Antenna Length Calculator

The Antenna Length Calculator turns any operating frequency into the correct physical antenna length. It supports the four most common resonant designs — the half-wave dipole, the quarter-wave vertical / ground plane, the 5/8-wave vertical, and the full-wave loop — and reports the length in both meters and feet/inches, together with each dipole leg or radial, the free-space wavelength, the matching radio band, and an animated diagram showing the standing wave on the element.

How to Calculate Antenna Length from Frequency

An antenna is resonant when its length is a specific fraction of the wavelength for your frequency. The calculation is three short steps: find the wavelength, take the antenna's fraction of it, then shorten slightly for the real-world velocity/end-effect factor.

Here fraction is 0.5 for a half-wave dipole, 0.25 for a quarter-wave vertical, 0.625 for a 5/8-wave vertical, and 1.0 for a full-wave loop. K is the velocity/end-effect factor — about 0.95 for thin bare wire. This is exactly why the famous ham-radio rule of thumb for a dipole is 468/f (feet) rather than the free-space 492/f.

Quick Antenna Length Formulas

These use the common 0.95 velocity factor. The calculator above lets you change that factor to match your conductor for a more exact cut.

Antenna Types Explained

What is the Velocity / End-Effect Factor?

Radio waves travel slightly slower along a wire than through free space, and the ends of an antenna add capacitance that makes it behave as if it were a little longer than it physically is. To land on resonance you therefore cut the conductor a few percent short. That correction is the velocity factor (also called the K factor or end-effect factor):

• 0.95 — thin bare wire (the classic default that gives 468/f).
• 0.92 — insulated or coated wire, which lowers the factor.
• 0.98 — thick aluminum tubing or elements.
• 1.00 — the theoretical value with no end effect (rarely used in practice).

Wavelength and Radio Bands

Because wavelength is inversely proportional to frequency, low bands need big antennas and high bands need tiny ones. A 3.5 MHz (80 m) dipole is over 40 m long, while a 2.4 GHz Wi-Fi quarter-wave is barely 3 cm. The calculator identifies the band your frequency falls in — ham, CB, FM broadcast, airband, marine, GMRS, Wi-Fi, and more — so you immediately know the context of your antenna.

How to Use This Calculator

1. Enter the frequency: Type your operating frequency and pick the unit (kHz, MHz, or GHz).
2. Choose the antenna type: Select a dipole, vertical, ground plane, 5/8-wave, or loop.
3. Set the velocity factor: Match it to your conductor (0.95 for bare wire is a safe default).
4. Click Calculate: Read the physical length in meters and feet, each leg or radial, the wavelength, the radio band, and the step-by-step math.

Frequently Asked Questions

How do I calculate antenna length from frequency?

First find the free-space wavelength: wavelength in meters equals 299.792458 divided by the frequency in MHz. Then multiply by the antenna's fraction of a wavelength (0.5 for a half-wave dipole, 0.25 for a quarter-wave) and by a velocity/end-effect factor of about 0.95 for real wire. The result is the physical length you cut.

What is the formula for a half-wave dipole?

A common rule of thumb is length in feet equals 468 divided by the frequency in MHz, or length in meters equals about 142.5 divided by the frequency in MHz. Each leg of the dipole is half of that total length. The 468 figure already includes the roughly 95% velocity factor for thin wire.

What is a quarter-wave vertical antenna length?

A quarter-wave vertical is length in feet equals 234 divided by the frequency in MHz, or about 71.25 divided by frequency in MHz for meters. It is half the size of a dipole and is worked against a ground plane or radials, which are usually cut about 5% longer than the vertical element.

What is the velocity factor and why does it matter?

The velocity or end-effect factor accounts for the fact that a real conductor makes an antenna resonate as if it were slightly longer than its physical length. Thin bare wire is about 0.95, insulated wire is a little lower, and thick tubing is a little higher. Using the right factor gives a length closer to resonance so you need less trimming.

Why is my antenna shorter than the wavelength?

An antenna is only a fraction of a full wavelength. A half-wave dipole is half a wavelength, and a quarter-wave vertical is a quarter. The physical length is also shortened slightly by the velocity factor, so a resonant antenna is always shorter than one full wavelength for that frequency.

Should I cut the antenna exactly to the calculated length?

Cut it slightly long and then trim while measuring SWR. Nearby objects, height above ground, wire thickness, and insulation all shift the resonant point, so the calculated length is an excellent starting point rather than an exact final value.

Additional Resources

• Dipole Antenna - Wikipedia
• Monopole (Quarter-Wave) Antenna - Wikipedia