Explanation/Reference:
Explanation:
If a signal is received twice at exactly the same time, the secondary wave adds its power to the primary wave, so the receiver gets twice the positive energy (positive crest) at the same instant, then twice the negative energy (negative crest) at the same instant. The result is that both waves add up to twice the amplitude (energy) of a single wave, and both signals are said to be in phase (both signals are said to have an angle of 0 degrees). This rare condition is called upfade. If the second signal negative energy (negative crest) reaches the receiver just when the first signal positive energy (positive crest) also reaches the receiver, both signals can cancel each other, resulting in no signal at all (this is the principle used in noise cancellation headsets).