Radiation pattern of a vertical half-wave dipole antenna. In this 3 dimensional perspective graph, the antenna is at the center, and the distance of the graph from the center at any angle is proportional to the energy density of the radio waves radiated at that angle by the antenna. This is called a omnidirectional antenna. The antenna radiates equal radio power in any azimuthal direction, and the radiation is maximum in horizontal directions and decreases monotonically to zero at the zenith or nadir, on the antenna's axis. Omnidirectional antennas are used by terrestrial radio communication systems in which the direction to the transmitter or receiver is unknown or changing, such as broadcasting, two-way radio transceivers, and wireless devices, since they radiate maximum power in all horizontal directions but little power up into the sky or down toward the Earth, where it would be wasted.