Calculator Inputs
Example Data Table
Use these sample values to test board geometry and coupled line behavior.
| Material | Er | Height mm | Width mm | Gap mm | Frequency GHz | Use Case |
|---|---|---|---|---|---|---|
| FR-4 | 4.30 | 0.80 | 1.45 | 0.25 | 2.40 | General RF coupler estimate |
| Rogers 4350B | 3.48 | 0.508 | 1.10 | 0.18 | 5.80 | Compact microwave layout |
| PTFE laminate | 2.20 | 0.787 | 2.30 | 0.35 | 1.00 | Low loss coupled section |
Formula Used
The calculator uses closed-form microstrip equations with an empirical coupled-line spacing estimate. These formulas are suitable for planning and comparison.
εeff = (εr + 1) / 2 + ((εr - 1) / 2) / √(1 + 12h / W)Z0 = 60 / √εeff × ln(8h / W + W / 4h)for narrow traces.Z0 = 120π / [√εeff × (W/h + 1.393 + 0.667ln(W/h + 1.444))]for wider traces.k ≈ 0.50 × e^(-0.88S/h) × (1 - e^(-0.78W/h)) × √(εeff / εr)Zeven = Z0 × √((1 + k) / (1 - k))Zodd = Z0 × √((1 - k) / (1 + k))Coupling dB = 20log10(k)λg = c / (f × √εeff)
How to Use This Calculator
Enter the dielectric constant, board height, copper thickness, trace width, spacing, line length, and operating frequency. Choose the same unit for all geometry inputs. Press calculate. The result area appears above the form. Review impedance, coupling, delay, wavelength, and synthesized dimensions. Use the graph to see how coupling changes with spacing. Export the table when you need design records.
Coupled Coplanar Microstrip Design Guide
Purpose
Coupled coplanar microstrip lines are used when two traces must share electric and magnetic fields. They appear in directional couplers, filters, baluns, delay sections, matching networks, and high speed layouts. The important variables are dielectric constant, board height, trace width, gap, copper thickness, length, and frequency. Small changes in gap can strongly change coupling.
Analysis Workflow
Start by entering the known board stack. Use the real laminate value, not only a catalog name. Then enter width and spacing from your layout. The calculator estimates single line impedance first. It then estimates the coupling coefficient. From that value it derives even mode and odd mode impedances. These values help you judge balance, mismatch, and coupling strength before detailed simulation.
Synthesis Workflow
For synthesis, enter a target impedance and coupling level. A smaller dB value means stronger coupling. For example, 6 dB is stronger than 20 dB. The tool solves a practical trace width for the target impedance. It then searches for a gap that gives the requested coupling coefficient. The target angle field estimates the line length for quarter wave or custom electrical sections.
Interpreting Results
Even mode impedance rises when the adjacent trace supports the same voltage direction. Odd mode impedance falls when the traces carry opposite voltages. The difference between them reflects coupling strength. Very tight gaps may be hard to fabricate. Very wide gaps may produce weak coupling and large structures. Always compare the synthesized values with your manufacturer design rules.
Practical Notes
Closed-form equations are fast. They are also approximate. Solder mask, ground pour openings, copper roughness, plating variation, frequency dispersion, and connector launch geometry can shift real results. Use this calculator during early design. Then confirm critical RF structures with a field solver, VNA measurement, or vendor stackup model. Keep dimensions consistent, and round final geometry to manufacturable values.
FAQs
1. What is a coupled coplanar microstrip line?
It is a pair of nearby microstrip traces on the same board layer. Their fields interact through the gap. This interaction creates even and odd propagation modes.
2. Why are even and odd impedances important?
They describe how the pair behaves under common and opposite signal conditions. Couplers, filters, and differential structures depend on their relationship.
3. Does a smaller gap always increase coupling?
Yes, a smaller edge gap usually increases field interaction. It also raises fabrication difficulty and may increase sensitivity to etching tolerance.
4. Can this replace an electromagnetic solver?
No. It is best for early estimates and quick comparison. Final RF layouts should be checked with simulation, measurement, or verified stackup data.
5. What unit should I use?
Select mm, mil, or inch. Keep every geometry input in the selected unit. Frequency is always entered in GHz.
6. What does target coupling dB mean?
It is the desired coupling magnitude. Lower positive numbers mean stronger coupling. A 10 dB target is stronger than a 20 dB target.
7. Why does dielectric constant affect coupling?
The dielectric controls field concentration and wave speed. It changes effective permittivity, impedance, guided wavelength, and coupling behavior.
8. Why include copper thickness?
Copper thickness changes effective trace width. That shift can affect impedance, especially on thin substrates or wide copper plating builds.