I recently had a problem where the SWR would "go crazy" frequently but
seemingly at random. After "getting lucky" in a trouble shooting
session I found a defective RG-8X jumper. In the middle of its length
there was a section about 3-4 inches long where the center conductor was
not centered in the dielectric. The center conductor "wandered" around
and was right at the edge of the dielectric and in one very small place
was right at the surface where it touched or nearly touched the foil
wrapped around the dielectric under the braid.
The jumper repetitively passed the ohm meter test showing good
conductivity everywhere it was supposed to and not conducting where it
shouldn't. However when a dummy load was attached to one end and the
antenna meter to the other the meter's indications were just weird and
uninterpretable. Of course the same test setup with another jumper
showed the expected 50 ohm reading independent of the freq. Subsequent
to these tests I cut the jumper in half and by chance found the defect
within an inch or so of the first piece I started cutting off little bits.
I saved a couple inch piece of just the dielectric with errant center
conductor showing the worst of the defect and an adjacent sample of a
couple inches showing a well centered center conductor at one end and
the other end with the center conductor at the edge of the dielectric.
I am keeping these samples as a reminder. I have had lots of coax
experience, nearly 55 years, but never had this problem before. I will
not have it again as all coax I buy with connectors installed will be
tested with the antenna meter and or TDR and ditto for coax I put
connectors on.
Prior to this experience I would have been skeptical that a jumper
passing the ohm meter test could be so weird with RF. Seeing is
believing. I hope reporting this incident may save someone some grief
should they encounter a "funny" section in a piece of coax.
Patrick NJ5G
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