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In this paper, a grounded stub-loaded microstrip periodic structure (SL-MPS) is presented and its transmission characteristics are investigated. A new method of suppression of modes is proposed through adding grounded via holes on the grounded SL-MPS for the design of bandpass filters (BPFs). In order to validate the feasibility of suppression of modes of grounded SL-MPS, two cases of BPFs with different dimensions are designed. For the case of BPF with suppression of adjacent modes, the simulated and measured results of BPFs demonstrate that the fundamental and 2nd high-order modes can be effectively suppressed. For the other case of BPF with suppression of multiple modes, the fundamental, 1st, 3rd and 4th high-order modes can be successfully suppressed. The measurements of these BPFs are reasonably in agreement with their simulations. Observed from the theoretical analysis and experimental results, the proposed method can effectively widen the stopband of BPF by suppressing unwanted modes.

A 60 GHz third-order on-chip bandpass filter (BPF) based on half-mode substrate integrated waveguide (HMSIW) cavity is synthesized using GaAs pHEMT technology. Two coupling slots are etched to divide the HMSIW cavity into three resonators, and then a third-order Chebyshev BPF is designed with predicted transmission zero, return loss and bandwidth through the synthesis method. The theoretical and extracted external quality factor and coupling coefficients are used to determine the dimensions of the BPF. For demonstration, a BPF sample with a bandwidth of 29.2% is fabricated, and its simulations and measurements are in good agreement.

Spoof surface plasmon polaritons (SSPPs) are developed in a rectangular waveguide (RWG) to realize a controllable band-pass propagation feature for millimeter-wave applications. The proposed SSPP structure consisting of a single row of corrugated metallic blocks, is embedded inside the closed structure of an RWG. The dispersion property of the RWG-based SSPP (RWG-SSPP) has tunable lower and upper cut-off frequencies, and a band-pass filter with flexible bandwidth has been designed using an RWG-SSPP structure. The proposed band-pass filter is based on an RWG-SSPP with an all-metal structure, which is of great potential for millimeter-wave applications due to small losses.

A half-mode substrate integrated waveguide (HMSIW) combined with spoof surface plasmon polariton (SSPP) structure is proposed to realize bandpass filter (BPF) characteristic and miniaturization, which is termed as the half-mode substrate integrated plasmonic waveguide (HMSIPW). Compared with the conventional HMSIW structure having identical cutoff frequency, this new design of HMSIPW not only supports SSPP modes, but also realizes a transversal size reduction of 19.4% and longitudinal reduction of more than 60%. Then, a diplexer based on two back-to-back placed HMSIPW BPFs is designed, and it has only one row of metallized via holes to further reduce the transversal size. The experimental prototypes of the filters and diplexer have been manufactured, and the measurement results agree well with simulation ones. Due to the size miniaturization and simple structure, the proposed designs will have many potentials in the integrated devices and circuits for wireless communication systems.