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Omni Antenna [1] have a broad propagation field, designed to be
basically uniform in all directions.
In practice, this is not spherical, but often more Smarties [2]-like,
wide, but not high. The higher the gain, the flatter the Smarties [3].
Typical gain of omni's are in the 3dbi-15dbi range.
One common and relatively easy to construct type of omnidirectional
antenna is the CoaxCollinear [4].
Some omni-directional Antenna [5] plans:
* Vertical Colinear [6] from Guerrilla.net [7]
* Easy Homebrew 2.4Ghz Omni [8]
* The Slim Jim [9] (not 2.4Ghz)
At this stage, we have not yet seen a homebrew omni with the
combination of decent gain and a suitable radiation pattern (the
Smarty is tilted up rather than be level) for the >1km links we're
trying to make at MW. Any results to the contrary would be much
appreciated!
If you want to buy a commercial omni antenna, D-Link sells an 8dbi
outdoor omni, as does Rojone [10]. That is, if you missed out on The
Incredible Bulk purchase of a few squillion lately. Higher gain ones
are available from several US vendors, but the propagation pattern
starts to get kinda thin and often has quite a deal of downtilt (which
is great if you're on the top of a hill, not so great otherwise). If
you really need that kind of gain, maybe you should consider a
YagiAntenna [11] or ParabolicAntenna [12].
Links:
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[1] http://melbournewireless.org.au/?Antenna
[2] http://melbournewireless.org.au/?Smarties
[3] http://melbournewireless.org.au/?Smarties
[4] http://melbournewireless.org.au/?CoaxCollinear
[5] http://melbournewireless.org.au/?Antenna
[6] http://www.guerrilla.net/reference/antennas/2ghz_collinear_omni/
[7] http://www.guerrilla.net
[8] http://wireless.gumph.org
[9] http://www.irational.org/sic/radio/omni-aerial.html
[10] http://www.rojone.com.au
[11] http://melbournewireless.org.au/?YagiAntenna
[12] http://melbournewireless.org.au/?ParabolicAntenna
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