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, 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. 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 (OMA).
Some planar antennas (constructed from printed circuit board) are omnidirectional antennas.
Read more about this topic: Omnidirectional Antenna
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