Module 11 - Microwave Principles Pages
i−ix,
1−1 to 1−10,
1−11 to 1−20,
1−21 to 1−30,
1−31 to 1−40,
1−41 to 1−50,
1−51 to 1−60,
1−61 to 1−68,
2−1 to 2−10,
2−11 to 2−20, 2−21 to
2−30, 2−31 to 2−40,
2−41 to 2−50,
2−51 to 2−60,
2−61 to 2−66,
3−1 to 3−10,
3−11 to 3−20,
AI−1 to AI−6,
Index−1 to Index−2,
Assignment 1 - 1−8,
Assignment 2 - 9−16
CHAPTER 1
WAVEGUIDE THEORY AND APPLICATION
LEARNING OBJECTIVES
Upon completion of this chapter
the student will be able to:
1. Describe the development of the various types
of waveguides in terms of their advantages and
disadvantages.
2. Describe
the physical dimensions of the various types of waveguides and explain the effects
of
those dimensions on power and frequency.
3. Explain the propagation of
energy in waveguides in terms of electromagnetic field theory.
4. Identify the
modes of operation in waveguides.
5. Explain the basic input/output methods
used in waveguides.
6. Describe the basic principles of waveguide plumbing.
7. Explain the reasons for and the methods of terminating waveguides.
8.
Explain the basic theory of operation and applications of directional couplers.
9. Describe the basic theory of operation, construction, and applications of
cavity resonators.
10. Describe the basic theory of operation of waveguide junctions.
11. Explain the operation of ferrite devices in terms of their applications.
INTRODUCTION TO WAVEGUIDE THEORY AND APPLICATION
That portion of the electromagnetic spectrum which falls between 1000 megahertz
and 100,000 megahertz is referred to as the MICROWAVE region. Before discussing
the principles and applications of microwave frequencies, the meaning of the term
microwave as it is used in this module must be established. On the surface, the
definition of a microwave would appear to be simple because, in electronics, the
prefix "micro" normally means a millionth part of a unit. Micro also means small,
which is a relative term, and it is used in that sense in this module. Microwave
is a term loosely applied to identify electromagnetic waves above 1000 megahertz
in frequency because of the short physical wavelengths of these frequencies. Short
wavelength energy offers distinct advantages in many applications. For instance,
excellent directivity can be obtained using relatively small antennas and low-power
transmitters. These features are ideal for use in both military and civilian radar
and
communication applications. Small antennas and other small components are
made possible by microwave frequency applications. This is an important consideration
in shipboard equipment planning where space and weight are major problems. Microwave
frequency usage is especially important in the design of shipboard radar because
it makes possible the detection of smaller targets.
1-1
Microwave frequencies present special problems in transmission, generation, and
circuit design that are not encountered at lower frequencies. Conventional circuit
theory is based on voltages and currents while microwave theory is based on electromagnetic
fields. The concept of electromagnetic field interaction is not entirely new, since
electromagnetic fields form the basis of all antenna theory. However, many students
of electronics find electromagnetic field theory very difficult to visualize and
understand. This module will present the principles of microwave theory in the simplest
terms possible but many of the concepts are still somewhat difficult to thoroughly
understand. Therefore, you must realize that this module will require very careful
study for you to properly understand microwave theory. Antenna fundamentals were
covered in NEETS, Module 10, Introduction to Wave Propagation, Transmission Lines,
and Antennas.
This module will show you the solutions to problems encountered
at microwave frequencies, beginning with the transmission of microwave energy and
continuing through to waveguides in chapter 1. Later chapters will cover the theory
of operation of microwave components, circuits, and antennas. The application of
these concepts will be discussed more thoroughly in later NEETS modules on radar
and communications.
Q-1. What is the region of the frequency spectrum
from 1,000 MHz to 100,000 MHz called?
Q-2. Microwave theory is based
upon what concept
WAVEGUIDE THEORY
The two-wire transmission line used in
conventional circuits is inefficient for transferring electromagnetic energy at
microwave frequencies. At these frequencies, energy escapes by radiation because
the fields are not confined in all directions, as illustrated in figure 1-1. Coaxial
lines are more efficient than two-wire lines for transferring electromagnetic energy
because the fields are completely confined by the conductors, as illustrated in
figure 1-2.
