Why AM & FM Broadcasts at Certain Frequencies?

[fran1942]

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.

 

[rcgldr]

I think it's mostly historical, not technical. Wiki articles:

http://en.wikipedia.org/wiki/AM_broadcasting

http://en.wikipedia.org/wiki/FM_broadcasting

 

[NascentOxygen]

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.

 

[RoshanBBQ]

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.