scope:

This handbook is intended for the electronic installations technician to be used as a guide for fabricating rectangular waveguide bends and twists used in the microwave region of the electronic spectrum.

Where bends and twists are required in equipment, it may not always be possible to obtain bends and twists of the exact lengths or radii required because they are not standard, and therefore are not carried in supply, or manufactured by industry as a stock item. By using the techniques described in this handbook such bends and twists can be fabricated in the field, and the equipment for which they are intended can be placed in service in a shorter time than purchased parts could be supplied.

This handbook describes the assembly and finishing of components fabricated from the two most commonly used materials, brass and aluminum. The processes and methods used in the fabrication of waveguide assemblies from other materials are similar to the procedures described herein. Careful attention should be paid, however, to brazing only the components made from similar metals to prevent the possibility of galvanic action between parts.

Waveguides are supplied in straight lengths of from 6 to 14 feet, but usually the length is 10 feet. The number of joints or couplings in a run should be kept to a minimum in order to obtain the most efficient electrical transmission. In an installation, there will be a need for bends, twists, and off-sets. Since all conditions cannot be met with standard fittings, it is desirable to be able to form bends and twists in the straight sections of the waveguide. These can be formed in the full straight lengths by the methods described in this handbook.

Proper identification of waveguide bends referenced herein is provided in figure 1.

For purposes of this handbook, the term twists means a waveguide configuration similar to those shown in figures 23 and 24. These illustrations should enable a technician to differentiate between twists and those waveguide assemblies that are constructed of ready-made elbows and sweep bends.

Bending of corrosion-resisting steel or magnesium waveguides is not permissible.

Ordinarily,bends and twists should not be sharper than those given in table I, but it is possible to make sharper bends and twists than specified herein without causing excessive attenuation, provided that reflections are avoided. This is done by forming the bend so that its electrical length (mean "L" in figure 2) is exact multiple of waveguides half wavelengths, and preferably at the midband of the frequency range at which it is to operate. This will cause the reflection at either end of the bend to cancel out. Similarly, twists may be so designed that their length is an exact multiple of waveguide half wavelengths.

Never make a bend or twist sharper than the specified minimum unless conditions make it absolutely necessary. Under no conditions should the radius of a bend or twist be less than one full wavelength of the center frequency for any waveguide. The reflections caused by poorly matched bends can result in a high standing wave ratio.

EXAMPLE A: The radii of preformed bends for RG 48/U is 7.5 inches, whereas, the radii of bends formed for RG 48/U TE₁₀ mode wavelength range (lowest to highest recommended operating frequency) is from 4.539 to 2.987 inches. The mean wavelength is 3.763 inches and a waveguide half wavelength is 1.8825 inches. Any multiple of 1.8825 inches that generates a radius greater than 7.5 inches shall be used as the mean electrical length of a formed bend for RG 48/U waveguide.

To determine the radius of a 90-degree bend the following calculations are made.