Measurement of the Loss of Unwanted Modes in Waveguide for Long-Distance Transmission

An important factor of waveguides suitable for long-distance transmission is the attenuation of unwanted modes in the waveguide. The paper describes one method of measuring this loss in a length of waveguide, and gives details of the apparatus and equipment used. Briefly, the method consists of using a suitable waveguide transducer to launch the particular parasitic mode into a length of waveguide under test that is terminated in a movable short-circuit. The reflected wave is continuously monitored at the input while the short-circuit is moved over a range of positions. The test is repeated with the length of waveguide out of circuit, and from a comparison of the data from the two tests the attenuation of the parasitic mode is calculated. Results are given with particular reference to the circular H12 mode in 2¿ in-diameter waveguide operating in the range 8¿9mm wavelength. The paper is concluded with a discussion of the accuracy and limitations of the method, and a brief description of the design of the mode transducer is included.