Module 11—Microwave Principles
Pages i - ix
1-1 to 1-10
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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
21 to 2-30
2-31 to 2-40
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AI-1 to AI-6
, Index-1 to Index-2
Assignment 1 - 1-8
Assignment 2 - 9-16
Figure 1-76.—Faraday rotation.
Q-59. Ferrite devices are useful in microwave applications because they possess what properties?
Q-60. Which of the two types of electron motion (orbital movement and electron spin) is more important
in the explanation of magnetism?
Q-61. The interaction between an external field and the binding force
of an atom causes electrons to do what?
Q-62. The resonant frequency of electron wobble can be changed
by variation of what force?
Q-63. Rotating the plane of polarization of a wavefront by passing it
through a ferrite device is called what?
This chapter has presented
information on waveguide theory and application. The information that follows summarizes the important points of
WAVEGUIDES are the primary methods of transporting microwave energy. Waveguides have fewer
losses and greater power-handling capability than transmission lines. The physical size of the waveguides becomes
too great for use at frequencies less than 1000 megahertz. Waveguides are made in three basic shapes, as shown in
the first illustration. The wide, or "a," dimension determines the frequency range of the waveguide, and the
narrow, or "b," dimension determines power-handling capability as shown in the second illustration. Waveguides
handle a small range of frequencies both above and below
the primary operating frequency. Energy is transported through waveguides by the interaction of
electric and magnetic fields, abbreviated E FIELD and H FIELD, respectively. The density of the E field varies at
the same rate as the applied voltage. If energy is to travel through a waveguide, two BOUNDARY CONDITIONS must be
met: (1) An electric field, to exist at the surface of a conductor, must be perpendicular to the conductor, and
(2) a varying magnetic field must exist in closed loops parallel to the conductors and perpendicular to the
WAVEFRONTS travel down a waveguide by reflecting from the side walls in a zigzag
pattern, as shown in the figure. The striking angle, or angle of incidence (θ), is the same as the angle of
reflection (θ), causing the reflected wavefront to have the same shape as the incident wavefront. The velocity of
wavefronts traveling down a waveguide is called the GROUP VELOCITY because of the zigzag path of these wavefronts.
The group velocity is slower than the velocity of wavefronts through space.
The MODES in waveguides are divided into two categories: (1) the TRANSVERSE ELECTRIC
(TE) mode and (2) the TRANSVERSE MAGNETIC (TM) mode. Subscripts are used to complete the
description of the various TE and TM modes. The dominant mode for rectangular waveguides is shown in
WAVEGUIDE INPUT/OUTPUT METHODS are divided into three basic categories: PROBES, LOOPS,
and SLOTS. Size, shape, and placement in the waveguide are critical factors in the efficiency of all three
WAVEGUIDE/IMPEDANCE MATCHING is often necessary to reduce reflections
caused by a MISMATCH between the waveguide and the load. Matching devices called IRISES, shown in the
illustration, are used to introduce either capacitance or inductance (or a combination of both) into a waveguide.
Conductive POSTS and SCREWS can also be used for impedance matching in waveguides.
WAVEGUIDE TERMINATIONS prevent standing waves at the end of a waveguide system. They
are usually specially constructed HORNS or absorptive loads called DUMMY LOADS.
refers to the bends, twists, and joints necessary to install waveguides. E bends, H bends, and twists must have a
radius greater than two wavelengths. The CHOKE JOINT, shown in the figure, is most often used to connect two
pieces of waveguide. The ROTATING JOINT is used when a waveguide must be connected to a rotating load such as an
DIRECTIONAL COUPLERS are devices that permit the sampling of the energy in a
waveguide. Directional couplers may be constructed to sample energy in one direction only or in both directions.
The energy removed by the directional coupler is a small sample that is proportional to the magnitude of the
energy in the waveguide. An example of a directional coupler is shown in the illustration.
A RESONANT CAVITY is any space completely enclosed by conductive walls that can contain
oscillating electromagnetic fields and can possess resonant properties. Several cavity shapes are shown in the
WAVEGUIDE JUNCTIONS are of several basic types. The T-JUNCTION may be either of the E-
TYPE or the H-TYPE. The effect on the input energy depends upon which arm is used as the input. The MAGIC-T HYBRID
JUNCTION, shown at the right, is a combination of the E- and H-type T junctions.
FERRITE DEVICES combine magnetic properties with a high resistance to current flow.
Ferrites are constructed from compounds of ferrous metal oxides to achieve the desired characteristics. The fact
that the spin axes of electrons will wobble at a natural resonant frequency when subjected to an external magnetic
field is the basic principle of operation of ferrite devices. The position of a typical ferrite device within a
waveguide is shown in the figure.
ANSWERS TO QUESTIONS Q1. THROUGH Q63.
A-1. Microwave region.
Electromagnetic field theory.
A-3. The electromagnetic fields are completely confined.
A-5. Copper loss.
A-6. Skin effect.
A-8. Physical size.
characteristics of the dielectric of a capacitor.
A-10. A shorted quarter-wave section called a metallic
A-11. The "a" dimension.
A-12. The bus bar becomes wider.
A-13. Energy will no longer pass
through the waveguide.
A-14. The interaction of the electric and magnetic fields.
relative strength of the field.
A-16. Magnetic lines of force must form a continuous closed loop.
A-17. The H lines cancel.
A-18. The field must be perpendicular to the conductors.
A-20. The angles are equal.
A-21. Cutoff frequency.
A-24. Mode of operation.
A-25. Dominant mode.
A-26. 1.71 times the diameter.
Transverse electric (TE) and transverse magnetic (TM).
A-31. Size and shape.
A-33. Standing waves that cause power losses, a reduction in power-handling capability, and
an increase in frequency and sensitivity.
A-34. Metal plates.
A-36. As a
A-38. Characteristic impedance.
A-39. Absorb all energy
without producing standing waves.
A-42. Greater than 2
A-43. Choke joint.
A-44. Improperly connected joints or damaged inner surface.
Sampling energy within a waveguide.
A-46. 1/4 wavelength.
A-47. Absorb the energy not directed at
the pick-up probe and a portion of the overall energy.
A-48. The wavefront portions add.
reflected energy adds at the absorbent material and is absorbed.
A-50. Size and shape of the cavity.
A-51. Probes, loops, and slots.
A-52. The area of maximum H lines.
A-53. E-type and H-type.
A-54. The junction arm extends in a direction parallel to the H lines in the main waveguide.
A-56. Low power-handling capability and power losses.
A-57. Basic E-type junctions.
A-58. High-power duplexes.
A-59. Magnetic properties and high resistance.
A-60. Electron spin.
A-61. Wobble at a natural resonant frequency.
A-62. The applied magnetic field.
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