Wavelength & Antenna Length Calculator — Ham Radio

Calculate Antenna Dimensions

Frequency (MHz)

How Antenna Length Is Calculated

The relationship between frequency and wavelength is: λ (meters) = 300 / f (MHz). A full-wave antenna is one complete wavelength long. A half-wave dipole is the most common antenna type — it is λ/2 long and self-resonant without a ground plane. A quarter-wave vertical is λ/4 long and requires a ground plane (radials) to complete the antenna system.

In practice, antennas are cut slightly shorter than theoretical length due to the velocity factor of wire and tubing — typically 95–97% of theoretical. The calculator above uses the theoretical free-space value. When cutting a physical antenna, shorten by about 3–5% and trim to resonance using an SWR meter.

Common Antenna Lengths at Key Ham Frequencies

Band	Freq (MHz)	Half-Wave (ft)	Quarter-Wave (ft)
6 meters	52.0	9.5 ft	4.7 ft
2 meters	146.0	3.4 ft (40.5 in)	1.7 ft (20.3 in)
1.25 meters	223.0	2.2 ft (26.5 in)	1.1 ft (13.3 in)
70 cm	440.0	1.1 ft (13.5 in)	6.7 in
10 meters	28.5	16.5 ft	8.2 ft
20 meters	14.2	33.0 ft	16.5 ft
40 meters	7.15	65.6 ft	32.8 ft

Why is my antenna slightly shorter than the calculator says?

Physical antennas are slightly shorter than free-space wavelength because the velocity of RF in wire and tubing is about 95–97% of the speed of light. This is called the velocity factor. For most practical wire antennas, multiply the theoretical length by 0.95 as a starting point and trim to resonance. Mobile whip antennas use loading coils to electrically lengthen a short physical element — they appear shorter than theory predicts.
Does it matter if I cut a dipole a little long versus a little short?

Start long and trim shorter. You can always remove wire; you cannot add it back without splicing. Cut 2–3% longer than calculated, measure SWR, trim 1/4 inch at a time from each end symmetrically, and remeasure. Most wire dipoles reach 1.5:1 SWR within a few trim cycles.
What is a 5/8-wave antenna and why is it common on vehicles?

A 5/8-wave vertical has slightly more gain than a quarter-wave vertical (about 3 dB over a quarter-wave) because its radiation pattern is compressed toward the horizon — exactly where you want it for mobile communication. The trade-off is that a 5/8-wave antenna is not self-resonant and requires a matching network at the base. Most commercial mobile antennas labeled "5/8 wave" include this network internally.