Volvo 240 1991 Owner's Manual page 46

Volvo 1991 240 Model
Because of the limited range of the U (FM) senders and the fact that these waves are very reflective this
problem usually occurs with U (FM) reception. If the sender is blocked by buildings or mountains
distortion can result.
Distortion
The reason why U (FM) but not M/L (AM) is audible in covered parking areas, under bridges etc. is that
U (FM) signals reflect against solid objects such as buildings. Because these waves are very reflective
multi-path distortion can result. This distortion is the result of the reflected signal and the direct signal
reaching your antenna at slightly different times causing a cancellation of all signals. This problem
occurs largely in built-up areas.
pg. 34 Cross modulation, reasons for distortion
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