Inradio communication, anomnidirectional antenna is a class ofantenna which radiates equal radio power in all directions perpendicular to anaxis (azimuthal directions), with power varying with angle to the axis (elevation angle), declining to zero on the axis.^[1][2] When graphed in three dimensions(see graph) thisradiation pattern is often described asdoughnut-shaped. This is different from anisotropic antenna, which radiates equal power inall directions, having aspherical radiation pattern. 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.

Omnidirectional antennas are widely used forradio broadcasting antennas,^[3] and in mobile devices that use radio such ascell phones,FM radios,walkie-talkies,wireless computer networks,cordless phones,GPS, as well as for base stations that communicate with mobile radios, such as police and taxi dispatchers and aircraft communications.

Theradiation pattern of a simple omnidirectional antenna, a vertical half-wavedipole antenna. In this graph the antenna is at the center of the "donut," ortorus. Radial distance from the center represents the power radiated in that direction. The power radiated is maximum in horizontal directions, dropping to zero directly above and below the antenna.

Animation of an omnidirectionalhalf-wave dipole antenna transmittingradio waves. The antenna in the center is two vertical metal rods, with an alternating current applied at its center from a radio transmitter(not shown). Loops of electric field(black lines) leave the antenna and travel away at thespeed of light; these are the radio waves. This diagram only shows the radiation in a single plane through the axis, the radiation is symmetrical about the antenna's vertical axis.

The most common omnidirectional antenna designs are themonopole antenna, consisting of a vertical rod conductor mounted over a conductingground plane, and verticaldipole antenna, consisting of two collinear vertical rods. Thequarter-wave monopole andhalf-wave dipole both have verticalradiation patterns consisting of a single broad lobe with maximum radiation in horizontal directions, so they are popular. The quarter-wave monopole, the most compact resonant antenna, may be the most widely used antenna in the world. The five-eighth wave monopole – length 0.625λ , 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.

Common types of low-gain omnidirectional antennas are thewhip antenna,"Rubber Ducky" antenna,ground plane antenna, vertically orienteddipole antenna,discone antenna,mast radiator, horizontalloop antenna (sometimes known colloquially as a 'circular aerial' because of the shape) and thehalo antenna.

Higher-gain omnidirectional antennas can also be built. "Higher gain" in this case means that the antenna radiates less energy at higher and lower elevation angles and more in the horizontal directions. High-gain omnidirectional antennas are generally realized usingcollinear dipole arrays. These consist of multiplehalf-wave dipoles mountedcollinearly (in a line), fed in phase.^[4]

• The coaxial collinear (COCO) antenna uses transposed coaxial sections to produce in-phase half-wavelength radiators.^[5]

• AFranklin array uses short U-shaped half-wavelength sections whose radiation cancels in the far-field to bring each half-wavelength dipole section into equal phase.

• Another type is the omnidirectionalmicrostrip antenna (OMA).^[6]

Omnidirectional radiation patterns are produced by the simplest practical antennas,monopole anddipole antennas, consisting of one or two straight rod conductors on a common axis. Antenna gain (G) is defined as antenna efficiency (e) multiplied by antennadirectivity (D) which is expressed mathematically as:${\displaystyle G=eD}$. A useful relationship between omnidirectional radiation patterndirectivity (D) indecibels and half-powerbeamwidth (HPBW) based on the assumptionof a${\displaystyle \sin (b\theta )/b\theta }$ pattern shape is:^[7]

${\displaystyle D\approx 10{\log }_{10}\left(\frac{101.5}{\text{HPBW}-0.00272{\text{HPBW}}^2}\right)\text{ dB}.}$

• Choke ring antenna
• Directional antenna

• Radio frequency antenna types
• Antennas (radio)

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