Dipole Antenna Length Calculator
Calculate the total and element lengths for a half-wave dipole antenna at any frequency for amateur radio.
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:
| Band | Frequency (MHz) | Total Length (m) | Total Length (ft) | Each Leg (ft) |
|---|---|---|---|---|
| 160m | 1.85 | 77.1 | 253.0 | 126.5 |
| 80m | 3.60 | 39.6 | 130.0 | 65.0 |
| 40m | 7.15 | 19.9 | 65.5 | 32.7 |
| 20m | 14.175 | 10.1 | 33.0 | 16.5 |
| 15m | 21.225 | 6.7 | 22.0 | 11.0 |
| 10m | 28.5 | 5.0 | 16.4 | 8.2 |
| 6m | 50.5 | 2.8 | 9.3 | 4.6 |
| 2m | 146.0 | 0.98 | 3.2 | 1.6 |
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.