dipole antenna length calculator

What this dipole antenna length calculator does

This calculator estimates the physical cut length for a center-fed half-wave dipole antenna. You enter your frequency and a velocity factor, and the tool returns:

• Total dipole length in feet and meters
• Length of each leg (half of total)
• A quarter-wave reference length

It is intended for quick planning of HF dipole antennas and general amateur radio wire designs. As always, cut a little long and trim while tuning for best SWR.

Dipole length formula explained

Base formula (free space approximation)

A common practical formula for a half-wave dipole is:

Total length (ft) = 468 / frequency (MHz)

This gives a useful starting point for a straight, center-fed wire dipole. Real-world factors like insulation, nearby objects, wire diameter, and height above ground usually shift the resonant point.

Velocity factor adjustment

If your wire is insulated or has different propagation characteristics, the effective electrical length changes slightly. A simple correction is:

Adjusted length = Base length × Velocity Factor

• Typical bare copper wire: around 0.95 to 0.98
• Heavily insulated wire: can be lower, often around 0.90 to 0.96

How to use this calculator

1. Enter your target frequency.
2. Choose the correct frequency unit (Hz, kHz, MHz, or GHz).
3. Set a velocity factor (start with 0.95 if unsure).
4. Click Calculate Length.
5. Cut the antenna slightly longer than shown, then trim to tune.

Example: 20 meter band dipole

Suppose you want to build a dipole for 14.2 MHz and you use a velocity factor of 0.95. The calculator gives an adjusted total length near 31.35 feet (about 9.56 meters), which means each leg starts around 15.67 feet.

From there, install the antenna, measure SWR, and trim both legs equally in small steps until resonance is where you want it.

Practical installation tips

1) Height matters

Dipoles closer to ground tend to show lower resonant frequency than expected. Raising the antenna can shift resonance upward and improve radiation performance.

2) Keep both legs symmetrical

Equal leg lengths help maintain balanced current distribution and predictable performance. Always trim both sides the same amount.

3) Account for nearby metal

Gutters, masts, utility lines, and building structures can detune the antenna. Try to keep the wire clear of conductive objects when possible.

4) Use a current balun at the feedpoint

A 1:1 current balun helps reduce common-mode current on the coax and often improves pattern stability and noise behavior.

Frequently asked questions

Is this exact for every antenna?

No. It is an excellent starting estimate, not a final tuned value. Final length always depends on installation details.

Why do I need velocity factor?

Velocity factor compensates for how the wave propagates in your specific conductor environment. Insulation and construction materials can make electrically “longer” behavior, requiring a shorter physical cut.

Can I use this for multiband dipoles?

You can use it for each target band section as an initial design step, but multiband traps, fan dipoles, and loading methods require additional design constraints.

Final note

A dipole is one of the most effective, low-cost antennas you can build. Use this calculator to get close quickly, then let measurement and trimming get you to a clean match at your operating frequency.