Design CPW, GCPW, and CBCPW transmission lines for RF, mmWave, and 5G applications. Calculate impedance, effective permittivity, and optimize geometry for frequencies up to 100+ GHz.
GCPW (Grounded Coplanar Waveguide) Cross-Section
Choose the right CPW variant based on your frequency, isolation, and manufacturing requirements
Signal with coplanar grounds only, no bottom ground plane
Chip probing, flip-chip, simple RF
CPW with bottom ground plane and stitching vias
Most PCB RF designs, 5G, mmWave
Conductor-Backed CPW, bottom ground without top via stitching
Power amplifiers, thermal management
Simplified: Z₀ ≈ (60/√εeff) × ln(2(1+√k)/(1-√k)) for practical use
Rule of thumb: Z₀_gcpw ≈ 0.7 - 0.85 × Z₀_cpw
28GHz, 39GHz, and 60GHz antenna feeds and transitions. GCPW provides low loss at mmWave.
77GHz ADAS radar systems using GCPW for antenna array feeds and power dividers.
On-wafer testing and flip-chip mounting. CPW provides excellent probe-pad transitions.
CPW-to-microstrip, CPW-to-waveguide, and CPW-to-coax transitions for system integration.
At 28GHz with εr=3.5, λg ≈ 5.7mm → via pitch ≤ 0.57mm
Close enough for mode suppression, far enough to avoid field disturbance
Reduces via inductance; 8-10mil diameter typical for mmWave
Always verify with field solver; these are starting points
In GCPW without via stitching, a parasitic parallel-plate mode can propagate between the coplanar ground and bottom ground, causing resonances and loss. Use via fences to short the grounds together and suppress this mode, especially above 10GHz.
| Parameter | GCPW | Microstrip | Notes |
|---|---|---|---|
| Frequency Range | DC to 100+ GHz | DC to ~40 GHz | CPW scales better at mmWave |
| Dispersion | Lower | Higher | Better pulse shape at high freq |
| Component Mounting | Direct (no via) | Via to ground | Easier SMT integration |
| Ground Definition | Local (coplanar) | Requires via | Better for RF transitions |
| Routing Density | Lower | Higher | CPW needs coplanar grounds |
| Design Complexity | Higher | Lower | Via fences required |
A coplanar waveguide is a transmission line where the signal trace and ground conductors are on the same layer, with the signal trace between two ground areas separated by gaps. It offers easy integration with surface-mount components and good performance at mmWave frequencies. The electric field is concentrated in the gaps.
Standard CPW has ground planes only on the signal layer (coplanar grounds). GCPW (Grounded CPW) adds a ground plane on the layer below, connected to the coplanar grounds via vias. GCPW provides better shielding, lower radiation losses, and more consistent impedance, making it preferred for most PCB applications.
CPW is preferred when: (1) operating above 10GHz where microstrip losses increase, (2) needing easy connection to surface-mount components without vias, (3) designing transitions between different transmission line types, (4) wanting reduced dispersion at high frequencies, or (5) requiring tight ground definition near signal traces.
CPW impedance is controlled by the ratio of signal width (W) to gap width (G): wider gaps or narrower signal = higher impedance. For GCPW, the substrate height (H) also matters: thinner substrate = lower impedance. Typical dimensions for 50Ω on FR-4: W=10mil, G=5mil, or adjust based on your substrate.
Via fences are rows of ground vias placed along both sides of a GCPW line, connecting the coplanar grounds to the bottom ground plane. They suppress parallel-plate modes, reduce crosstalk, and improve isolation. Space vias at λ/10 or closer for frequencies above 10GHz to prevent mode conversion.
Compare CPW with traditional microstrip for lower frequency designs.
Shielded inner layer traces with full ground plane coverage.
100Ω differential pairs for high-speed serial interfaces.
Complete reference of impedance equations and design formulas.
Complete guide from DC to mmWave frequencies.
Grounding vias for GCPW mode suppression.
Surface roughness and conductor losses at RF.
Complete A-Z reference of RF and PCB terms.