Coplanar Stripline Impedance Calculator

Calculator

Signal strip width

Width unit

Slot gap

Gap unit

Substrate height

Height unit

Copper thickness

Thickness unit

Relative dielectric constant

Frequency

Frequency unit

Line length

Length unit

Conductor conductivity S/m

Loss tangent

Target impedance, optional

Example Data Table

Example	Width	Gap	Height	Dielectric	Frequency	Approx. impedance	Guided wavelength
RF sensor line	0.90 mm	0.15 mm	0.80 mm	4.20	2.40 GHz	51.57 ohms	79.51 mm
Fast digital coupon	0.24 mm	0.10 mm	0.20 mm	3.70	10.00 GHz	71.31 ohms	20.50 mm
Microwave module	0.55 mm	0.08 mm	0.50 mm	3.38	5.80 GHz	50.04 ohms	35.63 mm

Formula Used

The calculator uses conformal mapping for a coplanar stripline style geometry. The main ratio is k = W / (W + 2S). The complementary value is k' = sqrt(1 - k²).

For finite substrate height, k2 = sinh(πW / 4h) / sinh(π(W + 2S) / 4h). Complete elliptic integrals are evaluated with the arithmetic geometric mean method.

The effective dielectric constant is estimated as εeff = 1 + ((εr - 1) / 2) × (K(k2) / K(k2')) × (K(k') / K(k)).

Characteristic impedance is estimated as Z0 = 30π / sqrt(εeff) × K(k') / K(k). Frequency then gives wavelength, phase, delay, skin depth, and approximate loss.

How to Use This Calculator

Enter the center strip width and slot gap.
Add substrate height, copper thickness, and dielectric constant.
Enter frequency and physical line length.
Use copper conductivity and loss tangent for loss estimates.
Add a target impedance when you want error checking.
Press Calculate and review the result table.
Download the CSV or PDF report when needed.

Understanding Coplanar Stripline Impedance

Coplanar stripline design places a signal conductor beside return conductors on the same copper layer. This shape is useful on radio, microwave, sensor, and fast digital boards. It keeps fields near the surface. It also helps when parts must connect without vias.

Why Geometry Matters

Impedance depends on width, gap, substrate height, dielectric constant, and copper thickness. A wider signal trace lowers impedance. A wider gap raises impedance. A higher dielectric constant lowers wave speed and usually lowers impedance. Thickness changes the effective edge shape, so the calculator includes a practical correction.

Design Use

The calculator helps during early layout work. You can enter board data, frequency, and line length. The result gives characteristic impedance, effective dielectric constant, velocity, guided wavelength, delay, and estimated loss. These values support matching, timing, and review tasks before simulation.

Frequency Effects

Impedance from the conformal formula is mainly geometry based. Frequency is used for wavelength, phase, skin depth, and loss estimates. At high frequency, copper roughness, plating, solder mask, and launch pads can change real results. Use the output as a strong starting point, not a final compliance proof.

Loss Review

The loss estimate combines a simple conductor term with a dielectric term. It is useful for comparison between design choices. It is not a replacement for field solving or test coupons. For critical microwave boards, confirm with a solver and measured stackup data.

Practical Layout Tips

Keep the gap consistent. Avoid neck downs near pads. Place ground stitching close to transitions when your structure needs a stable return path. Confirm that fabrication limits support your chosen gap and copper weight. Small errors can shift impedance more than expected.

Interpreting Results

Target impedance is optional. When entered, the calculator reports difference and percentage error. This makes tuning faster. Adjust the width or gap and calculate again. Export the data for notes or approval records.

Final Check

Use manufacturer dielectric values at your operating frequency. Ask for finished copper thickness, not only base copper. Check solder mask policy, because mask above the line can increase effective dielectric loading. Combine this tool with sound layout review and measurement.

Document assumptions clearly, especially when several board revisions share one impedance target across prototypes.

FAQs

What does this calculator estimate?

It estimates characteristic impedance, effective dielectric constant, signal velocity, guided wavelength, delay, phase, skin depth, and approximate loss for a coplanar stripline style trace.

Is the result exact for every circuit board?

No. It is an engineering estimate. Real boards can vary because of glass weave, solder mask, copper roughness, plating, fabrication tolerance, and launch geometry.

What units can I use?

You can enter geometry in meters, centimeters, millimeters, micrometers, mils, or inches. Frequency accepts hertz, kilohertz, megahertz, and gigahertz.

Why is copper thickness included?

Copper thickness changes the effective conductor edge. The calculator applies a practical correction so thick copper can shift impedance more realistically.

What is effective dielectric constant?

It is the dielectric value seen by the propagating field. Part of the field travels in air, and part travels in substrate material.

How should I use target impedance?

Enter your desired impedance, such as 50 ohms. The result will show the difference and percentage error, which helps you tune trace width or gap.

Can this replace a field solver?

No. Use this calculator for fast planning and comparison. Use a field solver or test coupon for critical microwave, compliance, or production signoff work.

Why does frequency matter here?

The impedance formula is mostly geometry based. Frequency is used for wavelength, phase, delay, skin depth, and the approximate conductor and dielectric loss values.