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Old 21st Mar 2019, 5:08 am   #505
Radio Wrangler
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Default Re: Audiophoolery?

It looks like wikipedia reflects common usage rather than

Faraday invented his screen to demonstrate magnetic coupling between two coils was not capacitive as some others had suggested. The whole point was that it segregated electric and magnetic fields.

While Maxwell's equations show that you cannot, for alternating fields, affect one without affecting the other, it is quite possible to have a significant imbalance in the relative strengths of the two components in the near field of an object.

It is possible to make E-field and H-field probes which measure each component individually.

It is possible to generate one field disproportionately more strongly than the other, at least in the near-field.

An electromagnetic wave-pair comes winging along in free space, many lambda from its source. The two waves are in equilibrium, each inducing into the other, settled at 377 volts per metre for every ampere per metre.

It hits a Faraday shield which shunts the electric field component. On the far side of the shield, there is very little electric field wave, but the magnetic wave continues unabated. As the magnetic wave moves further beyond the shield, it induces a growing electric wave accompanying it, and that wave strts inducing back into the magnetic wave until the pair balance in equilibrium with 377 (V/m)/(A/m)

On the input side of the shield the shunted electric wave is reflected with phase reversal, and as the electric wave travels, it similarly induces a magnetic wave and they too eventually in the far field of the screen settle as an equilibrium pair

Seen in the far-field, Faraday screens don't look like screens at all. In the close field they do something useful, principally to electric field components.

A lot of the things we do are in the near field.

If you're building a sensitive receiver, then you want a proper screening box to put it in, not a Faraday screen.

If you're building a transformer, a Faraday screen between windings may be rather desirable to block capacitive coupling of rubbish on the mains, but you don't want a magnetic screen, that would ruin the transformer operation.

It is very advantageous to have separate names for general and field-specific screens.

David
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