A Wideband Directional Coupler With Stable Coupling Factor Based on Spoof Surface Plasmon Polaritons

Abstract

This letter presents a broadband miniaturized directional coupler based on two parallel spoof surface plasmon polariton (SSPP) transmission lines (TLs). The design leverages the intrinsic dispersion and slow wave characteristics of SSPPs to significantly compress the guided wavelength, thereby achieving substantial miniaturization of the device. To balance the phase velocities of the odd and even modes in the coupled microstrip lines, the capacitive parasitic patches are introduced into the coupling region, resulting in a flatter coupling response. Furthermore, the lumped resistors are incorporated between the parasitic patches and the TL to enable flexible control of the coupling factor while simultaneously enhancing the directivity across a wide operating bandwidth. Experimental results demonstrate that the proposed coupler achieves a directivity of at least 14 dB over a 64% fractional bandwidth (2.64–5.1 GHz), with a coupling coefficient maintained at $10~\pm ~0.9$ dB. Owing to its wide bandwidth, high directivity, compact size, and stable coupling performance, the coupler meets the stringent requirements of modern wireless systems for high-performance components.