An omnidirectional antenna is an antenna system which radiates power uniformly in one plane with a directive pattern shape in a perpendicular plane. An antenna is a Transducer designed to transmit or Receive electromagnetic waves In other words antennas convert electromagnetic waves into This pattern is often described as "donut shaped"
The only 3 dimensional omnidirectional antenna is the unity gain isotropic antenna, a theoretical construct derived from actual antenna radiation patterns and used as a reference for specifying antenna gain and radio system effective radiated power. An isotropic radiator is a theoretical Point source of waves which exhibits the same magnitude or properties when measured in all directions In the field of antenna design the term radiation pattern most commonly refers to the directional (angular dependence of radiation from the antenna or other In Electronics, gain is a measure of the ability of a circuit (often an Amplifier) to increase the power or Amplitude of a In radio Telecommunications, effective radiated power or equivalent radiated power (ERP is a standardized theoretical measurement of radio frequency (RF Antenna gain (G) is defined as antenna efficiency (e) multiplied by antenna directivity (D) which is expressed mathematically as: G = eD. In electromagnetics, directivity is a figure of merit for an antenna. A useful relationship between omnidirectional radiation pattern directivity (D) in decibels and half-power beamwidth (HPBW) based on the assumption of a sinbθ / bθ pattern shape is:[1]
Practical antennas approach omnidirectionality by providing uniform radiation or response only in one reference plane, usually the horizontal one parallel to the earth's surface. In electromagnetics, directivity is a figure of merit for an antenna. The decibel ( dB) is a logarithmic unit of measurement that expresses the magnitude of a physical quantity (usually power or intensity relative to In Telecommunication, the term beamwidth has the following meanings 1
Common low gain omnidirectional antennas are the whip antenna, a vertically orientated dipole antenna, the discone antenna, and the horizontal loop (or halo) antenna (Sometimes known colloquially as a 'circular aerial' because of the shape). A whip antenna is the most common example of a Monopole antenna, an antenna with a single Driven element and a Ground plane. A dipole antenna, developed by Heinrich Rudolph Hertz around 1886, is an antenna with a center- fed Driven element for transmitting A discone antenna is a version of a Biconical antenna in which one of the cones is replaced by a disc
Higher gain omnidirectional antennas are the Coaxial Colinear (COCO) antenna[2] and Omnidirectional Microstrip Antenna (OMA)[3].
Omnidirectional antennas are generally realized using colinear 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[4]. The Coaxial Colinear or COCO antenna uses transposed coaxial sections to produce in-phase half-wavelength radiatiors. 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.
See also
References
- ^ McDonald, Noel, "Omnidirectional Pattern Directivity in the Presence of Minor Lobes: Revisited," IEEE Transactions on Antennas and Propagation Magazine, Volume 41, No. A choke ring antenna is a particular form of Omnidirectional antenna for use at high frequencies 2 April 1999, pp. 63-68
- ^ Judasz, T. and Balsley, B. , "Improved Theoretical and Experimental Models for the Coaxial Colinear Antenna," IEEE Transactions on Antennas and Propagation, Vol. 37 No. 3, March 1989, pp. 289-296
- ^ Bancroft R, "Design Parameters of an Omnidirectional Planar Microstrip Antenna," Microwave and Optical Technology Letters, Vol 47. No. 5 December 5, 2005 pp. 414-418.
- ^ Johnson, R and Jasik, H Ed. , "Antenna Engineering Handbook," McGraw Hill, 1984 page 27-14.
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