Abstract-This paper presents a design of a new type of in-line pseudo-elliptic filters implemented in substrate integrated waveguide (SIW) technology. To realize transmission zeros in in-line topology, frequency-dependent couplings were used. Such dispersive couplings were implemented as shorted stubs. The design process starts with the generation of a suitable starting point. To this end, an approximation of SIW as a rectangular waveguide is used and a fast electromagnetic solver based on mode-matching technique is utilized (µWave Wizard). The next step is the optimization process of a filter in a full-wave 3D EM simulator Ansoft HFSS. The Optimization routine employs a cost function involving zeros and poles of the rational functions approximating scattering parameters. To increase the speed of convergence, a built-in derivative calculation feature was used and zeros and poles and their derivatives with respect to design parameters were extracted using the vector fitting algorithm. Experimental validation of the method is demonstrated by a third-order filter with asymmetric response and a fifth-order filter with two transmission zeros in addition to an asymmetric response. The experimental results show good agreement between the simulated and measured data.

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An asymmetric GCPW discontinuity is proposed to provide frequency‐dependent coupling in microwave bandpass filters. Wider and narrow sections introduce, respectively, the capacitive and inductive component to the equivalent circuit representing coupling. By selecting the dimensions of the discontinuity and width of the inductive window in substrate integrated waveguide, an additional transmission zero can be introduced at prescribed positions. A distinct feature of the proposed structure is a small footprint and low loss. As an example, a bandpass filter with a central frequency of 4.6 GHz and a 200 MHz passband is designed, and its performance is measured. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:1818–1821, 2015