A Wideband Filtering Rat-race Ring Coupler Based on Double-sided Parallel Striplines with Excellent Isolation

A wideband filtering rat-race ring coupler based on double-sided parallel striplines (DSPS) with excellent isolation is proposed. By integrating a λ/8 coupled line and a λ/8 open circuited stub at each port of the coupler, wideband bandpass filtering characteristics have been achieved. For obtaining an equal power division, the coupler is designed based on a core ring with equal characteristic impedance for each branch. To reduce the circuit size, DSPS swap structure is used to replace the half-wavelength transmission line. To verify the proposed method, a DSPS filtering coupler with a size of 0.36λ
g
× 0.48λ
g
is designed and fabricated. From the measured results, identical insertion losses of about 3.2 dB for the output ports are obtained for equal power division. The measured impedance bandwidth for return loss greater than 10 dB is 80% (0.6–1.4 GHz) with amplitude balance better than ±0.5 dB and phase balance within ±5°. At the same time, the measured isolation is better than 30 dB at all measurement frequencies from 0 to 3 GHz.