About the Antenna Length Calculator
This antenna length calculator tells you how long to cut the wire or tubing for a resonant antenna at your chosen frequency. Pick a half-wave dipole, a quarter-wave vertical (ground plane), a 5/8-wave vertical or a full-wave loop, enter the frequency in MHz, and you get the total length in metres, feet and inches, the length of each dipole leg, and the free-space wavelength.
It is aimed at amateur radio operators building wire antennas for HF and VHF, CB users, and anyone making a simple antenna for FM, ISM-band or Wi-Fi experiments. A table shows the same antenna type across popular bands so you can plan a multi-band station quickly.
A real antenna resonates slightly shorter than the free-space fraction of a wavelength because of end effect and conductor thickness, so dipole, vertical and 5/8-wave lengths are multiplied by a shortening factor K — 0.95 by default, which reproduces the familiar 468 ÷ f(MHz) feet rule for a half-wave dipole. Loops resonate slightly longer than one wavelength and use the 1005 ÷ f rule. Always cut a little long and trim while watching SWR.
With the default inputs, the total antenna length is 0.975 m. Change any value above to recalculate instantly.
How to use the antenna length calculator
- 1Enter the frequency you want the antenna to resonate on, in MHz.
- 2Choose the antenna type: dipole, quarter-wave, 5/8-wave or loop.
- 3Adjust the shortening factor K if you use thick tubing or insulated wire.
- 4Read the total length and the length of each leg or element.
- 5Cut slightly long and trim while measuring SWR.
Formula and method
The free-space wavelength is the speed of light divided by frequency: with f in MHz, λ = 299.792458 ÷ f metres. A resonant dipole is half a wavelength long, a ground-plane vertical a quarter, and a 5/8-wave vertical 0.625 λ. Because currents near the ends of a real conductor slow down (end effect) and thicker elements look electrically longer, the physical length is multiplied by a shortening factor K, typically 0.95 for wire.
With K = 0.95 the half-wave dipole works out to 467.2 ÷ f feet, essentially the classic 468 ÷ f rule found in ARRL handbooks. A closed full-wave loop behaves the other way and resonates about 2% longer than one wavelength, so the calculator uses the standard 1005 ÷ f(MHz) feet formula for loops. These are starting points: height, nearby objects and insulation all shift resonance.
- λ
- Free-space wavelength (m)
- c
- Speed of light, 299,792,458 m/s
- f
- Frequency in MHz
- K
- Shortening / end-effect factor
- L
- Physical antenna length
Worked examples
Half-wave dipole for 146 MHz (2 m band)
λ = 299.792 ÷ 146 = 2.053 m. Half of that is 1.027 m, and × 0.95 gives 0.975 m (about 3 ft 2.4 in) in total — two legs of 48.8 cm each, fed in the middle.
20 m band dipole at 14.2 MHz
λ = 299.792 ÷ 14.2 = 21.11 m, so a dipole is 0.5 × 21.11 × 0.95 = 10.03 m or 32.9 ft — within a couple of inches of the 468 ÷ 14.2 = 32.96 ft rule. Each leg is about 16.45 ft.
Quarter-wave CB whip at 27.185 MHz
Channel 19 is 27.185 MHz, so λ = 11.03 m. A quarter wave is 2.757 m and × 0.95 gives 2.62 m (8.59 ft), close to the familiar 102-inch CB whip.
40 m full-wave loop at 7.1 MHz
Loops use 1005 ÷ f: 1005 ÷ 7.1 = 141.55 ft (43.14 m) of wire. Shaped as a square, each side is about 10.79 m.
Frequently asked questions
What is the formula for a dipole antenna length?+
A half-wave dipole is about 468 ÷ frequency in MHz feet (or 142.6 ÷ f metres) in total, split into two equal legs. That is half the free-space wavelength multiplied by a shortening factor of about 0.95.
Why is an antenna shorter than half a wavelength?+
End effect: capacitance at the open ends of the conductor makes it act electrically longer than it physically is. Thicker conductors and insulation increase the effect, so the physical length is typically 93–97% of the free-space value.
How long should a quarter-wave antenna be?+
Roughly 234 ÷ f(MHz) feet, or 71.3 ÷ f metres. For 146 MHz that is about 48.8 cm; it needs a ground plane — a metal car roof or three or four radials — to work properly.
Is a 5/8-wave antenna better than a quarter wave?+
A 5/8-wave vertical concentrates more energy at low radiation angles, giving roughly 2–3 dB more gain toward the horizon than a quarter wave. It is longer and needs a base matching network to present 50 Ω.
How do I tune a wire antenna after cutting it?+
Cut it a few percent long, raise it to its final height and measure SWR across the band. If the lowest SWR is below your target frequency the antenna is too long, so trim both legs equally in small steps.