Why do AM and FM broadcasts use their assigned frequency ranges?

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fran1942
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Hello, looking at the attached graph of noise voltage to frequency, I am trying to understand (from a noise point of view) why AM and FM broadcasts at the frequencies they do ?

Can anyone offer any suggestions please ?

Thank you.
 
Attachments
  • Log-log plot of noise voltage Vn against frequency F(Hz), showing 1/f noise falling at low frequencies, a flat thermal and shot noise floor around 1k to 10M, and transit time noise rising at high frequencies. Shaded bands mark the AM broadcast range (521 to 1612 kHz) and the FM broadcast range (88 to 108 MHz).
    graph.jpg
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Discussion

Everything is a compromise. AM frequencies were probably chosen to exploit the extended over the horizon range provided by bending and reflections, while keeping to a manageable antenna size.
 
FM has a better signal to noise ratio, so it looks like it is in a place with higher noise (to make use of its resilience toward noise -- it would be a waste to use it between 1k to 10M). AM seems situated in the dip since it isn't particularly resilient against noise.
 

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