I think the reason is less interesting: when you start to go out of range, your radio thinks there is only a mono signal, so it switches to mono mode. Same rea

I think the reason is less interesting: when you start to go out of range, your radio thinks there is only a mono signal, so it switches to mono mode. Same reason "static" on your TV is black and white. Your TV didn't detect a color signal, so it is looking for the black and white one. (For anyone old enough to remember static on their tv :)