A compact wideband ring coupler utilizing a pair of transitions for phase inversion

A new wideband ring coupler utilizing a pair of ultra-wideband planar transitions is introduced in this letter. The two types of transitions (in-phase and inverted phase transitions) are coplanar waveguide-to-parallel stripline transitions, and very similarly structured except for the phase inversion part. The area of the proposed ring coupler is only 41.4% of the conventional ring coupler. The measured bandwidth of the implemented ring coupler with more than 10 dB return loss is 92.5% with the Σ port excitation, and 81.3% with the Δ port excitation. Also, due to good matched properties of the transitions, the ring coupler shows less than ± 0.7 dB amplitude mismatch, less than 4 ° phase difference, and more than 30 dB isolation over the operating frequencies.