On Tue, 9 Oct 2012 10:32:35 -0400, "J. C. Mc Laughlin"
wrote:

** I will look at these sources. Ebay, well, it is Ebay. . . Thanks.

Good, fast, cheap. Pick any two.
eBay is fast and cheap at the expense of good. I buy quite a bit of
stuff on eBay and have had a good success rate. However, about once
or twice a year, I get a lemon. For cheap stuff like coax cable, it's
worth the risk.

What are expected to be the two
most important factors are a need not to have non-linear effects (no Fe and
attention to avoiding braid issues - you are familiar with the extensive
testing of harmonic production in/with some coax) AND a desire to minimize
the probability of center-conductor migration (staying away from foam)

PIM (passive intermod):
http://en.wikipedia.org/wiki/Intermodulation#Passive_Intermodulation_.28PIM.29
Also watch out for cadmium and nickel plated coaxial connectors.
Heliax and silver plated copper are the best so far. I recently spent
a weekend on a mountain top with a borrowed PIM tester.
http://www.kaelus.com/Kaelus/media/Site/QR%20Code%20Files/PIM-Testing-Guidelines_Brochure.pdf
http://boonton.com/products/pim-testers/pim21
We had the time to go through almost every coax cable in the building.
According to the tester, with the antennas connected, everything
needed to be replaced. With the dummy load, most everything looked
much better. I eventually determined that we were actually measuring
rectification from nearby antennas and the tower. Banging on the
antennas with a mallet and tightening tower screws fixed that. Most
of the IM came from stainless fasteners on zinc plated tower hardware
which was later replaced.

For an RF polluted radio site, PIM testing is a really good idea.
However, I just don't see it being very useful for ham radio unless
you're doing something that requires full duplex operation.

** A story from long ago: I saw center-conductor migration in RG8 type
(solid dielectric) in a piece of coax that was in a horizontal, outdoor
cable trough. This was a long time ago and it is possible that a
manufacturing defect was involved. Today, one would scan the long piece of
coax and discover such a latent defect if it existed.

I still use a home made TDR (time domain reflectometer) for testing
cables. Old habits and methods die slowly.

I usually have the opposite problem with coaxial cable. For example,
I would solder connectors onto a 100ft roll of RG8/u while it was
still rolled tight on the spool. Bad idea. When straightened, the
center conductor becomes slightly longer than the shield. The stiff
center conductor tries to push the center pins out from the
connectors. In one case, I found a mangled N connector pair where the
center pin had shoved its way into the mating receptacle.

I've seen what I suspected was center conductor migration on long
horizontal coax runs, but have never done a post mortem on the coax.
By the time it gets to the point where it's shorted, other problems
(i.e. losses) usually inspire a pre-emptive replacement. These days,
I just happen to have a roll of RG6/u with a messenger wire attached,
which eliminates all the stresses.

Good luck, whatever you're building.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558