> Date: Fri, 10 Apr 1998 09:51:45 -0400 (EDT)
> From: Kc4zum <Kc4zum@aol.com>
This might be a good time to point something important out.
> BTW, my grounding now is RG-8 coax with a .01 mfd disc capacitor on each end,
The coax ground with the .01 caps, incorrectly described as a
"shielded ground", was published years ago. Unfortunately many
things that make it into print, even when totally incorrect, take on
a life of their own. Some well known wire suppliers even sold kits
for "shielded grounds", and not just on April 1..
Despite the published articles, coaxial "shielded" grounds are
impossible to make. Impossible because current in a ground lead
is, by definition, common mode current. Common mode high frequency
(or high slew rate currents, like lightning) flow over the OUTSIDE of
the shield only.
The center conductor, if common mode current is above VLF, does next
to nothing. Virtually all RF current (even lightning) travels over
the outside of the shield. You might as well toss the center
conductor in the trash for all the good it does.
Now here's the real "rub".
Any woven or braided conductor has much higher RF resistance and
much lower current handling capability than a smooth conductor. The
rapidly changing current is forced to the outer surface (skin
effect), where it flows between the thousands of overlays where the
wire weaves in and out. The connection along the surface is not only
longer because of all the hills and valleys, it's also through
hundreds and thousands of tiny pressure connections. Even a low angle
weave braid (with wires that closely parallel the length of the
cable) has several times the RF resistance of a similar sized and
shaped smooth conductor!
If you try to use braiding (even from RG-8 cable) as a ten meter tank
lead in an high power amplifier, you'll generally find it gets hot and
often even MELTS. Yet a #10 solid wire, even though much smaller than
the braid, will run cool.
If you examine the lowest loss cables for a given diameter and
dielectric, you'll see the lowest loss cables all use smooth
The "way" RF current flows through the braid is the reason woven
braid cables have such a dramatic increase in loss if the braid
becomes tarnished. It also explains why open wire lines, even
when green with corrosion, still provide good performance (unless
they use woven conductors) and why coax with woven shielding has
significant loss increase with age and contamination...even if the
dielectric has no moisture ingress. (We often incorrect attribute the
increased loss to changes in the dielectric, but that is largely
NEVER use woven or braided connections if low resistance,
minimum loss, and maximum current handling is important.
73, Tom W8JI
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