2-26
Most ionospheric absorption occurs in the lower regions of the ionosphere where ionization density
is greatest. As a radio wave passes into the ionosphere, it loses some of its energy to the free electrons and
ions. If these high-energy free electrons and ions do not collide with gas molecules of low energy, most of
the energy lost by the radio wave is reconverted into electromagnetic energy, and the wave continues to
be propagated with little change in intensity. However, if the high-energy free electrons and ions do
collide with other particles, much of this energy is lost, resulting in absorption of the energy from the
wave. Since absorption of energy depends on collision of the particles, the greater the density of the
ionized layer, the greater the probability of collisions; therefore, the greater the absorption. The highly
dense D and E layers provide the greatest absorption of radio waves.
Because the amount of absorption of the sky wave depends on the density of the ionosphere, which
varies with seasonal and daily conditions, it is impossible to express a fixed relationship between distance
and signal strength for ionospheric propagation. Under certain conditions, the absorption of energy is so
great that communicating over any distance beyond the line of sight is difficult.
FADING
The most troublesome and frustrating problem in receiving radio signals is variations in signal
strength, most commonly known as FADING. There are several conditions that can produce fading.
When a radio wave is refracted by the ionosphere or reflected from the Earth's surface, random changes in
the polarization of the wave may occur. Vertically and horizontally mounted receiving antennas are
designed to receive vertically and horizontally polarized waves, respectively. Therefore, changes in
polarization cause changes in the received signal level because of the inability of the antenna to receive
polarization changes.
Fading also results from absorption of the rf energy in the ionosphere. Absorption fading occurs for a
longer period than other types of fading, since absorption takes place slowly.
Usually, however, fading on ionospheric circuits is mainly a result of multipath propagation.
Multipath Fading
MULTIPATH is simply a term used to describe the multiple paths a radio wave may follow between
transmitter and receiver. Such propagation paths include the ground wave, ionospheric refraction,
reradiation by the ionospheric layers, reflection from the Earth's surface or from more than one
ionospheric layer, etc. Figure 2-21 shows a few of the paths that a signal can travel between two sites in a
typical circuit. One path, XYZ, is the basic ground wave. Another path, XEA, refracts the wave at the E
layer and passes it on to the receiver at A. Still another path, XFZFA, results from a greater angle of
incidence and two refractions from the F layer. At point Z, the received signal is a combination of the
ground wave and the sky wave. These two signals having traveled different paths arrive at point Z at
different times. Thus, the arriving waves may or may not be in phase with each other. Radio waves that
are received in phase reinforce each other and produce a stronger signal at the receiving site. Conversely,
those that are received out of phase produce a weak or fading signal. Small alternations in the
transmission path may change the phase relationship of the two signals, causing periodic fading. This
condition occurs at point A. At this point, the double-hop F layer signal may be in or out of phase with the
signal arriving from the E layer.
