[Antennas] Question on distorting coax shape

Wed Dec 19 19:46:40 EST 2012

Back at the Jerrold Electronics School for CATV engineers (many years 
ago), they taught us that one cable discontinuity seldom caused a 
problem unless it was severe. This assumes the source and load is 
fairly well matched to begin with.
We were taught it generally takes at least two physical or manufactured 
discontinuities to cause what is called a structural return loss 
problem. This would normally would cause a narrow band mis-match as a 
function of the distance between the two mis-matches and the velocity 
factor of the cable. This often revealed itself as a "suck out", or 
higher losses at a particular frequency or harmonics of that frequency.
 In other words, in our AR narrow band applications it might not cause 
an issue at all. The problem could be analyzed or characterized by a 
return loss measurement, usually requiring a broadband sweep generator 
and a good return loss bridge. I made many such measurements and still 
have my old KDBV-75 75 ohm bridge.
It seems if you could get access at both ends, you could simply check 
the resultant cable losses at your frequencies(s) of interest with the 
manufacturers specs.
73,
Ray, W4BYG
VSWR as aOn 12/19/2012 6:58 PM, John Hensley wrote:
 I'm looking for some test data, preferably a chart or two, which might 
show how the technical specifications of coax (impedance, frequency 
efficiency, et al) are affected when the coax shape is deformed, e.g. as 
the coax cross sectional shape is distorted or changed. For example, 
imagine a worker dropping coax down from a tower and, using a tie wrap 
gun, over tightens the tie wrap so that the coax is squeezed out of 
shape. Or, imagine the coax is flattened at a point because of some 
other event. QSO via private or list email is okay. Thanks, John W5JV 
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