MadSci Network: Physics |
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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