__________________________________________________________ Radio Wave Propagation
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. Because 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.
2-81. 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.
2-82. 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 adjust to the polarization changes.
2-83. Fading also results from absorption of the RF energy in the ionosphere.
Absorption fading occurs for a longer period than other types of fading,
because absorption takes place slowly. Usually, however, fading on
ionospheric circuits is mainly a result of multipath propagation.
2-84. 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, and others. 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.