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 (λ) 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)
Advanced: The Velocity Factor (Vf)
The Velocity Factor (Vf) is a crucial concept for precise antenna calculations. It represents the speed of radio waves in a specific conductor relative to their speed in a vacuum. It's always a value less than 1.
Using the correct Vf for your antenna wire or transmission line adjusts the calculated physical length to match the required electrical length, resulting in a more resonant and efficient antenna. If you don't know the exact Vf, a value of 0.95 to 0.97 is a good starting point for typical copper wire antennas.
| Material / Cable Type | Typical Velocity Factor |
|---|---|
| Bare/Enameled Copper Wire | 0.95 - 0.97 |
| Insulated Wire (e.g., THHN) | 0.92 - 0.95 |
| RG-8/U, RG-213 Coax | 0.66 |
| RG-58/U Coax | 0.66 |
| 450-ohm Ladder Line | 0.90 |
