[TowerTalk] measuring old coax, return loss ?

[Jim Brown]

On 5/8/2024 9:42 AM, Jamie WW3S wrote:
> What is a good way to test older coax?

With a good, and well calibrated, Vector Network Analyzer 1) measuring 
S21 to see loss; and 2) making a TDR measurement with a sweep to at 
least 500 MHz.

Deterioration in old coax is assumed, but rarely true unless it's gotten 
wet. More than ten years ago, I helped N6RZ's widow dispose of his 
station and massive stash. It included a lot of coax. I didn't have that 
measurement capability then. I took a lot of it to an NCCC meeting for 
giveaway to members. All of them turned their noses up at it, so I 
brought it home and built stubs from it.

Measuring those stubs, I found that all but one of them had a very deep 
notch where it resonated; the only one that didn't came from a piece of 
coax that had been badly stored, and had gotten wet. The braid showed 
significant corrosion.

Water commonly enters coax at connectors that are poorly waterproofed, 
and at nicks in the outer jacket due to some sort of abrasion. I had 
water enter coax at a home-brew center insulator for a high wire dipole. 
I discovered it when seeing water coming out it at the arrestor panel. 
This was about 150 ft of a good RG11, and the VNA saw a mild increase in 
the loss (this was after maybe a year), and the braid was badly discolored.

73, Jim K9YC