Dipole Antenna Length Calculator

The half-wave dipole is the reference antenna of amateur radio, and the arithmetic behind it is the first calculation almost every new HF operator runs. Two lengths of wire, an insulator at each end, a feedpoint in the middle, and a formula that has not changed since the 1930s. This page gives you the number, the chart for every band, and the parts of the job the number does not cover.

What the 468 actually is

A full wavelength in free space, in feet, is 984 divided by the frequency in MHz. Half of that is 492. If a dipole were an infinitely thin wire floating in a vacuum, 492 divided by frequency would be the answer and this page would end here.

It is not, for two reasons. The first is end effect: the capacitance between the wire ends and everything nearby, including the insulators and the support ropes, makes the antenna behave electrically longer than it physically is. The second is the wire's own diameter, which has the same effect and gets stronger as the wire gets fatter relative to the wavelength. Together they add about five percent, so the physical wire has to be about five percent shorter to resonate where you want it. That is where 468 comes from, and why fatter conductors and thicker insulation move the number.

The important consequence is that 468 is a starting point, not an answer. It is accurate to within a few percent for typical 12 to 14 AWG wire in the clear, which is close enough to land inside a band. It is not close enough to guarantee resonance in the part of the band you want, which is why the trimming step below is not optional.

Dipole length by band, at a glance

The frequencies below are the centre of the most-used portion of each band rather than the band centre, because a 40 m dipole cut for 7.150 MHz is usable across the whole band while one cut for 7.000 MHz is not. All lengths are for bare wire before trimming.

How to cut and trim a dipole so you only do it once

The procedure below is the whole job, and the order matters more than the arithmetic does. Nearly every dipole that ends up resonant in the wrong place got there by being trimmed at the wrong height.

1. Cut each leg three percent long. On 40 m that is about a foot per leg, on 20 m about six inches, on 10 m about two inches. Do not cut to the calculated length and hope.
2. Fold the excess back rather than cutting it. Twist the surplus back along the leg and secure it with a loop. Folded wire changes the resonance slightly less than the same length extended, but the point is that the decision stays reversible.
3. Raise the antenna to its final height and final shape. A dipole tuned at eight feet and then hoisted to forty will move, often by more than the width of the band segment you wanted. If it is going up as an inverted V, tune it as an inverted V.
4. Measure, do not guess. Sweep it with a NanoVNA H4 vector network analyzer or an antenna analyser and find the actual minimum. An in-line SWR meter tells you the match at one frequency; an analyser tells you where the dip really is and which way to move.
5. Trim both legs equally. The rule of thumb is that a one percent change in length moves resonance about one percent in frequency, in the opposite direction. Too low in frequency means the antenna is too long. Take the same amount off each side to keep the pattern symmetrical and the feedpoint balanced.
6. Re-measure after every cut. Two small trims beat one large one, and wire that has been cut short cannot be uncut.

What you actually need to build one

A dipole is the cheapest real antenna in the hobby, and the parts list is short. The wire itself matters less than people expect; the insulators, the rope and the feedpoint are what decide whether it is still up in three years.

• Wire. Stranded 14 AWG is the general-purpose answer. A Paladin 14 AWG stranded copper wire, 500 ft$74.50 spool builds a full 80 m dipole with enough left over for a second antenna and a set of radials, which works out cheaper than buying two commercial antennas. For a single band, a short run of XRDS-RF 14 AWG stranded copper wire, 25 ft is enough.
• End insulators. Nylon dog bone antenna insulators, 10-pack$29.97 gives you ten, which is four antennas' worth. Nylon does not shatter in cold the way ceramic can.
• A centre insulator with a choke. Coax feeding a dipole directly will carry common-mode current on the outside of the braid, which turns the feedline into part of the antenna and puts RF in the shack. A Radiowavz B11A 1:1 air core balun at the feedpoint, or twelve turns of coax on an FT-240-31 ferrite toroid core, fixes it.
• Support rope. QNR 3/16 in polyester antenna rope, 500 ft$49.73 in polyester outlasts the cheap nylon that turns to dust in two seasons of sunlight.
• A way to get the rope up. A Forester arborist throw line kit$21.99 puts a line over a branch fifty feet up, safely, from the ground. This is the correct answer instead of a ladder.
• Coax. For a run under about 75 feet on HF, JEFA Tech 240-series flex coax, 50 ft with PL-259$49.99 is plenty. Longer or higher-power runs want MOOKEERF RG-213 coax, 100 ft with UHF male.

