# Dipole Calculator

Calculate half-wave dipole antenna length from frequency. Get total length, each leg, and quarter-wave dimensions for ham radio and RF design.

## What this calculates

A half-wave dipole is the most fundamental antenna in radio communications. Its length is directly determined by the operating frequency. The classic formula gives the total length in feet as 468 divided by frequency in MHz (using the standard 0.95 velocity factor for bare wire). This calculator lets you adjust the velocity factor and gives you both the full dipole and quarter-wave vertical dimensions.

## Inputs

- **Frequency** (MHz) — min 0 — Center frequency for the antenna band.
- **Velocity Factor** — min 0.1, max 1 — Accounts for wire thickness and end effects. 0.95 for bare wire, 0.82 for insulated wire.

## Outputs

- **Total Dipole Length** (ft) — Total tip-to-tip length of the half-wave dipole
- **Total Dipole Length** (m) — Total length in meters
- **Each Leg (from center)** (ft) — Length of each element from the feedpoint
- **Each Leg (from center)** (m) — Each leg in meters
- **Full Wavelength** (m) — Full wavelength at this frequency
- **Quarter Wave (ground plane)** (ft) — Quarter-wave vertical antenna length

## Details

The formula for a half-wave dipole is:

Length (feet) = 492 x VF / Frequency (MHz)

Where VF is the velocity factor. The often-cited "468 / f" formula uses a built-in velocity factor of 0.95, which works for typical bare copper wire. Insulated wire is electrically shorter (VF around 0.82), so the physical antenna is shorter too.

The velocity factor accounts for several real-world effects:

  - Wire diameter: Thicker wire has a slightly lower VF

  - End effects: The antenna acts a bit longer than its physical length at the tips

  - Insulation: Wire insulation slows the wave, reducing VF to 0.80-0.85

  - Height above ground: Nearby ground affects the effective length slightly

A dipole's feedpoint impedance is approximately 73 ohms, close to the 75-ohm coax standard and a reasonable match to 50-ohm coax with an SWR around 1.5:1. The radiation pattern is a donut shape, with maximum radiation broadside to the wire and nulls off the ends.

For a quarter-wave vertical (ground plane antenna), use half the dipole length for the vertical element. The ground plane radials are the same length as the vertical element.

## Frequently Asked Questions

**Q: Why is the formula 468/f and not 492/f?**

A: The number 492 comes from converting the speed of light into half-wavelengths measured in feet per MHz: 983.57/2 = 491.8, rounded to 492. In practice, a wire antenna resonates a few percent shorter than the theoretical half wavelength due to end effects and wire diameter. Multiplying 492 by a typical 0.95 velocity factor gives 467.4, rounded to 468. This calculator lets you set the velocity factor precisely rather than relying on the 468 shortcut.

**Q: What is the velocity factor for my wire?**

A: For bare copper or aluminum wire, use 0.95 as a starting point. For insulated wire (PVC jacket), use 0.80 to 0.85. For ladder line, around 0.92 to 0.95. The exact value depends on wire gauge, insulation thickness, and height above ground. In practice, cut the antenna a few percent long and trim to resonance using an SWR meter or antenna analyzer.

**Q: Does a dipole need to be perfectly straight and horizontal?**

A: No. An inverted-V dipole (center high, ends angled down) works well and needs only one support point. A sloper (angled from high to low) adds some directivity. Even a bent dipole works, though it changes the radiation pattern somewhat. The antenna will still resonate near the same frequency regardless of shape; just the radiation pattern and feedpoint impedance shift slightly.

**Q: How do I convert this to a quarter-wave vertical?**

A: Take one leg of the dipole (half the total length) and mount it vertically. Add at least four ground radials at the base, each the same length as the vertical element, spreading out horizontally. The feedpoint impedance of a quarter-wave vertical over a perfect ground is about 36 ohms, making it a decent match to 50-ohm coax.

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Source: https://vastcalc.com/calculators/physics/dipole
Category: Physics
Last updated: 2026-04-08