Compact capacitive compensated directional coupler using planar artificial transmission lines

A novel compact directional coupler based on planar artificial transmission lines is proposed. Interdigital capacitors are introduced to compensate for the odd-mode phase velocity in order to achieve a high directivity. The size of this proposed coupler is about 25% that of a conventional one. A 20 dB directional coupler at 900 MHz has been designed and fabricated. The directivity reaches 36 dB at the centre frequency. The design, simulation and measurements are presented.