How to use the dipole antenna calculator
- Select Frequency to wavelength or Wavelength to frequency as the input mode.
- Enter the frequency value and choose kHz, MHz or GHz, or enter a wavelength in meters for reverse mode.
- Set the velocity factor. Use 0.95 for typical bare copper wire or 1.0 for theoretical free-space values.
- Read the computed wavelength, full-wave, half-wave dipole length, quarter-wave antenna length and individual arm sizes below.
How the dipole antenna calculator works
The core formula converts between frequency and wavelength using the speed of light in a vacuum: wavelength = 299,792,458 / frequency_in_Hz. Once the free-space wavelength is known, physical lengths are derived by multiplying by the velocity factor.
A half-wave dipole has a total length of wavelength / 2 * velocity_factor. Each arm of that dipole is exactly half of the total. A quarter-wave vertical or monopole element uses wavelength / 4 * velocity_factor.
For example, at 144 MHz with a velocity factor of 0.95, the free-space wavelength is 2.0819 m. The half-wave dipole total is 2.0819 / 2 * 0.95 = 0.9889 m, and each arm is 0.4944 m long.
Common frequencies and antenna lengths
This table shows approximate values computed with a velocity factor of 0.95, which is standard for bare wire antennas.
| Band | Frequency | Wavelength | Half-wave dipole total | Each arm |
|---|---|---|---|---|
| AM broadcast | 1 MHz | 299.79 m | 142.40 m | 71.20 m |
| HF 20 m | 14.2 MHz | 21.11 m | 10.03 m | 5.01 m |
| VHF 2 m | 144 MHz | 2.082 m | 0.989 m | 0.494 m |
| UHF 70 cm | 435 MHz | 0.689 m | 0.327 m | 0.164 m |
| Wi-Fi 2.4 GHz | 2400 MHz | 0.1249 m | 0.059 m | 0.030 m |
Understanding the velocity factor
The velocity factor accounts for the fact that a real half-wave wire resonates slightly shorter than the free-space calculation suggests. The 0.95 factor used for bare wire is an empirical shortening (end effect) factor rather than a pure wave-speed ratio, and it varies with wire thickness and height above ground. For bare copper or aluminum wire, the factor is typically around 0.95. Insulated wire may drop to 0.90 or lower depending on the dielectric material.
If you set the velocity factor to 1.0, this dipole antenna calculator returns pure free-space dimensions. That is useful for academic work, but building an actual antenna requires the reduced lengths. Always measure and trim during installation, because nearby objects and ground effects shift the resonant point.
Half-wave dipole versus quarter-wave antenna length
A half-wave dipole consists of two collinear arms fed at the center. Its total length is one half of the adjusted wavelength. This is the most common wire antenna because it offers a predictable radiation pattern and roughly 73 ohms feed-point impedance in free space.
A quarter-wave antenna length refers to a single radiating element, often mounted over a ground plane or radial system. It is half the size of a dipole and presents about 36 ohms impedance. Many mobile and base-station verticals use this design. When people search for quarter wave antenna length, they usually need the single-element dimension, which this tool provides alongside the per-arm dipole measurement.
Using the reverse mode as a wavelength to frequency calculator
Switch the input mode to provide a wavelength in meters instead of a frequency. The tool divides the speed of light by your wavelength to return the corresponding frequency in Hz, kHz, MHz and GHz. This wavelength to frequency calculator function is handy when you have measured a physical structure and need to determine what band it resonates on.
For instance, entering 2.0819 m returns approximately 144 MHz, confirming the structure suits the 2-meter amateur band. You can then read off the recommended dipole and arm lengths directly.
Worked examples
| Example | Input | Result |
|---|---|---|
| FM broadcast at 100 MHz | Input mode: frequency; Frequency: 100; Unit: MHz; Velocity factor: 0.95 | Wavelength: 2.9979 m (9.8357 ft); Full-wave element: 2.8480 m (9.3439 ft); Half-wave dipole total: 1.4240 m (4.6720 ft); Half-wave dipole each arm: 0.7120 m (2.3360 ft); Quarter-wave element: 0.7120 m (2.3360 ft); Quarter-wave each arm: 0.3560 m (1.1680 ft) |
| 2-meter amateur band (144 MHz) | Input mode: frequency; Frequency: 144; Unit: MHz; Velocity factor: 0.95 | Wavelength: 2.0819 m (6.8304 ft); Full-wave element: 1.9778 m (6.4888 ft); Half-wave dipole total: 0.9889 m (3.2444 ft); Half-wave dipole each arm: 0.4944 m (1.6222 ft); Quarter-wave element: 0.4944 m (1.6222 ft); Quarter-wave each arm: 0.2472 m (0.8111 ft) |
| Wi-Fi at 2.4 GHz | Input mode: frequency; Frequency: 2.4; Unit: GHz; Velocity factor: 0.95 | Wavelength: 0.1249 m (0.4098 ft); Full-wave element: 0.1187 m (0.3893 ft); Half-wave dipole total: 0.0593 m (0.1947 ft); Half-wave dipole each arm: 0.0297 m (0.0973 ft); Quarter-wave element: 0.0297 m (0.0973 ft); Quarter-wave each arm: 0.0148 m (0.0487 ft) |
| Reverse: 2 meter wavelength | Input mode: wavelength; Wavelength (meters): 2; Velocity factor: 0.95 | Frequency: 149.8962 MHz, 149896.229 kHz, 149,896,229 Hz; Wavelength: 2.0000 m (6.5617 ft); Full-wave element: 1.9000 m (6.2336 ft); Half-wave dipole total: 0.9500 m (3.1168 ft); Half-wave dipole each arm: 0.4750 m (1.5584 ft); Quarter-wave element: 0.4750 m (1.5584 ft); Quarter-wave each arm: 0.2375 m (0.7792 ft) |
FAQ
What formula does this dipole antenna calculator use?
It divides the speed of light (299,792,458 m/s) by the frequency in hertz to get the free-space wavelength. Physical lengths are then multiplied by the velocity factor.
Why is the velocity factor not 1.0 for real antennas?
Electromagnetic waves travel slightly slower along a metal conductor than through empty space. Bare wire typically has a velocity factor near 0.95, while insulated wire can be lower.
How do I find the quarter wave antenna length?
Divide the free-space wavelength by four, then multiply by the velocity factor. The result is the length of a single radiating element used over a ground plane.
What is the difference between half-wave dipole length and arm length?
The half-wave dipole length is the total end-to-end measurement. Each arm is exactly half of that total, extending from the center feed point outward.
Can I use this as a wavelength to frequency calculator?
Yes. Switch the input mode to wavelength, enter the distance in meters, and the tool returns the corresponding frequency across all common units.
Does this tool upload my data anywhere?
No. All calculations run locally in your browser. Nothing is sent to any server.
Is this antenna length calculator accurate for VHF and UHF bands?
The math is exact for free space. At VHF and UHF frequencies, physical construction tolerances and nearby objects matter more, so always verify with an SWR meter after building.