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WiFi Antenna

Click:245Date:2011-09-22Information Source:

WiFi technology is considered to be one of the most successful technology standards ever devised. In less than a decade, WiFi has gone from complete obscurity to pervasive in many homes and offices. One of the most important pieces of a well-designed WiFi device is the antenna. To better understand this simple, but critical technology, four technical categories and one legal aspect need to be considered.

Omni-directional vs. Directional
Understanding the use and design criteria for the different antenna types used in WiFi devices is fundamental. Anomni-directional antenna is designed to allow the radio waves to disperse in a full 360 degree pattern. The typical radiation pattern emitted by any antenna is measured using horizontal and vertical graphs to show how the radio waves will travel from the antenna, or where the antenna will be most sensitive to receiving any RF transmitted from another device. Omni-directional antennas produce a donut-shaped pattern, technically referred to as toroidal, which can be compressed in the vertical dispersion as the antenna gain is increased. Directional antennas focus the antennas beamwidth in both the vertical and horizontal planes to produce a targeted focus which is useful for many reasons.

Increasing Gain
Increased gain translates into increased range, but at a cost. As mentioned earlier, when an omni-directional antenna is designed to increase its gain on the horizontal plane, while its sensitivity increases outward, this is at the detriment of its vertical range. In a scenario where a WiFi access point is located in the middle of a one floor office, a higher gain omni-directional antenna would make perfect sense, as there is no floor above or beneath it which needs coverage.

Positioning
In the section above, the use of an omni-directional antenna was described as being placed in the center of the room, and the topic was contained as to how changing the vertical dynamics of such an antenna could be used to cover a broader horizontal plane over a larger vertical coverage area. In contrast, the use of a directional antenna placed in the corner of a large, one floor, office is a legitimate option. Conversely, placing a directional antenna in the middle of a large, one floor, office would be an exceptionally bad design, as only one sector of the office would have coverage, with the other areas surrounding the antenna having spotty coverage at best.

Power Output
The radio output power is a fixed limit, but the amount of gain an antenna provides is variable. It is critical that a WiFi access point be paired with an antenna that is appropriate for its intended purpose. It is theoretically possible to use a high output access point with an very high gain antenna and create a situation where the emitted radio signal could approach being a health hazard in the most extreme cases.

Legal Limitations
The governing regulations covering WiFi in the United States is what is known as the FCC's Part 15 rules. This document sets the limits of output power, technically referred to as EIRP, as well as how much power the radio itself can emit. It is illegal to substitute antennas that are not "type and like" for what has been approved by the FCC through their certification process.

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