Antenna Length Calculator

Resonant antenna formulas

In free space, wavelength is the ratio of the speed of light to the operating frequency:

For practical antennas, builders often need quarter-wave or half-wave segments. Applying a velocity factor $v_f$ that captures dielectric loading leads to

where $n$ equals 4 for quarter-wave monopoles and 2 for half-wave dipoles. This calculator uses 300 m/µs as the speed of light when frequency is given in megahertz, delivering lengths directly in meters.

Example lengths

The table compares quarter-wave and half-wave lengths for common amateur bands assuming a velocity factor of 1.0. Adjust the form to see how insulation or tubing changes each value.

Remember that ground-plane spacing, feed matching, and conductor diameter further influence performance. Use the calculator as a starting point before trimming elements during tuning.

What the calculator is actually computing

Most “antenna length” questions are really “how long is a conductor for a fraction of a wavelength at my operating frequency?” The wavelength $\lambda$ is the physical distance a radio wave travels during one cycle. In free space, that distance depends only on the speed of light $c$ and frequency $f$. When you ask for a quarter-wave or half-wave, you are choosing a convenient resonant fraction:

• Half-wave (λ/2): common for dipoles and many wire antennas.
• Quarter-wave (λ/4): common for monopoles and verticals when you have a ground plane or counterpoise.
• Eighth-wave (λ/8): sometimes used as a short element when physical constraints matter.

The calculator’s output is the geometric length of those fractions in meters, after optionally multiplying by a velocity factor.

Introduction: Velocity factor: why “free-space length” is not the final answer

In the field, many antennas are not bare wire in free space. Conductors might be insulated, placed close to other materials, or built from tubing and mounting hardware that changes the effective electrical length. A velocity factor $v\_f$ is a simple way to model that the wave travels more slowly (or the antenna appears electrically longer) compared to free space. For coaxial cable, velocity factor is a well-defined manufacturer specification. For antennas, “velocity factor” is more like a practical trim factor: you compute a starting length, then tune based on SWR and resonance checks.

Typical starting points:

• 1.00: free-space estimate or thin wire far from other objects.
• 0.95–0.98: many real-world wire antennas (a common “cut a bit long, then trim” range).
• 0.80–0.95: antennas strongly affected by insulation, mounting hardware, or nearby dielectric materials.

If you do not have a measured factor for your build, start with 1.00, cut slightly long, and trim down while measuring.

End effects, diameter, and nearby objects

Even with a good starting factor, physical reality matters:

• End effects: current does not abruptly stop at the wire tip; the effective electrical length is influenced by the surrounding field.
• Conductor diameter: thicker elements can broaden bandwidth and slightly shift resonance.
• Mounting and proximity: nearby metal (masts, gutters), walls, and roof materials can detune an antenna.
• Ground and counterpoise: quarter-wave verticals depend heavily on the quality and geometry of the return path.

This is why “the right length” is usually a range plus a tuning process. The calculator provides a transparent first approximation.

Quick unit conversions (meters ↔ feet/inches)

Many builders think in feet and inches. Use these conversions to translate the output:

Worked example: cutting a 2 m band quarter-wave

Suppose you want a quarter-wave vertical for 146 MHz (a common 2 m band frequency). Free-space wavelength is about $\frac{300}{146}$ ≈ 2.055 m, so a quarter-wave is about 0.514 m. Converting to inches gives 0.514 × 39.37 ≈ 20.2 inches. Many builders would cut a bit long (for example, 21 inches), then trim down while watching SWR or resonance until the antenna centers on the desired frequency.

Limitations and assumptions: Safety and build notes

Any antenna project should be approached with practical safety constraints:

• Keep clear of power lines and follow local electrical safety rules.
• Use appropriate grounding and lightning protection where relevant.
• Confirm your radio system (transmitter power, feedline, connector quality) before blaming “length” for performance.

Finally, remember that antenna performance is not only about resonance. Pattern, polarization, height above ground, losses in the feedline, and matching networks often dominate real-world results. Length is the start of the conversation, not the finish.

Dipole vs. vertical: same wavelength, different setup

A half-wave dipole is two quarter-wave elements end to end, fed in the middle. In an ideal free-space model, each leg is about $\frac{\lambda }{4}$ and the total tip-to-tip span is about $\frac{\lambda }{2}$. A quarter-wave vertical, by contrast, is one quarter-wave element referenced to a ground system or counterpoise. That means a vertical build is often physically shorter, but its performance depends heavily on the quality of the return path (radials, vehicle body, roof metal, etc.).

When tuning, the same principle applies: resonance depends on electrical length. If your vertical is too long, resonance shifts lower than your target frequency; if it is too short, resonance shifts higher. With a dipole, you generally trim both legs equally to keep the feedpoint balanced.

Common starting lengths (quick reference)

Here are rough quarter-wave starting points for a few popular frequencies (free-space, velocity factor 1.0). Treat these as “cut long then trim” values rather than final, guaranteed lengths:

FAQ: tuning and troubleshooting

Why does my measured resonance differ from the calculator? Nearby metal, insulation, element diameter, and your ground/counterpoise system all shift electrical length. Use the calculator to cut an initial length, then tune.

Should I include a matching network in the length? Matching networks change impedance and can slightly shift resonance. Tune the radiator first, then optimize matching.

Do I need a velocity factor? If you don’t know your trim factor, leave it at 1.0 and trim during testing. If you have a known factor from a proven design, using it gets you closer on the first cut.

What if I want feet and inches? Multiply meters by 3.28084 to get feet, or by 39.37 to get inches. The calculator provides meter outputs because the wavelength formula naturally yields SI units.

Beyond quarter-wave: 5⁄8-wave and shortened builds

You will sometimes see vertical designs described as “5⁄8-wave” or “3⁄8-wave.” These lengths can change the radiation pattern and, with the right matching, can provide a lower takeoff angle in some setups. They are also popular because they offer a compromise between height, pattern, and bandwidth. However, these designs are more sensitive to matching and the local environment than a simple quarter-wave. Use them when you have a specific goal (pattern shaping, mounting constraints, or a proven design), and expect to spend more time tuning than you would with a basic quarter-wave radiator.

How to use this calculator

1. Enter Frequency (MHz) using the unit or time period shown by the field.
2. Enter Velocity factor (0–1, optional) using the unit or time period shown by the field.
3. Run the calculation and compare the output with a second scenario before acting on it.