Broadband filtering balun employing spoof surface plasmon polariton and substrate integrated waveguide

Abstract

In this paper, two filtering baluns exploiting a combination of spoof surface plasmon polariton (SSPP) and substrate integrated waveguide (SIW) techniques are proposed. The characteristic of out-of-phase between two output ports can be achieved by employing the multilayered SIW structures. The sub-wavelength periodic slots in improved dumbbell-shapes are etched on the top and bottom surfaces of SIW transmission lines (TLs). The dispersion characteristics and transmission performance of the SSPP-SIW unit cells are meticulously investigated. The lower and upper cut-off frequency can be controlled independently by adjusting the physical sizes of the SIW and SSPP structures. For validation, two cases of filtering baluns are designed, fabricated and measured. The measured and simulated results have reached satisfactory agreements. The experimental results exhibit that the developed prototype can achieve a fractional bandwidth (FBW) of>66%. Furthermore, HMSIW is implemented to integrate with SSPP configuration to decrease the circuital area of SSPP-SIW filtering balun by 50%.