From a frequency, get the physical length for full-, half-, quarter- and ⅝-wave antennas, adjusted by a velocity factor for real wire. An antenna works best when its length is a simple fraction of the wavelength: a half-wave dipole (two quarter-wave legs around the feed) and a quarter-wave vertical or whip are the most common, while ⅝-wave verticals give a little extra gain at the horizon.