Dipole Antenna Calculator

A dipole antenna is the most fundamental type of radio antenna, consisting of two conductive elements extending in opposite directions from a central feedpoint. In its simplest form — the half-wave dipole — the total length equals one half of the operating wavelength. The half-wave dipole is a reference standard against which all other antennas are compared, and it is widely used at every frequency from HF (shortwave) through VHF and UHF by amateur radio operators, broadcast stations, and wireless equipment manufacturers. The free-space wavelength λ is determined by the speed of light and the operating frequency: λ = c / f, where c ≈ 299.792458 m/s and f is in MHz, giving λ in metres. At 146.52 MHz (2-metre amateur band), λ ≈ 2.047 m. A free-space half-wave dipole would be about 1.024 m long from tip to tip, with each arm measuring approximately 0.512 m. In practice, antennas are made from physical conductors that have finite diameter, insulation, or proximity to ground and other objects. These effects cause the effective electrical length of the antenna to differ from the physical length. The velocity factor (vf) accounts for this: it is the ratio of the wave propagation speed in the actual conductor to the speed of light in free space. For bare wire in free space, vf ≈ 0.97–0.99; for insulated wire, vf drops to 0.88–0.95 depending on the insulation dielectric constant. The feedpoint impedance of a half-wave dipole in free space is approximately 73 Ω (purely resistive at resonance), which matches well with 75 Ω coaxial cable. A quarter-wave monopole over a perfect ground plane has an impedance of about 36 Ω (half of the dipole value), making it a good match for 50 Ω coaxial feed via a matching section. These are theoretical values; actual impedance depends on height above ground, conductor diameter, and nearby structures. Dipoles are used in yagi-uda directional arrays, folded dipoles for higher impedance matching, sleeve dipoles for coaxial feed, and as reference elements in antenna gain calculations. Knowing the correct physical length for a given frequency and velocity factor ensures the antenna resonates at the desired frequency, providing maximum radiation efficiency and minimum reflected power on the feedline. This calculator gives you the key dimensions instantly so you can cut your elements to the right length before assembly.