09-01-2016, 02:10 PM
INTRODUCTION
Radio wave propagation is the study of the transfer of energy at radio frequencies from one point, a transmitter, to another, a receiver. Radio waves are part of the broad electromagnetic spectrum that extends from the very low frequencies which are produced by electric power facilities up to the extremely high frequencies of cosmic rays. Between these two extremes are bands of frequencies that are found in every day uses: audio frequencies used in systems for the reproduction of audible sounds, radio frequencies, infrared light and ultraviolet light and x-rays. All electromagnetic waves propagate at the same velocity, regardless of the frequency. Light is an electromagnetic
wave, and thus the propagation velocity is often referred to as “the speed of light” ©, which for a vacuum is approximately 3 2 108 m/sec. The velocity of any wave is dependent upon the medium in which it is travelling, but for simplicity is usually
considered with respect to a vacuum. The frequency of a wave is defined in terms of the number of cycles per second or hertz (Hz) and is related to the wavelength (l) by the expression f 4 c/l. Radio frequencies are generally considered to be that portion of the electromagnetic spectrum below the infrared frequencies. At present, the practical upper limit of radio frequencies is roughly 100 GHz.1 Within the radio frequency spectrum are bands of frequencies that have been allocated to the broadcast service. The following discussions and methods will apply particularly to these bands of the radio frequency spectrum. The AM band frequency allotments are contained in what is referred to as medium frequencies (MF), 300 kHz to 3 MHz. The FM band frequencies and a
portion of the TV band are contained in the VHF band which extends from 30 MHz to 300 MHz. The remaining TV allocations are contained in the UHF band of 300 MHz to 3 GHz. Allocations for broadcast auxiliary services such as remote pickup, studio/transmitter links, intercity relays, MDS, and ITFS are interspersed within the MF, VHF, UHF and SHF (super high frequency) bands. Table 2.1-1 illustrates some of
the allotments assigned to the broadcast service.