Calculation Results:
How It Works: The Theory
This calculator helps you determine the physical length to cut an antenna for a specific radio frequency. The calculation is based on the fundamental relationship between frequency, wavelength, and the speed of light.
The Basic Formula
The wavelength (?) of a radio wave is its speed (the speed of light, c) divided by its frequency (f).
However, an antenna's physical length is slightly shorter than its electrical length due to an electrical phenomenon called the "end effect." Radio waves also travel slightly slower in a wire than in a vacuum. To account for this, we use a correction factor (k), which typically ranges from 0.94 to 0.98.
The Practical Formula
For a common half-wave dipole antenna, a simplified and very popular formula in feet is:
This formula has the correction factor built-in. Our calculator uses a more precise method by allowing you to adjust the Velocity Factor for even greater accuracy.
Common Antenna Types
Half-Wave Dipole
This is one of the most common and fundamental antenna types. It consists of two elements, each a quarter-wavelength long, making the total length a half-wavelength. It's typically fed in the center.
<-- 1/4 Wave --> Feed <-- 1/4 Wave -->
Total Length = 1/2 Wavelength
Quarter-Wave Vertical
Also known as a Marconi antenna, this type is essentially half of a dipole. It uses a single radiating element that is a quarter-wavelength long, mounted perpendicular to a conductive surface called a ground plane.
Radiator = 1/4 Wavelength
(Requires Ground Plane)
Element length correction factor (k)
The input k is an empirical radiator-length correction, not the propagation velocity factor of a coaxial feed line. Do not enter RG-58, RG-213 or ladder-line velocity factors when sizing a wire radiator.
The calculator starts from the exact vacuum wavelength λ = c/f and multiplies it by the chosen fraction and k. At 146.520 MHz with k = 0.95, the corrected full-, half- and quarter-wave lengths are about 1.944 m, 0.972 m and 0.486 m.
Use the result as a starting cut length. Conductor diameter, insulation, height, nearby metal, ground and feed geometry shift resonance; cut slightly long and trim while measuring with an antenna analyzer or SWR meter.
| Free-space reference | k |
|---|---|
| Free-space reference | 1.000 |
| Typical thin-wire starting factor | 0.95 |
| Measured or modelled custom factor | 0.10–1.00 |
Official references: NIST — speed of light in vacuum; ARRL — Single-Band Dipoles.
