Practical_Antenna_Handbook_0071639586

468 P a r t V I : A n t e n n a s f o r O t h e r F r e q u e n c i e s
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 quarter-Âwavelength
circular slot cut at a distance of one quarter-wavelength from the waveguide aperture.
The two flanges are shown in side view in Fig. 20.14A, and the slotted end view is
shown in Fig. 20.14B. The method for fitting the two ends together is shown in the
oblique view in Fig. 20.14C.
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. 20.15. 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 TM 01 mode is chosen. 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 20.16 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.
The rectangular input waveguide of Fig. 20.16 operates in the TE 10 mode that is
most efficient for rectangular waveguide. The E-field lines of force couple with the circular
segment, thereby setting up a TM 01 mode wave. The TM 01 mode has the required
symmetry to permit coupling across the junction, where it meets another transition
zone and is reconverted to TE 10 mode.
Waveguide Coupling Methods
Except possibly for the case where an oscillator exists inside a waveguide, it is necessary
to have some form of input or output coupling in a waveguide system. There are
three basic types of coupling used in a microwave waveguide: capacitive (or probe), inductive
(or loop), and aperture (or slot).
Capacitive coupling is shown in Fig. 20.17. This type of coupling uses a vertical
radiator inserted into one end of the waveguide. Typically, the probe is a quarter-
Stationary section
Choke joint
Rotating
section
Figure 20.15 Basic rotating joint.
Rotating joint