Module 11 − Microwave Principles
Pages 1-41 through 1-40
Figure 1-64, view (B), illustrates cross-sectional views of the E-type T junction
with inputs fed into the various arms. For simplicity, the magnetic lines that are
always present with an electric field have been omitted. In view (K), the input
is fed into arm b and the outputs are taken from the a and c arms. When the E field
arrives between points 1 and 2, point 1 becomes positive and point 2 becomes negative.
The positive charge at point 1 then induces a negative charge on the wall at point
3. The negative charge at point 2 induces a positive charge at point 4.
These charges cause the fields to form 180 degrees out of phase in the main waveguide;
therefore, the outputs will be 180 degrees out of phase with each other. In view
(L), two in-phase inputs of equal amplitude are fed into the a and c arms. The signals
at points 1 and 2 have the same phase and amplitude. No difference of potential
exists across the entrance to the b arm, and no energy will be coupled out. However,
when the two signals fed into the a and c arms are 180 degrees out of phase, as
shown in view (M), points 1 and 2 have a difference of potential. This difference
of potential induces an E field from point 1 to point 2 in the b arm, and energy
is coupled out of this arm. Views (N) and (P) illustrate two methods of obtaining
two outputs with only one input.
Figure 1-64. - E fields in an E-type T junction.
H-TYPE T JUNCTION. - An H-type T junction is illustrated in
figure 1-65A. It is called an H-type T junction because the long axis of the "b"
arm is parallel to the plane of the magnetic lines of force in the waveguide. Again,
for simplicity, only the E lines are shown in this figure. Each X indicates an E
line moving away from the observer. Each dot indicates an E line is moving toward
In view (1) of figure 1-65B, the signal is fed into arm b and in-phase outputs
are obtained from the a and c arms. In view (2), in-phase signals are fed into arms
a and c and the output signal is obtained from the b arm because the fields add
at the junction and induce E lines into the b arm. If 180-degree-out-of- phase signals
are fed into arms a and c, as shown in view (3), no output is obtained from the
b arm because the opposing fields cancel at the junction. If a signal is fed into
the a arm, as shown in view (4), outputs will be obtained from the b and c arms.
The reverse is also true. If a signal is fed into the c arm, outputs will be obtained
from the a and b arms.
Figure 1-65A. - E fields in an H-type junction. H-TYPE T JUNCTION.
Figure 1-65B. - E fields in an H-type junction.
Fields for Various Inputs
Magic-T Hybrid Junction
A simplified version of the magic-T hybrid junction is shown in figure
1-66. The magic-T is a combination of the H-type and E-type T junctions. The most
common application of this type of junction is as the mixer section for microwave
radar receivers. Its operation as a mixer will be discussed in later NEETS modules.
At present, only the fields within the magic-T junction will be discussed.
Figure 1-66. - Magic-T hybrid junction.
If a signal is fed into the b arm of the magic-T, it will divide into two out-of-phase
components. As shown in figure 1-67A, these two components will move into the a
and c arms. The signal entering the b arm will not enter the d arm because of the
zero potential existing at the entrance of the d arm. The potential must be zero
at this point to satisfy the boundary conditions of the b arm. This absence of potential
is illustrated in figures 1-67B and 1-67C where the magnitude of the E field in
the b arm is indicated by the length of the arrows. Since the E lines are at maximum
in the center of the b arm and minimum at the edge where the d arm entrance is located,
no potential difference exists across the mouth of the d arm.
Figure 1-67A. - Magic-T with input to arm b.
Figure 1-67B. - Magic-T with input to arm b.
Figure 1-67C. - Magic-T with input to arm b.
In summary, when an input is applied to arm b of the magic-T hybrid junction,
the output signals from arms a and c are 180 degrees out of phase with each other,
and no output occurs at the d arm.
The action that occurs when a signal is fed into the d arm of the magic-T is
illustrated in figure 1-68. As with the H-type T junction, the signal entering the
d arm divides and moves down the a and c arms as outputs which are in phase with
each other and with the input. The shape of the E fields in motion is shown by the
numbered curved slices. As the E field moves down the d arm, points 2 and 3 are
at an equal potential. The energy divides equally into arms a and c, and the E fields
in both arms become identical in shape. Since the potentials on both sides of the
b arm are equal, no potential difference exists at the entrance to the b arm, resulting
in no output.
Figure 1-68. - Magic-T with input to arm d.
When an input signal is fed into the a arm as shown in figure 1-69, a portion
of the energy is coupled into the b arm as it would be in an E-type T junction.
An equal portion of the signal is coupled through the d arm because of the action
of the H-type junction. The c arm has two fields across it that are out of phase
with each other. Therefore the fields cancel, resulting in no output at the c arm.
The reverse of this action takes place if a signal is fed into the c arm, resulting
in outputs at the b and d arms and no output at the a arm.
Figure 1-69. - Magic-T with input to arm a.
