e. Finally, characterization of the earth's environment and its
effect on propagation would be incomplete without the inclusion of
radio noise, whose origins are both natural and man-made.
Natural
radio noise is caused by atmospheric storms and cosmic radiation,
which originates outside the atmosphere. Actually, only a portion of
the cosmic noise reaches the surface of the earth because of the
screening effect of the ionosphere.
The origin and spectrum
occupancy of man-made noise vary. Its level can occasionally exceed
that of natural noise, especially in areas that are densely populated
or highly industrialized.
f. In summary, the earth's propagation environment is dynamic and
time varying.
Some facets of the time-varying character are known
and predictable, while others are random.
Which of these facets is
most important to propagation depends on the particular application?
38. Principal Modes of Propagation.
a. As discussed in the preceding paragraphs, the earth's unique
propagation environment imposes constraints on the utilization of the
radio frequency spectrum.
For example, the presence of the
ionosphere prohibits employment of frequencies lower than about 15
megahertz during the day and 3.5 megahertz at night for communication
with deep-space probes.
Lower frequencies cannot penetrate the
ionosphere. Similarly, use of frequencies above 10 gigahertz for the
same purpose may be limited by the strong absorption phenomena in the
troposphere, primarily caused by water vapor and oxygen.
b. Although the earth's environment imposes these constraints, it
also
provides
many
alternatives
and
novel
possibilities
for
communications, depending on one's experience, knowledge, and
ingenuity. Where natural alternatives are inadequate or undesirable,
man-made alternation or additions to the earth's environment have
sometimes proven satisfactory.
c. As an enumeration of all presently available alternatives
would neither be practicable nor useful for this synopsis, the
discussion that follows is limited to the major modes of radio wave
propagation: line of sight (LOS); refraction mode; scatter mode;
surface wave; and waveguide and mixed mode.
LESSON 4 - EXERCISE 1
Complete items 1 through 5.
1. What is the lowest frequency that will penetrate the ionosphere
during the daylight hours for skywave transmission?
a.
15 megahertz.
c.
40 megahertz.
b.
20 megahertz.
d.
3.5 megahertz.
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