The antenna length calculator computes the resonant length for quarter-wave, half-wave, and full-wave antennas at any frequency. Enter your target frequency to get exact cut lengths for dipoles, verticals, and yagis.
Calculate Antenna Length
How to Calculate Antenna Length
The fundamental formula for wavelength is λ = c/f (speed of light / frequency). For practical wire antennas, a velocity factor of 0.95-0.97 accounts for the slower propagation speed in wire compared to free space.
Common Amateur Radio Band Lengths
2m band (146 MHz): half-wave dipole ≈ 38.7 inches (98 cm). 70cm band (446 MHz): quarter-wave monopole ≈ 6.5 inches (16.5 cm). 40m band (7.1 MHz): half-wave dipole ≈ 65.8 feet (20 m). These are starting points — final trimming and SWR measurement determines exact resonant length.
Frequently Asked Questions
How do I calculate antenna length?
For a half-wave dipole, length = 468 / frequency(MHz) feet (or 143 / frequency meters). For a quarter-wave monopole (vertical), use 234 / frequency(MHz) feet. These formulas include a velocity factor of ~0.95 for practical wire antennas.
What is a quarter-wave antenna?
A quarter-wave monopole antenna is 1/4 the wavelength long, used with a ground plane (radials). It's the most common type for mobile, FM, and amateur radio antennas. Quarter-wave antennas are omnidirectional and have ~0 dBd gain over a dipole (2.15 dBi over isotropic).
What is a half-wave dipole?
A half-wave dipole is the fundamental antenna reference — two equal elements each 1/4 wavelength long, fed at the center. It's omnidirectional broadside and has ~2.15 dBi gain. All antenna gain is measured relative to an isotropic radiator (dBi) or relative to a dipole (dBd).
Does antenna length affect performance?
A perfectly resonant antenna (exact right length) has a low SWR (1:1 ideal) and transfers maximum power. An antenna too long or short has high SWR, which reflects power back into the transmitter. Most commercial antennas trim length during manufacture for a specific frequency.
Is this antenna calculator free?
Yes, completely free with no signup required.