What is coplanar waveguide (CPW)?
Coplanar waveguide (CPW) has a center conductor flanked by ground conductors on the same surface, separated by gaps. Grounded CPW (GCPW, also called conductor-backed CPW) adds a bottom ground plane. Impedance is set mainly by the strip width-to-gap ratio, so thin, low-impedance lines are possible regardless of substrate thickness, which suits mmWave circuits, MMICs and wafer probing.[1][2]
Why is CPW popular at high frequencies?
- Shunt components and grounds connect without vias.
- Lower dispersion and radiation than microstrip of comparable size.
- Ground-signal-ground (GSG) probes contact it directly for on-wafer measurement.
What are the pitfalls?
- Unwanted modes: in GCPW, the top grounds and bottom plane form a parallel-plate waveguide; rows of vias (a “via fence”) spaced well under λ/4 in the dielectric suppress it.
- The two coplanar grounds must stay at the same potential; air bridges or bond wires tie them at discontinuities in ungrounded CPW.
- Impedance calculations require conformal mapping or field solvers.
How is CPW impedance calculated?
Classic CPW impedance uses complete elliptic integrals of the ratio k = S/(S + 2W) for strip width S and gap W. For GCPW the ground plane lowers the impedance. Most designers use a field solver or the simulator’s line calculator.[2]
Frequently asked questions
Is GCPW the same as microstrip with ground pour?
Similar; if the gap is large compared with substrate height, GCPW behaves like microstrip.
What via spacing is used for fences?
Commonly λ/10 to λ/20 in the dielectric at the highest frequency.
Is CPW used in RF ICs?
Yes, along with microstrip-like lines on the top metal.
See also
References
Online sources accessed October 21, 2026; printed works cited by edition. See all sources.
- “Microwave Engineering, 4th ed.”. D. M. Pozar, Wiley. 2012.
- “Foundations for Microwave Engineering, 2nd ed.”. R. E. Collin, McGraw-Hill / IEEE Press. 1992 / 2001.