Comparative analysis of waveguide bandpass filters with cylindrical and rectangular resonators

This work is devoted to a comparative analysis of millimeter-wave waveguide bandpass filter designs incorporating different resonator types. A comprehensive numerical and experimental study was conducted on sixth-order Ka-band bandpass filters (center frequency 29.7 GHz) implemented using rectangular and circular cross-section cavity resonators. It was established that cylindrical resonators possess an unloaded Q-factor nearly twice as high as that of rectangular resonators, which theoretically ensures lower insertion loss. To increase the steepness of the frequency response roll-off, symmetrically located transmission zeros were synthesized in both structures. Experimental results demonstrated that while the insertion loss at the center frequency is comparable for both configurations, the bandpass filters with cylindrical resonators exhibit a smaller variation of insertion loss across the passband. A key finding is the identification of a trade-off between electrical, mass-dimensional, and technological parameters. The structure with cylindrical resonators offers a significant advantage in size and weight but is critically sensitive to manufacturing tolerances, complex to assemble, and difficult to tune (requiring adjustment in three planes), leading to a 35-40% increase in cost. In contrast, bandpass filters with rectangular resonators are more manufacturable, easier to tune (in a single plane), possess high reproducibility, and are significantly cheaper to produce. The results of this work provide microwave equipment designers with clear criteria for selecting a filter topology. Bandpass filters with cylindrical resonators are recommended for applications with stringent weight and size constraints, whereas filters based on rectangular resonators represent the optimal solution for commercial mass production.