Miniaturized Branch-Line Coupler with Harmonic Suppression for RFID Applications using Artificial Transmission Lines

A miniaturized branch-line coupler based on a newly proposed planar artificial transmission line (ATL) is presented in this paper for UHF RFID applications. This planar artificial transmission line consists of quasi-lumped elements and their discontinuities, and can synthesize microstrip lines with a wide variety of characteristic impedances and electrical lengths. The measurement results agree well with the simulation ones. The experiments demonstrate that the proposed hybrid has a compact size of 24.4 mm times 25.8 mm, or equivalently 0.12lambda_g times 0.13lambda_g at the operation frequency, 915 MHz. This corresponds to a 73% size reduction as compared to that of a conventional branch-line coupler. The wideband response reveals that the proposed hybrid is well-behaved with no spurious harmonics up to 11.1 GHz.