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Re: Radio waves...



On Tue, 11 Mar 1997, Dwight K. Souder wrote:

Greetings everyone! A student asked a question about radio waves
that I wasn't to sure about. The students were figuring out the
wavelengths and frequency of their favorite radio stations. A student
asked if the wavelengths remain the same, then how is the signal
transmitted so that it includes the information that we hear.
The way I understand the question is that, for example, when
people talk, we fluctuate the wavelengths and amplitude in such a way
that it forms the words we hear. If a DJ is talking (or music is
playing), what would the radio waves look like so that they transmit the
sounds.
My response to the student was that I believe the amplitudes my
fluctuate which is responsible for transmitting the signal, but I would
check. Am I right or wrong? Any help would be appreciated.

The trick is that the radio station does not have a licence for a precise
frequency, but for a frequency band. That is all the frequencies between
two values. This strikes me as a good place to lay out some of the ideas
they will eventually use if they ever study fourier analysis.

Only an infinitely long wave has a precise frequency. Anything else is
made up of a superposition of many waves of different frequencies. If you
have a long "wavetrain", that is one which is many wavelengths long, the
breakdown into frequency components shows up as a narrow spike whose width
is the "bandwidth". This is the case for radio transmissions. If you are
in 100's of kHz or higher, and the changes associated with audible sounds
are at most about 10 kHz, then the radio station can get by with a
frequency band in this general range (I don't have FCC rules in front of
me to look up typical values).

Two general classes of radio transmission are amplitude modulation and
frequency modulation. In the former, you transmit a particular frequency
and vary the amplitude with the amplitude of the sound. In the latter you
keep the amplitude constant and vary the frequency with the amplitude of
the sound. In either case, the ballpark of the frequency associated with
the changes in the signal give the ballpark of the bandwidth the station
is using.

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| Doug Craigen |
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| If you think Physics is no laughing matter, think again .... |
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