Performance optimization of capacitively compensated directional couplers

In this paper, a new design approach for microstrip directional couplers using capacitive compensation are presented. The focus of this work is small print size, ultra-low transmission loss, low coupling and high directivity over wide bandwidth. Modified structures for the compensation capacitors were developed, where the geometrical shapes, lengths and positions of the structures can be optimized for specific goals. The concept has been applied to a single-directional coupler with a geometrical coupling length of 8.5 mm. The measured performance data of the coupler were around -30 dB coupling factor, 0.07 dB transmission loss and 20 dB directivity within 152.5 % fractional bandwidth.