Dipole Antenna Calculator

Where 468 comes from

A half-wave dipole is two equal wires fed in the middle, together half a wavelength long. In free space, half a wavelength in feet is 491.8 divided by the frequency in megahertz, because the wavelength in metres is 299.79 ÷ f. Real wire antennas resonate a little shorter than that, because the field at the wire ends and the wire’s thickness make it behave as if it were longer, the end effect. The classic handbook formula, length in feet = 468 ÷ f, builds in a 5% shortening. For 14.200 MHz in the 20 m band it gives 32.96 ft, 10.05 m, or 16.48 ft for each leg from the centre insulator; the free-space half wave would be 34.63 ft. The same formula gives 65.45 ft for 7.150 MHz and 124.8 ft for 3.750 MHz.

Cut long, then trim

The formula is a starting point, not a final length. Insulated wire is shorter at resonance than bare wire, by a few per cent, because the insulation slows the wave. Height above ground, nearby trees and gutters, the angle of the wires in an inverted-V and the feed-line all move the resonance. The standard practice is to cut each leg 3% to 5% long, hoist the antenna to its working height and measure the SWR across the band. If the lowest SWR is below the frequency you want, the antenna is too long; shorten both legs equally. As a rule of thumb, at 14 MHz each inch removed from each leg raises the resonance by roughly 70 kHz. A common mistake is to trim at ground level, where the resonance is quite different from the installed antenna.

Verticals and loops

A quarter-wave vertical is half a dipole standing over a ground plane: 234 ÷ f feet, 16.48 ft at 14.2 MHz, or 19.2 inches for the 2 m band at 146 MHz. The missing half is provided by radials or by the ground; elevated ground planes usually use two to four radials cut about 5% longer than the radiator, 17.3 ft at 14.2 MHz, while ground-mounted verticals use many radials whose length matters less. A full-wave loop is about 1,005 ÷ f feet around, 70.8 ft at 14.2 MHz, or 17.7 ft on each side of a square loop. Loops are quieter on receive in many locations and do not need a ground system, but they are larger and present an impedance of about 100 Ω, so they are often fed through a quarter-wave matching section.

Feed-point impedance and height

A dipole in free space has a feed-point impedance of about 73 Ω, a good match for 50 Ω or 75 Ω coax with an SWR near 1.5:1. Close to the ground the impedance drops and the radiation pattern changes: at a quarter-wavelength height, about 17 ft on 20 m, much of the signal goes straight up, which is useful for nearby contacts and less so for long distance. Raising a 20 m dipole to half a wavelength, about 35 ft, lowers the main radiation angle. A balun at the feed point keeps current off the outside of the coax and keeps the pattern symmetrical.

How to use it

1. Enter the frequency you use most, in megahertz.
2. Choose a half-wave dipole, a quarter-wave vertical or a full-wave loop.
3. Read the total length, the length of each leg, radial or side, and the free-space wavelength.
4. Cut 3% to 5% long and trim at working height for the lowest SWR.