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Re: [RFI] shielding question

To: "Jim Brown" <jimbrown.enteract@rcn.com>,"RFI List" <rfi@contesting.com>
Subject: Re: [RFI] shielding question
From: "Tom Rauch" <w8ji@contesting.com>
Reply-to: Tom Rauch <w8ji@contesting.com>
Date: Mon, 22 Dec 2003 18:40:29 -0500
List-post: <mailto:rfi@contesting.com>
I think K1TTT just needs to ground all the shields at the active electronics
component and make the common ground point for the equipment the power line
safety ground at the outlet strip where power is distributed.

I do that here by using a power strip with CATV fittings that are sold as
"surge protectors" and have the CATV fittings bonded internally to the
safety ground.

I add  .01uF VDE/UL/CSA 250VAC line-bypass type capacitors from each power
line terminal to the safety ground inside the outlet strip. Everything
connects to this common ground point.

This technique alone is many times more effective than anything else you can
do, because ALL RF currents can loop across without going into sensitive
devices. It also prevents or greatly reduces noise egress.


I think his choice of a shielded cable is excellent, although it may not be

Even with a drain wire inside, I've found every foil covered cable I have
tested to be a very good RF shield at medium to VHF frequencies. There can
be some shield overlay problems that hurt UHF performance, but I've never
seen a foil overlay problem at VHF or lower.

Skin effect pretty much keeps all common mode currents on the outside
surface of the foil. The wires inside never enter the equation. It is only
at low frequencies (where the foil is not several skin depths thick) where
current on the drain wire might become an issue.

At *very* high frequencies (where the inductance of the shield wrap and
stray C from not having a solid bond between foil overlays) can cause a
resonant effect that makes the shield ineffective.

I can't imagine any type of foil shield being bad or useless at HF, drain
wire inside or not.

> And even if that receiver had a 2-prong plug, the path from the
> receiver to earth is likely to be some significant fraction of a
> wavelength away from earth. If it happens to be some odd multiple of a
> quarter wave at some frequency, it will look pretty much like an open
> circuit at that frequency.

Only in a lossless system with a zero-ohm end termination.  Once the wire or
ground has loss, or if it has multiple paths, the impedance can be far from
what we imagine and a length change often won't make a huge difference.

All that aside, what do we care anyway? As Jim points out it is only the
shield's bond to the system ground that is important.

> The principal reason for that 3-prong plug is to provide a safety
> ground for the equipment, both to prevent a shock hazard, and to
> provide a return path so that current faults to blow a fuse.

It certainly is a safety ground only! It has nothing predictable or special
to do with RF grounding.

73 Tom

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