2-44 MULTIPATH FADINGoccurs when a transmitted signal divides and takes more than one path to a receiver and some of the signals arrive out of phase, resulting in a weak or fading signal. Some TRANSMISSION LOSSES that affect radio-wave propagation are ionospheric absorption, ground reflection, and free-space losses. ELECTROMAGNETIC INTERFERENCE (emi), both natural and man-made, interfere with radio communications. The MAXIMUM USABLE FREQUENCY (muf) is the highest frequency that can be used for communications between two locations at a given angle of incidence and time of day. The LOWEST USABLE FREQUENCY (luf) is the lowest frequency that can be used for communications between two locations.
