Documentation sections

Coplanar Waveguide Calculator (CPW & GCPW)

POSThttps://rftools.io/api/py/v1/calculate

Slug coplanar-waveguide. Calculate coplanar waveguide impedance for CPW and grounded CPW (GCPW/CBCPW). Get Z₀, effective dielectric constant, propagation delay, and the gap width for 50 Ω.

Inputs

Send every one of these in inputs, in the units shown. Most are required: a call without one is refused (400) and costs nothing; one the calculator can do without takes the default shown. An input outside its range is still computed and flagged in the provenance.

Inputs of coplanar-waveguide
Name	Unit	Range	Default	Description
structurenumber	none	0 … 1	1	Grounded CPW adds a ground plane on the far side of the substrateOne of: 1 Grounded CPW (GCPW) · 0 CPW (no backing plane)
traceWidthnumber	mm	0.02 … 10	1	Centre Conductor Width
gapWidthnumber	mm	0.02 … 10	0.3	Edge-to-edge spacing between the centre conductor and each side ground
substrateHeightnumber	mm	at least 0.05	0.508	Substrate Height
dielectricConstnumber	none	at least 1	3.55	Dielectric Constant
copperThicknessnumber	μm	at least 0	35	Copper Thickness
frequencynumber	GHz	0.001 … 300	2.4	Used only for the guided wavelength; the impedance does not depend on it in this model

Outputs

Returned in values, by key. A value that is unbounded or undefined is null.

Outputs of coplanar-waveguide
Field	Unit	Description
impedanceheadline	Ω	Characteristic Impedance
effectiveDielectric	none	Effective Dielectric Constant
propagationDelay	ps/mm	Propagation Delay
gapFor50	mm	Gap for 50 Ω
wavelengthAt1GHz	mm	Guided Wavelength at 1 GHz
guidedWavelength	mm	Guided Wavelength at f
quarterWavelength	mm	Quarter Wavelength at f

Calling it

Calculate: one call, the request and response in full, and its errors.
Batch: this calculator with others, or a sweep, in one request.
Solve for a target: the value of one input that brings an output to a target.

curl -X POST https://rftools.io/api/py/v1/calculate \
  -H "X-API-Key: $RFTOOLS_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "slug": "coplanar-waveguide",
    "inputs": {
      "structure": 1,
      "traceWidth": 1,
      "gapWidth": 0.3,
      "substrateHeight": 0.508,
      "dielectricConst": 3.55,
      "copperThickness": 35,
      "frequency": 2.4
    }
  }'

import rftools

client = rftools.Client()  # reads RFTOOLS_API_KEY, or the key `rftools login` saved

result = client.calculate("coplanar-waveguide", {
    "structure": 1,
    "traceWidth": 1,
    "gapWidth": 0.3,
    "substrateHeight": 0.508,
    "dielectricConst": 3.55,
    "copperThickness": 35,
    "frequency": 2.4,
})
print(result.values)
print(result.warnings, result.provenance.formula_ref)

const response = await fetch("https://rftools.io/api/py/v1/calculate", {
  method: "POST",
  headers: {
    "X-API-Key": process.env.RFTOOLS_API_KEY,
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    "slug": "coplanar-waveguide",
    "inputs": {
      "structure": 1,
      "traceWidth": 1,
      "gapWidth": 0.3,
      "substrateHeight": 0.508,
      "dielectricConst": 3.55,
      "copperThickness": 35,
      "frequency": 2.4
    }
  }),
});
console.log(response.status, await response.json());

package main

import (
	"bytes"
	"fmt"
	"io"
	"net/http"
	"os"
)

