Half-wave dipole antenna calculator
Enter the frequency you want the antenna to resonate on and keep or adjust the shortening factor (0.95 is the usual starting point for wire). The calculator gives the total dipole length, the length of each leg either side of the feed point, the quarter-wave length and the wavelength.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Enter the frequency, for example 14.2 MHz for the 20 m band or 7.1 MHz for 40 m.
- Keep the shortening factor at 0.95 for bare wire, or use your own value if you have measured one.
- Read the total length and the length of each leg in metres and feet. Cut slightly long and trim.
- Wavelength λ = c / f, with c = 299,792,458 m/s
- Half-wave dipole length L = k × λ / 2
- Each leg = L / 2
- Quarter-wave vertical (monopole) = k × λ / 4
With k = 0.95 and f in MHz: L = 142.4 / f m = 467 / f ft. The often quoted 468 / f ft corresponds to k ≈ 0.951.
Worked example
For 14.2 MHz: λ = 299.792458 / 14.2 = 21.112 m. Half of it is 10.556 m, and with k = 0.95 the dipole is 10.028 m (32.9 ft) end to end. Each leg is 5.014 m (16.45 ft), and a quarter-wave vertical would also be 5.014 m.
At 146 MHz (2 m band) the dipole is 0.975 m and the quarter-wave whip 0.488 m.
Tips
- Height and surroundings matter. Nearby ground, trees and roofs detune the antenna; expect to trim a few percent.
- Trim both legs equally so the feed point stays in the centre; cutting one side only shifts the impedance as well as the frequency.
- Use your own factor next time. After trimming, divide the final length by λ/2: that k fits your wire and setup for other bands.
Frequently asked questions
How long is a half-wave dipole?
Half a wavelength times a shortening factor: L = k × c / (2f). With k = 0.95 and f in MHz, L ≈ 142.4 / f metres, close to the classic 468 / f feet. For 14.2 MHz that is about 10.03 m (32.9 ft).
Why is a dipole shorter than half a wavelength?
End effect: the wire has a finite thickness and the charge bunches at the ends, so it resonates as if it were a little longer than it is. Thicker conductors need a smaller factor; 0.95 is the usual starting value for wire.
How long should each side of the dipole be?
Half the total length, measured from the feed point to the tip. For a 10.03 m dipole each leg is 5.01 m. The gap at the centre insulator is part of the total.
How do I trim a dipole?
Cut it a little long, measure the resonant frequency or SWR, then shorten both legs equally. If resonance is too low, the antenna is too long: shorten by the ratio of measured to wanted frequency.