What is the periodic increase and decrease of received radio strength called?

The periodic increase and decrease of received radio strength is known as fading. This phenomenon occurs due to the varying conditions in the atmosphere and the physical environment that affect the radio signal as it propagates. Fading can be caused by several factors, including multipath propagation, where signals arrive at the receiver via multiple paths due to reflection, refraction, or scattering. The constructive and destructive interference of these multiple signals leads to variations in signal strength, resulting in the characteristic rises and falls of received signal levels.

Understanding fading is crucial for designing reliable communication systems, as it can impact the quality of the received signal and the overall performance of the radio communication link. By recognizing the effects of fading, engineers can implement techniques such as diversity reception, equalization, and adaptive modulation to mitigate its impact.