func main() {
	body := []byte(`{
	  "slug": "coplanar-waveguide",
	  "inputs": {
	    "structure": 1,
	    "traceWidth": 1,
	    "gapWidth": 0.3,
	    "substrateHeight": 0.508,
	    "dielectricConst": 3.55,
	    "copperThickness": 35,
	    "frequency": 2.4
	  }
	}`)
	req, err := http.NewRequest("POST", "https://rftools.io/api/py/v1/calculate", bytes.NewReader(body))
	if err != nil {
		panic(err)
	}
	req.Header.Set("X-API-Key", os.Getenv("RFTOOLS_API_KEY"))
	req.Header.Set("Content-Type", "application/json")
	res, err := http.DefaultClient.Do(req)
	if err != nil {
		panic(err)
	}
	defer res.Body.Close()
	out, _ := io.ReadAll(res.Body)
	fmt.Println(res.Status, string(out))
}

application/json

{
  "slug": "coplanar-waveguide",
  "values": {
    "impedance": 48.1394112493146,
    "effectiveDielectric": 2.5005578371519546,
    "propagationDelay": 5.274699818636543,
    "gapFor50": 0.4136988368795982,
    "wavelengthAt1GHz": 189.58424827642423,
    "guidedWavelength": 78.99343678184343,
    "quarterWavelength": 19.748359195460857
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:coplanar-waveguide",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "R. N. Simons, \"Coplanar Waveguide Circuits, Components and Systems\", Wiley 2001, ch. 2",
    "assumptions": [
      {
        "code": "quasi-static",
        "text": "Quasi-static conformal-mapping model: Z₀ and εr_eff are the low-frequency values; dispersion is not modelled."
      },
      {
        "code": "semi-infinite-grounds",
        "text": "The coplanar grounds on either side are treated as infinitely wide; the width of a real ground pour is not an input."
      },
      {
        "code": "grounds-equipotential",
        "text": "Grounded CPW: the coplanar grounds and the backing plane are one equipotential, as if densely stitched with vias; the parallel-plate mode between them is not modelled."
      },
      {
        "code": "ungrounded-open-below",
        "text": "Ungrounded CPW: the substrate has height h with air below it and no other conductor nearby."
      },
      {
        "code": "bare-surface",
        "text": "One homogeneous substrate of relative permittivity εr with air above the conductors: no soldermask or cover layer."
      },
      {
        "code": "thickness-correction",
        "text": "Copper thickness t widens the strip by Δ = (t/π)(1 + ln(2h/t)) and narrows each gap by Δ/2 for Z₀; its sidewall capacitance, in air, lowers εr_eff through a correction fitted to a 2-D electrostatic solver (max 0.52% on εr_eff across ½–2 oz copper)."
      },
      {
        "code": "lossless",
        "text": "Lossless: conductor and dielectric losses are neglected."
      }
    ],
    "validRange": {
      "status": "inside",
      "bounds": {
        "structure": {
          "min": 0,
          "max": 1,
          "unit": ""
        },
        "traceWidth": {
          "min": 0.02,
          "max": 10,
          "unit": "mm"
        },
        "gapWidth": {
          "min": 0.02,
          "max": 10,
          "unit": "mm"
        },
        "substrateHeight": {
          "min": 0.05,
          "max": null,
          "unit": "mm"
        },
        "dielectricConst": {
          "min": 1,
          "max": null,
          "unit": ""
        },
        "copperThickness": {
          "min": 0,
          "max": null,
          "unit": "μm"
        },
        "frequency": {
          "min": 0.001,
          "max": 300,
          "unit": "GHz"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "structure": 1,
      "traceWidth": 1,
      "gapWidth": 0.3,
      "substrateHeight": 0.508,
      "dielectricConst": 3.55,
      "copperThickness": 35,
      "frequency": 2.4
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "structure"
      ],
      "msg": "Input should be a valid number, unable to parse string as a number",
      "input": "abc"
    }
  ]
}

An input sent as text. Nothing was counted.

{
  "detail": "More than 120 requests in 60 seconds from this address.",
  "errorKind": "rate_limited",
  "limit": 120,
  "windowSeconds": 60,
  "upgradeUrl": "https://rftools.io/pricing/"
}

Too many requests in a minute, with Retry-After.

Same inputs on the calculator pageCoplanar Waveguide Calculator (CPW & GCPW), at these values