Compact coupled line quadrature hybrid coupler with enhanced balance bandwidth

A single stage quadrature hybrid coupler with enhanced flat balance over a wide bandwidth is presented in this article. The present coupler is realized by replacing the transmission line sections of the conventional two-branch coupler by their equivalent parallel coupled microstrip lines (PCML). The impedance controlled wide-bandpass response produced by the PCML is utilized to achieve flat balanced response over wider bandwidth that is comparable to that of a four-branch conventional coupler. A lossless transmission line model is used to derive the design equations along with design graphs. A prototype microstrip coupler having a wide ±1.0 dB amplitude balance of 71% is fabricated at 2.4 GHz to validate theoretical predictions.