Waveguide bandpass filter with adjustable TZs using planar insert technology

A simple, compact planar insert loaded waveguide bandpass filter (BPF) with multiple transmission zeros (TZs) is presented in this paper. The planar consists of a circular shape etched part in which three asymmetrical stubs have been placed. The two asymmetrical stub lengths are used to adjust the location of transmission zeros (TZs) independently. The third stub length and width are used to control centre frequency and 3-dB bandwidth without interfering with the location of TZs. A weak coupling simplified equivalent model is also presented, which is useful for understanding the operational behaviour of passband and TZs. To demonstrate the idea, second-order BPF is designed with four TZs. To validate the 3D model, a planar insert and a waveguide are fabricated and measured. The measured results exhibit a two-pole BPF with a centre frequency at 9.947 GHz, a 3-dB bandwidth from 9.8469 to 10.049 GHz and a minimum insertion loss (IL) of 0.7 dB. The total length of the proposed filter is 0.26 λg.