Antenna Length Calculator
Find the physical length of a quarter-wave, half-wave or full-wave antenna for any frequency, with the velocity factor built in.
How it works
λ (m) = 300 ÷ f(MHz) · Length = λ × fraction × velocity factor · (¼λ ≈ 234 ÷ f in feet)
A resonant antenna is cut to a fraction of the signal wavelength. The free-space wavelength is the speed of light divided by frequency, but a real wire is slightly shorter than the theoretical length because of the end effect — the velocity factor (about 0.95 for thin wire) accounts for it. The classic ham-radio shortcut, 234 ÷ f in MHz for a quarter-wave in feet, is exactly this formula with the 0.95 factor baked in.
Worked example
For 145 MHz, λ = 300 ÷ 145 = 2.069 m. A quarter-wave is 2.069 × 0.25 × 0.95 = 0.491 m — about 19.3 inches, matching the 234 ÷ 145 = 1.61 ft rule of thumb.
Frequently asked questions
Why is the antenna shorter than a pure quarter wavelength?
The electromagnetic wave travels slightly slower along a wire than in free space, and the open ends add capacitance (the "end effect"). Multiplying by a velocity factor of roughly 0.95 shortens the element to its true resonant length.
Which length should I build — quarter or half wave?
A quarter-wave vertical needs a ground plane or radials and is the most common mobile/base whip. A half-wave dipole is fed in the centre and needs no ground plane. Full-wave loops are used for quieter reception and specific gain patterns.
Does this work for any frequency?
Yes — the formula is frequency-agnostic, from AM broadcast to Wi-Fi. Just enter the frequency in MHz. At microwave frequencies the element becomes very short, so mechanical tolerance matters more.
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This calculator is for educational purposes. Double-check important results before acting on them.