Wideband Planar Hybrid With Ultralow Amplitude Imbalance

We present a new design approach for the 90\ub0 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\ub0 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 \ub18.5\ub0. Experimental verification of the hybrid shows excellent agreement with simulations