Optimizing Grounded Coplanar Waveguide RF Feedlines for Enhanced Wi‑Fi Performance

Editor's Note: Wireless PCB design can derail even the best plans if the RF feedline isn’t engineered correctly. This article, adapted from EEWeb, details how Arira Design’s Signal Integrity Group tackled a 5 GHz grounded coplanar waveguide (GCPW) that was under‑impedance and causing Wi‑Fi degradation.

During a recent project, the team was asked to re‑design an existing 5 GHz GCPW that had an impedance of roughly 38 Ω, well below the target 50 Ω. Initial measurements revealed a range of design oversights that can easily go unnoticed until late in development.

• Inadequate accounting for the solder mask’s dielectric effect on trace impedance.
• Failure to include PCB etch‑back in the impedance calculation.
• Incorrect cutout in a neighboring non‑reference ground plane.

After a thorough simulation review, the coplanar geometry was refined to meet the 50 Ω requirement. The updated PCB yielded a measurable improvement in Wi‑Fi throughput and reliability, as confirmed by the client.

The article covers the original geometry, the impact of the three design flaws, and the final optimized layout. E‑field plots illustrate both intentional and unintended coupling in grounded coplanar designs. Readers are expected to be familiar with basic CPW and GCPW concepts.

Grounded Coplanar Waveguides

GCPWs are increasingly popular for Wi‑Fi and Bluetooth integration on modern boards. Compared with traditional microstrip, they offer several compelling advantages:

• Lower loss: More of the electric field propagates through air rather than the lossy FR‑4 substrate, enabling the use of cost‑effective boards at 5 GHz.
• Superior isolation: Tighter field confinement reduces cross‑talk with adjacent traces.
• Design flexibility: Impedance is primarily governed by the gap between the signal trace and the adjacent ground, allowing wider trace widths without compromising performance.
• Reduced copper‑surface‑roughness loss: Current concentrates along the trace edges where the copper surface is smoother, unlike microstrip where current sits on the rougher bottom.
• Efficient matching component placement: The proximity of the ground plane lets series/parallel matching components be mounted directly between the trace and the coplanar ground, eliminating parasitic via effects.

While a number of free calculators exist for estimating GCPW impedance, they often cannot handle complex copper geometries or nearby structures. For accurate design, electromagnetic simulation remains essential.