AM and FM Frequency Selection
Country: United States
Date: Summer 2009
Why is FM signal broadcast not in the
lower band region (kHz) and AM in the upper band
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
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.
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.
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Update: June 2012