Monolithically 3-D Printed Hemispherical Resonator Waveguide Filters With Improved Out-of-Band Rejections

This paper provides a comprehensive presentation for the RF design and implementation of novel millimeter-wave waveguide bandpass filters (BPFs) that are composed of compact hemispherical resonators. The proposed hemispherical resonator features a high unload quality factor and a volume 50% smaller than a spherical one and more significantly is much less degenerate than the latter due to its structural asymmetry. With proper geometrical configurations, the hemispherical resonator BPF can perform significantly improved out-of-band rejections compared with the spherical resonator BPFs of the same orders. The second-and fourth-order waveguide-fed BPFs are designed at X and Ka bands with flexible inter-resonator coupling geometries. The proof-of-concept Ka-band filters are monolithically prototyped with a hightemperature-resistant ceramic-filled photosensitive resin by using a fast and low-cost stereolithography (SLA)-based additive manufacturing technique. A proprietary electroless nickel/copper/silver plating process is reported for surface metallization of the utilized commercially available resin. The Ka-band filters demonstrate in the passbands small insertion losses (0.43-1 dB), good return losses (mostly > 10-17 dB), and small frequency shifts (0.01%-0.47%), which validates excellent fabrication accuracy and reliability of the SLA printing and the metal plating. Characterization and quantification of surface morphology for SLA-printed samples are performed by employing contact profilometry and scanning electron microscopy.