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386 Microwave waveguides and antennas
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Waveguide terminations 387
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388 Microwave waveguides and antennas
Permanent plate at end Contact is at point of minimum current /4
Removable /4 wave end piece Removable short circuit Actual short here Virtual short here /4 Thumb screw 19-12 End terminations
Half wave channel Adjustable plunger
quarter-wavelength from the end This point is a minimum-current node, so I 2R losses in the contact resistance become less important The adjustable short circuit is shown in Fig 19-12C The walls of the waveguide and the surface of the plunger form a half-wavelength channel Because the metallic end of the channel is a short circuit, the impedance reflected back to the front of the plunger is zero ohms, or nearly so Thus, a virtual short exists at the points shown By this means, the contact (or joint) resistance problem is overcome
Waveguide joints and bends
Joints and bends in any form of transmission line or waveguide are seen as impedance discontinuities, and so are points at which disruptions occur Thus, improperly formed bends and joints are substantial contributors to a poor VSWR In general, bends, twists, joints, or abrupt changes in waveguide dimension can deteriorate the VSWR by giving rise to reflections Extensive runs of waveguide are sometimes difficult to make in a straight line Although some installations do permit a straight waveguide, many others require directional change This possibility is especially likely on shipboard installations Figure 19-13A shows the proper ways to bend a waveguide around a corner In each case, the radius of the bend must be at least two wavelengths at the lowest frequency that will be propagated in the system The twist shown in Fig 19-13B is used to rotate the polarity of the E and H fields by 90 This type of section is sometimes used in antenna arrays for phasing
Waveguide joints and bends 389 the elements As in the case of the bend, the twist must be made over a distance of at least two wavelengths When an abrupt 90 transition is needed, it is better to use two successive 45 bends spaced one-quarter wavelength apart (see Fig 19-13C) The theory (behind this kind of bend) is to cause the interference of the direct reflection of one bend, with the inverted reflection of the other The resultant relationship between the fields is reconstructed as if no reflections had taken place Joints are necessary in practical waveguides because it simply isn t possible to construct a single length of practical size for all situations Three types of common joints are used: permanent, semipermanent, and rotating To make a permanent joint the two waveguide ends must be machined extremely flat so that they can be butt-fitted together A welded or brazed seam bonds the two sections together Because such a surface represents a tremendous discontinuity, reflections and VSWR will result unless the interior surfaces are milled flat and then polished to a mirror-like finish A semipermanent joint allows the joint to be disassembled for repair and maintenance, as well as allowing easier on-site assembly The most common example of this class is the choke joint shown in Fig 19-14 One surface of the choke joint is machined flat and is a simple butt-end planar flange The other surface is the mate to the planar flange, but it has a quarterwavelength circular slot cut at a distance of one-quarter wavelength from the waveguide aperture The two flanges are shown in side view in Fig 19-14A, and the slotted end view is shown in Fig 19-14B The method for fitting the two ends together is shown in the oblique view in Fig 19-14C
At least 2
/4 /4
Narrow dimension Wide dimension Side view
19-13 Bends in waveguide must be gentle
390 Microwave waveguides and antennas
Flanges Short circuit
/4 /4 Flanges
19-14 Choke joint
Rotating joints are used in cases where the antenna has to point in different directions at different times Perhaps the most common example of such an application is the radar antenna The simplest form of rotating joint is shown in Fig 19-15 The key to its operation is that the selected mode is symmetrical about the rotating axis For this reason, a circular waveguide operating in the TM01 mode is selected In this rotating choke joint, the actual waveguide rotates but the internal fields do not (thereby minimizing reflections) Because most waveguide is rectangular, however, a somewhat more complex system is needed Figure 19-16 shows a rotating joint consisting of two circular waveguide sections inserted between segments of rectangular waveguide On each end of the joint, there is a rectangular-to-circular transition section In Fig 19-16, the rectangular input waveguide operates in the TE10 mode that is most efficient for rectangular waveguide The E-field lines of force couple with the circular segment, thereby setting up a TM01 mode wave The TM01 mode has the required symmetry to permit coupling across the junction, where it meets another transition zone and is reconverted to TE10 mode
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