Dipole Calculator - Antenna Length, Leg & Wavelength

A dipole calculator turns one operating frequency into the geometric properties of a half-wave dipole antenna: total length L, leg length l, free-space wavelength, half wavelength, and quarter wavelength. Adding a conductor diameter refines the length with the diameter-based shortening factor k.

• • Half-wave dipole homework: Compute the antenna length, leg length, and wavelength for an electromagnetics or antennas course.
• • FM or HF antenna build: Size a real wire dipole for the FM broadcast band (88-108 MHz) or an amateur HF band such as 14, 21, or 28 MHz.
• • Satellite or microwave link: Use the GHz unit selector to size a millimetre-scale dipole for a Wi-Fi, ISM, or satellite link at 2.4, 5, or 30 GHz.
• • Cut-to-length trim plan: Compare the 468 / f shortcut against the diameter-adjusted length to plan a trim-to-resonance build.

The dipole calculator covers the textbook half-wave dipole, which is the simplest resonant wire antenna and the building block for many Yagi, vertical, and beam designs. It does not model folded dipoles, off-centre-fed designs, orbaluns directly, because those change the feed-point impedance and require an extra parameter the simple geometric model does not capture.

Every output comes from one of two closed-form relationships evaluated with the NIST CODATA 2018 speed of light, so you can quote the same numbers a textbook or an ARRL handbook would use. The frequency input is shared by every output, and the optional diameter input only activates the adjustment factor branch.

Once you have the wavelength, the Diffraction Calculator takes the same wavelength to compute single-slit minima, grating maxima, and the Airy disk angle, which is the next concept in any wave-physics unit.

The calculator takes the frequency, converts it to hertz, and evaluates two closed-form formulas: free-space wavelength lambda = c / f and practical half-wave dipole length L = 468 / f (MHz). The leg length is l = L / 2. With a conductor diameter, the half-wavelength-to-diameter ratio R drives an adjustment factor k = 0.9787 - 11.86497 / (1 + (R / 0.000449)^1.7925)^0.3.

• f: Operating frequency in the selected unit (Hz, kHz, MHz, or GHz); drives every output.
• c: Speed of light in vacuum, 299,792,458 m/s exactly (NIST CODATA 2018).
• 468: Half-wave dipole constant in ft * MHz, equivalent to c / 2 * 0.95 * ft/m with a velocity factor of about 0.95.
• R: Half-wavelength-to-diameter ratio, equal to (c / 2f) / diameter in metres.
• k: Diameter-based shortening factor from the simplified Rothammel-style fit.

All formulas run in one pure JavaScript function. The unit selectors only affect the frequency and diameter conversions; every output is computed in SI base units and converted to metres or feet for display.

According to NIST CODATA 2018 - Speed of Light, the speed of light in vacuum is c = 299,792,458 m/s exactly, which sets the free-space wavelength lambda = c / f for any input frequency

If you need to convert between Hz, kHz, MHz, and GHz before sizing the antenna, the Frequency Calculator uses the same prefixes to convert any frequency value, which keeps the input consistent across radio and audio bands.

Four ideas hold the calculator together: a wavelength set by the speed of light, a half-wave dipole length set by 468 / f, a velocity factor that explains the gap between free-space and practical length, and a diameter-based shortening factor for thicker conductors.

These four concepts share a common thread: every output is set by the frequency. Once you pick f, the wavelength, the dipole length, and the optional diameter-adjusted length are all fixed by the same input.

The standing-wave pattern along the dipole is a half-sine at resonance, and the Harmonic Wave Equation Calculator solves y(x,t) = A sin(kx) cos(omega*t) for amplitude, wave number, and angular frequency using the same wavelength and frequency.

Enter an operating frequency, choose the unit, and read the seven outputs. Add an optional conductor diameter to activate the diameter-adjusted length.

1. 1 Pick the frequency unit: Choose MHz for VHF/UHF amateur and FM, kHz for AM and HF amateur, GHz for microwave and satellite links, or Hz for audio.
2. 2 Enter the operating frequency: Type the frequency. Defaults to 100 MHz, the centre of the FM broadcast band.
3. 3 Read the total antenna length: The headline output is the half-wave dipole length in metres; the leg length directly below it is half of that.
4. 4 Check the wavelength row: Use the free-space wavelength and half wavelength as the geometric reference; the quarter wavelength is useful for a quarter-wave vertical or radial.
5. 5 Add a conductor diameter for a refinement: For a thick wire or tube, enter the diameter and unit; the adjustment factor k and adjusted length appear in the last two rows.
6. 6 Cut wire slightly long and trim: Cut the antenna 2-5% longer than the calculated length, then trim a centimetre at a time while monitoring SWR until resonance is reached.

For a quick FM broadcast check, leave the unit on MHz and enter 100. The total length reads 1.426 m, the leg length 0.713 m, and the wavelength 2.998 m. Switch to 3.7 MHz and the length jumps to 38.55 m. For a 14 MHz HF dipole using 2 mm copper wire, set the diameter to 2 mm; the adjusted length reads about 10.46 m.

If you only know the wire in AWG rather than millimetres, the Wire Gauge Calculator converts AWG to millimetres or inches so the conductor-diameter input matches the wire you actually have on hand.

The dipole calculator packages five textbook formulas and one diameter correction behind one input, so a homework problem, an FM build, and an HF link check all share the same numbers.

• • Six outputs from one frequency: Total length, leg length, free-space wavelength, half wavelength, quarter wavelength, and optional diameter-adjusted length all come from the same frequency input.
• • Four frequency units: Hz, kHz, MHz, and GHz selectors let you enter any radio frequency without first dividing by 10^6.
• • Metres and feet together: Length outputs come in metres and feet, matching amateur radio handbooks and engineering datasheets side by side.
• • Optional diameter refinement: Adding a conductor diameter in mm, cm, or in switches on the Rothammel-style k and the adjusted length, avoiding the systematic overestimate of 468 / f for thick tubing.
• • Cross-validation friendly: Worked examples match the Omni Calculator 3.7 MHz reference, the ARRL 468 / f convention, and the NIST CODATA speed of light.

For a one-off homework problem the calculator gives a defensible numerical answer in seconds; for a real build it gives the leg length in feet and the diameter-adjusted length in metres without re-typing the constants.

Two input factors change every output, the unit selector changes the displayed magnitude, and three antenna-physics caveats keep the numbers honest about the regimes where the simple model applies.

• • The calculator covers the simple half-wave dipole. Folded dipoles, off-centre-fed designs, Yagi-driven elements, and end-fed wires change the feed-point impedance and resonant length in ways L = 468 / f does not capture.
• • The 468 / f shortcut assumes a thin wire in free space. Real antennas are close to ground and near other conductors, which pull the resonant length away from the free-space value.
• • The Rothammel-style k factor is an empirical fit, not an exact formula. For precise work, model the antenna in 4nec2 or EZNEC and tune by trimming while watching SWR.

Free-space wavelength ignores the refractive index of air, ground effects, and nearby objects, so the practical length of a real antenna is always a small correction away from the calculator's prediction.

According to ARRL Antenna Book (25th edition), the practical half-wave dipole length uses a velocity factor of about 0.95, which produces the 468 / f_MHz shortcut for length in feet

To check how the velocity factor changes for a wire on a different medium, the Wave Speed Calculator computes v = f * lambda from the same wavelength and frequency and helps you see why the wire slows the wave slightly compared to free space.