Slow-Wave Line Coupler With Interdigital Capacitor Loading

In this study, a full-wave-based circuit model of an interdigital capacitor in the form of a coplanar waveguide (CPW) configuration is studied by using numerical thru-reflect-line calibration techniques, which can easily be combined with commercial electromagnetic simulation software. Subsequently, a slow-wave line coupler is proposed and realized for size miniaturization by loading different interdigital capacitors onto a coupled CPW line, which has equal phase velocities for both odd and even modes. An experimental prototype is fabricated, and measured results show a good agreement with simulated ones, thereby demonstrating an excellent performance.