Module 11—Microwave Principles
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Assignment 1 - 1-8
Assignment 2 - 9-16
corrosion is not taking place. Any waveguide that is exposed to the weather should be painted and all joints
sealed. Proper painting prevents natural corrosion, and sealing the joints prevents moisture from entering the
Moisture can be one of the worst enemies of a waveguide system. As previously discussed, the dielectric in
waveguides is air, which is an excellent dielectric as long as it is free of moisture. Wet air, however, is a very
poor dielectric and can cause serious internal arcing in a waveguide system. For this reason care is taken to
ensure that waveguide systems are pressurized with air that is dry. Checking the pressure and moisture content of
the waveguide air may be one of your daily system maintenance duties.
More detailed waveguide installation and
maintenance information can be found in the technical manuals that apply to your particular system. Another good
source is the Electronics Installation and Maintenance Handbooks (EIMB) published by Naval Sea Systems Command.
Installation Standards Handbook EIMB, NAVSEA 0967-LP-000-0110, is the volume that deals with waveguide
installation and maintenance.
Q-41. What is the result of an abrupt change in the size, shape, or dielectric of a waveguide?
Q-42. A waveguide bend must have what minimum radius?
Q-43. What is the most common type of waveguide
Q-44. What is the most likely cause of losses in waveguide systems?
The discussion of waveguides, up to this point, has been concerned only with the transfer of energy from one point
to another. Many waveguide devices have been developed, however, that modify the energy in some fashion during
transit. Some devices do nothing more than change the direction of the energy. Others have been designed to change
the basic characteristics or power level of the electromagnetic energy.
This section will explain the basic
operating principles of some of the more common waveguide
devices, such as DIRECTIONAL COUPLERS, CAVITY
RESONATORS, and HYBRID JUNCTIONS.
The directional coupler is a
device that provides a method of sampling energy from within a waveguide for measurement or use in another
circuit. Most couplers sample energy traveling in one direction only. However, directional couplers can be
constructed that sample energy in both directions. These are called BIDIRECTIONAL couplers and are widely used in
radar and communications systems.
Directional couplers may be constructed in many ways. The coupler
illustrated in figure 1-53 is constructed from an enclosed waveguide section of the same dimensions as the
waveguide in which the energy is to be sampled. The "b" wall of this enclosed section is mounted to the "b" wall
of the waveguide from which the sample will be taken. There are two holes in the "b" wall between the sections of
the coupler. These two holes are 1/4λ apart. The upper section of the directional coupler has a wedge of
energy-absorbing material at one end and a pickup probe connected to an output jack at the other end. The
absorbent material absorbs the energy not directed at the probe and a portion of the overall energy that enters
Figure 1-53.—Directional coupler.
Figure 1-54 illustrates two portions of the incident wavefront in a waveguide. The waves travel down the
waveguide in the direction indicated and enter the coupler section through both holes. Since both portions of the
wave travel the same distance, they are in phase when they arrive at the pickup probe. Because the waves are in
phase, they add together and provide a sample of the energy traveling down the waveguide. The sample taken is only
a small portion of the energy that is traveling down the waveguide. The magnitude of the sample, however, is
proportional to the magnitude of the energy in the waveguide. The absorbent material is designed to ensure that
the ratio between the sample energy and the energy in the waveguide is constant. Otherwise the sample would
contain no useful information.
Figure 1-54.—Incident wave in a directional coupler designed to sample incident waves.
The ratio is usually stamped on the coupler in the form of an attenuation factor.
The effect of a
directional coupler on any reflected energy is illustrated in figure 1-55. Note that these two waves do not travel
the same distance to the pickup probe. The wave represented by the dotted line travels 1/2λ further and arrives at
the probe 180 degrees out of phase with the wave represented by
the solid line. Because the waves are 180 degrees out of phase at the probe, they cancel each other
and no energy is induced in the pickup probe. When the reflected energy arrives at the absorbent material, it adds
and is absorbed by the material.
Figure 1-55.—Reflected wave in a directional coupler.
A directional coupler designed to sample reflected energy is shown in figure 1-56. The absorbent material and
the probe are in opposite positions from the directional coupler designed to sample the incident energy. This
positioning causes the two portions of the reflected energy to arrive at the probe in phase, providing a sample of
the reflected energy. The sampled transmitted energy, however, is absorbed by the absorbent material.
Figure 1-56.—Directional coupler designed to sample reflected energy.
