Ham Radio Wire Antenna Length Calculator

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 = 142.65 / Frequency_MHz

The constant is 142.65 rather than 150 because of the velocity factor. Radio waves travel about 95% of the speed of light along a wire, thanks to the wire’s diameter and to end effects. 142.65 metres is exactly 468 feet, so this is the same formula as the 468/f every American handbook prints, just carried across in one step.

Quarter-wave vertical or radial:

Length_meters = 71.32 / Frequency_MHz (the classic 234/f in feet)

End-fed half-wave (EFHW):

Length_meters = 142.65 / Frequency_MHz (same as dipole, but fed at one end)

Inverted-V dipole:

Each leg = (142.65 / 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

Worked Example

Building a 40-meter band dipole for 7.15 MHz:

• Total length: 142.65 / 7.15 = 19.95 meters (65.5 feet)
• Each leg: 19.95 / 2 = 9.98 meters (32.7 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 (American Wire Gauge, 1.63 mm) wire, the 142.65 constant works well. For very thin wire (22 AWG), use 143.65. For thick wire (10 AWG), use 141.65.

That whole spread is 1.4%, which on 20 metres is about 14 centimetres out of ten metres. It is real, but it is smaller than the error you introduce by running the antenna near a gutter, so do not agonise over the gauge box. Cut long, hang it where it is actually going to live, and trim it there.

Height Matters

A horizontal 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).

A quarter-wave vertical is the exception, and it is the reason this calculator stops quoting a minimum height when you pick one. A ground-mounted vertical is supposed to sit on the ground: the radial field under it is the other half of the antenna. What it wants instead of height is copper, and the quarter-wave vertical calculator works out how much.