A wideband rat-race coupler using stepped impedance resonator

A wideband rat-race coupler using stepped impedance resonator (SIR) has been proposed in this paper. The proposed coupler operates over frequency range of 1.70 GHz to 2.40 GHz with 35% fractional bandwidth, centered at 2.0 GHz. The measured amplitude and phase imbalances between the two out-of-phase output ports are within 0.5 dB and 180±10°. To achieve this, each branch of a conventional rat-race coupler is replaced by the SIRs. The values of the impedances of the lines are computed mathematically first and later realized using microstrip lines. The proposed design offers 75% more wider bandwidth than the conventional rat race coupler, without increasing the overall foot-print. The measured reflection co-efficient is below −10 dB with isolation better than 20 dB in the desired band. The prototype has been fabricated and measured and the results are compared with the simulated data to validate the theory.