36. Radio Wave Propagation.
This lesson we will discuss propagation of electromagnetic waves
in the earth's natural environment; thus, transmission using man-made
waveguides and coaxial cables is not included.
Emphasis is on the
fundamental technical factors that limit, or otherwise influence,
spectrum utilization for various communications services. Even with
these
restrictions,
the
scope
of
a
complete
discussion
of
electromagnetic propagation is immense. Consequently this lesson is
devoted to an abbreviated introductory discussion of the earth's
propagation environment and a summary of the major modes of
propagation in natural environments.
37. The Earth's Propagation Environment.
a.The characteristics of all radio frequency transmission paths used
in connections with earth bound terminals reflect the unique nature
of the earth's propagation environments. Some communications systems
would not be possible without this environment, while for others it
limits their performance. Thus, some communications systems used in
an earth environment may not function on the moon, while others may
function better.
b. The earth's atmosphere may be the most important feature of
the propagation environment. Figure 14 is an idealized sketch of the
stratified nature of the atmosphere.
Part of the upper envelope,
known as the ionosphere, is characterized by the presence of free
electrons
in
sufficient
concentration
to
affect
radio
wave
propagation. The free electrons result primarily from ionization of
gases by ultraviolet radiation and x-radiation from the sun.
Ionization is the gain or loss of electrons by an atom of a gas
molecule.
Ionospheric ions are primarily positive, having lost
electrons; this results in an increased concentration of free
electrons.
The various regions of the ionosphere are distinguished
by the relative concentration of free electrons, which varies between
night and day and is to some extent affected by solar flares and
other natural phenomena. Looking at other pertinent characteristics
of the atmosphere the troposphere contains most of the neutral gases
of the atmosphere as well as water vapor.
The temperature in the
troposphere normally decreases uniformly with height; elsewhere in
the atmosphere, it is either constant, as in parts of the
stratosphere, or increases with height, as in the ionosphere.
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