Miniaturization of a 90° Hybrid Coupler with Improved Bandwidth Performance

This letter presents a compact hybrid coupler having wider bandwidth compared to a conventional 90° branch line coupler. Coupling between two closely spaced branches is utilized for the improvement. Closed form expressions are derived using a lossless transmission line theory. Further, graphs are presented to determine size of the coupler with the required bandwidth specifications. In the bandwidth calculation, input matching, isolation, phase and amplitude imbalances, all conditions are considered simultaneously. A coupler is fabricated on a 1.58 mm thick FR4 substrate. It occupies 33% area of a conventional branch line coupler. The measured fractional bandwidths for the input matching, isolation, phase and amplitude imbalance conditions are 19.6%, 12.4%, 41.9% and 35.2%, respectively, which is higher than a conventional one.