[TowerTalk] Evaluating coaxial cable not from a dealer

[Jim Brown]

On 3/6/2021 3:32 PM, Steve K8JQ wrote:
> Occasionally someone will post here or on other amateur radio forums 
> coaxial cable for sale. Often unused, left over from a project, on a 
> reel, pickup only, as-is where-is, cash. If one were to pursue such a 
> purchase, what would one look for and/or do to validate that the cable 
> is good?
> 
> An eyeball physical inspection of the cable is an obvious thing to do. 
> One can see the outer layer of cable on the reel and perhaps both ends 
> of the cable. Is that enough?

If it's on a spool, I would expect it to look like it was spooled at a 
factory. Certainly I would look for the mfr's printed ID on the cable, 
and I'd want it to be a well-known mfr.
> 
> Should one make some electrical tests before handing over the cash? What 
> tests and with what test equipment? A few spot frequencies? Wideband 
> sweeps? Antenna analyzer? VNA? 

Certainly a TDR with a good analyzer. Should be a very wideband sweep, 
50-500 MHz is what I usually use with my VNWA-3E and DG8SAQ's software. 
I wouldn't want to see any bumps except at the ends.

If both ends are available, I would make sweeps with the far end open 
and shorted, then export to ZPlots https://ac6la.com/zplots1.html to get 
Zo, VF, and loss. To get the length, I would study the sweeps to see 
where the lowest stub open/short frequencies land, then apply expected 
VF for foam or solid dielectric to get close on the length. These sweeps

Transmitter with power meter? Something
> else? What would you do if both ends are not available or do not have 
> connectors?

A TDR with a good analyzer will be "good enough" with a short clip-lead 
adapter to the exposed end of the cable, and it will get you in the 
ballpark for a ZPlots calc. Often, if the cable is still factory-wound, 
the far end will stick out of a hole in the spool, so it can be shorted.

> Is it OK to transport a reel with a few hundred feet (or more) of cable 
> on its side or should the reel be "standing up" in the bed of a pickup 
> truck?

I've always laid it on its side so that it doesn't roll around.
> 
> What else to look for, think about, consider?

Yes. Don't buy into the urban myth that old coax is no good. 10-12 years 
ago, I helped the widow of an SK neighbor get rid of all the stuff from 
his substantial station and huge stash of parts. It included a LOT of 
coax and hard line. I bought the hard line myself, and put some of the 
coax on a table at an NCCC meeting for "giveaways." Everyone turned 
their noses up at it, so I took it home, built a lot of stubs from it, 
and measured them. With the exception of one short piece that had 
obvious water penetration and corrosion, all of the stubs produced the 
steep nulls to be expected from very good coax. Most of it was at least 
20 years old, most identified as Belden or Times of Amphenol, and 
RG8/RG213 construction.

Yes, contaminating jackets can cause degradation, but lots of top 
quality coax has some form of non-contaminating jacket.

73, Jim K9YC
> 
> Steve, K8JQ
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