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Re: Question from grade 11 student



*** Resending note of 04/30/96 09:41
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(John D. Sample) To: phys-l@atlantis.cc.uwf.edu Subject: Re: Radio in a metal
cage Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" X-Sender:
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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, andI >> >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

Here is my guess. FM receivers usually cut off when the signal gets too weak
so you don't hear a bunch of static between stations. AM receivers are usually
much cheaper in design and don't do this, but have an automatic gain control
to compensate for weaker signals. Probably in both cases the signal in the
cage is severely attenuated. In the AM case the receiver does its best to pull
in the signal however weak. The FM receiver cuts off because the signal is too
weak to bother with.

J. D. Sample (501) 698-4625
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Newer FM receivers certainly have this feature, however, I had an FM receiver
many years ago without the cutoff--the signal just got noisier and noisier.
You might look for an FM receiver without this cutoff to test this idea.