Question about nighttime AM reception

[radiojay1]

Can someone please tell what this is and why it happens to distant AM signals at night? I notice that the station will be clear, then it will become louder and distorted and then become clear again. Also on some stations the signal will fade out completely for a few seconds or minutes and come back clear again, without the distortion.

 

[KeithE4]

Can someone please tell what this is and why it happens to distant AM signals at night? I notice that the station will be clear, then it will become louder and distorted and then become clear again. Also on some stations the signal will fade out completely for a few seconds or minutes and come back clear again, without the distortion.

Two words: The ionosphere.

Specifically, the "D" layer, which is the lowest layer of the ionosphere, about 50 miles above the Earth. It absorbs radio signals during the day and almost totally disappears at night to allow them to travel longer distances. It affects radio signals below 10 MHz.

 

[radio-outsider]

If you are at the location where you get both groundwave and skywave at night at near equal strength, you will get phase distortion as the skywave path flucuates in length, at first being "in phase" with the groundwave, and then shifting to "out of phase". Years ago WLW lopped of the top of their tower to minimize phase distortion in Indianapolis and Columbus. Living in Indianapolis, WLS was a decent groundwave signal in the day, but the phase distortion made nighttime listening aggravating.