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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. Figure 2.1-1
shows the ranges of various bands within the electromagnetic
spectrum in terms of frequency and wavelength.
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. the allocations for auxiliary services may change from
time to time as the needs of various services for radio
frequencies change and as technology for equipment
improves.
QUANTIFYING PROPAGATION
The energy that is emitted from a transmitter may take
many different paths before it is received. The path
that the radio wave will take depends on many factors,
some of which include: frequency, antenna type and
height, atmospheric conditions and terrain. Radio
waves that propagate along the surface of the earth
are commonly referred to as ground waves. All radio
waves have some ground wave component, however,
because the earth is a lossy medium