Home › Engineering & Automotive › Antenna Length Calculator

Antenna Length Calculator

Computes the free-space wavelength for a frequency and the physical wire length of a half-wave dipole (with the length of each leg), a quarter-wave vertical, a full-wave loop or a 5/8-wave radiator, applying a velocity (end-effect) factor as in the 468/f ft rule.

When to use

You are cutting wire or tubing for an amateur-radio, CB, FM, Wi-Fi, LoRa or ISM-band antenna and need the starting length for a given frequency.

Do not use when: You need a Yagi, patch, helical or loaded (shortened) antenna design, impedance matching, or just the wavelength without an antenna (use wavelength-frequency).

Formula

wavelength_m = 299.792458 / frequency_mhz; half_wave_dipole: total_length_m = wavelength_m / 2 × velocity_factor (≈ 468 / frequency_mhz ft at 0.95), each_leg_m = total_length_m / 2; quarter_wave_vertical: wavelength_m / 4 × velocity_factor (≈ 234 / f ft); full_wave_loop: wavelength_m × 1.02 (≈ 1005 / f ft, velocity_factor not applied); five_eighths_wave: 0.625 × wavelength_m × velocity_factor

Free-space lengths scaled by an empirical shortening factor (0.95 for thin wire, per the ARRL 468/f rule) that accounts for end effect; loops are electrically longer than free-space wavelength, hence the 1005/f ft (1.02 λ) rule. Results are starting lengths: cut slightly long, then trim for minimum SWR, because height above ground, wire diameter, insulation and nearby objects shift resonance by a few percent.

Inputs

Parameter	Type	Unit	Required	Description
`frequency_mhz`	number	MHz	yes	Operating (centre) frequency in megahertz, e.g. 146 (2 m band), 27.185 (CB channel 19), 433.92 (ISM), 2437 (Wi-Fi channel 6). Range: > 0, ≤ 1000000
`antenna_type`	enum: half_wave_dipole \| quarter_wave_vertical \| full_wave_loop \| five_eighths_wave		default half_wave_dipole	Radiator geometry.
`velocity_factor`	number		default 0.95	Shortening factor for end effect and conductor thickness: 0.95 for thin wire (the classic 468/f ft rule), 1.0 for the theoretical free-space length, 0.66–0.85 for coaxial-cable elements (coax dielectric). Range: ≥ 0.3, ≤ 1

Outputs

Output	Type	Unit	Description
`wavelength_m`	number	m	λ = 299.792458 / frequency_mhz.
`wavelength_ft`	number	ft	λ in feet (1 ft = 0.3048 m).
`total_length_m`	number	m	Overall radiator length (tip to tip for a dipole, total wire for a loop).
`total_length_ft`	number	ft	Overall length in feet.
`total_length_in`	number	in	Overall length in inches.
`each_leg_m`	number	m	Length of each of the two dipole legs (half of the total; dipole only).
`each_leg_ft`	number	ft	Each dipole leg in feet (dipole only).
`each_leg_in`	number	in	Each dipole leg in inches (dipole only).
`formula_used`	string		Length expression applied for the chosen antenna type.

Example

Half-wave dipole for 146 MHz (2 m band), vf 0.95: {"frequency_mhz":146,"antenna_type":"half_wave_dipole","velocity_factor":0.95} → {"wavelength_m":2.0534,"wavelength_ft":6.7368,"total_length_m":0.9754,"total_length_ft":3.2,"total_length_in":38.4,"each_leg_m":0.4877,"each_leg_ft":1.6,"each_leg_in":19.2}

Quarter-wave vertical for CB channel 19 (27.185 MHz): {"frequency_mhz":27.185,"antenna_type":"quarter_wave_vertical"} → {"wavelength_m":11.0279,"total_length_m":2.6191,"total_length_ft":8.5929,"total_length_in":103.11}

GET https://tttkmbb.com/api/v1/calculate/antenna-length?frequency_mhz=146&antenna_type=half_wave_dipole&velocity_factor=0.95

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/antenna-length (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/antenna-length · Markdown: https://tttkmbb.com/engineering/antenna-length.md · JSON definition: https://tttkmbb.com/engineering/antenna-length.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="antenna-length"
OpenAPI operationId: calculate_antenna_length
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

Wikipedia – Dipole antenna (reference)
ARRL Antenna Book for Radio Communications (American Radio Relay League) – dipole and loop length formulas (textbook)
Wikipedia – Velocity factor (reference)

FAQ

Why is the dipole shorter than half a wavelength?

Capacitance at the wire ends (end effect) and the finite conductor diameter make a wire resonate at a length about 5 % shorter than λ/2 in free space, which the 0.95 velocity factor and the 468/f ft rule build in.

Does the velocity factor apply to the loop?

No. A full-wave loop resonates when its total wire length is slightly longer than one wavelength, so the calculator uses the empirical 1.02 λ (1005/f ft) rule and ignores velocity_factor for loops.

Does a quarter-wave vertical need anything else?

Yes, a ground plane: radials or a conductive surface of about the same length as the radiator form the other half of the antenna. The 5/8-wave vertical also needs a base loading coil to match 50 Ω.

Related calculators

Wavelength & Frequency Calculator — Wavelength for any frequency and propagation speed.
LC Resonance Calculator — Tuned circuits and traps used with antennas.
Length Converter — Convert the cut length between metres, feet and inches.