Figure 1-1. - Fields confined in two directions only.
1-2
Figure 1-2. - Fields confined in all directions.
Waveguides are the most efficient way to transfer electromagnetic
energy. WAVEGUIDES are essentially coaxial lines without center conductors. They
are constructed from conductive material and may be rectangular, circular, or elliptical
in shape, as shown in figure 1-3.
Figure 1-3. - Waveguide shapes.
Waveguide Advantages
Waveguides have several advantages
over two-wire and coaxial transmission lines. For example, the large surface area
of waveguides greatly reduces COPPER (I2R) LOSSES. Two-wire transmission lines have
large copper losses because they have a relatively small surface area. The surface
area of the outer conductor of a coaxial cable is large, but the surface area of
the inner conductor is relatively small. At microwave frequencies, the current-carrying
area of the inner conductor is restricted to a very small layer at the surface of
the conductor by an action called SKIN EFFECT.
1-3
Skin effect was discussed in NEETS, Module 10, Introduction to Wave Propagation,
Transmission Lines, and Antennas, Chapter 3. Skin effect tends to increase the effective
resistance of the conductor. Although energy transfer in coaxial cable is caused
by electromagnetic field motion, the magnitude of the field is limited by the size
of the current-carrying area of the inner conductor. The small size of the center
conductor is even further reduced by skin effect and energy transmission by coaxial
cable becomes less efficient than by waveguides. DIELECTRIC LOSSES are also lower
in waveguides than in two-wire and coaxial transmission lines. Dielectric losses
in two-wire and coaxial lines are caused by the heating of the insulation between
the conductors. The insulation behaves as the dielectric of a capacitor formed by
the two wires of the transmission line. A voltage potential across the two wires
causes heating of the dielectric and results in a power loss. In practical applications,
the actual breakdown of the insulation between the conductors of a transmission
line is more frequently a problem than is the dielectric loss.
This breakdown
is usually caused by stationary voltage spikes or "nodes" which are caused by standing
waves. Standing waves are stationary and occur when part of the energy traveling
down the line is reflected by an impedance mismatch with the load. The voltage potential
of the standing waves at the points of greatest magnitude can become large enough
to break down the insulation between transmission line conductors.
The dielectric
in waveguides is air, which has a much lower dielectric loss than conventional insulating
materials. However, waveguides are also subject to dielectric breakdown caused by
standing waves. Standing waves in waveguides cause arcing which decreases the efficiency
of energy transfer and can severely damage the waveguide. Also since the electromagnetic
fields are completely contained within the waveguide, radiation losses are kept
very low.
Power-handling capability is another advantage of waveguides. Waveguides
can handle more power than coaxial lines of the same size because power-handling
capability is directly related to the distance between conductors. Figure 1-4 illustrates
the greater distance between conductors in a waveguide.
Figure 1-4. - Comparison of spacing in coaxial cable and a circular waveguide.
In view of the advantages of waveguides, you would think that waveguides
should be the only type of transmission lines used. However, waveguides have certain
disadvantages that make them practical for use only at microwave frequencies.
1-4
Waveguide Disadvantages
Physical size is the primary lower-frequency
limitation of waveguides. The width of a waveguide must be approximately a half
wavelength at the frequency of the wave to be transported. For example, a waveguide
for use at 1 megahertz would be about 500 feet wide. This makes the use of waveguides
at frequencies below 1000 megahertz increasingly impractical. The lower frequency
range of any system using waveguides is limited by the physical dimensions of the
waveguides.
Waveguides are difficult to install because of their rigid, hollow-pipe
shape. Special couplings at the joints are required to assure proper operation.
Also, the inside surfaces of waveguides are often plated with silver or gold to
reduce skin effect losses. These requirements increase the costs and decrease the
practicality of waveguide systems at any other than microwave frequencies.
Q-3. Why are coaxial lines more efficient at microwave frequencies than two-wire
transmission lines?
