2-28
Ground Reflection Loss
When propagation is accomplished via multihop refraction, rf energy is lost each time the radio wave
is reflected from the Earth's surface. The amount of energy lost depends on the frequency of the wave, the
angle of incidence, ground irregularities, and the electrical conductivity of the point of reflection.
Free space Loss
Normally, the major loss of energy is because of the spreading out of the wavefront as it travels away
from the transmitter. As the distance increases, the area of the wavefront spreads out, much like the beam
of a flashlight. This means the amount of energy contained within any unit of area on the wavefront will
decrease as distance increases. By the time the energy arrives at the receiving antenna, the wavefront is so
spread out that the receiving antenna extends into only a very small fraction of the wavefront. This is
illustrated in figure 2-22.
Figure 2-22.—Free space loss principle.
ELECTROMAGNETIC INTERFERENCE (EMI)
The transmission losses just discussed are not the only factors that interfere with communications.
An additional factor that can interfere with radio communications is the presence of
ELECTROMAGNETIC INTERFERENCE (EMI). This interference can result in annoying or impossible
operating conditions. Sources of emi are both man-made and natural.
Man-Made Interference
Man-made interference may come from several sources. Some of these sources, such as oscillators,
communications transmitters, and radio transmitters, may be specifically designed to generate radio
frequency energy. Some electrical devices also generate radio frequency energy, although they are not
specifically designed for this purpose. Examples are ignition systems, generators, motors, switches,
relays, and voltage regulators. The intensity of man-made interference may vary throughout the day and
drop off to a low level at night when many of these sources are not being used. Man-made interference
may be a critical limiting factor at radio receiving sites located near industrial areas.
