[TowerTalk] Coax - NOS

[Jim Brown]

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