Inverted V, sloper, or flat top

Very few dipoles end up as a horizontal flat top, because that requires two supports of equal height in the right places. The two common compromises behave differently enough to matter.

The inverted V is the one most people build, because it needs a single support in the middle and the ends can come down to fence posts. Keep the included angle at the apex above about 90 degrees. Below that the two legs begin to cancel each other, the feedpoint impedance falls away, and efficiency goes with it. A V that droops to 60 degrees is a compromise antenna pretending to be a dipole.

A bent dipole, where one leg turns a corner because the lot ran out, works better than its reputation. It is not symmetrical and its pattern is lopsided, but it radiates. An antenna in the air on a compromised layout beats a perfect antenna that never went up, and this is the single most useful thing to know when your lot is 50 feet wide and the band you want is 40 m.

Height is what decides whether the dipole works for DX

Length sets the resonant frequency. Height above ground sets where the energy goes, and it is the factor that most often disappoints someone who cut the wire correctly and still cannot work anyone far away.

A horizontal dipole a quarter wavelength above ground fires most of its energy nearly straight up. That is excellent for regional contacts out to a few hundred miles, which is called near-vertical incidence skywave and is exactly what a state emergency net wants. It is close to useless for working Europe. Raise the same antenna to half a wavelength and the main lobe drops to about 30 degrees. At a full wavelength it drops to about 15 degrees, which is the range that reaches across an ocean.

In feet, a half wavelength is 492 divided by the frequency in MHz. On 20 m that is 35 feet, which is achievable in most yards. On 40 m it is 69 feet, which usually is not, and on 80 m it is 138 feet, which effectively never is. This is why 20 m is the band most people work the world on from a normal lot, and why an 80 m dipole at 35 feet is a regional antenna no matter how carefully it was trimmed. Run the numbers for your own supports on the antenna height calculator.

Common mistakes that this calculator cannot fix

• Trimming at ground level. The most common error, and it wastes an afternoon every time.
• No common-mode choke at the feedpoint. The symptoms are a signal report that changes when you touch the radio, RF burns on a microphone, and an SWR reading that moves when you coil the coax. The antenna is fine; the feedline joined it.
• Coax lying against the wire. Bring the feedline away from a horizontal dipole at right angles for at least a quarter wavelength where you can. On an inverted V, straight down the mast is fine.
• Trusting an SWR meter to find resonance. A tuner can make almost anything read 1 to 1 at the radio while the antenna is nowhere near resonant and the feedline is heating up. Measure at the antenna with an analyser.
• Soldering only, with no mechanical strain relief. A soldered joint carrying the full tension of a 130 foot span will fail. Tie the mechanical load into the insulator and let the solder carry only the current.
• Ignoring nearby metal. Gutters, chain-link fences and aluminium siding within a few feet of the wire detune it and absorb power. If the antenna measures strangely, look at what is next to it before you look at the wire.

How this compares to the alternatives

A dipole is not automatically the right first antenna. It needs two supports and a clear span, which is exactly what a small lot does not have. The honest comparison:

• Against an end fed half wave, the dipole is cheaper, has no transformer to lose power in, and is a genuinely balanced antenna. The end fed needs only one support and a ground-level feedpoint, which is why it wins on a small lot. Compare them properly on dipole versus end fed half wave.
• Against a vertical, the dipole is quieter on receive and needs no radial field, but it is a low-angle antenna only if you can get it high. A vertical is low-angle from the moment it goes up. See vertical versus dipole.
• Against a multiband commercial wire antenna, a monoband dipole beats it on the one band it is cut for, every time, and costs a tenth as much. The wire antenna roundup covers the cases where the convenience is worth paying for.