Coplanar Strips Overview
Coplanar strips (CPS) is a planar transmission line structure used in microwave and RF circuit design. It consists of two parallel conductors placed side by side on the same surface of a dielectric substrate. Unlike microstrip, there is no ground plane on the bottom. Both signal conductors sit on the same side of the board.
This structure supports a quasi-TEM mode of propagation. The electromagnetic field is concentrated in the gap between the two conductors and in the substrate beneath them. Because both conductors are on the same layer, coplanar strips lines are easy to fabricate and integrate into planar circuits.
Coplanar strips are commonly used in balanced transmission line applications. They work well in antenna feed networks, balun designs, and millimeter-wave integrated circuits. They are also a natural fit for circuits that require a differential signal path, where two conductors carry equal and opposite signals.
The key design parameters are the conductor width (W), the gap between the conductors (S), and the substrate height (h) and dielectric constant (εr). A wider gap produces higher impedance. A higher dielectric constant pulls more of the field into the substrate and lowers the effective permittivity seen by the signal.
The characteristic impedance (Zo) tells you how the line resists the flow of electrical energy. The effective permittivity (εr eff) describes how fast the signal travels along the line relative to free space. Both values must be known before a design can move forward.
This calculator computes Zo and εr eff based on your input parameters. It uses established closed-form models suited for standard PCB substrates. Enter your values and get your results in seconds.