Dipole Antenna Length Calculator

The half-wave dipole antenna

The dipole is the most fundamental antenna in amateur radio. It consists of two equal-length wire elements connected to a feedline at the center. A half-wave dipole resonates at a frequency where its total length is approximately one-half wavelength.

Basic formula (in free space):

Wavelength (meters) = 300 / Frequency (MHz)

Half-wave dipole length = Wavelength / 2

Practical formula with velocity factor:

In practice, wire antennas are shorter than the free-space calculation due to end effects and the conductor’s velocity factor. The standard correction factor is 0.95 (5% shorter).

Dipole total length (meters) = 142.65 / Frequency (MHz)

Each leg = 71.325 / Frequency (MHz)

In feet: Total length (feet) = 468 / Frequency (MHz)

Common ham band dipole lengths:

Wire gauge correction:

Thicker wire has a slightly lower velocity factor, making the antenna a tiny bit shorter. For most practical purposes the difference is negligible (less than 1%), but if you are using very thick wire (10 AWG or larger), reduce the calculated length by about 1%.

Height and feedpoint impedance:

A half-wave dipole in free space has a feedpoint impedance of 73 ohms. When mounted over real ground, this changes:

• At 1/4 wavelength height: ~50 ohms (good match to 50-ohm coax)
• At 1/2 wavelength height: ~90 ohms
• Below 1/4 wavelength: impedance drops, often below 30 ohms

For best performance, mount the dipole at least 1/2 wavelength above ground. A 40m dipole should be at least 10 meters (33 feet) high.

Tuning: Cut the wire 2–3% longer than calculated, then trim in small increments (1 cm at a time) while checking SWR with an antenna analyzer. Trim equal amounts from both legs. Target SWR below 1.5:1 at your desired operating frequency.