Balanced Directional Coupler Structure with Insensitive Isolation for Load Impedance

In this paper, a balanced directional coupler structure with lumped elements is presented as a new method to solve Tx-to-Rx leakage problem. An analysis for the characteristics of the balanced directional coupler is performed to introduce that the proposed structure is insensitive for varying load impedance. The balanced directional coupler structure consists of two Wilkinson power dividers and two contra-directional couplers implemented with lumped elements. The validity of the proposed structure is confirmed by comparisons with the measured results obtained from a single contra-directional coupler. The fabricated balanced directional coupler has 58 dB of isolation for matched load impedance and less than 45 dB for mismatched load impedance at 910 MHz, the center frequency of UHF RFID.