Closed-form design method for tight parallel-coupled microstrip coupler with ultra-wideband performance and practical dimensions

A closed-form method for designing a microstrip coupler that has a simple planar structure, tight coupling, practical dimensions, and ultra-wideband performance is presented. According to the proposed method, the coupled microstrip structure is divided into three sections. A theoretical model based on the even-odd mode analysis of four-port networks is derived and used to find the optimum length and coupling factor for each of those sections to get an ultra-wideband performance. According to the model, the central section is designed to have a tight coupling by utilising a slotted ground plane and one lumped capacitor, whereas the two side sections have loose coupling. The presented method is validated by building a compact 3 dB coupler that has more than 116% fractional bandwidth.