Waveguide plumbing is really only hollow metal piping, the installation and the physical handling have many similarities to ordinary plumbing. In light of this fact, the bending, twisting, joining, and installation 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 PLUMBING 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 the figure below. 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.
Gradual E bend.
Another common bend is the gradual H bend (the figure below). 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.
Gradual H bend.
A sharp bend in either dimension may be used if it meets certain requirements. Notice the two 45-degree bends in the figure below; the bends are 1/4 wavelength apart. The reflections that occur at the 45-degree bends cancel each other, leaving the fields as though no reflections have occurred.
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 the next figure below. The twist must be gradual and greater than 2 wavelengths.
The flexible waveguide (the last figure) 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.
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