What Are Coplanar Waveguides?

Coplanar waveguides (CPW) are planar transmission line structures used to carry microwave and millimeter-wave signals on a single surface of a dielectric substrate. Unlike microstrip lines, which place the ground plane beneath the substrate, a CPW positions the signal conductor and its two flanking ground electrodes on the same face of the dielectric. This coplanar geometry simplifies the attachment of shunt circuit elements, reduces parasitic inductance in ground connections, and makes the structure well suited for integration with monolithic microwave integrated circuits (MMICs).

The CPW configuration was introduced by Cheng P. Wen in a 1969 paper published in IEEE Transactions on Microwave Theory and Techniques, where it was proposed specifically for building nonreciprocal components such as gyrators and isolators. The structure draws on classical electromagnetic waveguide theory and conformal mapping, a mathematical technique that transforms the CPW cross-section into a parallel-plate geometry to derive closed-form expressions for characteristic impedance and effective dielectric constant.

Electromagnetic Propagation

A coplanar waveguide supports a quasi-TEM mode: the electromagnetic field is concentrated mainly in the gap between the central strip and the ground conductors, with a portion extending into the air above the substrate. Because the wave travels partly through the dielectric and partly through air, the effective permittivity is an average of the two media. True TEM propagation is not supported at nonzero frequencies; both the electric and magnetic fields develop longitudinal components, producing a hybrid mode. At frequencies above roughly 20 GHz the CPW becomes the preferred planar medium because its dispersion characteristics remain more predictable than those of microstrip, and its ground planes provide better isolation between adjacent signal lines.

Conformal Mapping and Characteristic Impedance

The primary analytical tool for CPW design is conformal mapping, which converts the two-dimensional cross-section of the line into a geometry where potential distributions can be computed using complete elliptic integrals. The ratio of the strip width to the gap width determines the characteristic impedance, which typically ranges from 20 to 150 ohms in practical designs. This geometric flexibility allows designers to hit a target impedance across a wide range of substrate materials, including low-loss ceramics, semiconductors such as gallium arsenide, and CMOS silicon. Characterization of CPW behavior on silicon at millimeter-wave frequencies has been an active area of IEEE research since the late 1990s, driven by the demand for on-chip transmission lines in RF integrated circuits.

Variants and Fabrication

Several variants extend the basic CPW geometry. Grounded coplanar waveguide (GCPW) adds a backside metal plane to reduce substrate radiation losses and improve shielding. Conductor-backed CPW is common in flip-chip packaging. Finite-ground CPW narrows the ground electrodes to reduce the coupling of surface waves into the substrate at high frequencies. All variants are fabricated using standard photolithographic processes on printed circuit boards or semiconductor wafers, and the single-surface geometry avoids the through-substrate vias required for some other line types. Engineering LibreTexts provides a practical treatment of CPW impedance formulas and design charts used in coursework and industrial design.

Applications

Coplanar waveguides have applications in a wide range of microwave and millimeter-wave systems, including:

• Monolithic microwave integrated circuits (MMICs) for radar and wireless communication
• Superconducting quantum circuits, where the CPW geometry defines resonator and qubit coupling structures
• Millimeter-wave transceivers for 5G infrastructure and automotive radar at 77 GHz
• On-wafer probing and calibration in microwave measurement systems
• Phased-array antennas requiring low-loss feed networks above 20 GHz