Could someone explain to me why amplitude modulated radio stations transmit further distances in the evening? I have read somewhere that there is a change in the atmosphere that allows this at night time

 

My guess would be that there is no electromagnetic interferrence from the sun but i'm unsure of the specifics of how this really works.

Why doesn't it affect FM transmissions?

 

Cheers.

I did look into this once, trying to dredge up the knowledge from my memory.

Part of it is that the sun is quite loud in shortwave/am frequencies. Also FM is quite directional, so you can get higher gain antennas more easily and/or align your receiving arial with the polarization of the transmitting one.

But more significant has something to do with the ionization of the ionosphere.

 

Edit: I recall after a bit of help from the internet. This goes into some detail http://radiojove.gsfc.nasa.gov/education/educ/radio/tran-rec/exerc/iono.htm

 

TLDR version is the more free electrons there are, the more reflective the ionosphere is (free electrons are what makes metal shiny). During the day there is more ionizing radiation knocking electrons off of their atoms. This results in the atmosphere being more reflective.

FM isn't effected because it travels fairly close to line of sight.

AM and shortwave will bounce off of the ionosphere as they travel (which is why you can pick them up from much further away).

Edited December 9, 201114 yr by Schrödinger's hat