Wideband Planar Hybrid With Ultralow Amplitude Imbalance

We present a new design approach for the 90° directional coupler with very low amplitude imbalance. The primary feature of this quadrature coupler is the introduction of a controllable ripple in the operational band for achieving a better overall amplitude balance. This design concept is demonstrated through a 90° branch-line hybrid for the 4-7.9 GHz band (65% fractional bandwidth) using microstrip transmission lines. Our simulations indicate that the amplitude imbalance of the designed hybrid is better than 0.3 dB over the most of the 4-7.9 GHz band with a phase imbalance better than ±8.5°. Experimental verification of the hybrid shows excellent agreement with simulations.