Why AM & FM Broadcasts at Certain Frequencies?

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Discussion Overview

The discussion revolves around the reasons behind the specific frequency allocations for AM and FM broadcasting, particularly from a noise perspective. Participants explore historical, technical, and practical considerations influencing these choices.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant suggests that the choice of AM frequencies is primarily historical rather than technical, referencing external sources for further reading.
  • Another participant proposes that AM frequencies were selected to take advantage of the extended over-the-horizon range due to bending and reflections, while also considering antenna size.
  • A different viewpoint indicates that FM broadcasting operates in a frequency range that offers a better signal-to-noise ratio, suggesting that its placement in higher noise environments is intentional to leverage its resilience against noise.
  • It is noted that AM frequencies are situated in a noise dip, which may relate to its lower resilience against noise compared to FM.

Areas of Agreement / Disagreement

Participants express differing views on the reasons for frequency selection, with some attributing it to historical factors and others to technical compromises related to noise characteristics. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Participants do not fully explore the assumptions behind their claims, such as the specific definitions of noise or the implications of frequency selection on broadcast quality.

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.
 

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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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