A 3-D-Printing-Compatible In-Line Polarization-Rotated Waveguide Filter Based on Shaped Spherical Resonators

This paper reports on a new design concept of millimeter-wave polarization-rotated waveguide filters taking advantage of high rotational symmetry of spherical and cylindrical cavities. The filter is based on geometrically shaped high-quality-factor spherical resonators that are coupled with cylindrical cavities in an in-line configuration. The polarization rotation is realized by step-rotating the shaped spherical resonators with appropriate angle intervals. The proposed filter features enhanced structural compatibility with additive manufacturing technology. Flexible design of arbitrary polarization rotation angles between 0° and 90° can be achieved without producing any distortion of the filter geometry. A $Ka$-band fifth-order bandpass filter prototype with a polarization rotation angle of 90° is designed and monolithically manufactured by employing metallic 3-D printing process. The 3-D printed filter demonstrates passband return and insertion losses of over 20 dB and lower than 0.8 dB, respectively.