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The front wheels of a car will stay in step with each other but only on a dead straight road and only if the amount of tyre wear is identical. On a bend, the outer wheel travels further and faster so it gets ahead (out of phase) of the other wheel.
Waves are similar. They can be in or out of step with each other.
Radio signal fading on frequencies below 30MHz often occurs because the signals can arrive by multiple paths. The signals, reflected off the ionosphere, do not always arrive in phase.
A similar effect can be noticed on weak VHF radio signals. In this case the very regular fading is caused by additional signals reflected off aircraft and less frequently ships. Reflections off vehicles tend to be more random and cause a fading effect called flutter.
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