Coplanar Waveguide Low-Pass Filter Based on Butterfly-Shaped Spoof Surface Plasmon Polaritons with Compact Size and Constant-Width Transition.

This paper proposes a spoof surface plasmon polariton (SSPP) low-pass filter by etching butterfly-shaped cells on the middle line of a coplanar waveguide (CPW). The transmission and dispersion characteristics of the structure are analyzed. This design has a perfect low-pass transmission with tunable cutoff frequency. The angle and radius of the butterfly grooves are two variables that can adjust the cutoff frequency. The existence of two parameters is helpful to design a constant-width transition between the CPW and CPW-SSPP structures. It can be obtained by varying the angle of the butterfly grooves, while their radius is fixed. The introduced butterfly SSPP achieves a cutoff frequency reduction of 33% in comparison with its conventional rectangular counterpart with the same lateral size. It also has less phase velocity and more slow-wave effects than the rectangular SSPPs. Consequently, both transverse and longitudinal dimensions can be reduced by using the butterfly grooves. Moreover, the transition length is significantly reduced. The measured results show a 3-dB bandwidth from 0 to 10 GHz, a return loss of greater than 13.76 dB, an insertion loss of less than 1.6 dB, and an out-of-band rejection of higher than 15 dB from 10.2 to 14.5 GHz. The proposed circuit with the compact size, low loss, groundless structure, excellent slow-wave effect, and compact transition with excellent mode conversion can be applicable to the future communication systems. [ABSTRACT FROM AUTHOR]