Design of Coplanar Waveguide Artificial Transmission Lines and Its Application to a Miniaturized Branch-Line Coupler

Abstract

This paper presents a compact size $90^{°}$ hybrid coupler designed by the use of coplanar waveguide (CPW) artificial transmission line (ATL) for microwave applications. The ATLs comprise a periodic structure made from open-ended stub lines that form a discontinuity and capacitive loading to the transmission lines. The electrical length and line impedance of the conventional transmission lines are modified by using the proposed CPW ATLs. A detailed mathematical analysis of the proposed ATLs and the coupler is explained in the paper. A full wave electromagnetic simulation and fabrication of the coupler are carried out at a central frequency of 2.1GHz to validate the proposed size reduction method. The measured bandwidth and phase difference between the through and the coupled port are 670 MHz or 31.9$\%$ fractional bandwidth and ${90}^{°}\pm 1^{°}$, respectively. The transmission losses to the through and the coupled port are 3.71 and 3.68 dB, respectively. A conventional quadrature CPW hybrid is also simulated and fabricated to compare the size with the proposed structure. The proposed design offers a size reduction of 74.87$\%$ when compared to the conventional CPW line-based coupler.