Re: [SI-LIST] : Cable Shield Coverage

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From: Doug McKean (dmckean@corp.auspex.com)
Date: Fri Jul 21 2000 - 12:17:33 PDT


Peter Baxter wrote:
>
> Hi,
>
> Most round coaxial cables have 100% shield coverage to reduce EMI. The
> shield also carries the return current.
>
> Now imagine a flat ribbon cable with 3V 100MHz digital signals going
> down. The cable is organised ground/signal/ground.
>
> Wrapped around the flat ribbon cable is a "faraday cage" shield. Is is
> grounded to mains earth, at one end only. No digital signal current
> flows through it.
>
> Can this faraday cage shield have, say 80% coverage, (crosshatch,
> interwoven) and still contain RF emissions, or will is effectiveness be
> reduced dramatically?

Any type of pig tail connection (where the center
conductor is exposed from the shield) from the coax
to the associated circuitry will be your weakest link.

Take one end of ANY coax cable, hook it up to a
signal generator or RF output of a spectrum analyzer.
Cut the center conductor of the other end clean flat
with the shield. You *should* get no radiation.

As the shield is stripped back from the center conductor,
more and more radiation will result. I.e., a 2 inch long
exposed center conductor will have roughly twice as much
radiation as a 1 inch long stub. The exposed stub is
attempting to use the outside of the conductor as a
ground plane in a crude form of an antenna.

If this is in fact the construction you described, be
very careful how much shield is stripped back exposing
the ribbon cable (electrically speaking) for whatever
connection being done. Otherwise, it won't matter if
there's 80% coverage or 100% coverage.

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