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How long should a half-wave dipole be?

Cut a half-wave dipole, a quarter-wave vertical or a 5/8-wave whip for any frequency, with the end-effect shortening factor taken into account.

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Half-wave dipole, total length

32.9

Cut slightly long and trim both legs equally while checking the SWR
Each leg of the dipole
16.45
Quarter-wave vertical
16.45
5/8-wave vertical
41.13
Wavelength in free space
69.27

The simplest effective antenna

The half-wave dipole is the reference antenna of radio: two equal lengths of wire fed in the middle, together about half a wavelength long. It is cheap, efficient and easy to build, which makes it a favorite of ham radio operators for the HF bands. This calculator gives the cutting length of a dipole and of quarter-wave and 5/8-wave verticals for any frequency.

The formulas

The wavelength in free space is λ = c / f, or 299.79 divided by the frequency in MHz, in meters. A real dipole resonates a little shorter than λ/2 because of the end effect, so a shortening factor k, about 0.95 for thin wire, is applied:

  • Half-wave dipole: L = k × λ / 2, about 468 / f(MHz) feet or 142.5 / f(MHz) meters.
  • Each leg: half of that, measured from the center insulator.
  • Quarter-wave vertical: k × λ / 4, about 234 / f feet, used with radials or a ground plane.
  • 5/8-wave vertical: k × 0.625 × λ, which gives a little more gain at low angles but needs a matching network.

Worked example

For the 20-meter band at 14.2 MHz, λ = 21.1 m. A half-wave dipole is 0.95 × 21.1 / 2 = 10.03 m, or 32.9 feet, made of two legs of about 16.5 feet. A quarter-wave vertical for the same frequency is about 16.5 feet tall.

Tuning in practice

  1. Cut each leg 2 to 3% long and fold the extra wire back at the ends.
  2. Raise the antenna to its final height; nearby objects and height above ground shift resonance.
  3. Measure the standing wave ratio (SWR) across the band with an antenna analyzer.
  4. If the lowest SWR is below your target frequency, the antenna is too long: shorten both legs equally. Removing about 1% of the length raises resonance by about 1%.

Common band lengths

Band Frequency Dipole length
40 m 7.15 MHz 65.5 ft (19.9 m)
20 m 14.2 MHz 33.0 ft (10.0 m)
10 m 28.5 MHz 16.4 ft (5.0 m)
CB 27.2 MHz 17.2 ft (5.2 m)
2 m 146 MHz 3.2 ft (0.98 m)

Feeding the dipole

A dipole’s impedance at resonance is about 73 ohms in free space and often 50 to 70 ohms in practice, a good match for 50-ohm coaxial cable. A 1:1 current balun at the feed point keeps the coax shield from radiating. In an inverted-V configuration, with the center higher than the ends, the impedance drops closer to 50 ohms and the antenna may need to be slightly longer.

Frequently asked questions

What is the formula for a dipole antenna length?

A half-wave dipole is about 468 divided by the frequency in MHz, in feet, or 142.5 divided by the frequency, in meters. That is half the free-space wavelength multiplied by a shortening factor of about 0.95.

Why is a dipole shorter than half a wavelength?

Because of the end effect: the capacitance at the wire ends and the wire thickness make the antenna electrically longer than its physical length. The factor is typically 0.95 for thin wire and lower for thicker elements.

How do I tune a dipole?

Cut it a few percent long, hang it in its final position and measure the SWR across the band. If the lowest SWR is below your target frequency, shorten both legs by the same amount; if it is above, the antenna is too short.

What length for the 20-meter band?

At 14.2 MHz, a half-wave dipole is about 32.9 ft (10.0 m) long, or two legs of about 16.5 ft (5.0 m) each, fed in the center with a balun.

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Results are estimates provided for information purposes. They do not replace professional advice.

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