Coplanar Strips (CPS) Calculator
Coplanar strips are two parallel conductors on the same layer with no backside ground — a balanced (differential) transmission line. They feed dipole, printed Yagi and spiral antennas and connect balanced mixers and baluns.
What are coplanar strips (coplanar stripline)?
Coplanar strips (CPS), also called coplanar stripline, are two parallel conductors of width W on the same side of a substrate, separated by spacing S, with no ground plane. The strips carry equal and opposite currents, so CPS is a balanced line, the planar counterpart of a twin-lead feeder. Its characteristic impedance is usually high because the capacitance between narrow strips is small; wider strips or a smaller spacing lower it.
CPS is used to feed printed dipoles, folded dipoles and other balanced antennas, in differential circuits and at the balanced port of a balun. Keep ground planes and large metal away from the strips, or the line becomes unbalanced and both impedance and radiation change. To connect a CPS-fed antenna to an unbalanced 50 Ω system such as microstrip or GCPW, add a balun; the narrowband balun calculator helps size one. Results use conformal mapping for zero-thickness conductors on a finite substrate.
How to use
- Enter laminate εr, substrate height h and frequency f.
- Enter the spacing S between the two strips.
- Known width: enter W and press Analyze to get Zo (balanced impedance between the strips).
- Known target: enter Zo (e.g. 100 Ω) and press Synthesize to find the strip width W.
Theory and formulas
Conformal mapping for coplanar strips on a finite substrate (zero-thickness conductors; K: complete elliptic integral of the first kind):
k1 = sinh(πS/4h) / sinh(π(S + 2W)/4h)
εeff = 1 + (εr − 1)/2 · [K(k′)/K(k)] · [K(k1)/K(k1′)]
Zo = 120π / √εeff · K(k)/K(k′)
kv = 1/√εeff λ/4 = kv · c / (4f)
Design tips
- CPS impedance is usually 80–250 Ω and 50 Ω is hard to reach. To connect to an unbalanced 50 Ω system, add a balun (see the narrowband balun calculator).
- This tool: FR-4 (εr 4.4), h = 1.6 mm, S = 0.2 mm gives about 100 Ω at W ≈ 0.86 mm.
- Keep ground planes and large metal away; otherwise the line becomes unbalanced and both impedance and radiation change.
- Keep both strips symmetric and equal in length to stay balanced and minimize common-mode current.
FAQ
- CPS vs CPW?
- CPW is one signal trace between two grounds (unbalanced); CPS is two equal strips (balanced) with no separate ground.
- Why is CPS impedance high?
- The capacitance between the strips is small, so impedance is mostly above 100 Ω — a good match for dipoles (73 Ω and up) and folded dipoles (about 300 Ω).
- What are S and W?
- S is the spacing between the inner edges of the strips; W is the width of each strip — see the cross-section.
Microstrip vs CPW vs GCPW vs coplanar strips
Pick the calculator that matches your board stack-up. The structures differ mainly in where the return (ground) conductor is:
| Line type | Conductors and ground | Typical use | Calculator |
|---|---|---|---|
| Microstrip | Trace on the top layer over a solid ground plane on the other side of the dielectric | General RF routing and 50 Ω feeds on FR-4; easy to fabricate and tune | Open Microstrip calculator |
| Coplanar waveguide (CPW) | Signal trace between two ground planes on the same layer; no backside ground | Ceramic or single-sided boards, many shunt parts, GSG probe measurements | Open Coplanar waveguide calculator |
| Grounded CPW (GCPW / CPWG) | CPW plus a backside ground plane, top grounds stitched with vias | RF modules, antenna feeds and dense layouts needing low radiation and crosstalk | Open Grounded CPW calculator |
| Coplanar strips (CPS) | Two equal parallel strips, balanced, no ground plane | Printed dipole and folded-dipole feeds, baluns, differential lines | This page |
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