The eyes and ears of a ship or shore station depend
on sophisticated, highly computerized electronic
systems. The one thing all of these systems have in
common is that they lead to and from antennas. Ships
operators who must communicate, navigate, and be
ready to fight the ship 24 hours a day depend on you
to keep these emitters and sensors operational.
In this volume, we will review wave propagation,
antenna characteristics, shore-based and shipboard
communications antennas, matching networks, antenna
tuning, radar antennas, antenna safety, transmission
lines, connector installation and weatherproofing,
waveguides, and waveguide couplings. When you
have completed this chapter, you should be able to
discuss the basic principles of wave propagation and
the atmospheres effects on wave propagation.
THE EARTHS ATMOSPHERE
While radio waves traveling in free space have
little outside influence to affect them, radio waves
traveling in the earths atmosphere have many
influences that affect them. We have all experienced
problems with radio waves, caused by certain
atmospheric conditions complicating what at first
seemed to be a relatively simple electronic problem.
These problem-causing conditions result from a lack
of uniformity in the earths atmosphere.
Many factors can affect atmospheric conditions,
either positively or negatively. Three of these are
variations in geographic height, differences in
geographic location, and changes in time (day, night,
To understand wave propagation, you must have
at least a basic understanding of the earths atmosphere.
The earths atmosphere is divided into three separate
regions, or layers.
They are the troposphere, the
stratosphere, and the ionosphere. These layers are
illustrated in figure 1-1.
Almost all weather phenomena take place in the
troposphere. The temperature in this region decreases
rapidly with altitude. Clouds form, and there may be
a lot of turbulence because of variations in the
temperature, pressure, and density. These conditions
have a profound effect on the propagation of radio
waves, as we will explain later in this chapter.
The stratosphere is located between the troposphere
and the ionosphere. The temperature throughout this
region is almost constant and there is little water vapor
present. Because it is a relatively calm region with
little or no temperature change, the stratosphere has
almost no effect on radio waves.
This is the most important region of the earths
atmosphere for long distance, point-to-point communi-
cations. Because the existence of the ionosphere is
directly related to radiation emitted from the sun, the
movement of the earth about the sun or changes in
the suns activity will result in variations in the
ionosphere. These variations are of two general types:
(1) those that more or less occur in cycles and,
therefore, can be predicted with reasonable accuracy;
and (2) those that are irregular as a result of abnormal
behavior of the sun and, therefore, cannot be predicted.
Both regular and irregular variations have important
effects on radio-wave propagation. Since irregular
variations cannot be predicted, we will concentrate
on regular variations.
The regular variations can be divided into four
main classes: daily, 27-day, seasonal, and 11-year.
We will concentrate our discussion on daily variations,