Branch-Line Coupler Based on Edge-Coupled Parallel Lines With Improved Balanced Response

A conventional branch-line coupler is flat-balanced, in terms of magnitude and phase, in a narrow frequency band due to the quarter-wave length requirement in the transmission lines. In this paper, a novel branch-line structure is presented in order to overcome this limitation. Loading the horizontal transmission lines with a pair of open-end series stubs, the magnitude and phase balance becomes flatter in a wider frequency range. However, the proposed structure cannot be fabricated, for which an equivalent circuit using edge-coupled parallel lines has been used. The measurement of this device shows a relative bandwidth of 82.5% for 2 dB magnitude and 72% for plusmn5^deg phase balance condition.