1-2
and understand those concepts. Through ample use of illustrations and a step-by-step transition from the
simple to the complex, we will help you develop a better understanding of wave propagation. In this
chapter, we will discuss propagation theory on an introductory level, without going into the technical
details that concern the engineer. However, you must still use thought and imagination to understand the
new ideas and concepts as they are presented.
To understand radio wave propagation, you must first learn what wave propagation is and some of
the basic physics or properties that affect propagation. Many of these properties are common everyday
occurrences, with which you are already familiar.
WHAT IS PROPAGATION?
Early man was quick to recognize the need to communicate beyond the range of the human voice. To
satisfy this need, he developed alternate methods of communication, such as hand gestures, beating on a
hollow log, and smoke signals. Although these methods were effective, they were still greatly limited in
range. Eventually, the range limitations were overcome by the development of courier and postal systems;
but there was then a problem of speed. For centuries the time required for the delivery of a message
depended on the speed of a horse.
During the latter part of the 19th century, both distance and time limitations were largely overcome.
The invention of the telegraph made possible instantaneous communication over long wires. Then a short
time later, man discovered how to transmit messages in the form of RADIO WAVES.
As you will learn in this chapter, radio waves are propagated. PROPAGATION means "movement
through a medium." This is most easily illustrated by light rays. When a light is turned on in a darkened
room, light rays travel from the light bulb throughout the room. When a flashlight is turned on, light rays
also radiate from its bulb, but are focused into a narrow beam. You can use these examples to picture how
radio waves propagate. Like the light in the room, radio waves may spread out in all directions. They can
also be focused (concentrated) like the flashlight, depending upon the need. Radio waves are a form of
radiant energy, similar to light and heat. Although they can neither be seen nor felt, their presence can be
detected through the use of sensitive measuring devices. The speed at which both forms of waves travel is
the same; they both travel at the speed of light.
You may wonder why you can see light but not radio waves, which consist of the same form of
energy as light. The reason is that you can only "see" what your eyes can detect. Your eyes can detect
radiant energy only within a fixed range of frequencies. Since the frequencies of radio waves are below
the frequencies your eyes can detect, you cannot see radio waves.
The theory of wave propagation that we discuss in this module applies to Navy electronic equipment,
such as radar, navigation, detection, and communication equipment. We will not discuss these individual
systems in this module, but we will explain them in future modules.
Q1. What is propagation?
PRINCIPLES OF WAVE MOTION
All things on the earth—on the land, or in the water—are showered continually with waves of
energy. Some of these waves stimulate our senses and can be seen, felt, or heard. For instance, we can see
light, hear sound, and feel heat. However, there are some waves that do not stimulate our senses. For
