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Wire antenna dimensions calculator

The cheapest, and often the most effective, antenna is a piece of wire cut to size. Enter the frequency and I’ll calculate the lengths for five classic designs — a dipole, a quarter‑wave vertical, an end‑fed (EFHW), a full‑wave loop and a Windom/OCFD — together with diagrams and the length of each leg, radial and perimeter.

dipole · GP · EFHW · loop · OCFD 160 m – 70 cm legs, radials, perimeter diagrams
Calculate your antenna How I work it out
A mast with amateur radio antennas against a sunset sky
the antenna is the heart of the station — the rest is coax and a radio

Your antenna

// enter the resonant frequency — all the dimensions will work themselves out

The band centre or your working frequency. You can enter any value.
0.950
A wire’s resonant length is shorter than in free space (end effect + insulation). Thinner/insulated wire = a smaller k.
Wavelength · starting point
42.2m
λ = 42.22 m  ·  f = 7.100 MHz  ·  band 40 m  ·  k = 0.950
!

This is a starting dimension, not the final one. Height above ground, the surroundings (roof, gutters, trees) and wire thickness all shift the resonance. Cut the wire 2–3% longer, measure SWR with an analyser and shorten by ~2–3 cm at a time until you hit the minimum. It’s better to tune by trimming than by adding.

Under the bonnet

How I work out the dimensions

No magic — just the basic resonance formulae. The result is approximate, but close enough that after you build it there’s only light touch‑up tuning left.

Equations

  • Wavelength: λ = 299.79 / f (metres, f in MHz). That’s the distance the wave travels in one period.
  • Half‑wave dipole: total length = 0.5 · λ · k, each leg is half. Fed in the middle, ~50–70 Ω, 1:1 balun. An inverted‑V comes out ~2–3% shorter.
  • Quarter‑wave GP: vertical radiator = 0.25 · λ · k + 4 radials of the same length. Dropping the radials to ~45° raises the impedance to ~50 Ω.
  • End‑Fed Half‑Wave (EFHW): wire = 0.5 · λ · k, end‑fed through a 49:1 transformer + a short counterpoise ~0.05 λ. Works on harmonics → multiband.
  • Full‑wave loop: circumference ≈ 1.02 · λ (loops resonate a little longer). Side of a delta = circumference/3, side of a square = circumference/4. ~110 Ω — match with a 75 Ω quarter‑wave section or a 4:1 balun.
  • Windom / OCFD: like a dipole (0.5 · λ · k), but fed off‑centre (~⅓ of the length) through a 4:1 balun — this lets it work on several harmonic bands.
Antenna mast — illustration for the technical section wire, insulators and a good earth — the rest is tuning
✓

k in practice: for 1–2 mm bare wire take 0.95–0.96; for thin insulated wire (insulation slows the wave) go down to 0.92–0.94. On VHF/UHF, where the antenna is tubing or thick wire, closer to 0.96–0.97. Not sure — leave 0.95 and fine‑tune after you build it.

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