Coplanar Waveguide: A Surface Strip Transmission Line Suitable for Nonreciprocal Gyromagnetic Device Applications

TLDR

A coplanar waveguide is a thin metallic strip on a dielectric slab flanked by two ground electrodes, offering a built‑in circularly polarized magnetic vector at the air‑dielectric boundary that makes it suitable for nonreciprocal magnetic devices, while its configuration allows easy shunt connections in hybrid circuits and monolithic integration, and its characteristic impedance is largely independent of substrate thickness, enabling compact low‑loss high‑dielectric‑constant designs for low‑frequency microwave systems. The authors calculated the characteristic impedance, phase velocity, and attenuation of a one‑side‑electrode coplanar waveguide on a dielectric substrate. Experimental measurements on resonant isolators and differential phase shifters fabricated on low‑loss high‑dielectric‑constant substrates confirm the practical viability of the coplanar waveguide, and the calculated impedance, phase velocity, and attenuation agree well with these preliminary results.

Abstract

A coplanar waveguide consists of a strip of thin metallic film on the surface of a dielectric slab with two ground electrodes running adjacent and parallel to the strip. This novel transmission line readily lends itself to nonreciprocal magnetic device applications because of the built-in circularly polarized magnetic vector at the air-dielectric boundary between the conductors. Practical applications of the coplanar waveguide have been experimentally demonstrated by measurements on resonant isolators and differential phase shifters fabricated on low-loss dielectric substrates with high dielectric constants. Calculations have been made for the characteristic impedance, phase velocity, and ripper bound of attenuation of a transmission line whose electrodes are all on one side of a dielectric substrate. These calculations are in good agreement with preliminary experimental results. The coplanar configuration of the transmission system not only permits easy shunt connection of external elements in hybrid integrated circuits, but also adapts well to the fabrication of monolithic integrated systems. Low-loss dielectric substrates with high dielectric constants may be employed to reduce the longitudinal dimension of the integrated circuits because the characteristic impedance of the coplanar waveguide is relatively independent of the substrate thickness; this may be of vital importance for Iow-frequency integrated microwave systems.