Unfortunately, when a signal is applied to any arm of a magic-T, the flow of
energy in the output arms is affected by reflections. Reflections are caused by
impedance mismatching at the junctions. These reflections are the cause of the two
major disadvantages of' the magic-T. First, the reflections represent a power loss
since all the energy fed into the junction does not reach the load which the arms
feed. Second, the reflections produce standing waves that can result in internal
arching. Thus the maximum power a magic-T can handle is greatly reduced.
Reflections can be reduced by using some means of' impedance matching that does
not destroy the shape of' the junctions. One method is shown in figure 1-70. a post
is used to match the H plane, and an iris is used to match the E plane. Even though
this method reduces reflections, it lowers the power- handling capability even further.
Figure 1-70. - Magic-T impedance matching.
HYBRID RING. - a type of hybrid junction that overcomes the
power limitation of the magic-T is the hybrid ring, also called a RAT RACE. The
hybrid ring, illustrated in figure 1-71A, is actually a modification of the magic-T.
It is constructed of rectangular waveguides molded into a circular pattern. The
arms are joined to the circular waveguide to form E-type T junctions. Figure 1-71B
shows, in wavelengths, the dimensions required for a hybrid ring to operate properly.
Figure 1-71A. - Hybrid ring with wavelength measurements.
Figure 1-71B. - Hybrid ring with wavelength measurements.
The hybrid ring is used primarily in high-powered radar and communications systems
to perform two functions. During the transmit period, the hybrid ring couples microwave
energy from the transmitter to the antenna and allows no energy to reach the receiver.
During the receive cycle, the hybrid ring couples energy from the antenna to the
receiver and allows no energy to reach the transmitter. Any device that performs
both of these functions is called a DUPLEXER. a duplexer permits a system to use
the same antenna for both transmitting and receiving. Since the only common application
of the hybrid ring is as a duplexer, the details of hybrid ring operation will be
explained in later NEETS modules concerning duplexers.
Q-53. What are the two basic types of T junctions?
Q-54. Why is the H-type T junction so named?
Q-55. The magic-T is composed of what two basic types of T junctions?
Q-56. What are the primary disadvantages of the magic-T?
Q-57. What type of junctions are formed where the arms of a hybrid ring meet
the main ring?
Q-58. Hybrid rings are used primarily for what purpose?
A FERRITE is a device that is composed of material that causes it to have useful
magnetic properties and, at the same time, high resistance to current flow. The
primary material used in the construction of ferrites is normally a compound of
iron oxide with impurities of other oxides added. The compound of iron oxide retains
the properties of the ferromagnetic atoms, and the impurities of the other oxides
increase the resistance to current flow. This combination of properties is not found
in conventional magnetic materials. Iron, for example, has good magnetic properties
but a relatively low resistance to current flow. The low resistance causes eddy
currents and significant power losses at high frequencies (You may want to review
NEETS, Module 2, Introduction to Alternating Current and Transformers, Chapter 5).
Ferrites, on the other hand, have sufficient resistance to be classified as semiconductors.
The compounds used in the composition of ferrites can be compared to the more
familiar compounds used in transistors. As in the construction of transistors, a
wide range of magnetic and electrical properties can be produced by the proper choice
of atoms in the right proportions. An example of a ferrite device is shown in figure
Figure 1-72. - Ferrite attenuator.
Ferrites have long been used at conventional frequencies in computers, television,
and magnetic recording systems. The use of ferrites at microwave frequencies is
a relatively new development and has had considerable influence on the design of
microwave systems. In the past, the microwave equipment was made to conform to the
frequency of the system and the design possibilities were limited. The unique properties
of ferrites provide a variable reactance by which microwave energy can be manipulated
to conform to the microwave system. At present, ferrites are used as Load IsOLATORS,
Phase SHIFTERS, VARIABLE ATTENUATORS, ModulatorS, and Switches in microwave systems.
The operation of ferrites as isolators, attenuators, and phase shifters will be
explained in the following paragraphs. The operation of ferrites in other applications
will be explained in later NEETS modules. Ferromagnetism is a continuation of the
conventional domain theory of magnetism that was explained in NEETS, Module 1, Matter,
Energy, and Direct Current. a review of the section on magnetism might be helpful
to you at this point.
The magnetic property of any material is a result of electron movement within
the atoms of the material. Electrons have two basic types of motion. The most familiar
is the ORBITAL movement of the electron about the nucleus of the atom. Less familiar,
but even more important, is the movement of the electron about its own axis, called
You will recall that magnetic fields are generated by current flow. Since current
is the movement of electrons, the movement of the electrons within an atom create
magnetic fields. The magnetic fields caused by the movement of the electrons about
the nucleus have little effect on the magnetic properties of a material. The magnetic
fields caused by electron spin combine to give a material magnetic properties. The
different types of electron movement are illustrated in figure 1-73. In most materials
the spin axes of the electrons are so randomly arranged that the magnetic fields
largely cancel out and the material displays no significant magnetic properties.
