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Name: Leonoy
Status: Other
Grade: Other
Location: GA
Country: United States
Date: Summer 2009

Why is FM signal broadcast not in the lower band region (kHz) and AM in the upper band region (MHz/GHz)?

Hi Leonoy,

There are several answers to your question. The simplest is that FM (or Frequency Modulation) transmission was a later development. Thus, by the time FM radio was developed, most of the lower frequency bands were already in use by much older AM and shortwave transmissions.

It is not practical for AM and FM signals to be transmitted on the same band, due to interference of these incompatible signal types. AM transmission regulations are very old, and were created to reflect the very limited sound fidelity possible a century ago. Thus AM radio has a maximum of only 5kHz audio frequency response, which was thought to be excellent back then, but sounds rather muffled to us now. That is why AM radio does not have very good sound quality. This means that an AM radio station transmitting at (say) 1000kHz, actually uses the range of frequencies from 995kHz to 1005kHz (1000 +/-5 kHz). This limits the number of stations that will fit in the AM band.

By the time FM radio was a reality, people wanted better quality sound. Thus, FM radio stations were allowed a wider audio frequency response of up to 15kHz. This wider, and more lifelike-sounding frequency response uses up more "bandwidth" than an AM radio transmitter does.

Since technology had advanced significantly from when AM was first introduced, and transmitting and receiving at much higher frequencies was easily now possible, and since most of the lower frequency bands were already in use anyway, the answer was to assign a new higher frequency band only for FM radio. This new FM band solved the need for FM's greater bandwidth, and also it solved the interference problem that would have resulted if FM stations transmitted at frequencies closer to the AM band.


Bob Wilson


I have been thinking about this for some time.

I believe the reason why AM is in the lower band, the Medium Frequency (MF) frequency band in Thousands of KHz, is because AM is the simplest modulation technique and only MF oscillators were available for superheterodyning so it was invented first.

When technology was sophisticated enough to invent FM, Very High Frequency (VHF) oscillators were available (10s of MHz) and it was decided to broadcast FM signals at the higher frequency for a higher quality sound.

And it is hard to change a practice once it is in service.

It will take more research to ferret out the details, but this seems the most plausible reason.

Sincere regards,
Mike Stewart

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