This paper presents a class of wideband 180�° hybrid (rat race) couplers implemented by parallel-strip line. By replacing the 270�°\narm of a conventional 180�° hybrid coupler by a 90�° arm with phase inverter, the bandwidth of the coupler is greatly enhanced\nand the total circuit size is reduced by almost half. Simple design formulas relating the characteristic impedance of the arms and\npower division ration are derived. To demonstrate the concept, four couplers with different power division ratios of 1, 2, 4, and 8\nwere designed, fabricated, and tested. ??-parameters of the coupler are simulated and measured with good agreement. All working\nprototypes operate more than 112% impedance bandwidth with more than 25 dB port-to-port isolation and less than 5�° absolute\nphase imbalance. The proposed 180�° hybrid couplers can be employed as a wideband in-phase/differential power divider/combiner,\nwhich are essential for many RF and microwave subsystem designs.
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