Something else possibly interesting

I use to work for a company that amongst other things installed mobile
phone base systems. One of the mandatory tests was to sweep the coax
from the TX end after it had been terminated to the antenna. Return loss
(SWR) was the major criteria for the test pass but it also showed the
location of the impedance discontinuity. (ie in metres from the
feedpoint) In most cases this was a cable indentation where it wasnt
unwound from the drum carefully, or where the coiled stuff had been laid
down flat instead of being kept vertical. (ie was distorted by its own
weight) We did have a few cases however where the inside of the inner
core had slight amounts of corrosion causing the problem. This was
commonly LDF4-50 or LDF5-50 (foam coax a little less and a little more
than 1" dia and had an inner core "pipe" with an inside air space.

Cheers Bob


Sal M. Onella wrote:

Last year I had a case of a Navy shipboard satcom uplink transmission line
that failed its sweep test at one spot near the upper end of the frequency
range ... around 8 GHz. We had to replace it. (AN/WSC-6, for those who know
what that is.) We had the contractor "shoot" it three different times with
different test equipment each time -- always the same. The new line was
fine. Until then, I had never seen a defective cable that was quite so
frequency sensitive!