Transparent Branch-Line Coupler Using Micro-Metal Mesh Conductive Film

This letter presents the design of a novel transparent 3 dB Branch-Line Coupler (BLC) using a proprietary self-assembling nano-particle technology based Micro-metal Mesh (MM) conductive film. The transparent MM conductive film has a sheet resistance of 0.7 Ω/sq and a visible-light transmission (VBLT) of 75%, resulting in good conductivity and high transparency, respectively. The proposed transparent BLC is mounted on a 2 mm-thick soda-lime glass substrate of dielectric 5.7 and designed to operate at 2.45 GHz. A layer of MM film on the back of the glass serves as a ground. The transparent BLC delivered measured coupling and transmission values of 3.9 and 4.3 dB, respectively. The transparent BLC can be used as a building block for realizing a transparent Butler Matrix (BM) Beam Forming Network (BFN) for Inter/Intra-Vehicle Wireless Communication (IIVWC) in Intelligent Transport System (ITS). Besides being transparent, the proposed BLC has a measured fractional bandwidth of 38.8% and 0.25 mm profile excluding the glass substrate. These advantages greatly enhance the BLC's potential to be used also as a BM building component that can be mounted onto the glass surfaces of buildings for future 5G indoor wireless communications where beam forming is required.