The Characterization of a Ridged Half-Mode Substrate-Integrated Waveguide and Its Application in Coupler Design

The miniaturization of microwave and millimeter-wave components is necessary to meet demand for increasingly smaller integrated circuits. This paper explores the technique of capacitively loading the open side of a half-mode substrate-integrated waveguide (HMSIW) with a continuous ridge in order to lower the first-mode cutoff frequency and allow for substantial miniaturization. The full analytical calculation for the design of the first-mode cutoff frequency is presented, along with the extraction of the complex propagation constant and the analysis of the tradeoff between radiation and conductor losses. A comparison between the performance of the ridged HMSIW (RHMSIW) and HMSIW is simulated and measured, where the insertion loss of the RHMSIW is shown to be lower for frequencies up to 1.45 times the cutoff frequency f_c, with a total width approximately 0.45 times the width of the HMSIW. The near-field radiation pattern is also measured, clearly illustrating the reduced radiation loss of the RHMSIW. A 3-dB coupler using the RHMSIW technology is designed, fabricated, and measured. The RHMSIW coupler's performance is shown to be comparable to similar HMSIW and SIW designs in the literature, but with substantially improved miniaturization. The isolation of two parallel RHMSIWs in different configurations is also studied and compared with the HMSIW for use in applications requiring high-density routing or interconnection. It is shown that the capacitive ridge helps to reduce the crosstalk between neighboring transmission lines, allowing for the possibility of greater circuit densities, and further improving the miniaturization of microwave systems using the RHMSIW technology. To the best of our knowledge, the RHMSIW is the smallest waveguide-based transmission line with respect to wavelength presently in the literature.