[hpsdr] [Hpsdr] Call for Comments - ALEX

[Chris Albertson]

--- Ray J <ray at w9ray.org> wrote:

> ***** High Performance Software Defined Radio Discussion List *****
> 
> Yes I also fail to see how a non magnetic type material
> (aluminum)would 
> shield in/out magnetic fields...

Short answer:  Because E and M fields are coupled.

Actually what happens is not "blocking".  A new magnetic
field is created that is 180 degrees out of phase and
it cancels.

Longer answer....

First off, if the magnetic field is static, that is, it does not
change with time then you are right.  But a static field has
no effect on a radio pre-selector so we don't need to shield
out a static field.  Static magnetic fields are caused by things
like refrigerator magnets and the Earth's magnetic field or
maybe by a large nearby motor

If the magnetic field is non-static or varying with time and has
a frequency up in the RF range then it is coupled with an eletric
field.  Maxwell proved this a long time ago.  So if you believe
the M and E fields are coupled, then what happens is the E field
causes a current in the aluminum walls of the enclosure.  We all
know that a current will generate a magnetic field.  But we are in
luck because the generated magnetic field is exactly opposite of
the M field that is always coupled to the E field so inside the
box the two cancel. Faraday was the first to notice this effect
and we name this a "Faraday Cage" after him.

In the real world this is not perfect. The box has some electrical
resistance and it may have holes in it and so on.  But at RF
frequencies you get hundreds of db of attenuation.

A speaker enclosure cannot be a Faraday Cage.  If it were then
the sound could not get out. But speaker enclosures solve a different
problem.  The magnets in the speaksr can effect a nearby CRT so
they DO want to shield a static magnetic
field so you can put the speaker next to a CRT.  We don't care about
static fields because magnets have no effect on radios.
We don't need to shield the radio from strong light or xrays either.

OK there are some patological ways strong light could effect a
glass bead diode and xrays can zap electronics and a strong magnet
right on a small signal toroid could saturate it but these are
way out in left feild and unlikely to ever hapen except in a
lab test or science experiment



Chris Albertson
  Home:   310-376-1029  chrisalbertson90278 at yahoo.com
  Office: 310-336-5189  Christopher.J.Albertson at aero.org
  KG6OMK/AG


       
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