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 <inline-formula> <tex-math notation="LaTeX">$X$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$Ka$ </tex-math></inline-formula> band s with flexible inter-resonator coupling geometries. The proof-of-concept <inline-formula> <tex-math notation="LaTeX">$Ka$ </tex-math></inline-formula>-band filters are monolithically prototyped with a high-temperature-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 <inline-formula> <tex-math notation="LaTeX">$Ka$ </tex-math></inline-formula>-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.