If a radio transmission follows two or more different paths during propagation, the received signal may degrade due to fading. What other type of degradation can occur?

Multipath propagation occurs when a transmitted signal bounces off various layers of the ionosphere or physical objects, taking two or more different paths to reach the receiver. Because these paths have different lengths, the signals arrive at slightly different times. Radio waves have a specific phase, which is a measurement of where the wave is in its alternating cycle. When signals arrive at different times, they arrive out of phase. As the receiver combines these out-of-phase signals, it results in phase distortion, which can make voice signals sound hollow, mushy, or completely distorted. This multipath delay does not change the actual frequency of the signal (like a Doppler shift would), nor does it increase the background noise floor. The degradation is strictly related to the timing and phase of the arriving wave fronts.