How do I check if old coax is still usable?
 

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?

It's OK AFAICT with a VOM.

73, Bill W1AC
--
Bill Horne

(Remove QRM from my address for direct replies.)

How do I check if old coax is still usable?
 

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?

It's OK AFAICT with a VOM.

73, Bill W1AC

Bill
Apply a megaohmmeter to test for any breakdown in the dialectic between
the center conductor and the shield and between the shield and ground.
A test voltage of 500 would be the highest I would go and I think three
hundred would tell you all there is to know about the integrity of the
insulation. You already know that the condition of the copper is
essentially unchanged.
--
Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous
for general use." Thomas Alva Edison

How do I check if old coax is still usable?
 

"Tom Horne" wrote in message
...
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?

It's OK AFAICT with a VOM.

73, Bill W1AC

Bill
Apply a megaohmmeter to test for any breakdown in the dialectic between
the center conductor and the shield and between the shield and ground.
A test voltage of 500 would be the highest I would go and I think three
hundred would tell you all there is to know about the integrity of the
insulation. You already know that the condition of the copper is
essentially unchanged.
--
Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous
for general use." Thomas Alva Edison

Buried coax is prone to moisture unless the conduit was able to drain and
has air space. If you think it may have had moisture in it, replace it. I
like to replace at 10 years maximum because of contamination from the
chemicals in the dialectric.

How do I check if old coax is still usable?
 

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

How do I check if old coax is still usable?
 

O.M's
Bill simply try it satisfied use it disappointed use it as grounding wire
in all the replies of this subject is usefull stuff to experimentals
in practical sense chemical chances of the dielectrical between inner and
outer is telling
after 10 - 15 years depending the manifactor
In coastal envoriments the salt in the air reduce this
moisture between inner and outer is a silent dange rust reduce the
conductivity and moisture the impendance
Jim told you a way to measure some qualities I do the same when i got
unknown cable for JOTA use
A practcal tip old coaxcable is excelent to use as a dummyload
so again a lot of things to experiment
55 + 73 de PA0RAB ruud
schreef in bericht
...
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