Documentation sections

Asymmetric Stripline Impedance Calculator

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

Slug asymmetric-stripline. Calculate the characteristic impedance of an offset (asymmetric) stripline where the trace sits closer to one reference plane than the other. Compares against the centred case.

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 asymmetric-stripline
Name	Unit	Range	Default	Description
traceWidthnumber	mm	0.02 … 10	0.15	Trace Width
heightToNearPlanenumber	mm	0.01 … 5	0.2	Dielectric thickness between the trace and the closer reference plane
heightToFarPlanenumber	mm	at least 0.01	0.6	Dielectric thickness between the trace and the farther 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 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 asymmetric-stripline
Field	Unit	Description
impedanceheadline	Ω	Characteristic Impedance
centredImpedance	Ω	Same plane-to-plane spacing, trace on the centreline
offsetPenalty	%	Impedance Shift from Offset
propagationDelay	ps/mm	Propagation Delay
planeSpacing	mm	Plane-to-Plane Spacing
asymmetryRatio	none	Asymmetry Ratio
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": "asymmetric-stripline",
    "inputs": {
      "traceWidth": 0.15,
      "heightToNearPlane": 0.2,
      "heightToFarPlane": 0.6,
      "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("asymmetric-stripline", {
    "traceWidth": 0.15,
    "heightToNearPlane": 0.2,
    "heightToFarPlane": 0.6,
    "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": "asymmetric-stripline",
    "inputs": {
      "traceWidth": 0.15,
      "heightToNearPlane": 0.2,
      "heightToFarPlane": 0.6,
      "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": "asymmetric-stripline",
	  "inputs": {
	    "traceWidth": 0.15,
	    "heightToNearPlane": 0.2,
	    "heightToFarPlane": 0.6,
	    "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": "asymmetric-stripline",
  "values": {
    "impedance": 57.98713714970441,
    "centredImpedance": 65.51810716918695,
    "offsetPenalty": -11.494486554740305,
    "propagationDelay": 6.83602972157465,
    "planeSpacing": 0.8350000000000001,
    "asymmetryRatio": 2.9999999999999996,
    "guidedWavelength": 60.95155867325418,
    "quarterWavelength": 15.237889668313546
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:asymmetric-stripline",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "S. B. Cohn, “Characteristic Impedance of the Shielded-Strip Transmission Line”, IRE Trans. MTT-2, 1954 (exact centred strip and thickness increment); off-centre fringing fitted to rftools.io’s 2-D method-of-moments solver (src/lib/pcb/__tests__/solver), within 0.75% over W/b 0.05–1.2 and a near-plane gap of 0.05–0.5 of b.",
    "assumptions": [
      {
        "code": "quasi-static",
        "text": "Z₀ comes from the electrostatic capacitance per unit length of a TEM line, with no frequency dependence; higher-order modes are not modelled."
      },
      {
        "code": "homogeneous-dielectric",
        "text": "One homogeneous dielectric fills both sides of the trace (core and prepreg share a single εr), so εeff = εr and the propagation delay is √εr/c."
      },
      {
        "code": "infinite-planes",
        "text": "Both reference planes are solid and extend well beyond the trace; plane splits, voids and side walls are not modelled."
      },
      {
        "code": "fitted-offset-correction",
        "text": "The extra edge fringing of an off-centre trace, which superposing two half-striplines misses, is a correction fitted to a 2-D method-of-moments solver over W/b 0.05–1.2 and a near-plane gap of 0.05–0.5 of b = h₁ + h₂ (max residual 0.75%); outside that range it is extrapolated."
      },
      {
        "code": "rectangular-trace",
        "text": "The trace is a rectangle of thickness t, which enters through Cohn’s fringing increment (exact for wide strips); a trapezoidal etch profile is not modelled."
      },
      {
        "code": "lossless",
        "text": "Lossless: conductor and dielectric losses are neglected."
      }
    ],
    "validRange": {
      "status": "inside",
      "bounds": {
        "traceWidth": {
          "min": 0.02,
          "max": 10,
          "unit": "mm"
        },
        "heightToNearPlane": {
          "min": 0.01,
          "max": 5,
          "unit": "mm"
        },
        "heightToFarPlane": {
          "min": 0.01,
          "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": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "traceWidth": 0.15,
      "heightToNearPlane": 0.2,
      "heightToFarPlane": 0.6,
      "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 pageAsymmetric Stripline Impedance Calculator, at these values