MadSci Network: Physics
Query:

Re: Why does my walkman reception change on the bus?

Date: Sat Dec 20 11:40:54 2003
Posted By: Madhu Siddalingaiah, Physicist, author, consultant
Area of science: Physics
ID: 1070085694.Ph
Message:

Hi Gavin,

That's an interesting question.

It sounds to me like there is some kind of transmitter operating at the
same frequency as the station you are listening to on your walkman. There
are strict regulations on the frequencies that can be used for radio
transmission. The FM broadcast band covers the frequencies from 88-108MHz
in most countries. Since this is a widely used band, it is unlikely
anything was intentionally designed to operate within that band.

However, there are sources which could cause interference within the FM
band. There are two possibilities that come to mind. First, in the US,
there is an unlicensed band at roughly 49MHz. This band is typically used
for remote control toys, walkie-talkies, even baby monitors and inexpensive
cordless phones. So how is it that a 49MHz transmitter could interfere with
FM at 100MHz? Even though a transmitter may operate at 49MHz, it can also
produce spurious transmissions at harmonics or multiples of the fundamental
frequency. This means there is some radiation at 98MHz, 157MHz and so. The
power at those harmonics is rather small, but it is there.

Given some small radiation at within the FM band, how can it overpower a
large broadcast station? The answer is that the broadcast station antenna
is far away, probably much farther than the baby monitor or other 49MHz
transmitter that might be operating in a house near one of the bus stops.
The closer you are to the baby monitor, the more likely some interference
will occurr.

The other possibility of interference is a low power intentional
transmitter operating within the FM band. Again, in the US, unlicensed very
low power transmitters can operate within the FM band as long as they meet
strict guidelines. Have you heard of a toy called "Mr. Microphone"? It's a
low power transmitter operating within the FM band. You can receive its
transmission using any FM radio, but it's only useful within 100 feet or less.

What you might do is perform a simple experiment to determine the source of
interference. When you get to a stop where interference occurrs, take note
of the frequency you are listening to. Retune your walkman to see if there
are other frequencies where interference occurrs. If it's only one
frequency, then the likely suspects are transmitters at that frequency or
integer fractions of that frequency. If you find that interference
occurrs on several frequencies, there might be some equipment
malfunctioning near a bus stop that is producing broadband radiation.

If you find the interference really annoying, contact your government
agency responsible for regulating radio communications. In the US, it's the
Federal Communications Commission (FCC). That agency will determine if the
interference is within legal limits, and if it's not, they will locate the
source and make certain it is eliminated or reduced to within legal limits.

I hope this answers your question and good luck!



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