Dipole Antenna Length Calculator
A half-wave dipole is the workhorse antenna of amateur and broadcast radio, and its length is set almost entirely by the operating frequency. This calculator takes a frequency in megahertz and returns the total length of the dipole along with the length of each of its two legs, using the standard metric rule of thumb that a practical half-wave dipole measures about 143 divided by the frequency in megahertz. That constant, slightly below the free-space figure of 150, allows for the velocity factor of real wire and the end effect that makes a physical conductor resonate a few percent shorter than a mathematical half wavelength. Enter the frequency you plan to operate on and the calculator gives a starting length you can cut, then split into two equal quarter-wave legs fed at the center. The result is a design length, not a guaranteed in-situ dimension, because height above ground, nearby objects and wire insulation all shift the resonant point; you finish the job by measuring standing wave ratio in place and trimming both legs equally. Every figure here is computed deterministically from the formula shown below, with a worked example that reconciles exactly to the calculator so you can follow each step.
A half-wave dipole length in meters is about 143 / f(MHz), split into two equal legs. At 7 MHz the total length is 20.43 m, with each leg 10.21 m.
Dipole antenna length formula
L = 143 / f
L = total dipole length in meters
f = frequency in megahertz (MHz)
each leg = L / 2
The free-space half-wavelength is 150 / f. The constant is reduced to about 143 to account for the velocity factor of real wire and the end effect, which together make a physical dipole resonate a little shorter than a mathematical half wavelength.
Worked example
Design a half-wave dipole for the 40-meter band at a frequency of 7 megahertz.
- Total length L = 143 / 7 = 20.43 meters
- Each leg = 20.43 / 2 = 10.21 meters
- Cut two equal wires of 10.21 meters and feed at the center
The total comes to 20.43 meters with each leg 10.21 meters. These are the calculator's default inputs, so the result above matches the widget exactly.
Common band examples
Total half-wave dipole length using L = 143 / f.
| Frequency (MHz) | Total length (m) | Each leg (m) |
|---|---|---|
| 3.5 | 40.86 | 20.43 |
| 7 | 20.43 | 10.21 |
| 14 | 10.21 | 5.11 |
| 21 | 6.81 | 3.40 |
| 28 | 5.11 | 2.55 |
Measurement units and constants: US National Institute of Standards and Technology (NIST).
Dipole antenna length calculator: frequently asked questions
What is a half-wave dipole antenna?
A half-wave dipole is a straight conductor cut to roughly half the wavelength of the signal it transmits or receives. It is fed at the center and split into two equal legs, each about a quarter wavelength long. The half-wave length is the most common resonant antenna because it presents a convenient feed impedance near 73 ohms and radiates efficiently broadside to the wire.
Why use 143 instead of 150 in the formula?
In free space the half-wavelength in meters equals 150 divided by the frequency in megahertz. Real wire has a velocity factor below the speed of light and an end effect from its finite thickness, so a practical dipole is cut a few percent shorter. The widely used metric rule of thumb replaces 150 with about 143, giving a length close to resonance before trimming. The exact figure depends on wire diameter and height above ground.
How long is each leg of the dipole?
Each leg is half of the total dipole length. The calculator divides the total by two so you can cut two equal pieces of wire. After building the antenna, measure the standing wave ratio and trim both legs by the same small amount to fine-tune resonance for your installation.
Does height above ground change the length?
Yes. Ground proximity, nearby objects, wire insulation and the support structure all shift the resonant point. The 143/f formula gives a strong starting length, but the final dimension is found by measuring standing wave ratio in place and trimming. This calculator gives the design length, not a guaranteed in-situ figure.
What is the dipole length formula?
Total length in meters equals 143 divided by the frequency in megahertz. Each leg equals half of that total. For example, at 7 megahertz the total is 143 divided by 7, about 20.43 meters, with each leg about 10.21 meters.
Official sources
- Measurement units, physical constants and the speed of light: US National Institute of Standards and Technology (NIST). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.