Attenuation

 
Anyone who has tried to listen to a radio while driving in rural areas knows that
signals get weaker the farther you get from the source.This weakening of a signal
is known as attenuation.There are several factors that cause attenuation, but to see
how distance alone can cause attenuation first consider the example of propagation
in free space. Unlike audio waves (which are pressure waves and must have a medium to
propagate), EM waves do not require a medium to propagate and can travel
through the vacuum of space. In free space there are no other factors that cause
resistance to the signal, yet there is still attenuation because the signal density
diverges. Figure 2.8 demonstrates this phenomenon using light. Assume that each ray of light represents an equal amount of the total light
energy transmitted.You can see that the rays diverge as the distance from the
source increases.The result is a decrease in light intensity. Visible light waves are
high frequency EM waves; therefore, this analogy also applies to EM waves in the
RF spectrum. Since the waves are propagating in all directions, it is impossible to
collect them all back at the receiver.Thus the receiver receives only a small portion
of the energy transmitted, and this amount of energy received continually
decreases as the distance increases and the “rays” diverge further.The affects of
distance on the strength of EM waves in free space are given by the following
equation:
P (proportional to) 1/r2
where P is power and r is the distance from the source to the receiver.The
inverse square relationship means that when the distance doubles (r × 2), the
power received is reduced by a factor of four (22 = 4 Passing through objects further attenuates EM waves.The amount of attenuation
depends on the frequency of the wave and the thickness and composition of
the object through which the wave is passing. Some objects, like mountains,
attenuate 100 percent of the signal, thus blocking communication.This general
attenuation equation gets worse when obstacles such as rain, buildings, mountains,
and so on are placed in the path of the signal.The resulting affect for terrestrial
EM propagation can be estimated by the equation:
P (proportional to) 1/r3
In this approximation, as the distance between transmitter and receiver doubles
(r × 2), the power received is reduced by a factor of eight (23 = 8).
One way to minimize the amount the transmitted energy diverges is to use
a directional antenna that focuses the waves in a specific direction. Figure 2.9
illustrates the previous example but uses an analogy of a flashlight to represent a
directional antenna.
Assuming the omnidirectional light fixture in Figure 2.8 and the flashlight in
Figure 2.9 both transmitted the same amount of energy, it is easy to see that the
flashlight gets a stronger signal to a receiver that is the same distance away.
Directional antennas are discussed in more depth later in this chapter.
EM waves do not penetrate the earth well.Therefore, for most land-based to
land-based communications, the distance of the horizon is the ultimate constraint
to the distance a signal can propagate. By elevating the transmitter and receiver
on mountains and/or towers, however, you extend the horizon. Figure 2.10 illustrates
how towers can extend the horizon. Rain Attenuation
Rain attenuation is the attenuation to a signal due to precipitation.This affects
high frequency waves more than low frequency waves because high frequency
waves do not penetrate water as well.
The phenomenon of rain attenuation has been used to the advantage of
some systems, for example, weather radar.Water droplets in the air that signify
rain or clouds reflect and attenuate the high frequency radar signal differently
than the surrounding air.This allows the radar system to paint a picture of the
moisture.