Waveguide Theory 12
Since waveguides are really only hollow metal pipes, the installation and the physical handling of waveguides have many similarities to ordinary plumbing. In light of this fact, the bending, twisting, joining, and installation of waveguides is commonly called waveguide plumbing. Naturally, waveguides are different in design from pipes that are designed to carry liquids or other substances. The design of a waveguide is determined by the frequency and power level of the electromagnetic energy it will carry. The following paragraphs explain the physical factors involved in the design of waveguides.
WAVEGUIDE BENDS. - The size, shape, and dielectric material of a waveguide must be constant throughout its length for energy to move from one end to the other without reflections. Any abrupt change in its size or shape can cause reflections and a loss in overall efficiency. When such a change is necessary, the bends, twists, and joints of the waveguides must meet certain conditions to prevent reflections.
Waveguides may be bent in several ways that do not cause reflections. One way is the gradual bend shown in figure 1-46. This gradual bend is known as an E bend because it distorts the E fields. The E bend must have a radius greater than two wavelengths to prevent reflections.
Figure 1-46. - Gradual E bend.
Another common bend is the gradual H bend (figure 1-47). It is called an H bend because the H fields are distorted when a waveguide is bent in this manner. Again, the radius of the bend must be greater than two wavelengths to prevent reflections. Neither the E bend in the "a" dimension nor the H bend in the "b" dimension changes the normal mode of operation.
Figure 1-47. - Gradual H bend.
A sharp bend in either dimension may be used if it meets certain requirements. Notice the two 45-degree bends in figure 1-48; the bends are 1/4l apart. The reflections that occur at the 45-degree bends cancel each other, leaving the fields as though no reflections have occurred.
Figure 1-48. - 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 1-49. The twist must be gradual and greater than 2l.
Figure 1-49. - Waveguide twist.
The flexible waveguide (figure 1-50) allows special bends which some equipment applications might require. It consists of a specially wound ribbon of conductive material, most commonly 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.
Figure 1-50. - Flexible waveguide.
WAVEGUIDE JOINTS. - Since an entire waveguide system cannot possibly be molded into one piece, the waveguide must be 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 sections of waveguide with very little power loss.
A cross-sectional view of a choke joint is shown in figure 1-51. The pressure gasket shown between the two metal surfaces forms an airtight seal. Notice in view (B) that the slot is exactly 1/4l from the "a" wall of the waveguide. The slot is also 1/4l deep, as shown in view (A), and because it is shorted at point (1), a high impedance results at point (2). Point (3) is 1/4l 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.
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.
Figure 1-52. - Rotating joint.
Distance "O" is 3/4l so that a high impedance will be presented to any unwanted modes. This is the most common design used for rotating joints, but other types may be used in specific applications.
WAVEGUIDE MAINTENANCE. - The installation of a waveguide system presents problems that are not normally encountered when dealing with other types of transmission lines. These problems often fall within the technician's area of responsibility. A brief discussion of waveguide handling, installation, and maintenance will help prepare you for this maintenance responsibility. Detailed information concerning waveguide maintenance in a particular system may be found in the technical manuals for the system.
Since a waveguide naturally has a low loss ratio, most losses in a waveguide system are caused by other factors. Improperly connected joints or damaged inner surfaces can decrease the efficiency of a system to the point that it will not work at all. Therefore, you must take great care when working with waveguides to prevent physical damage. Since waveguides are made from a soft, conductive material, such as copper or aluminum, they are very easy to dent or deform. Even the slightest damage to the inner surface of a waveguide will cause standing waves and, often, internal arcing. Internal arcing causes further damage to the waveguide in an action that is often self-sustaining until the waveguide is damaged beyond use. Part of your job as a technician will be to inspect the waveguide system for physical damage. The previously mentioned dents are only one type of physical damage that can decrease the efficiency of the system. Another problem occurs because waveguides are made from a conductive material such as copper while the structures of most ships are made from steel. When two dissimilar metals, such as copper and steel, are in direct contact, an electrical action called ELECTROLYSIS takes place that causes very rapid corrosion of the metals. Waveguides can be completely destroyed by electrolytic corrosion in a relatively short period of time if they are not isolated from direct contact with other metals. Any inspection of a waveguide system should include a detailed inspection of all support points to ensure that electrolytic corrosion is not taking place. Any waveguide that is exposed to the weather should be painted and all joints sealed. Proper painting prevents natural corrosion, and sealing the joints prevents moisture from entering the waveguide.
Moisture can be one of the worst enemies of a waveguide system. As previously discussed, the dielectric in waveguides is air, which is an excellent dielectric as long as it is free of moisture. Wet air, however, is a very poor dielectric and can cause serious internal arcing in a waveguide system. For this reason care is taken to ensure that waveguide systems are pressurized with air that is dry. Checking the pressure and moisture content of the waveguide air may be one of your daily system maintenance duties.
More detailed waveguide installation and maintenance information can be found in the technical manuals that apply to your particular system.
Another good source is the Electronics Installation and Maintenance Handbooks (EIMB) published by Naval Sea Systems Command. Installation Standards Handbook EIMB, NAVSEA 0967-LP-000-0110, is the volume that deals with waveguide installation and maintenance.
Q.41 What is the result of an abrupt change in the size, shape, or dielectric of a waveguide?
Q.42 A waveguide bend must have what minimum radius?
Q.43 What is the most common type of waveguide joint?
Q.44 What is the most likely cause of losses in waveguide systems?