An omnidirectional antenna is one that transmits and
receives power equally in all directions, in contrast to the
omnidirectional antenna that transmits and receives power preferentially
in a particular direction. In other words, the radiation pattern
of a direction antenna is spherical.
Sometimes, the term omnidirectional antenna is used for an antenna that is omnidirectional
in a particular plane. In this case, the radiation pattern
of the antenna is circular in that plane.
Directional antennas may
be either narrowband or broadband.