Geometric Parameter Tuning for Bandwidth and Filtering Control in Wideband Directional Coupler

In this study, four different directional couplers with increasing structural complexity (Coupler I, II, III, and IV) were designed and analyzed using the Ansys full-wave simulator. The designs, implemented on an FR-4 substrate, were derived from a fundamental configuration by adding and removing geometric features. A comparative analysis of the simulation results indicates that Coupler II exhibits superior performance for wideband applications, offering a return loss better than -15 dB, a flat coupling factor, and deep isolation below -40 dB over the wide frequency range of 1.5 GHz to 4.5 GHz. The focus of the study is a parametric analysis conducted on Coupler IV, which investigates the effect of the geometric parameter 'p'. This analysis revealed that the 'p' parameter is a powerful tool for precisely tuning the coupler's operating bandwidth, impedance matching, and filtering characteristics. It was observed that by increasing the 'p' value, the upper cut-off frequency of the operating band can be lowered, while simultaneously creating an adjustable notch (rejection band) in the transmission response. These results demonstrate that tuning the geometric parameter provides critical flexibility for optimizing directional couplers for specific application requirements, such as wideband, narrowband, or integrated filtering.