What Are Coplanar Transmission Lines?

Coplanar transmission lines are a family of planar microwave transmission structures in which the signal conductor and its associated ground conductors lie on the same surface of a dielectric substrate. This arrangement contrasts with microstrip, where the ground plane is on the opposite face of the substrate, and with stripline, where the conductor is buried between two ground layers. Placing signal and ground in the same plane allows probe access from above and makes it straightforward to mount shunt components between the signal strip and ground without drilling through the substrate, which simplifies the integration of series and shunt elements in monolithic microwave integrated circuits (MMICs).

Coplanar structures were first analyzed systematically by Cheng P. Wen in a 1969 paper in IEEE Transactions on Microwave Theory and Techniques that introduced the coplanar waveguide (CPW) as a transmission line suitable for incorporating non-reciprocal ferrite devices. The geometry has since become a standard medium for circuits operating above roughly 20 GHz, where substrate thickness tolerances degrade microstrip performance.

Coplanar Waveguide Structure

The canonical coplanar waveguide consists of a central signal strip flanked by two coplanar ground conductors, all patterned on the top surface of a dielectric substrate. The gap between the signal strip and each ground conductor, together with the strip width, determines the characteristic impedance of the line. A practical variant, finite ground coplanar (FGC) waveguide, uses ground conductors of finite rather than infinite width, which reduces parasitic coupling between adjacent lines and allows tighter circuit packing. Conductor-backed coplanar waveguide (CBCPW) adds a continuous ground plane on the bottom face of the substrate to prevent radiation into substrate modes, at the cost of introducing a parallel plate waveguide mode that must be suppressed with via-hole fences. The coplanar strip (CPS), a simpler variant consisting of just two parallel conductors on the same surface, supports a balanced mode and is used for differential signal routing in RF integrated circuits and antenna feed networks.

Electrical Characteristics and Substrate Effects

The electromagnetic field of a coplanar line distributes partly in the substrate and partly in the air above it, so the effective permittivity lies between the substrate permittivity and unity. For a standard CPW on a thick substrate, the effective relative permittivity is approximately the mean of the substrate and air values, typically written as (εᵣ + 1)/2. Because the critical dimensions, the strip width and gap width, are defined photolithographically on a single metallization layer, CPW impedance is largely insensitive to substrate thickness variations, which are difficult to control precisely in ceramic and semiconductor processes. Dispersion, the frequency dependence of phase velocity, is lower in CPW than in microstrip for the same substrate, making CPW preferable for broadband circuits and for millimeter-wave designs above 60 GHz. The Engineering LibreTexts microwave design reference documents the conformal mapping formulas used to calculate CPW characteristic impedance and effective permittivity from physical dimensions.

Applications in Microwave and Millimeter-Wave Circuits

Coplanar transmission lines are the medium of choice for MMICs fabricated on GaAs, InP, and GaN substrates, where the substrate is thin and its permittivity is high enough that microstrip dimensions become impractically narrow at millimeter-wave frequencies. CPW allows passive elements such as couplers, filters, and impedance transformers to be designed with larger, more easily fabricated feature dimensions. Research on composite microstrip-CPW transmission lines demonstrates how combining CPW and microstrip sections on a single substrate enables miniaturized filter designs that exploit the distinct dispersion characteristics of each medium. CPW is also used as the feed structure for patch and slot antennas in wideband applications, because its inherent shielding between adjacent ground conductors reduces spurious coupling between the feed and the radiating element.

Applications

Coplanar transmission lines have applications in a wide range of disciplines, including:

• Monolithic microwave integrated circuits for radar, point-to-point radio, and satellite transceivers
• Millimeter-wave imaging and sensing systems operating in the 60 GHz, 77 GHz, and sub-terahertz bands
• On-wafer microwave probing and network analyzer measurement of active and passive devices
• Wideband antenna feed networks and baluns in communication terminals
• Superconducting quantum circuits, where CPW resonators form coupling elements in qubit architectures