Documentation sections

Edge-Coupled Asymmetric (Offset) Stripline Calculator

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

Slug edge-coupled-internal-asymmetric. Calculate differential and common-mode impedance for an edge-coupled pair sitting off-centre between two reference planes — the usual case in an asymmetric stack-up.

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 edge-coupled-internal-asymmetric
Name	Unit	Range	Default	Description
traceWidthnumber	mm	0.02 … 10	0.13	Trace Width
traceSpacingnumber	mm	0.02 … 10	0.2	Trace Spacing (edge to edge)
heightBelownumber	mm	at least 0.02	0.2	Distance from the trace layer down to the nearer reference plane
heightAbovenumber	mm	at least 0.02	0.5	Distance from the trace layer up to the other reference plane
copperThicknessnumber	μm	at least 5	35	Copper Thickness
dielectricConstnumber	none	at least 1	4.2	Dielectric Constant
frequencynumber	GHz	0.001 … 300	2.4	Used only for the guided wavelength; the impedances do not depend on it in this model

Outputs

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

Outputs of edge-coupled-internal-asymmetric
Field	Unit	Description
diffImpedanceheadline	Ω	Differential Impedance
commonImpedance	Ω	Common-Mode Impedance
oddImpedance	Ω	Odd-Mode Impedance
evenImpedance	Ω	Even-Mode Impedance
uncoupledImpedance	Ω	Single Trace Without Its Partner
offsetRatio	none	\|h₁ − h₂\|/(h₁ + h₂); zero when the pair is centred
propagationDelay	ps/mm	Propagation Delay
inValidatedRange	none	1 when the geometry lies inside the range the model was checked against a field solver
guidedWavelength	mm	Guided Wavelength
quarterWavelength	mm	Quarter Wavelength

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": "edge-coupled-internal-asymmetric",
    "inputs": {
      "traceWidth": 0.13,
      "traceSpacing": 0.2,
      "heightBelow": 0.2,
      "heightAbove": 0.5,
      "copperThickness": 35,
      "dielectricConst": 4.2,
      "frequency": 2.4
    }
  }'

import rftools

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

result = client.calculate("edge-coupled-internal-asymmetric", {
    "traceWidth": 0.13,
    "traceSpacing": 0.2,
    "heightBelow": 0.2,
    "heightAbove": 0.5,
    "copperThickness": 35,
    "dielectricConst": 4.2,
    "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": "edge-coupled-internal-asymmetric",
    "inputs": {
      "traceWidth": 0.13,
      "traceSpacing": 0.2,
      "heightBelow": 0.2,
      "heightAbove": 0.5,
      "copperThickness": 35,
      "dielectricConst": 4.2,
      "frequency": 2.4
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "edge-coupled-internal-asymmetric",
	  "inputs": {
	    "traceWidth": 0.13,
	    "traceSpacing": 0.2,
	    "heightBelow": 0.2,
	    "heightAbove": 0.5,
	    "copperThickness": 35,
	    "dielectricConst": 4.2,
	    "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": "edge-coupled-internal-asymmetric",
  "values": {
    "diffImpedance": 103.02169184994771,
    "commonImpedance": 33.351055689617894,
    "oddImpedance": 51.51084592497386,
    "evenImpedance": 66.70211137923579,
    "uncoupledImpedance": 59.439310214642056,
    "offsetRatio": 0.4285714285714286,
    "propagationDelay": 6.83602972157465,
    "inValidatedRange": 1,
    "guidedWavelength": 60.95155867325418,
    "quarterWavelength": 15.237889668313546
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:edge-coupled-internal-asymmetric",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "S. B. Cohn, IRE Trans. MTT-3 (1955) for the centred case; offset correction fitted to the committed solver in src/lib/pcb/__tests__/solver.",
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "traceWidth": {
          "min": 0.02,
          "max": 10,
          "unit": "mm"
        },
        "traceSpacing": {
          "min": 0.02,
          "max": 10,
          "unit": "mm"
        },
        "heightBelow": {
          "min": 0.02,
          "max": null,
          "unit": "mm"
        },
        "heightAbove": {
          "min": 0.02,
          "max": null,
          "unit": "mm"
        },
        "copperThickness": {
          "min": 5,
          "max": null,
          "unit": "μm"
        },
        "dielectricConst": {
          "min": 1,
          "max": null,
          "unit": ""
        },
        "frequency": {
          "min": 0.001,
          "max": 300,
          "unit": "GHz"
        }
      },
      "outside": [],
      "model": {
        "description": "w/b 0.05–0.8, s/b 0.05–1.0, nearer gap 0.125–0.5 of the plane spacing",
        "worstCaseError": 0.019,
        "inside": true
      }
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "traceWidth": 0.13,
      "traceSpacing": 0.2,
      "heightBelow": 0.2,
      "heightAbove": 0.5,
      "copperThickness": 35,
      "dielectricConst": 4.2,
      "frequency": 2.4
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "traceWidth"
      ],
      "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 pageEdge-Coupled Asymmetric (Offset) Stripline Calculator, at these values