Uniplanar Compact 180 ^ Hybrid Coupler With Fast and Accurate Wide Power-Division Ratio Switching Ranges and Enhanced Bandwidth

In this article, a uniplanar compact 180 ^{\circ} hybrid coupler with fast and accurate wide power division ratio (PDR) switching ranges and enhanced bandwidth is proposed. The structure consists of two novel 90 ^{\circ} couplers, one wideband 90 ^{\circ} phase shifter (PS), and two tunable p-i-n diode-based PSs. The key uniqueness is that a symmetrical compact wideband 90 ^{\circ} coupler is presented as the core component. By using two three-folded-coupled lines and one unequal-width three-coupled line section, the proposed compact wideband 90 ^{\circ} coupler can be fabricated without patterned ground plane or any lumped element. Compared with conventional two-section branch-line couplers, this new coupler not only effectively reduces the circuit size by 57.76% but also improves the bandwidth from 37% to 48.45%. Then, compared with the varactor-based couplers, by adding p-i-n diodes at the end of the quarter-wavelength lines, the proposed 180 ^{\circ} tunable coupler finally features fast and accurate wide PDR switching ranges and wide bandwidth. Good performances in return loss, isolation, insertion loss, phase differences, and amplitude imbalances can be achieved for all states. For verification, a 90 ^{\circ} coupler and a tunable 180 ^{\circ} coupler are designed, simulated, and measured. The measured results match well with the simulated results, validating the proposed design theory.