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omni [1]
Omni Antenna [2] have a broad propagation field, designed to be
basically uniform in all directions.
In practice, this is not spherical, but often more Smarties [3]-like,
wide, but not high. The higher the gain, the flatter the Smarties [4].
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 [5].
Some omni-directional Antenna [6] plans:
* VerticalColinear [7] from Guerrilla.net [8]
* Easy Homebrew 2.4Ghz Omni [9]
* The Slim Jim [10] (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 [11]. 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 [12] or ParabolicAntenna [13].
Some more pictures ...
[http://www.xtreme.com.au/amplified-omni2.jpg]
[http://www.xtreme.com.au/amplified-omni.jpg]
Links:
------
[1]
http://www.hyperlinktech.com/web/copyrighted_images/hg2408u_thumb.jpg
[2] http://www.melbournewireless.org.au/?Antenna
[3] http://www.melbournewireless.org.au/?Smarties
[4] http://www.melbournewireless.org.au/?Smarties
[5] http://www.melbournewireless.org.au/?CoaxCollinear
[6] http://www.melbournewireless.org.au/?Antenna
[7] http://www.guerrilla.net/reference/antennas/2ghz_collinear_omni/
[8] http://www.guerrilla.net
[9] http://wireless.gumph.org
[10] http://www.irational.org/sic/radio/omni-aerial.html
[11] http://www.rojone.com.au
[12] http://www.melbournewireless.org.au/?YagiAntenna
[13] http://www.melbournewireless.org.au/?ParabolicAntenna
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