HesapHanem

Antenna Length Calculator

Find the right antenna length for a frequency.

How it is calculated

Radio waves travel at about 300 million m/s, so with the frequency in MHz the wavelength in metres is λ = 300 ÷ f. The tool shows the full wavelength, the half-wave length λ ÷ 2 (a classic dipole) and the quarter-wave length λ ÷ 4 (a ground-plane or whip antenna), each to four decimals in metres. The result is the length in free space; a real antenna is usually cut a few percent shorter because of the velocity factor of the wire and end effects, so trim while tuning. Frequency must be above zero.

Worked example

For the 2 m amateur band at 146 MHz: λ = 300 ÷ 146 = 2.0548 m. Half-wave = 2.0548 ÷ 2 = 1.0274 m. Quarter-wave = 2.0548 ÷ 4 = 0.5137 m, about 51 cm for a whip.

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Common mistakes

  • Entering the frequency in Hz or kHz; the tool needs MHz, so 146 MHz is 146, not 146000000.
  • Cutting the element exactly to the printed length; real antennas are normally shortened by a few percent and fine-tuned with an SWR meter.
  • Mixing up the quarter-wave figure for one whip element with the half-wave figure for the full dipole, which is made of two quarter-wave arms.
  • Forgetting that a quarter-wave whip needs a ground plane to work properly.
  • Using the middle of a band without checking: the length is exact for one frequency only.

About this calculator

Radio wavelength relates to frequency by λ = c/f; since the speed of light c ≈ 3×10⁸ m/s, this simplifies to the handy rule λ (meters) = 300 / f (MHz). From that wavelength, this calculator derives the three most common antenna lengths: full-wave (λ), half-wave (λ/2 — the classic dipole), and quarter-wave (λ/4 — the classic ground-plane monopole). These are free-space lengths; a real wire is usually cut a few percent shorter (a velocity factor of roughly 0.95) to account for end effects, so trim while tuning.

Amateur (ham) radio operators use these formulas to cut dipole and monopole wire antennas tuned to a specific band, and RF and broadcast engineers use the same relationship when designing antennas for transmitters and receivers across the spectrum, from HF ham bands up through VHF/UHF.

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Comments (1)

  • Editorial team

    The figures are free-space wavelengths using 300 divided by the frequency in MHz. A real wire is cut a little shorter, because the velocity factor and end effects shorten the resonant length, often by a few percent. So treat the result as a starting length and trim by measuring the SWR with an analyser. Also note the frequency must be in MHz, not kHz or GHz. Which band are you building for? Tell us the frequency you use.

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