On 11/28/2020 5:45 PM, Steve, W7WM wrote:
What is the current thinking on aging coax?
"Current thinking" is often wrong, the repetition of urban myths. See
below. :)
I have some 20+ year-old RG-8, still on the spool,l and some coax that has
been in conduit for 30 years, and coax that has been exposed to weather
(not wet though). None of my coax has water damage.
Is all of this 'past it's prime'? How much has this deteriorated?
The primary way that coax deteriorates is moisture inside the jacket,
which degrades the shield, and with some dielectric types (foam, air)
degrades the performance of the dielectric. Both mechanisms can increase
loss. W2VJN, who has done a lot of work and writing about stubs,
reported in the second revised edition of his book on Interstation
Interference that some LMR400 stubs he had stored in a garage for ten
years showed increased loss, which he speculated was due to oxidation
degrading contact between the foil and braid shields.
About 12 years ago, I helped N6RZ's widow get rid of his very large
stash of "stuff," including a lot of coax that had been in use in his
station. Some had been inside, some outside. It was all good quality
stuff, most of it 20 years old, all of it with a good copper braid
shield, no foil. I took a lot of it to a meeting of NCCC, a major
contest club, with a lot of very sharp guys. I laid it out on table for
them to take it for free. All turned up their noses, so I took it home
and made stubs out of it, as I did so, inspecting for moisture damage.
Only one piece was bad, and it had been stored badly in a shed.
The stubs I built were all stubs for harmonic suppression, and a deep
null is a good measure of their performance. All measured with nulls
comparable to brand new coax.
Bottom line -- if the coax has no physical damage or poorly waterproofed
connectors that could let moisture in, and it's copper with a good
copper braid shield, I'd keep it in service. If it's foil/braid shield,
I'd measure it using AC6LA's spreadsheet method and compare with the
datasheet.
73, Jim K9YC
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