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lambda

Antenna length calculator

Enter frequency and velocity factor: we compute the physical length from c = 299792458 m/s. Half-wave = 0.5 x vf x lambda (total tip-to-tip length), quarter-wave = 0.25 x vf x lambda, full-wave = vf x lambda, where vf is the velocity factor.

Inputs

Operating frequency. Examples: 27 MHz CB, 433 or 868 MHz LoRa, 2400 MHz Wi-Fi and BLE.

Frequency unit

MHz for HF and VHF wire antennas, GHz for multi-gigahertz radio modules.

Antenna type

The quarter-wave monopole needs a ground plane. The half-wave dipole is made of two quarter-wave legs.

The velocity factor shortens the antenna versus free space. 0.66 is for a velocity-loaded insulated element, not a bare wire merely fed by coax (which stays near 0.95 to 0.98). Pick by conductor or Custom value.

Dimensionless, between 0.3 and 1. 1 is ideal free space. For a PCB trace, about 1 / square root of the effective permittivity.

mm suits 868 MHz and 2.4 GHz IoT antennas. m for HF wire antennas.

feed L total L/2 per leg

Cut-and-trim starting length (not to scale)

Result

164.1 mm

Breakdown

lambda0 = 345.38 mm · L/2 = 82.03 mm · vf = 0.95

Theoretical resonant length. Real antennas need a velocity factor and trimming; ground plane, nearby metal and the feed shift resonance. Tune with a VNA.

Frequently asked questions

FAQ

What formula do you use for the antenna length?
We start from the speed of light c = 299792458 m/s (CODATA, exact value). The free-space wavelength is lambda = c / f. The physical length is then 0.25 x vf x lambda for a quarter-wave monopole, 0.5 x vf x lambda (tip to tip) for a half-wave dipole, and vf x lambda for a full-wave element, where vf is the velocity factor.
What is the velocity factor and why does it shorten the antenna?
The velocity factor (or end-effect factor) accounts for a wave traveling slower in a real conductor than in free space. It is about 0.95 for bare wire, about 0.93 for PVC-insulated wire, and about 0.66 for a coax-fed PE element. The lower the vf, the shorter the physical antenna. This is exactly the coefficient that 468/f or 150/f shortcuts hide; here it is explicit and editable.
Why do you separate total length from per-leg length for the dipole?
A half-wave dipole is made of two quarter-wave legs fed at the center. The total tip-to-tip length is 0.5 x vf x lambda; each leg is half of that, shown on the second result line. At 868 MHz with vf = 0.95 that is about 164 mm total and 82 mm per side. Cutting each side to the right size is the most common mistake.
Is the result ready to use or does it need tuning?
It is a cut-and-trim starting length, not a tuned final dimension. Conductor diameter, proximity of a ground plane, enclosure, connector and surrounding medium all shift the real resonance. We always recommend cutting slightly long, then trimming with a vector network analyzer (VNA) until SWR is minimized across the target band.
How do I adapt the calculation to a PCB antenna trace?
On a PCB the wave travels partly in the dielectric, so the velocity factor is about 1 / square root of the effective permittivity (eps_eff). Select Custom value and type that vf. Effective permittivity depends on the stackup, trace width and ground plane; for serious RF design, 3D simulation and a measured prototype remain essential.
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