Calculator Inputs
Example Data Table
| Trace Width | Left Gap | Right Gap | Substrate Height | Permittivity | Frequency | Use Case |
|---|---|---|---|---|---|---|
| 0.35 mm | 0.18 mm | 0.26 mm | 0.80 mm | 4.20 | 2.40 GHz | General RF routing |
| 0.22 mm | 0.12 mm | 0.18 mm | 0.50 mm | 3.66 | 5.80 GHz | Compact wireless board |
| 0.60 mm | 0.30 mm | 0.30 mm | 1.60 mm | 4.40 | 1.00 GHz | Balanced reference layout |
Formula Used
The calculator uses a quasi-static coplanar waveguide estimate with an offset correction.
Effective gap: s = square root of left gap multiplied by right gap.
Conformal ratio: k = effective width / (effective width + 2s).
Impedance: Z0 = 30π divided by square root of effective permittivity, multiplied by K(k') / K(k).
Offset correction: the impedance is adjusted by a small factor based on unequal side gaps.
Delay: delay per millimeter equals square root of effective permittivity multiplied by free-space delay per millimeter.
Loss: conductor loss uses skin resistance. Dielectric loss uses loss tangent and phase constant.
How to Use This Calculator
- Enter the signal trace width in millimeters.
- Enter the left and right gaps to the ground metals.
- Add substrate height, copper thickness, and relative permittivity.
- Enter frequency, line length, loss tangent, and conductivity.
- Set your target impedance, such as 50 ohms.
- Press Calculate to show results below the header.
- Use CSV or PDF export for documentation.
Offset Coplanar Waveguide Design Guide
An offset coplanar waveguide places the signal trace between two ground planes with unequal side gaps. This layout appears when routing around pads, vias, shields, connectors, or dense component fields. It is useful, but it changes field balance. The narrow gap carries stronger electric field. The wide gap carries less. That difference can shift impedance and may raise radiation.
Why Geometry Matters
The main inputs are signal width, left gap, right gap, substrate height, copper thickness, and dielectric constant. Width and gaps set the conformal mapping ratio. The dielectric constant sets wave speed. Thickness adds a small correction. Frequency affects skin depth and loss. Length converts the line data into total delay and attenuation.
Using the Estimate
This calculator treats the offset line as an asymmetric version of a conventional coplanar waveguide. It first builds an effective gap from both sides. Then it applies an offset correction from the gap imbalance. The result is an engineering estimate, not a field solver. It is best for early sizing, quick comparison, and documentation.
Reading the Results
Characteristic impedance shows how the line should match nearby ports. Effective permittivity shows how much the substrate slows the signal. Capacitance and inductance are per unit length values. Delay shows timing across the entered line length. Guided wavelength helps with quarter wave and phase checks. The mismatch section compares the calculated impedance with your target.
Practical Layout Advice
Keep the two gaps close when possible. Use ground via fences near transitions. Avoid sudden gap changes beside connectors. Check solder mask, copper roughness, and plating for final designs. For microwave work, verify the final geometry with an electromagnetic solver. Also confirm the stackup from the board supplier before release.
When Offset Is Acceptable
Small offset is often acceptable for short interconnects. Large offset needs closer review. Watch return current near slots and cutouts. Keep reference grounds continuous. Match launch dimensions to connector recommendations. Use the export buttons to save each trial. A saved record makes design reviews easier. It also helps compare board revisions, materials, and fabrication tolerances before ordering prototypes. Documenting assumptions prevents confusion when measurements arrive during bring up. Retune dimensions when lab data shows a consistent shift later again.
FAQs
What is an offset coplanar waveguide?
It is a coplanar waveguide where the signal trace has unequal gaps to the ground planes. This offset changes field balance, impedance, and transition behavior.
Is this calculator a field solver?
No. It gives a quasi-static engineering estimate. Use it for early design checks, then verify critical microwave layouts with electromagnetic simulation.
Why are left and right gaps separate?
Offset routing often has different ground clearances on each side. Separate gap inputs help estimate imbalance instead of assuming a centered trace.
What impedance target should I use?
Many RF systems use 50 ohms. Some digital, sensor, or custom networks may need another value. Use the target from your interface requirement.
Does copper thickness affect the result?
Yes. Thicker copper slightly changes effective width. It also affects conductor loss through surface resistance and current distribution.
What does effective permittivity mean?
It is the apparent dielectric constant seen by the wave. It controls velocity, delay, wavelength, phase, and line capacitance.
Why is my return loss low?
Low return loss means the calculated impedance differs from the target. Adjust trace width, gaps, or stackup to improve matching.
Can I use the exported report directly?
Yes, for review notes and design records. For production release, include stackup confirmation, tolerance checks, and simulation evidence.