Jim, if the braid and foil were in poor contact, they would constitute a
very low impedance transmission line with the foil acting as center conductor
and braid as outer conductor.
After a very short distance any rf on the braid would be closely coupled to
the foil where it would then flow to the open end of the foil and around
the end into the inner surface of the foil.
With partial contact between the braid and foil, the rf path would be a
combination of dc shorts between the two shields and the transmission line
effect. The rf would make it's way to the inner surface pretty easily in
either case. There could be a small difference in electrical length when
corrosion causes the d.c. shorts to degrade. I have actually seen this happen
in
432 MHz phasing lines due to water ingress corrosion between the braid and
foil. Water is not our friend!
73,
Gerald K5GW
In a message dated 7/17/2010 11:15:19 A.M. Central Daylight Time,
jim@audiosystemsgroup.com writes:
On Sat, 17 Jul 2010 11:45:43 -0400, Joe Subich, W4TV wrote:
>Connector installation techniques should always insure there is
>an adequate RF connection between the connector body and the
>INSIDE of the inner shield.
Interesting. Let's say I'm installing an Amphenol PL-259 on
something like LMR400 or Commscope 3227 (similar shield
construction). The connector is a tight screw-on fit to the
jacket, the braid and foil continue all the way to the point where
they butt into the hole for the center, and I solder all four
holes carefully, with the foil and braid (hopefully) in contact
under the solder. Would that make an adequate RF connection? I'd
guess that the answer depends on the frequency of interest. Or
would only a crimp-type connector be capable of an adequate
connection?
73, Jim K9YC
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