टंक न्यौपाने Tank Neupane: Omnidirectional antenna

An omnidirectional antenna is an antenna which radiates power uniformly 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 is often described as "donut shaped". 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-talkies, Wifi, cordless phones, GPS as well as for base stations that communicate with mobile radios, such as police and taxi dispatchers and aircraft communications.

Omnidirectional radiation pattern of a vertical dipole antenna. In this graph the antenna is at the center of the "donut". 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.

Types

Common types of low gain omnidirectional antennas are the whip antenna, "Rubber Ducky", ground plane antenna, vertically oriented dipole antenna, discone antenna, mast radiator and the horizontal loop antenna (or halo antenna) (Sometimes known colloquially as a 'circular aerial' because of the shape).

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^[1]. 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.
Types of higher gain omnidirectional antennas are the Coaxial Colinear (COCO) antenna^[2] and Omnidirectional Microstrip Antenna (OMA)^[3].
Some planar antennas (constructed from printed circuit board) are omnidirectional antennas.^[4]

Analysis
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. Antenna gain (G) is defined as antenna efficiency (e) multiplied by antenna directivity (D) which is expressed mathematically as:

G = e D

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

sin b θ / b θ

pattern shape is:^[5]

$D = 10\log_{10} {\left ({101.5\over {HPBW - 0.00272(HPBW)^2}}\right )} \;\; dB$

References

^ Johnson, R and Jasik, H Ed., "Antenna Engineering Handbook," McGraw Hill, 1984 page 27-14.
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.
^ Yanaga; Kurashima; Arita; Kobayashi. "A Planar UWB Monopole Antenna Formed on a Printed Circuit Board". (2003 ?) [1]
^ McDonald, Noel, "Omnidirectional Pattern Directivity in the Presence of Minor Lobes: Revisited," IEEE Transactions on Antennas and Propagation Magazine, Volume 41, No. 2 April 1999, pp. 63-68