Summary - Page 44r 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. ">

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 SUMMARY This chapter has presented information on waveguide theory and application. The information that follows summarizes the important points of this chapter. 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 electric field. 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 (q), is the same as the angle of reflection (q), 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 the figure. 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 input/output methods. 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. WAVEGUIDE PLUMBING 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 antenna. 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 illustration. 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.