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
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 the observer.
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 1-72.
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 ELECTRON SPIN.
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 properties.
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.
FERRITE ATTENUATORS.—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.
FERRITE ISOLATORS.—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.
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