Dipole Calculator
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.
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.