The electron spin axes within some materials, such as iron and nickel, can be caused
to align by applying an external magnetic field. The alignment of the electrons
within a material causes the magnetic fields to add, and the material then has magnetic
Figure 1-73. - Two types of electron movement.
In the absence of an external force, the axis of any spinning object tends to
remain pointed in one direction. Spinning electrons behave the same way. Therefore,
once the electrons are aligned, they tend to remain aligned even when the external
field is removed. Electron alignment in a ferrite is caused by the orbital motion
of the electrons about the nucleus and the force that holds the atom together. When
a static magnetic field is applied, the electrons try to align their spin axes with
the new force. The attempt of the electrons to balance between the interaction of
the new force and the binding force causes the electrons to wobble on their axes,
as shown in figure 1-74. The wobble of the electrons has a natural resonant WOBBLE
Frequency that varies with the strength of the applied field. Ferrite action is
based on this behavior of the electrons under the influence of an external field
and the resulting wobble frequency.
Figure 1-74. - Electron wobble in a magnetic field.
A ferrite attenuator can be constructed that will attenuate a particular microwave
frequency and allow all others to pass unaffected. This can be done by placing a
ferrite in the center of a waveguide, as shown in figure 1-72. The ferrite must
be positioned so that the magnetic fields caused by its electrons are perpendicular
to the energy in the waveguide. a steady external field causes the electrons to
wobble at the same frequency as the energy that is to be attenuated.
Since the wobble frequency is the same as the energy frequency, the energy in
the waveguide always adds to the wobble of the electrons. The spin axis of the electron
changes direction during the wobble motion and energy is used. The force causing
the increase in wobble is the energy in the waveguide. Thus, the energy in the waveguide
is attenuated by the ferrite and is given off as heat. Energy in the waveguide that
is a different frequency from the wobble frequency of the ferrite is largely unaffected
because it does not increase the amount of electron wobble. The resonant frequency
of electron wobble can be varied over a limited range by changing the strength of
the applied magnetic field.
An isolator is a ferrite device that can be constructed so that it allows microwave
energy to pass in one direction but blocks energy in the other direction in a waveguide.
This isolator is constructed by placing a piece of ferrite off-center in a waveguide,
as shown in figure 1-75. a magnetic field is applied by the magnet and adjusted
to make the electron wobble frequency of the ferrite equal to the frequency of the
energy traveling down the waveguide. Energy traveling down the waveguide from left
to right will set up a rotating magnetic field that rotates through the ferrite
material in the same direction as the natural wobble of the electrons. The aiding
magnetic field increases the wobble of the ferrite electrons so much that almost
all of the energy in the waveguide is absorbed and dissipated as heat. The magnetic
fields caused by energy traveling from right to left rotate in the opposite direction
through the ferrite and have very little effect on the amount of electron wobble.
In this case the fields attempt to push the electrons in the direction opposite
the natural wobble and no large movements occur. Since no overall energy exchange
takes place, energy traveling from right to left is affected very little.
Figure 1-75. - One-way isolator.
FERRITE Phase SHIFTER. - When microwave energy is passed through
a piece of ferrite in a magnetic field, another effect occurs. If the frequency
of the microwave energy is much greater than the electron wobble frequency, the
plane of polarization of the wavefront is rotated. This is known as the FARADAY
ROTATION EFFECT and is illustrated in figure 1-76. a ferrite rod is placed along
the axis of the waveguide, and a magnetic field is set up along the axis by a coil.
As a wavefront enters the section containing the ferrite, it sets up a limited motion
in the electrons. The magnetic fields of the wavefront and the wobbling electrons
interact, and the polarization of the wavefront is rotated. The amount of rotation
depends upon the length of the ferrite rod. The direction of rotation depends upon
the direction of the external magnetic field and can be reversed by reversing the
field. The direction of rotation will remain constant, no matter what direction
the energy in the waveguide travels, as long as the external field is not changed.
Pages 1-61 through 1-68
and Direct Current
||Alternating Current and Transformers
||Circuit Protection, Control, and Measurement
||Electrical Conductors, Wiring Techniques,
and Schematic Reading
||Generators and Motors
||Electronic Emission, Tubes, and Power Supplies
||Solid-State Devices and Power Supplies
||Wave-Generation and Wave-Shaping Circuits
||Wave Propagation, Transmission Lines, and
||Introduction to Number Systems and Logic Circuits
||- Introduction to Microelectronics
||Principles of Synchros, Servos, and Gyros
||Introduction to Test Equipment
||Radio-Frequency Communications Principles
||The Technician's Handbook, Master Glossary
||Test Methods and Practices
||Introduction to Digital Computers
||Introduction to Fiber Optics
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Series (NEETS) content is U.S. Navy property in the public domain.
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