Radio Frequencies

I never understand when a radio comes in and out of the feed. There are many times when the radio station works fine. Then I'll be stopped at a red light and the station will lose its frequency and go to another station. Then when I start driving again, it will go back to the first station. It makes no sense to me. It has happened a lot since I moved down here. I guess that will be one of life's many unsolvable mysteries.