File:Monopole antenna radiation patterns.svg

English: Graphs of the vertical radiation patterns of three monopole antennas of different lengths, mounted over a perfectly conducting ground. The antenna is assumed to be vertical. The graphs represent sections through the symmetrical omnidirectional pattern. The radial distance of the line from the origin at any elevation angle is proportional to the power density radiated at that elevation. The radial axis is graduated in power density relative to a quarter wave monopole, and decibels-isotropic. The quarter-wave monopole (0.25λ, blue) has a pattern identical to the upper half of a dipole antenna pattern. The half-wave monopole (0.5λ, green) radiates more of the power in horizontal directions. The maximum horizontal radiation for a monopole antenna is achieved at a length of 0.625λ (red). At lengths above a half wavelength (0.5λ) the radiation pattern divides into two lobes, with a second conical lobe directed into the sky. The maximum at 0.625λ occurs because at high angles the opposite phase radiation of the two lobes tends to cancel (interfere distructively) compressing more of the power into the horizontal lobe.