A police car radio experiances unusual interferance while driving, but not when parked.?

this is for school so try and make it so i could under stand the answer.

Assuming the interference is not due to a loose connection or some other loose component inside the radio, the biggest source of interference in mobile radios is due to "multi-path" signals reaching the receiver, or vice-versa (from the mobile radio to the base station).

Multipath is a phenomenon that is caused by two or more signal paths between transmitter and receiver. For example, if one signal reaches the receiver directly, and another one (or more) is reflected off a building or other large metal object, the two signal carriers can arrive at the receiver "out-of-phase". If the phase-shift between the two is just right, the reflected signal can subtract from the energy the receiving antenna collects.

Another problem has to do with low-signal strength zones, where a large building, mountain or hill blocks the transmitted signal.

You can read more about this by searching on "multipath" and/or "fading" - it is the subject of considerable research and engineering work, as one of the main complaints about cell phones (dropped calls) has to do with fading.
interference from the motors high tension ignition system maybe?

When the motor is turned off that source of interference would stop.
Another possible source of interference to the radio is from the vehicle's alternator. A test of whether the alternator or ignition is interfering with the radio is to see if the frequency of any noise from the radio changes in pitch with engine revs.

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