03-08-2012, 11:20 AM
Wave Propagation, Transmission Lines, and Antennas
[attachment=29988]
Wave Propagation
The methods used to propagate (transmit) waves through space are based
on the same physical principles today as they were 70 years ago. In this
chapter, we discuss propagation theory on an introductory level, without
going into the technical details that concern the engineer. Understanding
wave propagation requires you to use your imagination to visualize the
associated concepts and how they are used in practical application. To
help you in this process, this chapter includes many illustrations and
step-by-step descriptions that guide you from simple concepts to more
complex ideas. This chapter covers the fundamentals of wave propagation
and the physical factors that affect propagation. Many of the principles
discussed are observable in common, everyday occurrences with which
you are already familiar.
WHAT IS PROPAGATION?
1-2. 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 to deliver a message depended on
the speed of a horse.
1-3. During the latter part of the nineteenth 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.
PRINCIPLES OF WAVE MOTION
1-6. All things 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
example, radio waves, such as those received by our portable radio or
television sets, cannot be seen, heard, or felt. A device must be used to
convert radio waves into light (TV pictures) and sound (audio) for us to sense
them.
1-7. A wave can be defined as a disturbance (e.g., sound, light, radio waves)
that moves through a medium (e.g., air, water, vacuum). To help you
understand what is meant by “a disturbance that moves through a medium,”
picture the following illustration. You are standing in the middle of a wheat
field. As the wind blows across the field toward you, you can see the wheat
stalks bending and rising as the force of the wind moves into and across
them. The wheat appears to be moving toward you, but it isn’t. Instead, the
stalks are actually moving back and forth.
TRANSVERSE WAVES
1-13. To explain transverse waves, we again use our example of water waves.
Figure 1-3 is a cross-section diagram of waves viewed from the side. Notice
that the waves are a succession of crests and troughs. The wavelength (one
360-degree cycle) is the distance from the crest of one wave to the crest of the
next, or between any two similar points on adjacent waves. The amplitude of
a transverse wave is half the distance measured vertically from the crest to
the trough. Water waves are known as transverse waves because the motion
of the water is up and down, or at right angles to the direction in which the
waves are traveling. You can see this by observing a cork bobbing up and
down on water as the waves pass by; the cork moves very little in a sideways
direction.
MEDIUM
1-17. We have used the term medium in describing the motion of waves.
Because medium is a term that is used frequently in discussing propagation,
it needs to be defined so you will understand what a medium is and its
application to propagation.
1-18. A medium is the vehicle through which the wave travels from one point
to the next. The vehicle that carries a wave can be just about anything. An
example of a medium, already mentioned, is air. Air, as defined by the
dictionary, is the mixture of invisible, odorless, tasteless gases that
surrounds the earth (the atmosphere). Air is made up of molecules of various
gases (and impurities). We will call these molecules of air particles of air or
simply particles. Figure 1-6 can help you to understand how waves travel
through air. The object producing the waves is called the source—a bell in
this illustration. The object responding to the waves is called a detector or
receiver; in this case, the human ear. The medium is air, which is the means
of conveying the waves from the source to the detector. The source, detector,
and medium are all necessary for wave motion and wave propagation (except
for electromagnetic waves, which require no medium).
TERMS USED IN WAVE MOTION
1-19. There are a number of special terms concerning waves that you should
know. Many of the terms, such as cycle, wavelength, amplitude, and frequency,
were introduced in TC 9-60. We now discuss these terms in detail as they
pertain to wave propagation. Before we begin our discussion, however, note
that in figure 1-7, wave 1 and wave 2 have equal frequency and wavelength but
different amplitudes. The reference line (also known as rest position or point of
zero displacement) is the position that a particle of matter would have if it were
not disturbed by wave motion. For example, in the case of the water wave, the
reference line is the level of the water when no wave motion is present. With this
in mind, let us go on to our discussion of the four terms, as shown in figure 1-7.
[attachment=29988]
Wave Propagation
The methods used to propagate (transmit) waves through space are based
on the same physical principles today as they were 70 years ago. In this
chapter, we discuss propagation theory on an introductory level, without
going into the technical details that concern the engineer. Understanding
wave propagation requires you to use your imagination to visualize the
associated concepts and how they are used in practical application. To
help you in this process, this chapter includes many illustrations and
step-by-step descriptions that guide you from simple concepts to more
complex ideas. This chapter covers the fundamentals of wave propagation
and the physical factors that affect propagation. Many of the principles
discussed are observable in common, everyday occurrences with which
you are already familiar.
WHAT IS PROPAGATION?
1-2. 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 to deliver a message depended on
the speed of a horse.
1-3. During the latter part of the nineteenth 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.
PRINCIPLES OF WAVE MOTION
1-6. All things 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
example, radio waves, such as those received by our portable radio or
television sets, cannot be seen, heard, or felt. A device must be used to
convert radio waves into light (TV pictures) and sound (audio) for us to sense
them.
1-7. A wave can be defined as a disturbance (e.g., sound, light, radio waves)
that moves through a medium (e.g., air, water, vacuum). To help you
understand what is meant by “a disturbance that moves through a medium,”
picture the following illustration. You are standing in the middle of a wheat
field. As the wind blows across the field toward you, you can see the wheat
stalks bending and rising as the force of the wind moves into and across
them. The wheat appears to be moving toward you, but it isn’t. Instead, the
stalks are actually moving back and forth.
TRANSVERSE WAVES
1-13. To explain transverse waves, we again use our example of water waves.
Figure 1-3 is a cross-section diagram of waves viewed from the side. Notice
that the waves are a succession of crests and troughs. The wavelength (one
360-degree cycle) is the distance from the crest of one wave to the crest of the
next, or between any two similar points on adjacent waves. The amplitude of
a transverse wave is half the distance measured vertically from the crest to
the trough. Water waves are known as transverse waves because the motion
of the water is up and down, or at right angles to the direction in which the
waves are traveling. You can see this by observing a cork bobbing up and
down on water as the waves pass by; the cork moves very little in a sideways
direction.
MEDIUM
1-17. We have used the term medium in describing the motion of waves.
Because medium is a term that is used frequently in discussing propagation,
it needs to be defined so you will understand what a medium is and its
application to propagation.
1-18. A medium is the vehicle through which the wave travels from one point
to the next. The vehicle that carries a wave can be just about anything. An
example of a medium, already mentioned, is air. Air, as defined by the
dictionary, is the mixture of invisible, odorless, tasteless gases that
surrounds the earth (the atmosphere). Air is made up of molecules of various
gases (and impurities). We will call these molecules of air particles of air or
simply particles. Figure 1-6 can help you to understand how waves travel
through air. The object producing the waves is called the source—a bell in
this illustration. The object responding to the waves is called a detector or
receiver; in this case, the human ear. The medium is air, which is the means
of conveying the waves from the source to the detector. The source, detector,
and medium are all necessary for wave motion and wave propagation (except
for electromagnetic waves, which require no medium).
TERMS USED IN WAVE MOTION
1-19. There are a number of special terms concerning waves that you should
know. Many of the terms, such as cycle, wavelength, amplitude, and frequency,
were introduced in TC 9-60. We now discuss these terms in detail as they
pertain to wave propagation. Before we begin our discussion, however, note
that in figure 1-7, wave 1 and wave 2 have equal frequency and wavelength but
different amplitudes. The reference line (also known as rest position or point of
zero displacement) is the position that a particle of matter would have if it were
not disturbed by wave motion. For example, in the case of the water wave, the
reference line is the level of the water when no wave motion is present. With this
in mind, let us go on to our discussion of the four terms, as shown in figure 1-7.