It’s due to “multipath interference”. On its way to your radio antenna, a radio wave can also bounce off various things (buildings, trees, cars, etc.). The reflected versions of these multiple paths take a little longer to get to your antenna than the most direct one, so they are out of phase with the original. (The peaks and troughs don’t line up.) This can cause destructive interference to occur, degrading the radio signal. (It can also result in *constructive* interference.)
If you move your car just a little bit, you change which reflections are hitting your antenna, and the destructive interference can be increased or reduced.
It is due to multipath interference. Radio waves bounce off metallic objects like other cars, buildings, sign poles, etc. If they arrive at your car antenna out of phase with the direct (not reflected) radio signal, they will cancel it out. A few inches can move you into a location where they no longer cancel.
interference. radio is transmitted by electromagnetic wave, which gets reflected by all kind of obstacles, especially buildings. When two waves at different angles meet, they create an “interference pattern”, at some points the wave will be strong, at some other points it will be weak.
Your car is constantly moving through those interference patterns so you don’t notice it much (merely by short noise bursts), but when it stops you may stand on some location where it will be significantly weaker. You can roll your car a few cm to catch a better spot.
This is because the wavelength of FM radio is right around 3m depending on the frequency. So as you move up the antenna is able to receive the final again.