In radio communication, an omnidirectional antenna is an antenna which radiates radio wave

Radio Wave

Radio Wave may refer to:*Radio frequency*Radio Wave 96.5, a radio station in Blackpool, UK...

 power uniformly in all directions in one plane, with the radiated power decreasing with elevation angle above or below the plane, dropping to zero on the antenna's axis. This radiation pattern

Radiation pattern

In the field of antenna design the term radiation pattern most commonly refers to the directional dependence of the strength of the radio waves from the antenna or other source ....

 is often described as "doughnut shaped". Note that this is different from an isotropic antenna

Isotropic radiator

An isotropic radiator is a theoretical point source of electromagnetic or sound waves which radiates the same intensity of radiation in all directions. It has no preferred direction of radiation. It radiates uniformly in all directions over a sphere centred on the source...

, which radiates equal power in all directions and has a "spherical" 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 for radio broadcasting antennas, and in mobile devices that use radio such as cell phones, FM radios, walkie-talkie

Walkie-talkie

A walkie-talkie is a hand-held, portable, two-way radio transceiver. Its development during the Second World War has been variously credited to Donald L. Hings, radio engineer Alfred J. Gross, and engineering teams at Motorola...

s, wireless computer networks

Wireless network

Wireless network refers to any type of computer network that is not connected by cables of any kind. It is a method by which homes, telecommunications networks and enterprise installations avoid the costly process of introducing cables into a building, or as a connection between various equipment...

, cordless phones, GPS  as well as for base stations that communicate with mobile radios, such as police and taxi dispatchers and aircraft communications.

Types

Common types of low-gain omnidirectional antennas are the whip antenna

Whip antenna

A whip antenna is an antenna consisting of a single straight flexible wire or rod, often mounted above some type of conducting surface called a ground plane. The bottom end of the whip is connected to the radio receiver or transmitter. They are designed to be flexible so that they won't break...

, "Rubber Ducky"

Rubber Ducky antenna

The Rubber Ducky antenna is an electrically short monopole antenna which functions somewhat like a base-loaded whip antenna and is sealed in a rubber or plastic jacket to protect the antenna...

, ground plane antenna

Monopole antenna

A monopole antenna is a class of radio antenna consisting of a straight rod-shaped conductor, often mounted perpendicularly over some type of conductive surface, called a ground plane. The driving signal from the transmitter is applied, or for receiving antennas the output voltage is taken,...

, vertically oriented dipole antenna

Dipole antenna

A dipole antenna is a radio antenna that can be made of a simple wire, with a center-fed driven element. It consists of two metal conductors of rod or wire, oriented parallel and collinear with each other , with a small space between them. The radio frequency voltage is applied to the antenna at...

, discone antenna

Discone antenna

A discone antenna is a version of a biconical antenna in which one of the cones is replaced by a disc. It is usually mounted vertically, with the disc at the top and the cone beneath....

, mast radiator

Mast radiator

A mast radiator is a radio mast or tower in which the whole structure itself functions as an antenna. This design is commonly used for transmitting antennas operating at low frequencies, in the VLF, LF and MF ranges, in particular those used for AM broadcasting. The metal mast is electrically...

, horizontal loop antenna

Loop antenna

A loop antenna is a radio antenna consisting of a loop of wire, tubing, or other electrical conductor with its ends connected to a balanced transmission line...

 (sometimes known colloquially as a 'circular aerial' because of the shape), and the halo antenna

Halo antenna

A halo antenna, or halo, is a horizontally polarized, omni-directional 1/2 wavelength dipole antenna. It is shaped like a loop with a small break on the side of the loop directly opposite the feedpoint, so that the dipole ends do not meet...

.

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 using collinear dipole arrays. These arrays consist of half-wavelength dipoles with a phase shifting method between each element that ensures the current in each dipole is in phase. The Coaxial Colinear or COCO antenna uses transposed coaxial sections to produce in-phase half-wavelength radiators. A Franklin 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.

Types of higher gain omnidirectional antennas are the coaxial collinear (COCO) antenna and Omnidirectional Microstrip Antenna

Microstrip antenna

In telecommunication, there are several types of microstrip antennas the most common of which is the microstrip patch antenna or patch antenna...

 (OMA).

Some planar antennas (constructed from printed circuit board) are omnidirectional antennas.

Analysis

Omnidirectional radiation patterns are produced by the simplest practical antennas, monopole

Monopole antenna

A monopole antenna is a class of radio antenna consisting of a straight rod-shaped conductor, often mounted perpendicularly over some type of conductive surface, called a ground plane. The driving signal from the transmitter is applied, or for receiving antennas the output voltage is taken,...

 and dipole antenna

Dipole antenna

A dipole antenna is a radio antenna that can be made of a simple wire, with a center-fed driven element. It consists of two metal conductors of rod or wire, oriented parallel and collinear with each other , with a small space between them. The radio frequency voltage is applied to the antenna at...

s, consisting of one or two straight rod conductors on a common axis. Antenna gain (G) is defined as antenna efficiency (e) multiplied by antenna directivity

Directivity

In electromagnetics, directivity is a figure of merit for an antenna. It measures the power density the antenna radiates in the direction of its strongest emission, versus the power density radiated by an ideal isotropic radiator radiating the same total power.An antenna's directivity is a...

 (D) which is expressed mathematically as: . A useful relationship between omnidirectional radiation pattern directivity

Directivity

In electromagnetics, directivity is a figure of merit for an antenna. It measures the power density the antenna radiates in the direction of its strongest emission, versus the power density radiated by an ideal isotropic radiator radiating the same total power.An antenna's directivity is a...

 (D) in decibels and half-power beamwidth

Beamwidth

In telecommunication, the term beamwidth has the following meanings:1. In the radio regime, of an antenna pattern, the angle between the half-power points of the main lobe, when referenced to the peak effective radiated power of the main lobe....

(HPBW) based on the assumption
of a pattern shape is: