[HCARC] Antennas, Radios and Elmering

[Kerry Sandstrom]

Gary and all,

The losses in coaxial cable come from at least 3 causes.  One is loss do to 
the resistance of copper, aluminum, and or steel/iron.  Two is leakage 
through the shield, outer conductor of the coax.  Three is loss in the 
dielectric.  The dominant source in coaxial cable that hams are apt to use 
is dielectric losses.

RF power is transported in the electromagnetic fields in the cable. It 
doesn't travel down a wire like DC does.  in addition, depending on 
frequency, the RF fields are only in a thin region of the conductors.  The 
better the conductor the thinner the region.  The ways that manufacturers 
reduce the 'copper' losses is by plating a thin layer of silver on the 
copper wires.  The way thay they reduce dielectric losses is by using air as 
the dielectric or foam instead of solid polyethylene.  The way that they 
reduce leakage is by using a solid aluminum tube for the outer conductor or 
by using two layers of braid for the outer conductor or a sparse layer of 
braid and aluminum foil.  At low HF, any coaxial cable is useful and RG8/U 
and its relatives is the most common.  If you want to really hear about 
cable, talk to Chuck, KA1PM who used to work for Times Wire and Cable.  He 
can fill your head on the subject.

There are a couple things you need to know about cable.  The RG, Radio 
Guide, by the way, were defined by mil spec.  An RG designation was assigned 
and the physical dimensions, electrical properties and the materials were 
defined.  The RG standard includes waveguide, incidently.  This seemed to 
work for years until some of the manufacturers started cheating.  The 
government's approach to solving the problem was to define a new family of 
cables.  You will see these as MIL C17-XXXXXXX (RGYY/U).  I don't know who 
the culprits were, but Radio Shack/Tandy started selling RG cable a few 
years ago that had about half the copper that real cable had.  Now often you 
will see cable that is "RG8 Type". What this seems to mean is it has the 
basic dimensions of RG8/U but your guess is as good as mine on the rest. 
This is important in at least one area.  Coaxial cable gets exposed to the 
weather and particularly sunlight.  As I sure most of you realize, sunlight, 
particularly the ultra violet component destroys a lot of plastic.  Some 
years ago the cable companies started using a plastic jacket material called 
non-contaminating PVC.  This material has much less of a problem than the 
older PVC jacket material.  What happens is the black material, probably 
some form of carbon, migrates through the braid and contaminates the 
dielectric material which dramatically increases the cable losses.  Some of 
the RG standards called for non-contaminating jackets and all the MIL C17 
standards do.  Recommendation:  buy your cable from a recognized source and 
make sure you understand what you're getting.

By the way the "big boys", not hams, use stuff like RG-373/U which is 1/2" 
cable with teflon dielectric and jacket and silvered center conductor and 
double braid.  I think it runs around $15 a foot now.  in the old days 
VHF/UHF guys coveted RG-214/U which was the same thing with Polyethylene 
instead of teflon.  The common braids are tinned copper, copper and 
silvered.  Tinned copper is primarily very low frequency cable.  Copper 
braid is standard for HF and VHF while silvered is for UHF and microwaves.

That is probably more than any one wanted to know, but you ought to be at 
least exposed to it.  Yes, there is a reason for everything.

Kerry