Dipole Antenna Calculator

The Workhorse of Radio: The Half-Wave Dipole

In the world of amateur radio (Ham Radio), the half-wave dipole is legendary. It is simple, cheap, and surprisingly effective. Whether you are trying to talk across town on 2 meters or across the world on 20 meters, a properly cut piece of wire can do the job. Our Dipole Antenna Calculator helps you find the exact resonant length for your target frequency.

The Magic Number: 468

If you calculated the length of a radio wave purely based on the speed of light in a vacuum ($c = 300,000,000$ m/s), a half-wave would be $492 / f$ (in feet).
However, radio waves travel slower in wire than in a vacuum due to the "Velocity Factor." Additionally, "End Effects" (capacitance at the ends of the wire) make the antenna electrically longer than it is physically.
To correct for this, we typically reduce the length by about 5%. This gives us the standard formula used by hams for decades:

$$ L_{total} (feet) = \frac{468}{f (MHz)} $$
Where:
- $L_{total}$: The total length of the antenna wire (tip to tip).
- $f$: The frequency in Megahertz.

Metric Calculation

If you prefer meters, the formula is:
$$ L_{total} (meters) = \frac{143}{f (MHz)} $$

Construction Tips

1. Cut Long, Trim Later: The formula is an approximation. Nearby objects, ground conductivity, and wire thickness affect resonance. Always cut your wire a few inches longer than calculated. You can fold the ends back to tune it (shorten it) using an SWR meter or antenna analyzer. It is much harder to add wire back on!
2. Center Fed: The dipole is fed in the center. This means you have two equal "legs," each half the total length calculated above. Connecting your coax center conductor to one leg and the shield to the other creates the antenna.
3. Height Matters: For a dipole to work like a true dipole (broadside radiation), it should ideally be at least 1/2 wavelength above the ground. Lower heights will send more signal straight up (NVIS), which is good for local/regional contacts but bad for DX.

Common Dipole Types

- Flat Top: Strung horizontally between two supports. Standard pattern.
- Inverted V: Center is high, ends slope down. Requires only one tall support. Has a more omnidirectional pattern but slightly lower gain.
- Sloper: One end high, one end low. Can be directional.