Page:Electronics Technician - Volume 7 - Antennas and Wave Propagation - NAVEDTRA 14092.pdf/81

 A sharp bend in either dimension may be used if it meets certain requirements. Notice the two 45-degree bends in figure 3-50; the bends are 1/4 part. The reflections that occur at the 45-degree bends cancel each other, leaving the fields as though no reflections have occurred.

Figure 3-50.—Sharp bends.

Sometimes the electromagnetic fields must be rotated so that they are in the proper phase to match the phase of the load. This may be accomplished by twisting the waveguide as shown in figure 3-51. The twist must be gradual and greater than 2A.

Figure 3-51.—Waveguide twist.

The flexible waveguide (fig. 3-52) allows special bends, which some equipment applications might require. It consists of a specially wound ribbon of conductive material, the most commonly used is brass, with the inner surface plated with chromium. Power losses are greater in the flexible waveguide because the inner surfaces are not perfectly smooth. Therefore, it is only used in short sections where no other reasonable solution is available.

Waveguide Joints

Since an entire waveguide system cannot possibly be molded into one piece, the waveguide must be

Figure 3-52.—Flexible waveguide.

constructed in sections and the sections connected with joints. The three basic types of waveguide joints are the PERMANENT, the SEMIPERMANENT, and the ROTATING JOINTS. Since the permanent joint is a factory-welded joint that requires no maintenance, only the semipermanent and rotating joints will be discussed.

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 the sections of the waveguide with very little power loss.

A cross-sectional view of a choke joint is shown in figure 3-53. The pressure gasket shown between the two metal surfaces forms an airtight seal. Notice in view B that the slot is exactly 1/4λ from the "a" wall of the waveguide. The slot is also 1/4λ deep,as shown in view A, and because it is shorted at point 1, a high impedance results at point 2. Point 3 is 1/4λ 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 or loss.

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 3-54, 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 TM$0,1$ mode. 3-23