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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|