Antenna Length Calculator
Calculate antenna lengths for dipole, quarter-wave, 5/8-wave, and Yagi designs from frequency.
Covers HF, VHF, UHF, and WiFi in meters, feet, and inches.
Antenna Length is determined by the operating frequency. Antennas are typically sized as fractions of the wavelength of the signal.
Wavelength Formula:
λ = c / f
Where:
- λ (lambda) = Wavelength
- c = Speed of light = 299,792,458 m/s (≈ 300,000 km/s)
- f = Frequency in Hz
Common Antenna Types and Lengths:
| Type | Length | Formula (meters) | Use |
|---|---|---|---|
| Full wave | 1λ | 300 / f(MHz) | Loop antennas |
| Half-wave dipole | ½λ | 150 / f(MHz) | Most common HF antenna |
| Quarter-wave vertical | ¼λ | 75 / f(MHz) | Ground-plane, mobile |
| 5/8 wave | ⅝λ | 187.5 / f(MHz) | VHF/UHF mobile, higher gain |
The 0.95 shortening factor, and what it is not
A real wire antenna resonates slightly shorter than the mathematics says, because the electric field fringes past the physical ends and the wire has thickness. Multiplying by about 0.95 accounts for it. Thicker wire needs a little more shortening, thin wire a little less.
Practical length = Theoretical length × 0.95
This factor is frequently, and confusingly, called “velocity factor”. It is not the same thing. Velocity factor is a property of coaxial cable, the ratio of signal speed inside the line to the speed of light, and it runs 0.66 for common polyethylene coax up to about 0.85 for foam types. You use it to cut phasing lines and matching stubs, not antenna elements. Two different numbers, two different jobs, and mixing them up produces an antenna cut a third too short.
Common Frequencies and Antenna Lengths
Practical half-wave dipole lengths, with the 0.95 factor already applied:
| Application | Frequency | Half-wave dipole |
|---|---|---|
| AM Radio (centre) | 1 MHz | 142.5 m (467 ft) |
| 40m Ham Band | 7.15 MHz | 19.9 m (65.4 ft) |
| 20m Ham Band | 14.175 MHz | 10.1 m (33.0 ft) |
| FM Radio (centre) | 100 MHz | 1.43 m (4.7 ft) |
| 2m Ham Band | 146 MHz | 0.98 m (3.2 ft) |
| WiFi 2.4 GHz | 2,450 MHz | 5.8 cm (2.3 in) |
| WiFi 5 GHz | 5,800 MHz | 2.5 cm (1.0 in) |
The AM row is the reason nobody puts a resonant dipole on the broadcast band at home. At 142 metres you use a loaded vertical or a loop instead and accept the loss.
Practical Example: A 2m half-wave dipole at 146 MHz. Theoretical: 150 / 146 = 1.027 m. With the 0.95 shortening factor: 1.027 × 0.95 = 0.976 m (38.4 inches) end to end, so 0.488 m (19.2 in) per arm.
One caveat on the 5/8 wave
A 5/8-wave vertical gives a lower radiation angle and a couple of dB more gain toward the horizon than a quarter wave, which is why it is popular on VHF mobile. But it is not resonant at that length: it looks capacitive and needs a loading coil at the base to present a usable impedance. Cut one to the length below and connect it straight to coax and the SWR will be dreadful. The length is right; the matching network is the missing piece.
Tips:
- A dipole has two equal halves, so each arm is a quarter wavelength.
- Mount a dipole at least a quarter wavelength above ground if you can. Lower than that and the ground pulls the pattern upward and the feedpoint impedance drops.
- Thicker wire gives broader bandwidth and needs slightly more shortening.
- Indoors, nearby metal detunes everything. Cut long, measure, then trim.
- Trim in small steps from both ends of a dipole equally, checking SWR each time. You can always cut more off; you cannot put it back.
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This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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