design is also important for reducing multipath and other types of
interference. At frequencies below about 100 megahertz, the effects
of the ground other than multipath effects, become noticeable on the
antenna and its radiating properties.
Furthermore, ground-based
systems employing even lower frequencies are affected by an
additional mode of propagation, the so-called surface wave.
(The
surface wave is discussed further in paragraph 41.) The term ground
wave is often used to denote the ground hugging portion of those
lower
frequency
transmissions,
particularly
in
the
amplitude
modulation broadcast band.
Generally speaking, ground waves
represent a complex mixture of wave components and are not equivalent
to the surface wave per se.
d. Line-of-sight propagation can be extended to long earth
surface transmission through the use of relays.
The primary
characteristics of line-of-sight systems are wide emission band-
width, high reliability, and a reasonable immunity to interference as
a result of the high degree of antenna directivity normally employed
in line-of-sight systems.
When the circuit traverses inaccessible
land or very large bodies of water, other modes or variations of the
line-of-sight mode (e.g., satellite terminals) are often more
advantageous.
40. Refraction Mode.
a. The refraction mode of propagation (often called the skywave
mode) depends primarily on the ionosphere, although other atmospheric
layers do contribute to refraction.
Here, the incident ray is
normally bent in a downward direction, as depicted in figure 16. It
is sometimes useful to view this as a reflection from the mirror-like
layers that characterize the ionosphere, although this is an over-
simplification.
A wave penetrating the ionosphere at an oblique
angle is gradually bent away from its original direction of
propagation.
At suitable frequencies and heights, this redirection
is sufficient to return the wave to earth.
The lowest and highest
frequencies at which this reversal takes place are called critical
frequencies; they depend on the local properties of the ionosphere
and the angle of wave incidence.
Typically, the ionosphere will
support (i.e., refract back to earth) propagation up to above 40
megahertz.
The lower frequency limit is generally determined by
absorption in the lower portion of the ionosphere and is about 0.8
megahertz.
The lowest usable frequency is also limited by the
presence of atmospheric noise.
Distances which can reliably be
attained in this mode range, from a few hundred to a few thousand
miles, transmissions around the world are not uncommon.
Long-range
transmissions depend on multiple reflection between the ground and
the ionosphere, or between the "layers" of the ionosphere.
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