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Re: Radio in a metal cage





On Tue, 30 Apr 1996 dcruz@t1acc1.intel.com wrote:

Being an electronics tech I'll venture on an explanation.

I think in the FM radio the receiver receives the electric field portion
of the radio (electromagnetic) wave. That's why when you closed the
non-magnetic metal cage you put the FM radio in a Faraday shield. You
shielded the RF section from receiving the electric field of the FM
signal.

On the AM radio: They usually use ferite rod antennas. The ferite rod
antenna picks up the magnetic field portion of the AM signal. Try an iron
wire cage and the AM signal will be shielded out.

Regards, Dave Cruz <DCRUZ@T1ACC1.INTEL.COM>

Last week I did this demo to my pupils: I built a non-magnetic metal cage for
a radio. Kids loved to hear its music vanishing when I closed it. That was OK
for FM.

Suddenly one kid asked me to try it again with AM. I thought it would make no
difference at all, but OOPS!!!, the music did not vanished completely, and I
could not explain the reason...

Does anybody know it?

PS. The cage is made of aluminium wire mesh.(aprox. 4mm side squares)

I have to suspect that you have a scale problem. I have not done any
research, but I would suspect that the relevant quantity would be the
size of the box divided by the wavelength of the radiation. In other
words, I would suspect that the effect is more pronounced when the
wavelength of the signals is not too different from the size of the box.

If you have ever driven a car onto the roadway of a bridge that has steel
overhead with the radio on you can hear that there is a decrease in the
volume--even with AM. The bridge size is more nearly comparable to the
wavelength than your box.

Actually I'm just guessing here. I haven't looked up any radio
antenna-propagation theory.
Barlow Newbolt