Ham Radio Wire Antenna Length Calculator
Calculate the exact wire length for dipole, inverted-V, and end-fed antennas for any amateur radio frequency.
How Wire Antenna Lengths Are Calculated
Wire antennas are the most common and cost-effective antennas in amateur radio. The fundamental principle is that a wire resonates most efficiently when its length is a specific fraction of the operating wavelength.
Core Wavelength Formula
Wavelength (meters) = 300 / Frequency_MHz
This is derived from the speed of light (approximately 300,000,000 m/s) divided by the frequency in Hz, simplified for MHz.
Antenna Length Formulas
Half-wave dipole:
Length_meters = 143 / Frequency_MHz
The constant 143 (not 150) accounts for the velocity factor — radio waves travel about 95% of the speed of light along a wire due to the wire’s diameter and end effects.
Quarter-wave vertical or radial:
Length_meters = 71.5 / Frequency_MHz
End-fed half-wave (EFHW):
Length_meters = 143 / Frequency_MHz (same as dipole, but fed at one end)
Inverted-V dipole:
Each leg = (143 / Frequency_MHz) / 2
The apex angle affects resonant frequency. At a 90° apex angle, shorten each leg by about 3% compared to a flat dipole.
Common Amateur Band Reference
| Band | Frequency (MHz) | Half-Wave Dipole (m) | Half-Wave Dipole (ft) |
|---|---|---|---|
| 160m | 1.9 | 75.3 | 247 |
| 80m | 3.6 | 39.7 | 130 |
| 40m | 7.1 | 20.1 | 66 |
| 20m | 14.1 | 10.1 | 33 |
| 15m | 21.1 | 6.8 | 22 |
| 10m | 28.4 | 5.0 | 16 |
Worked Example
Building a 40-meter band dipole for 7.15 MHz:
- Total length: 143 / 7.15 = 20.0 meters (65.6 feet)
- Each leg: 20.0 / 2 = 10.0 meters (32.8 feet)
- Cut slightly long and trim for lowest SWR
Wire Gauge Effect
Thicker wire has a slightly lower velocity factor, making the antenna slightly shorter. For typical 14 AWG (1.63 mm) wire, the 143 constant works well. For very thin wire (22 AWG), use 142. For thick wire (10 AWG), use 144.
Height Matters
A dipole should ideally be at least a quarter wavelength above ground for best performance. On 40 meters, that means at least 10 meters (33 feet) high. Lower heights reduce efficiency and change the radiation pattern, favoring higher takeoff angles (better for local contacts, worse for long-distance DX).