How to use the dipole antenna length calculator
- Enter the frequency and choose megahertz or kilohertz. Use the middle of the range you want to operate in.
- Set the shortening factor. The default of 0.95 is an example; real wire resonates a little shorter than free space.
- Read the length of the whole dipole, the length of each leg and the wavelength.
- Use the table for wire lengths in meters and in feet and inches, including a quarter-wave vertical.
Formula
wavelength = speed of light / frequency, with c = 299,792,458 m/s
half-wave dipole length = wavelength / 2 x shortening factor
each leg = dipole length / 2
rule of thumb: length in feet = 468 / frequency in MHz
The 468 / f rule is the same calculation with a shortening factor of about 0.95, in feet.
A dipole for 14.2 MHz
At 14.2 MHz the wavelength is 299,792,458 / 14,200,000 = 21.112 m. Half of that is 10.556 m, and with a shortening factor of 0.95 the dipole is 10.028 m (32 ft 10.8 in) end to end, with each leg 5.014 m (16 ft 5.4 in). The 468 / f rule gives 32.958 ft, which agrees to within about an inch.
At 7.1 MHz the dipole is 20.06 m (65 ft 9.6 in). At 146 MHz it is only 0.975 m (3 ft 2.4 in). A quarter-wave vertical at 14.2 MHz is 5.014 m.
Why wire is shorter than a half wavelength
A half-wave dipole resonates when its electrical length is half a wavelength, but waves on a real wire travel a little slower than in free space and the ends of the wire add a small effect. Both make the wire resonate at a slightly shorter length than the free-space half wavelength. A factor of 0.95 is a common starting point, but height above ground, nearby metal, insulation and wire thickness all move it.
Cut a little long and trim in small steps while you check the match with an antenna analyzer or meter. It is much easier to remove wire than to add it.
Safety and related tools
Keep antennas and wires well away from power lines when you put them up, never work on them in a storm, and follow the rules for your station. For feed lines see the wire voltage drop calculator and the AWG wire gauge converter. Other energy tools: the solar string sizing calculator and the wind power output calculator.
Assumptions and limits
- The antenna is a straight, center-fed half-wave dipole made of thin wire.
- The shortening factor is between 0.90 and 1 and is an example value that you can change.
- Frequencies up to 30,000 MHz are accepted. The page converts kilohertz to hertz exactly.
- Lengths are for the wire itself, not including the center insulator or the tie-off at the ends.
Frequently asked questions
Where does 468 come from?
Half the speed of light is about 491.8 when you work in feet and megahertz. Multiplying by a shortening factor of about 0.95 gives 468. That is why the rule works for a simple dipole.
Does an inverted V need a different length?
It usually needs a little more wire than a flat dipole, depending on the angle between the legs. Start long and trim it, as you would for a straight dipole.
How do I use the quarter-wave figure?
It is the length of one element of a vertical antenna that works against a ground plane. The same shortening factor is applied, but it is a starting point to be tuned.
Does it matter if the wire is insulated?
Yes. Insulation slows the wave slightly, so insulated wire resonates a little shorter. Lower the shortening factor a little and trim to your measurements.