected power readings - they shouldn't be much different if the
cable is any good at all. (If you have any significant reflected power
when feeding the dummy load, it's clear the cable has water, an open
or a short someplace).

Finally move the wattmeter to the far end of the coax to be tested,
and put it just before the dummy load. Get one more set of forward and
reflected power readings. Compare with the readings when the wattmeter
is at the rig end of the cable. Compute the loss, convert it to dB,
and see how it compares to the expected value for the cable.

For example, suppose the cable is 50 feet of RG-8U and I do the test
on 40 meters. Suppose the wattmeter says 20 watts goes in but only 10
watts comes out, and almost no reflected power. That's half the power
(3 dB) lost in a cable that's supposed to be well under 1 dB per 100
feet at 7 MHz. Something's wrong.

73 de Jim, N2EY