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Re: [RFI] shielding answer

To: "Rfi" <rfi@contesting.com>
Subject: Re: [RFI] shielding answer
From: "Tom Rauch" <w8ji@contesting.com>
Reply-to: Tom Rauch <w8ji@contesting.com>
Date: Wed, 24 Dec 2003 16:23:28 -0500
List-post: <mailto:rfi@contesting.com>
> > Was your shielded speaker cable also twisted pair, or was it parallel
> > conductors?  It is well known that cable shields provide relatively
> > little magnetic shielding (Ott says almost none except in narrow
> > frequency ranges), but does provide shielding to the electric field.
> > Most LF interference is magnetically coupled, while the E-field
> becomes
> > more important as frequency increases.

I'm trying to follow this, but I have a problem.

When a time-varying electric field is taken to zero, so it the magnetic
field. When a time-varying magnetic field is taken to zero, so is the
electric field.

The idea we can "shield" or filter one field and not the other is the basis
of all sorts of antenna hokey-pokey and voo-doo, like the CFA or E-H antenna
or "magnetic loops" that only respond to magnetic fields and thus somehow
magically filter noise from signals that are on the same frequency.

Of course we can alter field impedance near the conductor (we still can't
reduce any particular field to zero or the other goes to zero), but even at
close distances things quickly change.

For example a small loop has a low field impedance near the antenna
(shielded or not) but 1/10th wl away a small "magnetic" loop actually
becomes electric field dominant. A magnetic loop is only "magnetic" within
20 feet or so on 80 meters, and it still has a strong electric field close
to the antenna near the point where the capacitor is placed! Put a shield
over it, and the shield has the same strong electric field across the shield
gap. Take that field to zero and the loop goes stone dead for either
magnetic or electric fields.

Conversely a small e-field probe becomes magnetic field dominant at the same
distance. Take the time-varying magnetic field near the antenna to zero and
it stops working also.

At a larger fraction of a wavelength away, the field impedance of any source
or sensitivity of any probe would be essentially equal.

73 Tom

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