An omnidirectional antenna is designed to provide a 360 degree radiation
pattern
. This type of
antenna is used when coverage in all directions from the antenna is required.
Omnidirectional antennas come in many different styles and shapes. Most
operated in the 2.4 GHz ranges, whereas a few operate in the 5 GHz range.
Omnidirectional antennas include dipoles, mast mount, pillar, and patch
antennas
. The
standard 2.14 dBi "Rubber Duck" is the most commonly used
omnidirectional antenna.
In a perfect world, all the omnidirectional
antennas would radiate perfectly in all directions. Unfortunately, this is not
the case. Notice the radiation pattern of each antenna as they are presented in
this module. The radiation patterns will be shown as a horizontal (H-plane)
radiation pattern, an Elevation (E-plane) radiation pattern, or both. Elevation
plane radiation is sometimes called vertical plane radiation as well.
The
H-plane is also referred to as the Azimuth Plane Pattern. This can be
envisioned by looking at a top view of the antenna, looking down
. Notice the
green area can be viewed as a tabletop, floor, or ground below the antenna. All
of the H-plane omnidirectional patterns are virtually the same, which is a 360
degree pattern
. However, keep
in mind that the gain determines the distance of the radiation. For instance,
the 12dBi mast mount will reach much further than the 2.2 dBi dipole.
The E-plane can be envisioned as the antenna perpendicular to the ground
. The E-Plane
radiation patterns can be significantly different between omnidirectional
antennas. Pay careful attention to these differences. Knowing the antenna
radiation patterns are very important when locating antennas during the test
and installation phase.
Everything about antenna choice involves a tradeoff. If maximum range is
desired, coverage must be traded. Do not forget that coverage is more than just
horizontal. There is a vertical aspect also. Most omnidirectional antennas
trade vertical coverage for additional range. Antenna coverage can be likened
to a balloon. Pressing on the top and bottom of the balloon makes a pancake.
This would give a very narrow vertical beamwidth, but a very large horizontal
coverage. This type of antenna design can deliver very long communications
distances. This design has one drawback, which is poor coverage below the
antenna, as pictured in Figure
. With high gain
omnidirectional antennas, this problem can be partially solved by designing in
something called downtilt. An antenna that uses downtilt is designed to radiate
at a slight angle, rather than at 90 degrees from the vertical element. This
does help for local coverage, but it reduces effectiveness of the long-range
ability. Cellular antennas use downtilt. The Cisco 12dBi omnidirectional
antenna has a downtilt of zero degrees, but can be mounted to provide downtilt.