Dipole Calculator

What the dipole calculator does

A half-wave dipole is the simplest practical antenna, and its length depends on the frequency you want it to work at. This calculator finds that length from the frequency, along with each leg, the wavelength, and useful fractions of it, with an option to account for the conductor's thickness.

Below is what a dipole antenna is, the equation behind it, why the famous number is 468, and a worked example.

How to use it

1. Enter the frequency you want the antenna to work at.
2. Optionally switch to advanced mode to enter the conductor diameter for a thickness correction.
3. Press Calculate for the antenna length and related figures, or Reset to clear them.

What a dipole antenna is

A dipole is the simplest and most widely used resonant antenna, consisting of two straight conductors laid end to end with a small gap in the middle, where the feed line connects. It is the antenna that most people picture when they think of one, and it is the starting point for understanding nearly all other antenna designs. A half-wave dipole, the most common kind, is cut so that its total length is about half the wavelength of the radio signal it is meant to transmit or receive, which makes it resonate efficiently at that frequency.

The length is the crucial dimension, because it determines the frequency at which the antenna works best. An antenna cut for one frequency will not perform well at a very different one, so building a dipole means cutting it to the right length for the band you want. This is why a calculator is so useful: it turns the desired frequency into the physical length you need to cut. The half-wave dipole is a favourite first antenna for amateur radio, shortwave listening, and do-it-yourself wireless projects because it is simple, effective, and easy to build, and this calculator gives the dimensions to start from.

The equation it uses

The length of a half-wave dipole in feet is found from a simple formula:

length (feet) = 468 ÷ frequency (MHz)

So a dipole's total length in feet is 468 divided by the operating frequency in megahertz. The calculator also works out the wavelength itself, from the speed of light divided by the frequency, and reports the half and quarter wavelengths, which are useful reference points in antenna work. Each of the two legs of the dipole is half the total length. In advanced mode, the calculator can apply a small correction based on the conductor's diameter, since thicker conductors need to be slightly shorter, refining the basic estimate.

Why the number is 468 and not 492

You might expect the length of a half-wave dipole to come straight from the wavelength, and in free space a half wavelength in feet would be about 492 divided by the frequency in megahertz. But a real dipole made of wire needs to be cut a little shorter than that, and the practical number used is 468 rather than 492. This is one of the best-known rules of thumb in antenna building.

The reason is an effect at the ends of the wire. A radio wave travels slightly slower along a real conductor than it does in free space, and capacitance at the open ends of the wire makes the antenna behave as if it were electrically a bit longer than its physical length. To resonate at the right frequency, the wire must therefore be cut about five percent shorter than the free-space half wavelength. Multiplying 492 by roughly 0.95 gives about 468, which is where the number comes from. This is why the calculator uses 468 for the practical length, giving a wire that will resonate close to the target, though the exact length is also affected by height above ground and nearby objects, so a little trimming is usually needed.

Wavelength, legs, and tuning

Alongside the antenna length, the calculator reports the full wavelength and its halves and quarters, because these crop up constantly in antenna work. The wavelength is the natural length scale of the signal, and many antenna dimensions are expressed as fractions of it. A half-wave dipole is about half a wavelength long overall, and each of its two legs is therefore about a quarter wavelength, which is why the quarter-wavelength figure is handy to have.

The leg length matters in practice because a dipole is built as two equal arms extending from the central feed point, so knowing the length of each leg tells you how much wire to cut on each side. Because the simple formula gives only a starting point, the usual approach is to cut the legs slightly long, then trim them while measuring, tuning the antenna to resonate exactly where you want. The calculator's figures get you close enough that only fine adjustment is needed, which saves wasting wire and makes the build straightforward. Having the wavelength and its fractions in front of you also helps when adapting the design or planning related antennas.

Units and precision

The calculator takes the frequency in units from hertz up to terahertz and returns the antenna length, each leg, the wavelength, and its half and quarter, in a choice of length units. The length uses the standard practical formula for a half-wave wire dipole, which already includes the typical end-effect shortening. The advanced mode applies a further correction for conductor thickness. The results are a well-established starting point; the final length depends on installation details like height and surroundings, so antennas are usually trimmed to resonance after building.

A worked example

Suppose you want a dipole for 7.15 megahertz, in the middle of the popular 40-metre amateur band.

The total length is 468 ÷ 7.15 ≈ 65.5 feet, with each leg about 32.7 feet. For reference, the full wavelength is the speed of light divided by the frequency, about 41.9 metres, and the free-space half wavelength would be about 68.8 feet, so the practical dipole is a few percent shorter, as expected from the end effect. You would cut each leg a little long and trim to resonance.

Questions people ask

How do you calculate dipole antenna length?

For a half-wave dipole, use length in feet = 468 ÷ frequency in megahertz. Each leg is half the total length.

Why 468 instead of 492?

Because a real wire dipole must be about five percent shorter than the free-space half wavelength, owing to end effects. Multiplying 492 by about 0.95 gives 468.

How long is each leg?

Each of the two legs is half the total length, which is about a quarter of the wavelength. The dipole is fed in the centre between the two legs.

Is the calculated length exact?

It is a close starting point. Height above ground and nearby objects shift the resonance, so antennas are usually cut a little long and trimmed to tune.

References

A quick note on where this comes from. The half-wave dipole length formula and the end-effect factor behind the number 468 are standard antenna engineering, documented by the ARRL and in antenna references. The underlying wavelength relationship is standard physics. The links are worth a quick click to confirm they land where you expect.

1. American Radio Relay League (ARRL), The ARRL Antenna Book, half-wave dipole length. https://www.arrl.org/antennas
2. Electronics Notes, Dipole Antenna Length Calculation. https://www.electronics-notes.com/articles/antennas-propagation/dipole-antenna/length-calculations-equation-formula.php
3. Wikipedia, Dipole antenna. https://en.wikipedia.org/wiki/Dipole_antenna