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[TowerTalk] Coax Loss Mechanisms

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Subject: [TowerTalk] Coax Loss Mechanisms
From: (Pat Barthelow)
Date: Thu, 25 Jun 1998 13:40:49 -0700 (PDT)

After discovering badly corroded shield inside an old RG-213 cable at the
old Ft. Ord Army MARS station, I replaced it with available 3/4" CATV
coax.  The 400 foot run terminated at our new 4 element long boom, 20m
Yagi at the top of the hill.  Even with the old RG-213, that antenna is
the sweetest antenna I had the luxury of owning and using regularly.  The
long boom, monoband design, 65 ft, and hilltop combines so well...When I
replaced the coax, I was surprised to NOT see dramatic improvements in
hearing. Some, but, not dramatic.  The coax shield was BLACK with
corrosion, and may have been only due to water intrusion (I found a splice
had been penetrated by water, even though it was taped fairly well.  On
the other hand, the braid may have been corroded due to contamination by
chemicals leaching from the jacket.  The cable was old, evidenced by the
dull, chalky black color of the exposed jacket.  The dielectric was foam.
I thought though, that copper antenna wire, corroded green for decades of
exposure, only has insignificant loss, as the additional fractions of an
ohm due to corrosion is small compared to the antenna radiation impedance.
        The question is, is corroded shield in old coax responsible for
significant additional loss, or is the loss created by moisture captured
by the porosity of the foam dielectric, or by lossy chemicals leached into
the dielectric from the jacket?  What is the mechanism for additional loss
in old, contaminated, or moisture penetrated coax? 

73, DX de Pat, AA6EG/N6IJ 
"The Contest Station from the Government"

Marina Amateur Radio Contest Station; N6IJ
599 DX Drive
Marina CA 93933

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