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Old 21st Mar 2019, 10:51 am   #509
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 3,119
Default Re: Audiophoolery?

Originally Posted by Radio Wrangler View Post

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. ...
Yes, this is exactly right.

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.
Even in the far-field they look like screens in the amplitude sense. As you say, there's a back-reflected wave outside the box and conservation of energy says it's taken its power from the incoming wave, leaving less to propagate beyond the screen.

A lot of the things we do are in the near field.
When I was building these I was concerned with GHz radio waves and 10-100 nanosecond pulses and the screened rooms were quite large (~10m scale) so I wasn't always in the near-field. But the cage shown on that vibration table certainly would have been NF inside. We lined the room with copper foil but we never thought the idea of using mumetal as well, or even a cheaper high-permability material, was realistic. I don't know if anyone does. Given that the building contained a particle accelerator I know that they used non-magnetic steel to reinforce the concrete though !

If you're building a sensitive receiver, then you want a proper screening box to put it in, not a Faraday screen.
Presumably that does have something to redirect the magnetic field too (shame there are no free monopoles to short it out ).


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