Circular waveguide TM11-mode resonators and their application to polarization-preserving bandpass and quasi-highpass filters

Circular waveguide filters with improved performance are presented. They are based on TM
11
-mode resonators which permit the filter to be smaller than standard TE
11
-mode filters, preserve all polarizations due to rotational symmetry, and are flexible with respect to the design of bandpass or highpass filters. In order to demonstrate their advantages, they are compared to a standard TE
11
-mode design. Design guidelines are presented, and performances of resulting bandpass and quasi-highpass filters are shown. The coupled-integral equation technique (CIET) is used as the main computational tool. All provisional and final designs are verified by comparison with the commercially available software package μWave Wizard.