RF choke impedance calculator
At 100 MHz, a 1 µH choke presents 628.318530718 Ω to the RF signal, which counts as moderate blocking. Enter frequency and inductance to check that the coil feeds DC to your device without letting RF leak into the supply. Free in your browser.
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- Your data stays on your device
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Worked example
- Frequency hz
- 100,000,000
- Inductance h
- 0.000001
- Impedance ohms
- 628.318530718
- Reactance ohms
- 628.318530718
- Ideal impedance
- j628.318530718 Ω
- Blocking level
- moderate
This RF choke impedance calculator estimates how strongly an ideal choke opposes an RF signal in a bias network. Enter the signal frequency in hertz and the choke inductance in henries to calculate inductive reactance in ohms using two pi times frequency times inductance. The result includes the ideal complex impedance, a simple blocking-level label, and the exact formula used. It is a fast first-pass design check, not a substitute for the manufacturer’s impedance curve, self-resonant frequency, current rating, or a complete parasitic model.
Why choke impedance matters in an RF bias network
An RF choke supplies direct current to an active device while discouraging the RF signal from escaping into the power rail. In the ideal inductive model, its opposition to alternating current rises with both frequency and inductance. The calculator evaluates the magnitude of that inductive reactance as two pi times frequency times inductance and reports it in ohms.
A larger value generally means less RF current enters the bias feed, provided the rest of the network presents a much lower signal-path impedance. For example, a choke that offers several thousand ohms at the operating frequency is usually more useful as an isolating element than one offering only a few tens of ohms. The blocking label is deliberately broad because effectiveness is relative: one kilohm looks large beside a fifty-ohm RF path, but it may not be large beside a high-impedance resonant node.
Compare the calculated result with the impedance around the choke, the desired isolation, and the acceptable supply-line leakage. This calculation answers the initial component-selection question quickly while keeping the underlying number visible for engineering judgment.
Enter consistent values and interpret the output
Convert megahertz to hertz by multiplying by one million, and convert microhenries to henries by multiplying by one millionth. Treat that label as a convenient screening cue, not as a pass or fail verdict for a circuit. A useful follow-up is to divide the calculated choke impedance by the characteristic or node impedance relevant to the RF signal.
The larger that ratio, the more the choke resembles an open circuit to RF in the simplified model. Zero frequency correctly produces zero reactance because an ideal inductor does not block steady DC after transients have settled.
Know when the ideal calculation stops being reliable
Real RF chokes depart from the simple j two pi f L model. Winding resistance adds loss, core material changes inductance with frequency and current, and interwinding capacitance creates a self-resonant frequency. Below self-resonance, the component may behave mostly inductively and this estimate can be a useful first check.
Near resonance, measured impedance can peak far above the elementary prediction. Above resonance, the choke can become predominantly capacitive, so increasing frequency no longer guarantees stronger blocking. DC bias can also reduce effective inductance or approach saturation, while package layout and pad capacitance alter the installed result.
After using this calculator, inspect the manufacturer’s impedance-versus-frequency graph at the intended bias current and temperature. Confirm that the operating band remains on the appropriate side of self-resonance, check current and voltage ratings, and include bypass capacitors and layout parasitics in simulation when isolation is critical. For a prototype, a network analyzer measurement of the mounted bias network is stronger evidence than nominal inductance alone.
Use the ideal answer to shortlist parts and catch obvious sizing errors; use device data and measurement to sign off the RF design.
What you can do with it
Choose a bias-feed choke
Compare candidate inductances at the operating frequency before checking their detailed manufacturer curves.
Check RF supply isolation
Estimate whether a choke looks large relative to a fifty-ohm path or another known circuit impedance.
Review a frequency change
Recalculate the ideal choke reactance when an existing bias network is reused in a different RF band.
FAQ
What formula does the calculator use?
It uses the ideal-inductor relation Z = j2πfL.
Does a higher result always mean better RF blocking?
In the ideal model, a higher impedance generally improves isolation. Real performance also depends on circuit impedance, parasitics, self-resonance, bias current, and layout.
Can I enter megahertz and microhenries directly?
Convert them first: multiply megahertz by 1,000,000 for hertz and microhenries by 0.000001 for henries.
Why can a real choke differ from this result?
Real components include winding resistance, core loss, capacitance, current dependence, and self-resonance that the ideal inductance formula does not model.
How do I know if my RF choke impedance is high enough?
Divide it by the impedance of the RF path or node around it. The larger that ratio, the more the choke looks like an open circuit to RF: 1 kΩ is large beside a 50-ohm line, but may not be beside a high-impedance resonant node.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/elec/rf-choke-impedance \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"frequency_hz":100000000,"inductance_h":0.000001}'const res = await fetch("https://api.kit.forhosting.com/elec/rf-choke-impedance", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"frequency_hz": 100000000,
"inductance_h": 0.000001
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/elec/rf-choke-impedance",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"frequency_hz": 100000000,
"inductance_h": 0.000001
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/elec/rf-choke-impedance", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"frequency_hz":100000000,"inductance_h":0.000001}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"frequency_hz":100000000,"inductance_h":0.000001}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/elec/rf-choke-impedance", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"frequency_hz": 100000000,
"inductance_h": 0.000001
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "elec.rf_choke_impedance",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
Enter consistent values and interpret the output
Provide frequency_hz as the RF frequency in hertz and inductance_h as the nominal choke inductance in henries. The response returns impedance_ohms and reactance_ohms with the same magnitude because an ideal inductor has no resistive component. The ideal_impedance field writes that result as jX ohms, showing the positive imaginary impedance expected from inductance. The blocking_level field groups the magnitude into low, moderate, high, or very high ranges. Missing, negative, non-finite, or excessively large inputs return a typed invalid_input error rather than a misleading numerical result.
Technical questions
What does the API request cost?
Each successful API request costs $0.002. The browser calculator runs locally for free, and invalid requests are not charged.