Dipole Antenna Calculator
Enter your target frequency to get the full dipole length, each leg length, and the free-space wavelength in both metric and imperial units. Enable the adjustment factor for more accurate results when you know the wire diameter, or switch to inverted-vee mode for the classic V-shaped antenna. The feedpoint impedance and quarter-wavelength are also shown. Results update as you type.
Formula
Worked example
20 m band centre, 14.175 MHz: wavelength = 299,792,458 / 14,175,000 = 21.15 m. Half wavelength = 10.57 m. Multiply by k = 0.9516: total dipole = 10.06 m (33.0 ft). Each leg = 5.03 m (16.5 ft).
How a half-wave dipole works
A half-wave dipole is two equal conductors arranged end to end and fed at the centre. At resonance each leg is one quarter of the operating wavelength, so the total length is half a wavelength. Voltage stands at the tips and current peaks at the feedpoint, giving the antenna a well-defined radiation pattern shaped like a flattened torus (doughnut) perpendicular to the wire. The free-space gain is 2.15 dBi - slightly better than an isotropic radiator. For most amateur and commercial HF applications, the dipole is the reference against which every other antenna is compared.
The 468 formula and why it is not exactly 492
The theoretical half-wavelength in free space would give a constant of 492 (in imperial units, 150 in metric). The real constant is lower - about 468 (ft) or 142.6 (m) - because of end effects and the finite thickness of practical conductors. Current in a physical wire does not quite stop at the tip; it extends a little beyond, making the wire behave as if it were slightly longer than it is. The correction is absorbed into an adjustment factor k, which for typical HF wire (1-3 mm diameter) is about 0.95. This calculator lets you compute k precisely from the actual wire or rod diameter when you need tighter accuracy.
Inverted vee versus flat-top horizontal dipole
An inverted vee hangs from a single central support with both legs drooping downward at an included angle of roughly 120 degrees. This makes it easier to install - only one tall support is needed instead of two - and gives a slightly more omnidirectional radiation pattern in the horizontal plane. The trade-off is that the feedpoint impedance drops to roughly 50-55 ohm (conveniently close to 50-ohm coax) and the resonant length is 2-3% shorter than the flat-top version. A horizontal dipole hung well above ground radiates a stronger signal in the two directions broadside to the wire and has nulls off the ends.
Feed impedance and matching
A free-space half-wave dipole has a feedpoint impedance of about 73 ohm resistive, which rises or falls with height above ground. At lambda/4 height it can dip to 50-60 ohm; at lambda/2 it rises above 85 ohm. Common coaxial cables are 50 ohm or 75 ohm. The closest standard match is a 4:1 balun followed by 75-ohm coax, or direct feed with 50-ohm coax accepting a small mismatch (SWR about 1.5:1). A 1:1 current (choke) balun at the feedpoint is always recommended to suppress common-mode currents that would otherwise travel back down the coax shield and distort the radiation pattern.
Amateur radio HF band dipole lengths
| Band | Frequency (MHz) | Total length (ft) | Total length (m) | Each leg (ft) |
|---|---|---|---|---|
| 160 m | 1.900 | 246.3 | 75.08 | 123.2 |
| 80 m | 3.750 | 124.8 | 38.04 | 62.4 |
| 60 m | 5.358 | 87.3 | 26.61 | 43.7 |
| 40 m | 7.150 | 65.5 | 19.94 | 32.7 |
| 30 m | 10.125 | 46.2 | 14.09 | 23.1 |
| 20 m | 14.175 | 33.0 | 10.07 | 16.5 |
| 17 m | 18.100 | 25.9 | 7.88 | 12.9 |
| 15 m | 21.225 | 22.1 | 6.72 | 11.0 |
| 12 m | 24.940 | 18.8 | 5.72 | 9.4 |
| 10 m | 28.500 | 16.4 | 5.00 | 8.2 |
Approximate end-to-end dipole length at the centre of each HF amateur band, using the standard 468/f (ft) formula. Trim to resonance after building.
Frequently asked questions
Why should I cut the antenna slightly longer than calculated?
The formula gives the ideal resonant length for a perfect conductor in free space. Real-world effects - proximity to trees, buildings, soil, insulation on the wire, and support ropes - can all shift the resonant frequency slightly. Starting 2-5% longer and trimming while measuring SWR lets you bring the antenna to resonance without having to splice on extra wire later.
What is the adjustment factor (k) and when should I use it?
The adjustment factor k corrects the ideal half-wavelength for the physical behaviour of a real conductor. End effects make the resonant length about 5% shorter than the theoretical value, and a thicker conductor shortens it further. For thin wire at HF (1-2 mm) the standard 468/f formula already embeds a good k. If you are using a thicker rod or tube - common at VHF/UHF - enter the diameter and let the calculator derive k precisely. The formula used is from NBS (NIST) research on short dipole antennas.
What impedance coax should I connect to a dipole?
A standard horizontal dipole has about 73 ohm feedpoint impedance, which is a reasonable match to 75-ohm coax (SWR ~1.03:1) and an acceptable match to 50-ohm coax (SWR ~1.46:1). An inverted vee drops to around 55 ohm, very close to 50-ohm coax. Regardless of the coax used, fit a 1:1 current balun at the feedpoint to prevent the shield from radiating and upsetting the antenna pattern.
Can I use this calculator for VHF or UHF dipoles?
Yes. Enter the frequency in MHz (or GHz using the unit selector) and the conductor diameter. At VHF and UHF, conductor diameter is a larger fraction of the wavelength so the adjustment factor matters more. For a 2.4 GHz Wi-Fi dipole driven element, for example, a 1 mm rod gives k around 0.96 whereas a 4 mm rod gives k around 0.93, a meaningful difference at those frequencies.
What is the difference between a dipole and a monopole?
A monopole is half a dipole - one quarter wavelength long - mounted over a ground plane or radial system that acts as the missing half. The monopole feedpoint impedance is half that of the dipole (~36 ohm). Vertical whips, car antennas, and many broadcast towers are monopoles. The quarter-wavelength output in this calculator gives you the monopole length directly.
What gain does a dipole antenna have?
A half-wave dipole has 2.15 dBi of gain relative to an isotropic radiator, or 0 dBd when used as the reference itself. Most antenna specs compare to the dipole (dBd), so a 3 dBd Yagi actually has 5.15 dBi. The dipole gain is a consequence of the radiation pattern concentrating signal broadside to the wire rather than in all directions equally.