Antenna Length Calculator
Calculate quarter-wave and half-wave antenna lengths from a given frequency.
Supports output in meters, centimeters, feet, and inches.
Antenna length is determined by the wavelength of the radio frequency you want to transmit or receive. Building an antenna to the correct length dramatically improves signal strength and efficiency — an incorrectly sized antenna wastes power and delivers poor performance.
Wavelength formula: λ (meters) = 300 ÷ Frequency (MHz)
Half-wave dipole antenna length: L = 142.5 ÷ Frequency (MHz) (in meters) Or: L = 468 ÷ Frequency (MHz) (in feet)
Quarter-wave monopole (vertical antenna) length: L = 71.25 ÷ Frequency (MHz) (in meters) Or: L = 234 ÷ Frequency (MHz) (in feet)
What each variable means:
- Frequency (MHz) — the radio frequency in megahertz. Higher frequency = shorter antenna.
- λ (lambda) — the wavelength; the physical distance one complete radio wave cycle occupies in free space.
- Half-wave dipole — the most common antenna type; two arms each 1/4 wavelength long, creating a balanced radiator.
- Quarter-wave monopole — single vertical element, requires a ground plane (radials) to act as the other half.
Worked examples:
FM Radio (98 MHz): Full wavelength = 300 ÷ 98 = 3.06 m Half-wave dipole = 468 ÷ 98 = 4.78 feet (1.46 m) total Quarter-wave = 234 ÷ 98 = 2.39 feet (0.73 m)
WiFi 2.4 GHz (2,400 MHz): Quarter-wave = 234 ÷ 2,400 = 0.098 feet = 1.17 inches (3.0 cm)
Ham Radio 40-meter band (7.1 MHz): Half-wave dipole = 468 ÷ 7.1 = 65.9 feet (20.1 m)
Velocity factor: Practical wire antennas are typically cut 2–5% shorter than the theoretical length due to the velocity factor of real-world wire and insulators. Use 0.95 as a starting multiplier and tune from there.