Documentation sections

Chassis Resonant Frequency

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

Slug chassis-resonance. Calculate the lowest resonant frequency and TE modes of a metallic enclosure. Identify cavity resonance problems for EMC shielding design.

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 chassis-resonance
Name	Unit	Range	Default	Description
lengthnumber	cm	at least 0.1	30	Length
widthnumber	cm	at least 0.1	20	Width
heightnumber	cm	at least 0.1	10	Height
bandLownumber	MHz	at least 0	30	Where the band you care about starts: the radiated-emissions range (30 MHz to 1 GHz), a clock's harmonics or a radio's band
bandHighnumber	MHz	at least 0	1000	Where the band ends

Outputs

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

Outputs of chassis-resonance
Field	Unit	Description
fLowestheadline	MHz	Lowest Resonant Frequency
f101MHz	MHz	TE₁₀₁ Mode
f110MHz	MHz	TE₁₁₀ Mode
wavelengthAtResonance	cm	Wavelength at f_min
f011MHz	MHz	The mode across the width and height: the lowest when the length is the shortest side
modesInBand	none	Resonant frequencies of the closed box inside the band, each (m, n, p) once (with no zero index it is two modes, TE and TM): a source there can excite the enclosure

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": "chassis-resonance",
    "inputs": {
      "length": 30,
      "width": 20,
      "height": 10,
      "bandLow": 30,
      "bandHigh": 1000
    }
  }'

import rftools

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

result = client.calculate("chassis-resonance", {
    "length": 30,
    "width": 20,
    "height": 10,
    "bandLow": 30,
    "bandHigh": 1000,
})
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": "chassis-resonance",
    "inputs": {
      "length": 30,
      "width": 20,
      "height": 10,
      "bandLow": 30,
      "bandHigh": 1000
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "chassis-resonance",
	  "inputs": {
	    "length": 30,
	    "width": 20,
	    "height": 10,
	    "bandLow": 30,
	    "bandHigh": 1000
	  }
	}`)
	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": "chassis-resonance",
  "values": {
    "fLowest": 900.7642327636539,
    "f101MHz": 1580.044987700612,
    "f110MHz": 900.7642327636539,
    "wavelengthAtResonance": 33.282011773513744,
    "f011MHz": 1675.8907880743768,
    "modesInBand": 1
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:chassis-resonance",
    "version": "api@3f2c1a9b04de",
    "formulaRef": null,
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "length": {
          "min": 0.1,
          "max": null,
          "unit": "cm"
        },
        "width": {
          "min": 0.1,
          "max": null,
          "unit": "cm"
        },
        "height": {
          "min": 0.1,
          "max": null,
          "unit": "cm"
        },
        "bandLow": {
          "min": 0,
          "max": null,
          "unit": "MHz"
        },
        "bandHigh": {
          "min": 0,
          "max": null,
          "unit": "MHz"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "length": 30,
      "width": 20,
      "height": 10,
      "bandLow": 30,
      "bandHigh": 1000
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "length"
      ],
      "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 pageChassis Resonant Frequency, at these values