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Phase

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Phase is a measure of the degree to which two waves or rotating systems are in step with each other.
Phase is measured in degrees or sometimes radians.

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.

If two waves are in step, they add together (constructive interference).
If the waves are out of step, they cancel each other out (destructive interference).

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.