A simple bidirectional coupler for sampling both transmitted and reflected energy can be constructed
mounting two directional couplers on opposite sides of a waveguide, as shown in figure 1-57.
Figure 1-57.—Bidirectional coupler.
Q-45. What is the primary purpose of a directional coupler?
Q-46. How far apart are the two
holes in a simple directional coupler?
Q-47. What is the purpose of the absorbent material in a
Q-48. In a directional coupler that is designed to sample the incident energy, what
happens to the two portions of the wavefront when they arrive at the pickup probe?
Q-49. What happens to
reflected energy that enters a directional coupler that is designed to sample incident energy?
In ordinary electronic equipment a resonant circuit consists of a
coil and a capacitor that are connected either in series or in parallel. The resonant frequency of the circuit is
increased by reducing the capacitance, the inductance, or both. A point is eventually reached where the inductance
and the capacitance can be reduced no further. This is the highest frequency at which a conventional circuit can
The upper limit for a conventional resonant circuit is between 2000 and 3000 megahertz. At
these frequencies, the inductance may consist of a coil of one-half turn, and the capacitance may simply be the
stray capacitance of the coil. Tuning a one-half turn coil is very difficult and tuning stray capacitance is even
more difficult. In addition, such a circuit will handle only very small amounts of current.
NEETS, Module 10,
Introduction to Wave Propagation explained that a 1/4λ section of transmission line can act as a resonant circuit.
The same is true of a 1/4λ section of waveguide. Since a waveguide is hollow, it can also be considered as a
By definition, a resonant cavity is any space completely enclosed by conducting walls that
contain oscillating electromagnetic fields and possess resonant properties. The cavity has many
advantages and uses at microwave frequencies. Resonant cavities have a very high Q and can be built to
handle relatively large amounts of power. Cavities with a Q value in excess of 30,000 are not uncommon. The high Q
gives these devices a narrow bandpass and allows very accurate tuning. Simple, rugged construction is an
Although cavity resonators, built for different frequency ranges and applications, have
a variety of
shapes, the basic principles of operation are the same for all.
One example of a cavity resonator is the
rectangular box shown in figure 1-58A. It may be thought
of as a section of rectangular waveguide closed at
both ends by conducting plates. The frequency at which the resonant mode occurs is 1/2λ of the distance between
the end plates. The magnetic and electric field patterns in the rectangular cavity are shown in figure 1-58B.
Figure 1-58A.—Rectangular waveguide cavity resonator.
Figure 1-58B.—Rectangular waveguide cavity resonator.
FIELD PATTERNS OF A SIMPLE MODE.
The rectangular cavity is only one of many cavity devices that are useful as high-frequency resonators.
Figures 1-59A, 1-59B, 1-59C, and 1-59D show the development of a cylindrical resonant cavity from an infinite
number of quarter-wave sections of transmission line. In figure 1-59A the 1/4λ section is shown to be equivalent
to a resonant circuit with a very small amount of inductance and capacitance. Three 1/4λ sections are joined in
parallel in figure 1-59B. Note that although the
current-carrying ability of several 1/4λ sections is greater than that of any one section, the
resonant frequency is unchanged. This occurs because the addition of inductance in parallel lowers the total
inductance, but the addition of capacitance in parallel increases the total capacitance by the same proportion.
Thus, the resonant frequency remains the same as it was for one section. The increase in the number of current
paths also decreases the total resistance and increases the Q of the resonant circuit. Figure 1-59C shows an
intermediate step in the development of the cavity. Figure 1-59D shows a completed cylindrical resonant cavity
with a diameter of 1/2λ at the resonant frequency.
Figure 1-59A.—Development of a cylindrical resonant cavity.
QUARTER-WAVE SECTION EQUIVALENT TO LC CIRCUIT.
Figure 1-59B.—Development of a cylindrical resonant cavity.
QUARTER-WAVE LINES JOINED.
Figure 1-59C.—Development of a cylindrical resonant cavity.
CYLINDRICAL RESONANT CAVITY BEING FORMED FROM QUARTER-WAVE SECTIONS.
Figure 1-59D.—Development of a cylindrical resonant cavity.
CYLINDRICAL RESONANT CAVITY.
There are two variables that determine the primary frequency of any resonant cavity. The first variable is
PHYSICAL SIZE. In general, the smaller the cavity, the higher its resonant frequency. The second controlling
factor is the SHAPE of the cavity. Figure 1-60 illustrates several cavity shapes that are commonly used. Remember
from the previously stated definition of a resonant cavity that any completely enclosed conductive surface,
regardless of its shape, can act as a cavity resonator.
