Omnidirectional antenna

Omnidirectional antennas oriented vertically are widely used for nondirectional antennas on the surface of the Earth because they radiate equally in all horizontal directions, while the power radiated drops off with elevation angle so little radio energy is aimed into the sky or down toward the earth and wasted.

The five-eighth wave monopole – length  0.625 λ , or ⁠ 5 / 8 ⁠ of a wavelength – is also popular, since at that length monopoles direct the greatest proportion of their radiated power horizontally, hence the best use of transmit power for long-distance communication.

High-gain omnidirectional antennas are generally realized using collinear dipole arrays.

These consist of multiple half-wave dipoles mounted collinearly (in a line), fed in phase.

A useful relationship between omnidirectional radiation pattern directivity (D) in decibels and half-power beamwidth (HPBW) based on the assumption of a

Radiation pattern of a ⁠ 3 / 2 ⁠ λ monopole antenna . Although the radiation of an omnidirectional antenna is symmetrical in azimuthal directions, it may vary in a complicated way with elevation angle, having lobes and nulls at different angles.
Vertical polarized VHF-UHF biconical antenna 170–1100 MHz with omnidirectional H-plane pattern