2. Obstacles to propagation.
a. Absorption of RF energy in the ionosphere result in loss of signal strength and reduced
transmission distances. Most ionospheric absorption occurs in the lower regions of the atmosphere
where ionization density is greatest. As a radio wave passes into the ionosphere, it loses energy to the
free electrons and ions. The highly dense D and E layers provide the greatest absorption of radio waves.
b. A radio signal will at times have variations in its strength. This is called fading. A radio
wave refracted by the ionosphere or reflected from the Earth's surface may suffer changes in its
polarization. This change in polarization results in weak signal reception. Fading is also caused by
absorption of the RF energy in the ionosphere.
c. There are other losses which affect the ionospheric propagation of radio waves, besides
energy losses in the atmosphere. These are ground-reflection loss and free space loss.
(1) Ground-reflection loss occurs when a transmitted signal is refracted off the ionosphere,
strikes the Earth, and is reflected back to the ionosphere. RF energy is lost each time the radio wave is
reflected from the 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.
(2) Free space loss occurs when a
receiver is picking up both the desired
traveling radio wave spreads out, much like a
flashlight's beam. Figure 3-8 shows the free
space loss principle. As distance increases, the
amount of energy contained in a wavefront will
decrease. By the time the energy is received at
the antenna, the wavefront is so spread out that
the antenna extends into only a very small
fraction of the wavefront.
d. Electromagnetic interference (EMI)
can significantly reduce the quality of
Figure 3-8. Free space loss principle.
communications. This is because the radio
transmission and electromagnetic radiation from an undesired source.
(1) Sources of EMI are manmade and natural. Examples of manmade EMI include assorted
radio transmitters that can cause mutual interference, and various electrical devices that generate
interfering signals, including ignition systems, generators, motors, and so forth. This is the reason you
must never transmit a radio signal across a signal site, or position your communications systems near
power lines. You can appreciate the severity of this type of interference the next time you listen to your
car radio while driving under electrical power lines. The intensity of the radiation from the
SS0130
3-8
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