Sections of waveguide must be taken apart for maintenance and repair. A semipermanent joint,
called a CHOKE JOINT, is most commonly used for this purpose. The choke joint provides good
electromagnetic continuity between sections of waveguide with very little power loss.
A cross-sectional view of a choke joint is shown in figures 1-51A and 1-51B. The pressure gasket
shown between the two metal surfaces forms an airtight seal. Notice in figure 1-51B that the slot is
exactly 1/4! from the "a" wall of the waveguide. The slot is also 1/4! deep, as shown in figure 1-51A, and
because it is shorted at point (1), a high impedance results at point (2). Point (3) is 1/4! from point (2).
The high impedance at point (2) results in a low impedance, or short, at point (3). This effect creates a
good electrical connection between the two sections that permits energy to pass with very little reflection
Figure 1-51A.Choke joint.
Figure 1-51B.Choke joint.
Whenever a stationary rectangular waveguide is to be connected to a rotating antenna, a rotating
joint must be used. A circular waveguide is normally used in a rotating joint. Rotating a rectangular
waveguide would cause field pattern distortion. The rotating section of the joint, illustrated in figure 1-52,
uses a choke joint to complete the electrical connection with the stationary section. The circular
waveguide is designed so that it will operate in the TM0,1 mode. The rectangular sections are attached as
shown in the illustration to prevent the circular waveguide from operating in the wrong mode.