An Inductive-Loading Method for Directivity Enhancement of Microstrip Coupled-Line Couplers

This letter demonstrates a series-inductive loading method for the directivity enhancement of microstrip coupled-line couplers. The method has an extreme structural simplicity that is based on two pairs of mutually coupled inductors. Furthermore, it shows little, if any, compromise in the performance, as verified with a 20-dB coupler at 2 GHz. Experimental results show a ±0.5-dB bandwidth of 42%, which is the widest reported for reactively loaded coupler. The demonstrated 20-dB directivity bandwidth of 147% is the highest reported for a 20-dB coupler. Most importantly, the peak directivity of 59.9 dB is the highest reported thus far, which is a clear indication of the effectiveness of the proposed method.