Half-Wave Dipole Length

Formula

Description

The half-wave dipole is the fundamental reference antenna in radio engineering, consisting of two quarter-wave elements fed at the center. The 0.95 factor accounts for the velocity factor and end effects that make the physical length slightly shorter than the free-space half wavelength. A half-wave dipole has a theoretical gain of 2.15 dBi and a feedpoint impedance of approximately 73Ω in free space, making it a reasonable match to 50Ω coaxial cable. It is the starting point for most amateur radio antenna projects.

Variables

• l — Total dipole length tip to tip (m)
• f — Resonant frequency (Hz)

Practical Notes

The classic ham radio dipole formula in feet is l = 468/f_MHz, which incorporates the 0.95 velocity factor. Height above ground significantly affects impedance and radiation pattern: at 0.5 wavelength height the pattern is mostly broadside, while lower heights tilt the pattern upward. An inverted-V dipole (apex up, ends drooping at 90-120°) is a practical compromise for limited space with only a slight reduction in performance.