Figure 1-60.—Several types of cavities.
Cavity resonators are energized in basically the same manner as waveguides and have a similar field
distribution. If the cavity shown in figure 1-61 were energized in the TE mode, the electromagnetic wave would
reflect back and forth along the Z axis and form standing waves. These standing waves would form a field
configuration within the cavity that would have to satisfy the same boundary conditions as those in a waveguide.
Modes of operation in the cavity are described in terms of the fields that exist in the X, Y, and Z dimensions.
Three subscripts are used; the first subscript indicates the number of 1/2λ along the X axis; the second subscript
indicates the number of 1/2λ along the Y axis; and the third subscript indicates the number of 1/2λ along the Z
Figure 1-61.—Rectangular cavity resonator.
Energy can be inserted or removed from a cavity by the same methods that are used to couple energy
into and out of waveguides. The operating principles of probes, loops, and slots are the same whether
in a cavity or a waveguide. Therefore, any of the three methods can be used with cavities to inject or remove
The resonant frequency of a cavity can be varied by changing any of three parameters: cavity volume,
cavity capacitance, or cavity inductance. Changing the frequencies of a cavity is known as TUNING. The mechanical
methods of tuning a cavity may vary with the application, but all methods use the same electrical principles.
AA mechanical method of tuning a cavity by changing the volume (VOLUME TUNING) is illustrated in figure 1-62.
Varying the distance d will result in a new resonant frequency because the inductance and the capacitance of the
cavity are changed by different amounts. If the volume is decreased, the resonant frequency will be higher. The
resonant frequency will be lower if the volume of the cavity is made larger.
Figure 1-62.—Cavity tuning by volume.
CAPACITIVE TUNING of a cavity is shown in figure 1-63A. An adjustable slug or screw is
placed in the area of maximum E lines. The distance d represents the distance between two capacitor plates. As the
slug is moved in, the distance between the two plates becomes smaller and the capacitance increases. The increase
in capacitance causes a decrease in the resonant frequency. As the slug is moved out, the resonant frequency of
the cavity increases.
Figure 1-63A.—Methods of changing the resonant frequency of a cavity.
CHANGING THE CAPACITANCE.
Figure 1-63B.—Methods of changing the resonant frequency of a cavity.
CHANGING THE INDUCTANCE.
INDUCTIVE TUNING is accomplished by placing a nonmagnetic slug in the area of maximum H
shown in figure 1-63B. The changing H lines induce a current in the slug that sets up an opposing H field. The
opposing field reduces the total H field in the cavity, and therefore reduces the total inductance. Reducing the
inductance, by moving the slug in, raises the resonant frequency. Increasing the inductance, by moving the slug
out, lowers the resonant frequency.
Resonant cavities are widely used in the microwave range, and many of the
applications will be studied in chapter 2. For example, most microwave tubes and transmitting devices use cavities
in some form to generate microwave energy. Cavities are also used to determine the frequency of the energy
traveling in a waveguide, since conventional measurement devices do not work well at microwave frequencies.
Q-50. What two variables determine the primary frequency of a resonant cavity?
Q-51. Energy can be
inserted or removed from a cavity by what three methods?
Q-52. Inductive tuning of a resonant cavity is
accomplished by placing a nonmagnetic slug in what area?
may have assumed that when energy traveling down a waveguide reaches a junction, it simply divides and follows the
junction. This is not strictly true. Different types of junctions affect the energy in different ways. Since
waveguide junctions are used extensively in most systems, you need to understand the basic operating principles of
those most commonly used.
The T JUNCTION is the most simple of the commonly used waveguide junctions. T junctions are divided into two
basic types, the E-TYPE and the H-TYPE. HYBRID JUNCTIONS are more complicated developments of the basic T
junctions. The MAGIC-T and the HYBRID RING are the two most commonly used hybrid junctions.
JUNCTION.—An E-type T junction is illustrated in figure 1-64, view (A). It is called an E-type T junction because
the junction arm extends from the main waveguide in the same direction as the E field in the waveguide.
Introduction to Matter, Energy, and Direct Current,
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,
, Introduction to Number Systems and Logic Circuits, Introduction
to Microelectronics, Principles of Synchros, Servos, and Gyros
Introduction to Test Equipment
, Radar Principles,
The Technician's Handbook,
Master Glossary, Test Methods and Practices,
Introduction to Digital Computers,
Magnetic Recording, Introduction to Fiber Optics