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Heretofore, tunable waveguide filters have consisted of cavities connected with quarter-wavelength coupling lines and tuned by means of a lumped reactance in the cavity. Such filters exhibit asymmetrical frequency response and a bandwidth proportional to the cube of frequency when tuned. A different method of tuning is to maintain constant guide wavelength as the filter is tuned. A filter so tuned will exhibit symmetrical pass-band response and essentially constant bandwidth. Various methods of varying guide wavelength are investigated, including (1) changing the guide width, (2) inserting a dielectric strip in the broad face of the guide, and (3) inserting a metal strip in the broad face of the guide. Measurements on a three-section filter tuned by a dielectric strip show that almost constant bandwidth with a straight-line frequency tuning curve can be obtained over at least a 12 per cent frequency range.