A radio transmission may follow two or more different paths during propagation, and this may result in phase differences at the receiver. What is the effect at the receiver?

When a radio signal takes multiple paths to reach a receiver—such as bouncing off different layers of the ionosphere or reflecting off buildings and terrain—this is called multipath propagation. Because these paths have different lengths, the signals take slightly different amounts of time to arrive. When these multiple signals reach the receiving antenna, their waveforms interact. If the waves arrive perfectly aligned (in phase), they combine to make a stronger signal. However, if they arrive out of phase, the peaks of one wave will align with the troughs of another, causing them to cancel each other out. This continuous shifting between constructive and destructive interference causes the received signal strength to rise and fall periodically. In amateur radio, this phenomenon is specifically referred to as fading.