Q-4. What kind of material must be used in the
construction of waveguides?
Q-5. The large surface area of a waveguide
greatly reduces what type of loss that is common in two-wire and coaxial lines?
Q-6. What causes the current-carrying area at the center conductor
of a coaxial line to be restricted to a small layer at the surface?
Q-7. What is used as a dielectric in waveguides?
Q-8. What is
the primary lower-frequency limitation of waveguides?
Developing
the Waveguide from Parallel Lines
You may better understand the transition
from ordinary transmission line concepts to waveguide theories by considering the
development of a waveguide from a two-wire transmission line. Figure 1-5 shows a
section of two-wire transmission line supported on two insulators. At the junction
with the line, the insulators must present a very high impedance to ground for proper
operation of the line. A low impedance insulator would obviously short-circuit the
line to ground, and this is what happens at very high frequencies. Ordinary insulators
display the characteristics of the dielectric of a capacitor formed by the wire
and ground. As the frequency increases, the overall impedance decreases. A better
high- frequency insulator is a quarter-wave section of transmission line shorted
at one end. Such an insulator is shown in figure 1-6. The impedance of a shorted
quarter-wave section is very high at the open-end junction with the two-wire transmission
line. This type of insulator is known as a METALLIC INSULATOR and may be placed
anywhere along a two-wire line. Note that quarter-wave sections are insulators at
only one frequency. This severely limits the bandwidth, efficiency, and application
of this type of two-wire line.
1-5
Figure 1-5. - Two-wire transmission line using ordinary insulators.
Figure 1-6. - Quarter-wave section of transmission line shorted at one end.
Figure 1-7 shows several metallic insulators on each side of a two-wire
transmission line. As more insulators are added, each section makes contact with
the next, and a rectangular waveguide is formed. The lines become part of the walls
of the waveguide, as illustrated in figure 1-8. The energy is then conducted within
the hollow waveguide instead of along the two-wire transmission line.
Figure 1-7. - Metallic insulators on each side of a two-wire line.
1-6
Figure 1-8. - Forming a waveguide by adding quarter-wave sections.
The comparison of the way electromagnetic fields work on a transmission
line and in a waveguide is not exact. During the change from a two-wire line to
a waveguide, the electromagnetic field configurations also undergo many changes.
These will be discussed later in this chapter. As a result of these changes, the
waveguide does not actually operate like a two-wire line that is completely shunted
by quarter-wave sections. If it did, the use of a waveguide would be limited to
a single-frequency wavelength that was four times the length of the quarter-wave
sections. In fact, waves of this length cannot pass efficiently through waveguides.
Only a small range of frequencies of somewhat shorter wavelength (higher frequency)
can pass efficiently.
As shown in figure 1-9, the widest dimension of a waveguide
is called the "a" dimension and determines the range of operating frequencies. The
narrowest dimension determines the power-handling capability of the waveguide and
is called the "b" dimension.
Figure 1-9. - Labeling waveguide dimensions.
1-7
NOTE
This method of labeling waveguides is not standard
in all texts. Different methods may be used in other texts on microwave principles,
but this method is in accordance with Navy Military Standards (MIL-STDS).
The
ability of a waveguide of a given dimension to transport more than one frequency
may be better understood by analyzing the actions illustrated in figure 1-10A, B,
and C. A waveguide may be considered as having upper and lower quarter-wave sections
and a central section which is a solid conductor called a BUS BAR. In figure 1-10A,
distance mn is equal to distance pq, and both are equal to one quarter-wavelength
(λ/4).
Figure 1-10A. - Frequency effects on a waveguide. NORMAL OPERATING FREQUENCY.
NOTE
Throughout NEETS, 1/4λ and λ/4 are both
used to represent one quarter-wavelength and are used interchangeably. Also, λ/2
and 3/2λ will be used to represent one half-wavelength and 1 1/2 wavelengths, respectively.
Distance np is the width of the bus bar. If the overall dimensions of the waveguide
are held constant, the required length of the quarter-wave sections DECREASES as
the frequency increases. As illustrated in figure 1-10B, this causes the width of
the bus bar to INCREASE. In theory the waveguide could function at an infinite number
of frequencies higher than the designed frequency; as the length of each
quarter-wave
section approaches zero, the bus bar continues to widen to fill the available space.
