Antenna length calculator
Type a frequency and get the wavelength in vacuum with the quarter-wave and half-wave element lengths: 100 MHz gives about 3 m, 75 cm and 1.5 m. A first cut for whips, dipoles and radio labs before trimming. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Frequency
- 100,000,000
- Frequency hz
- 100,000,000 m
- Wavelength m
- 2.99792458
- Quarter wave m
- 0.749481145
- Half wave m
- 1.49896229
This antenna length calculator takes an operating frequency in hertz and returns the free-space wavelength together with the classic quarter-wave and half-wave radiator lengths used in RF design, amateur radio, and wireless prototyping. The identities are fully deterministic: wavelength equals the vacuum speed of light divided by frequency, the quarter-wave length is one fourth of that wavelength, and the half-wave length is one half. With the exact SI speed of light three hundred million metres per second to the last defined digit, a one-hundred-megahertz carrier yields roughly three metres of wavelength, about seventy-five centimetres of quarter-wave element, and about one and a half metres of half-wave dipole. The engine uses only finite arithmetic with no network calls, no sampling, and no date-dependent shortcuts.
What quarter-wave and half-wave lengths mean
A free-space electromagnetic wave at frequency f has wavelength lambda equal to the speed of light c divided by f. Antenna elements are often sized as simple fractions of that wavelength so the standing-wave pattern and feed-point impedance fall into useful ranges. A quarter-wave monopole against a ground plane is approximately lambda over four long; a half-wave center-fed dipole is approximately lambda over two long.
Those free-space ideal lengths are the first cut every designer and every student learns before velocity factor, end-effect, and ground losses trim the physical wire.
Where antenna length formulas fit in design, study, and software
Students meet quarter-wave and half-wave lengths when a textbook introduces transmission and reception, when a lab asks for a simple whip or dipole for a given carrier, or when they convert between band-plan megahertz and physical element size before cutting wire. Teachers can pre-generate answer keys for VHF, UHF, and HF examples without trusting opaque calculator apps that silently switch between feet, inches, and velocity-factor-adjusted lengths.
Amateur radio operators and makers use the free-space cut as a starting point for 2 m, 70 cm, FM broadcast, and ISM experiments before trimming on a VNA. Developers call the same contract from scripts to seed deterministic test vectors for antenna widgets, to normalize user-entered frequencies before layout demos, or to satisfy programming-challenge statements that demand lambda over four and lambda over two rather than a hand-waved sketch.
Contest and homework practice often needs a fast, offline source of metre lengths so solutions can be checked without a network of RF simulators under time pressure. Product and education tools use the evaluation when a learner types a frequency and the UI must show wavelength plus both classic radiator fractions.
What you can do with it
Homework and lab checks
Compute free-space quarter-wave and half-wave lengths for textbook frequencies and verify lab whip or dipole cuts.
Amateur radio and maker builds
Convert band-plan megahertz into first-cut metre lengths before trimming a dipole or ground-plane antenna.
FAQ
What does the antenna length calculator cost?
It is free in your browser on this page.
What happens if frequency is zero?
Zero or negative frequency makes λ = c/f undefined, so the fractional antenna lengths cannot be computed.
Does this apply a velocity factor?
No. Lengths are free-space ideals. Multiply by a velocity factor yourself if you are sizing wire, coax stubs, or dielectric-loaded elements.
What unit is frequency in?
Hertz (Hz). For example, 100 MHz is 100000000 or 1e8. Optional precision (0–15, default 10) only affects displayed decimal places of the numeric outputs.
Do I need the quarter-wave or the half-wave length?
It depends on the antenna. A quarter-wave monopole works against a ground plane and is about a quarter of the wavelength long; a centre-fed half-wave dipole is about half. The calculator gives both.
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/antenna-length \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"frequency":100000000}'const res = await fetch("https://api.kit.forhosting.com/elec/antenna-length", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"frequency": 100000000
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/elec/antenna-length",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"frequency": 100000000
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/elec/antenna-length", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"frequency":100000000}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"frequency":100000000}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/elec/antenna-length", 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": 100000000
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "elec.antenna_length",
"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.
Limits
max_abs_frequency_hz | 1000000000000000 |
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
What quarter-wave and half-wave lengths mean and what the response contains
This calculator returns frequency_hz, wavelength_m, quarter_wave_m, half_wave_m, the constant speed_of_light_m_s used, the precision applied, a method tag quarter_half_wave, a short formula string, and the unit tag m for the length fields. There is no approximate symbolic CAS path and no random sampling: the engine either builds exact floating-point lengths inside the supported frequency range or rejects the input with invalid_input. Rejected cases include missing frequency, non-finite values, magnitudes above the documented ceiling, and any zero or non-positive frequency, because lambda equals c over f is undefined when f is not positive.
How frequency is read and how lengths stay reproducible
Enter SI hertz: one hundred megahertz is one times ten to the eight, not one hundred. Optional leading plus signs and scientific notation on numeric strings are allowed when they parse as finite decimals; embedded spaces, hex prefixes, and non-numeric tokens are rejected with a clear message so a bad cell does not become a wrong element length. Cross-checks that half-wave equals twice quarter-wave keep the fractional lengths consistent. The preferred field is frequency for f in hertz, but aliases f, freq, hz, and frequency_hz are accepted so spreadsheets, query strings, and typed forms can reuse the same contract. Zero frequency is invalid and fails with invalid_input, as is any negative value, because wavelength and the fractional lengths are not defined for non-positive operating frequency in this model. An optional precision field, with aliases decimals and dp, chooses how many decimal places to expose for the numeric outputs, defaulting to ten and capped at fifteen so JSON goldens stay stable. Pair the free browser run on this page with the priced API when you need the identical mathematical result inside an automated pipeline without re-implementing the c over f formulas yourself.
Where antenna length formulas fit in design, study, and software
On this page the work runs entirely in your browser; through the API each successful lookup costs $0.002, failed validation is not charged, and nothing is stored after the response returns. That split — free interactive exploration and a priced, identical API — matches the rest of the KIT tier-A catalog and lets you move from a classroom demo to an automated pipeline without changing the mathematical result. Pair it with RLC resonance or frequency calculators when you need related RF and electronics helpers, and with other electronics endpoints when you are building a small lab of free, deterministic calculators.
Integration use cases
Deterministic test fixtures
Generate reproducible wavelength, L_λ/4, and L_λ/2 numbers for unit tests, property checks, and CI assertions.
Technical questions
How are quarter-wave and half-wave lengths defined?
With f > 0 and c = 299792458 m/s, wavelength λ = c/f, quarter-wave L_λ/4 = c/(4f), and half-wave L_λ/2 = c/(2f), all in metres.