Rigorous Design of Compact High-Performance 3-D Microstrip Filters Using Inverse Scattering

In this letter, a novel design method for 3-D microstrip filters is proposed. The method is valid for any all-pole transfer function and employs an inverse scattering synthesis technique to generate a smooth characteristic impedance profile that is implemented by smoothly varying the substrate height and strip width. In this way, strongly enhanced characteristic impedance excursions can be implemented, allowing us to synthesize filters with very wide rejection bands and very compact physical dimensions. As an example of application, a Ku-band low-pass filter (LPF) with cut-off frequency of 11.8 GHz and rejection band up to 36 GHz is designed, featuring a length of 10.66 mm. The filter is fabricated through additive manufacturing (AM) and the measurements obtained confirm its excellent performance.