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Free PCB Coplanar Waveguide Impedance Calculator Electronics & RF
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Free PCB Coplanar Waveguide Impedance Calculator

Calculate characteristic impedance (Z0), effective dielectric constant (εeff), and propagation delay for Coplanar Waveguide with Ground (CPWG).

⚡ Substrate & Trace Geometry

Signal copper trace width
Spacing to top coplanar ground
Core/prepreg dielectric thickness

⚡ RF Impedance & Line Metrics

Characteristic Impedance (Z0) -- Ohms (Ω)
Effective Dielectric (εeff) -- Air + Substrate mix
Propagation Delay --
Target 50Ω Matching Deviation --
Phase Velocity (Vp) --
Capacitance per Length --
Ground Via Stitching Pitch --

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Why Coplanar Waveguide with Ground (CPWG) Dominates RF Design

In high-frequency printed circuit board design (Bluetooth, Wi-Fi 6E/7, 5G, and radar up to 30+ GHz), traditional microstrip traces suffer from radiation loss and parasitic crosstalk with adjacent circuits. Coplanar Waveguide with Ground (CPWG) surrounds the central signal conductor with side ground planes on the same layer, backed by a solid reference ground plane on the layer directly beneath.

Key Advantages of CPWG over Standard Microstrip

  • Superior Isolation: The coplanar ground shielding captures fringing electric field lines laterally, dramatically suppressing EMI radiation and cross-coupling.
  • Narrower Trace Widths: The capacitive coupling to side grounds allows 50(Omega) traces to be significantly narrower than traditional microstrips on thick substrates, facilitating easy routing into dense 0.4mm BGA pins and miniature RF connectors (SMA, U.FL).
  • Lower Dispersion: Propagation velocity remains virtually flat across a broader frequency spectrum.

The Critical Rule of Via Stitching

A coplanar ground plane that is not properly stitched to the underlying ground plane will resonate like a slot antenna! You must place grounding vias connecting the top ground pours to the bottom reference plane along the entire length of the trace. The spacing between via centers must never exceed one-tenth of a wavelength (( rac{lambda}{10})) at your highest harmonic frequency (typically spaced 3 to 5 times the gap width (S)).

Frequently Asked Questions

What is the difference between CPW and CPWG?

Standard Coplanar Waveguide (CPW) has no ground plane on the bottom of the board; it relies purely on the top side grounds. Coplanar Waveguide with Ground (CPWG or Ground-Backed CPW) adds a continuous solid ground plane on the layer directly beneath the dielectric, which significantly increases power handling and mechanical robustness.

Why does solder mask affect RF impedance?

Standard LPI liquid photoimageable solder mask has a high dielectric constant (er ≈ 3.8 to 4.2). Coating a CPWG trace in solder mask pulls the electric field into the mask, dropping trace impedance by 2Ω to 4Ω and increasing high-frequency insertion loss. High-precision RF lines are usually left as bare copper with immersion gold (ENIG) with solder mask cleared.

How does FR-4 compare to Rogers RO4350B for RF traces?

Standard FR-4 has a high loss tangent (dissipation factor tan delta ≈ 0.020) and an inconsistent dielectric constant that varies wildly across manufacturing batches and humidity. Rogers RO4350B has a microscopic loss tangent (tan delta ≈ 0.0037) and tightly controlled dielectric stability up to 40 GHz.

What happens if the gap S is much larger than substrate height H?

When the ground gap S is greater than roughly 2 to 3 times the substrate height H, the side ground coupling becomes negligible, and the CPWG mathematically degenerates into a standard microstrip line.

Why is 50 Ohms the universal RF impedance standard?

In 1930, researchers at Bell Labs proved that for coaxial air lines, minimum RF signal attenuation occurs at 77 Ohms, while maximum peak RF power-handling capacity occurs at 30 Ohms. 50 Ohms was selected as the perfect commercial engineering compromise between power handling and signal attenuation.