Compact High-Directivity Contra-Directional Coupler

This paper presents a novel design of a compact contra-directional coupler with high directivity for high-power monitoring in high frequency. Microstrip parallel coupled lines are widely used for directional couplers; however, they show poor directivity inherently. Their directivity has been improved by many works. However, the suggested approaches often result in other limitations, such as a weak structure for high-power monitoring, or a larger size to be integrated with other circuits. The design approach proposed in this study starts from a ring-type four-port network to avoid weak components that are vulnerable to high power, and uses a 60 degrees electrical length of coupled line for a compact size. The design equations for the initial dimensions are derived from the ring-type four-port network model. The weak coupling of the 20 dB coupler was designed and measured. The measurement shows 20 dB directivity from 12.8 GHz to 14.8 GHz, covering the Ku-band satellite uplink communication and peak directivity of about 45 dB. The coupler's active area is 4 mm by 5.5 mm; this is a compact size compared with other works.