Wireless Network Access Point (WNAP) Set Up Guide

or: How To Set Up a TWCN Node

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Appendix A : Antennas - Going the distance

Omni-Directionals

A straight wire is an omni-directional antenna. By modifying its length, diameter, type of conductor, sheathing, connectors, and amplification, we modify its gain. By design, omnis are point-to-multipoint, meaning one point distributes to many end points (devices).

Unfortunately, omni's don't make for great diagrams or topic discussion. If you want equal range in all degrees, choose an omni. It's as simple as that. Some are better than others, but you can find decent ones for under $100 USD.

Omni's can be interesting when you combine two of them to create a "dipole setup". Many dipoles have one antenna one for transmitting & receiving at the same time (like any regular omni) and one intended for reception only. The benefit to this is that weak antennas on PCMCIA devices can be picked up easier using the reception only antenna. Whether this actually works is anyone's guess. The author has not found any documentation or evidence to support this claim.

As the image below is meant to detail, the mathematical representation of a dipole is not a perfect sphere, but more the shape of a slightly squished donut. Again this is also a mathematical representation, and as with all such representations, there are no boundaries. However, you could conceive such boundaries as a threshold of operation. If you upped the amplification you move the threshold boundary out farther, but generally the shapes remain the same.

Omni-directionals can become even more interesting when you start making them into specific shapes like rectangles, or co-linears. For more information about all this, we recommend you consult the following documents:

13.5 dBi Omnidirectional Antenna, Horizontally Polarized, 2.4 GHz
http://www.winncom.com/cgi-bin/moreinfo.pl?ITCODE=WRO2400-135H

A 2.4Ghz Vertical Co-linear Antenna for 802.11 Applications
http://www.guerrilla.net/reference/antennas/2ghz_collinear_omni/

Omnis such as the "slotted waveguides" antenna (sometimes called "slotted wavelength"), or the previously mentioned co-linear have, in the words of Trevor Marshall, "a number of dipoles, one above the other, radiating their signals in phase so that the energy is concentrated along the axis of the dipole(s)". The image below is a represention of the distribution patterns from a slotted wavelength omni-directional. The important thing to note is how such antennas squish the ER distribution, but are still considered omni-directional.

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Title: Wireless Network Access Point (WNAP) Set Up Guide, or: How To Set Up a TWCN Node
Authors: Jason Hammerschmidt (ffff654.noSPAM.at.yahoo.com), and Jason Ramsay-Brown
Contributors: Emir Alikadic, Hugh Reilly
Version: 1.0, April 2nd, 2002.

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