A multi-layer design of 3-dB 180° wideband hybrid coupler

This paper presents a design of multi-layer wideband 3 dB 180
^o
hybrid coupler based on tapered coupled-line structure. The design is fully implemented on a four-layer printed circuit board. The transition between tightly coupled lines and uncoupled lines is fully characterized and laid out to ensure impedance and field matching. As a result, the proposed coupler obtains a return loss better than 15 dB, an insertion loss better than 4 dB, an isolation better than 20 dB, an amplitude imbalance within ±1dB, and a phase imbalance within ± 5
^o
over the range of 8 to 18 GHz. The hybrid coupler is designed to cover the X band Ku bands (8 – 18 GHz), utilized as balun for wideband antennas.