Design of compact filtering rat-race hybrid with λ/2-resonators

A novel compact filtering rat-race hybrid coupler with improved upper stopband is presented. Four coupled λ/2-resonator pairs are utilised to replace the λ/4- and 3λ/4-transmission lines in the conventional hybrid. Each coupled λ/2-resonator pair exhibits a bandpass filtering response and acts as a K-inverter. Eight resistors are placed at the centre of the λ/2-resonators to suppress the second-harmonic. For verification, a filtering rat-race hybrid is implemented with a centre frequency of 1.27 GHz and 20 dB rejection band of 2.73 f₀. Good in-phase and out-of-phase characteristics are obtained. The proposed device has a simple circuit topology, compact size and good performance.