# Omnidirectional antenna

The radiation pattern of a simple omnidirectional antenna, a vertical half-wave dipole antenna. In this graph the antenna is at the center of the "donut," or torus. 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

Radiation pattern of a 3λ/2 monopole antenna. Although the radiation of an omnidirectional antenna is symmetrical in azmuthal directions, it may vary in a complicated way with elevation angle, having lobes and nulls at different angles.

Common types of low-gain omnidirectional antennas are the whip antenna, "Rubber Duck" or "Rubber Ducky", ground plane antenna, vertically oriented dipole antenna, discone antenna, mast radiator, horizontal loop antenna (sometimes known colloquially as a 'circular aerial' because of the shape), and the halo 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 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 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[2] and Omnidirectional Microstrip Antenna (OMA).[3]

Some planar antennas (constructed from printed circuit board) are omnidirectional antennas.[4]

## Analysis

Vertical polarized VHF- UHF biconical antenna 170 – 1100 MHz with omni directional H-plane pattern

Omnidirectional radiation patterns are produced by the simplest practical antennas, monopole and dipole antennas, 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 (D) which is expressed mathematically as: $G = eD$. A useful relationship between omnidirectional radiation pattern directivity (D) in decibels and half-power beamwidth (HPBW) based on the assumption of a $\sin{b\theta} / {b\theta}$ pattern shape is:[5]

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

## References

1. ^ Johnson, R, Jasik, H, ed. (1984). Antenna Engineering Handbook. McGraw Hill. pp. 27–14.
2. ^ Judasz, T., Balsley, B. (March 1989). "Improved Theoretical and Experimental Models for the Coaxial Colinear Antenna". IEEE Transactions on Antennas and Propagation 37 (3): 289–296. doi:10.1109/8.18724.
3. ^ Bancroft R (December 5, 2005). "Design Parameters of an Omnidirectional Planar Microstrip Antenna". Microwave and Optical Technology Letters 47 (5): 414–8. doi:10.1002/mop.21187.
4. ^ Yanaga; Kurashima; Arita; Kobayashi (2003?). "A Planar UWB Monopole Antenna Formed on a Printed Circuit Board" (PDF).
5. ^ McDonald, Noel (April 1999). "Omnidirectional Pattern Directivity in the Presence of Minor Lobes: Revisited". IEEE Transactions on Antennas and Propagation 41 (2): 63–8.