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or: How To Set Up a TWCN Node
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Directional antennas come in a wide variety of types, shapes, and sizes. Each configuration has an intended application. For example sector directionals might be good for an office that rents just a corner of a larger office floor, where you could place a 90 degree sector directional in one corner and squish the distribution to target your office instead of radiating out into the streets. As another example, parabolic antenna types are used in long distance point-to-point (PtP) links.
Yagi antennas are primarily used to create focused, point-to-point links. They are considered more attractive than Parabolic antennas for shorter distances because they are more forgiving of misalignment. They also have much less wind-loading than panel antenna's, an obvious advantage in certain situations. One known disadvantage is that snow and ice can build up on the long tube which can distort the signal, and reduce antenna gain. In addition, they can be relatively expensive. It is possible for a single yagi to service a small cluster of mid-range devices, so long as they are located close enough to each other. Often Yagis will have side lobes, in effect making a kind of very narrow sector antenna (see below). Yagis are typically mast mounted.
Sector antenna's are designed for Point-to-Multipoint systems, and are usually mast mounted, or mounted on exterior/interior walls.
Sector antennas attempt to radiate in a specific area, shaped like a slice of pie. A 90-degree sector attempts to radiate in a shape like a quarter of a circle. To increase their gain, the signal is both flattened out, taking away focus from directly above and below and is further narrowed into a specific direction. As with an omni, the higher the gain for a given horizontal coverage, the narrower the signal must be. A key feature of using multiple Sector antennas over an omni is that the antenna can be mounted with a downward tilt, to push the focus toward the ground where end devices may be located. This also reduces the area in which the antenna will pick up unwanted noise from other devices, including antennas from adjacent coverage areas.
Be aware that a sector antenna does not simply stop its signal at the given area boundaries. The signal margins represents a 3dB reduction from the maximum focus. Thus, a 120-degree sector antenna can actually cover a 180-degree area with more than a 3dB reduction in strength at the 30 degree margin areas. Some vendors and distributors do not recommend 180 degree sectors.
Panel antennas are used for focused point-to-point links, but some people use them in point-to-multipoint applications as well. The primary advantage of panels is aesthetics; They are considered more attractive than parabolic antennas, and can seamlessly blend into many different environments. They can be mounted on masts, or in locations similar to sector antennas. Their major disadvantage is the large wind-loading created by their solid design, particularly a problem for large high-gain panel antennas. Be sure you know your antennas wind-loading factor before attempting to mount it. Panel antennas can be aligned for either vertical or horizontal polarity.
Parabolic antennas are used for very focused point-to-point links over long distances. The main advantage is the higher gain possible with their parabolic design which focuses the signals it receives to a single point for collection. The main disadvantage is that two parabolics have to be directly aligned for use. Parabolic antennas can be mounted for either vertical or horizontal polarity. They are typically designed for mounting on a pole only, but kits can be bought to mount in other places.
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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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