Dipole & Monopole Antenna Length Calculator

A resonant wire antenna's physical length is a fraction of its signal's free-space wavelength, λ = c ÷ f. A half-wave dipole (fed at its center, with two legs) is resonant at λ/2; a quarter-wave monopole (fed against a ground plane, a single element) is resonant at λ/4. Because a real wire is very slightly electrically longer than its physical length — a phenomenon called 'end effect,' the capacitive coupling of an unterminated wire end to its surroundings — the length actually cut is that free-space fraction multiplied by an end-effect factor, typically 0.90-0.98 for ordinary wire, with 0.95 a widely used default for common AWG 12-16 copper wire.

This calculator computes the free-space wavelength directly from the speed of light rather than starting from the rounded '468 ÷ f(MHz)' feet shortcut some references quote, which keeps it exact for any end-effect factor you enter. With the default 0.95 factor, it reproduces that classic 468 ft (dipole) / 234 ft (monopole) rule of thumb to within a small fraction of a foot, since 468 ≈ 492 × 0.95 and 234 ≈ 246 × 0.95, where 492 ft and 246 ft are themselves the half- and quarter-wavelength at exactly 1 MHz.

Real-world antennas typically need final on-air trimming — nearby objects, wire gauge, and height above ground all shift the true resonant length slightly from this formula's prediction — so treat the result as a solid starting cut length, not a guaranteed final one.