A 3-D Printed Compact Inline Waveguide Filter With Transmission Zeros Based on Strongly Coupled Novel Triple-Post Arrangement

This article presents a new class of compact, inline waveguide bandpass filters with sharp rejection capabilities at the upper stopband using a novel nonlinear frequency variant coupling (NFVC) inverter, which can be fabricated only by 3-D printing. The proposed NFVC inverter is composed of three closely spaced posts, including the middle one slanted, arranged at the center of the waveguide’s broad wall. Utilizing the novel slanted post, this inverter can provide two poles above three TZs. Additionally, one of the TZs can be located very close to both sides of the passband by changing the middle post’s slant angle. To demonstrate its usefulness, a fourth-order inline waveguide filter with an operating frequency of 7.9 GHz, a bandwidth (BW) of 1.5 GHz, and three TZs is designed, fabricated, and measured. As will be seen, conventional manufacturing methods, such as milling, are impractical for this structure. Furthermore, the measured results are in reasonable agreement with the electromagnetic (EM) simulated ones.