Antenna Length Calculator
Dipole and quarter-wave antenna lengths for any frequency.
About this tool
Enter a frequency in MHz or pick a band (amateur bands, CB, FM, LoRa, ADS-B or Wi-Fi). The tool gives the half-wave dipole length and each leg, quarter-wave and 5/8-wave verticals, and the free-space wavelength.
Cutting an antenna to length
Resonant antennas are sized as fractions of the wavelength, ฮป = 300 รท f metres with f in MHz. A half-wave dipole is two quarter-wave legs fed in the middle; a quarter-wave vertical works against a ground plane or radials; a 5/8-wave vertical gives a little more gain at low angles. Wire antennas resonate a few percent shorter than the free-space figure because of end effects, so the traditional dipole formula is 468 รท f feet, equivalent to a shortening factor of about 0.95, which is the default here. At 14.2 MHz (the 20 m band) that's a 10.0 m (32.9 ft) dipole with 5.0 m legs. Change the factor for thick elements or insulated wire. Honest limits: height above ground, nearby objects, insulation and the feed line all shift resonance, so cut slightly long and trim while measuring SWR with an antenna analyser. Transmitting on amateur bands requires a licence. The wavelength and frequency calculator and the reactance calculator are related.
Frequently asked questions
How long is a 20 m dipole?
About 10 m (33 ft) in total for 14.2 MHz, with two legs of about 5 m.
What is the dipole length formula?
468 รท frequency in MHz gives the total length in feet, or about 143 รท f in metres.
How long is a quarter-wave antenna?
About 234 รท f feet or 71.3 รท f metres. At 146 MHz that's about 49 cm.
Why is a real antenna shorter than half a wavelength?
End effects and the conductor's thickness make it resonate about 5% shorter than in free space.
Is my data sent anywhere?
No. Everything is calculated in your browser, and nothing is stored or sent.