Improved Capacitive Loading Method for Miniaturization of 0 dB Forward-Wave Directional Couplers

An improved capacitive loading method for miniaturization of 0 dB forward-wave directional couplers is demonstrated. The method enables to miniaturize the coupler to have a predetermined characteristic impedance, eliminating the need for additional impedance transformers and therefore allowing for a substantial improvement in the degree of miniaturization. Experimental results for a 0 dB coupler at 5.0 GHz are provided. The demonstrated miniaturization rate of 89.4% is higher than previous methods, along with the widest fractional bandwidth and the minimum insertion loss.