However, in practice, an upper frequency limit is caused by modes of operation,
which will be discussed later.
1-8
Figure 1-10B. - Frequency effects on a waveguide. INCREASING FREQUENCY.
Figure 1-10C. - Frequency effects on a waveguide. DECREASING FREQUENCY.
If the frequency of a signal is decreased so much that two quarter-wavelengths
are longer than the wide dimension of a waveguide, energy will no longer pass through
the waveguide. This is the lower frequency limit, or CUT-OFF FREQUENCY, of a given
waveguide. In practical applications, the wide dimension of a waveguide is usually
0.7 wavelength at the operating frequency. This allows the waveguide to handle a
small range of frequencies both above and below the operating frequency. The "b"
dimension is governed by the breakdown potential of the dielectric, which is usually
air. Dimensions ranging from 0.2 to 0.5 wavelength are common for the "b" sides
of a waveguide.
Q-9. At very high frequencies, what characteristics
are displayed by ordinary insulators?
Q-10. What type of insulator
works well at very high frequencies?
1-9
Q-11. The frequency range of a waveguide is determined by what dimension?
Q-12. What happens to the bus bar dimensions of the waveguide when the frequency
is increased?
Q-13. When the frequency is decreased so that two quarter-wavelengths
are longer than the "a" (wide) dimension of the waveguide, what will happen?
Energy Propagation in Waveguides
Since energy is transferred
through waveguides by electromagnetic fields, you need a basic understanding of
field theory. Both magnetic (H FIELD) and electric field (E FIELD) are present in
waveguides, and the interaction of these fields causes energy to travel through
the waveguide. This action is best understood by first looking at the properties
of the two individual fields.
E FIELD. - An electric field
exists when a difference of potential causes a stress in the dielectric between
two points. The simplest electric field is one that forms between the plates of
a capacitor when one plate is made positive compared to the other, as shown in figure
1-11A. The stress created in the dielectric is an electric field.
Electric fields
are represented by arrows that point from the positive toward the negative potential.
The number of arrows shows the relative strength of the field. In figure 1-11A,
for example, evenly spaced arrows indicate the field is evenly distributed. For
ease of explanation, the electric field is abbreviated E field, and the lines of
stress are called E lines.
Figure 1-11A. - Simple electric fields. CAPACITOR.
Figure 1-11B - Simple electric fields. TWO-WIRE TRANSMISSION LINE.
1-10
NEETS Table of Contents
- Introduction to Matter, Energy,
and Direct Current
- Introduction to Alternating Current and Transformers
- Introduction to Circuit Protection,
Control, and Measurement
- Introduction to Electrical Conductors, Wiring
Techniques, and Schematic Reading
- Introduction to Generators and Motors
- Introduction to Electronic Emission, Tubes,
and Power Supplies
- Introduction to Solid-State Devices and
Power Supplies
- Introduction to Amplifiers
- Introduction to Wave-Generation and Wave-Shaping
Circuits
- Introduction to Wave Propagation, Transmission
Lines, and Antennas
- Microwave Principles
- Modulation Principles
- Introduction to Number Systems and Logic Circuits
- Introduction to Microelectronics
- Principles of Synchros, Servos, and Gyros
- Introduction to Test Equipment
- Radio-Frequency Communications Principles
- Radar Principles
- The Technician's Handbook, Master Glossary
- Test Methods and Practices
- Introduction to Digital Computers
- Magnetic Recording
- Introduction to Fiber Optics
Related Pages on RF Cafe
- Properties of Modes in a Rectangular Waveguide
- Properties of Modes in a Circular Waveguide
- Waveguide & Flange Selection Guide
-
Rectangular & Circular Waveguide: Equations & Fields
-
Rectangular waveguide TE1,0 cutoff frequency calculator.
- Waveguide Component
Vendors
-
NEETS - Waveguide Theory and Application
- EWHBK, Microwave Waveguide
and Coaxial Cable
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