Locating Devices

If you want to extend your network coverage to a specific house, garage, workshop, or some other building, your best bet may be to set up a line-of-sight connection. This way, you can share your network between the two buildings without broadcasting to your entire neighborhood.

Map out your signal and determine where you ultimately want to be able to connect. From that point, you can determine if you need additional equipment and what type of equipment you need to extend coverage to each area.

Assuming that you want to avoid having to buy additional equipment, you also can try relocating your existing equipment to improve coverage to the areas that you desire. This may be as simple as moving an access point closer to a window so that you can extend coverage to an area outside, such as a deck. Of course, after moving an AP, you'll need to check that you still have sufficient signal reception in the area that AP covered before you moved it.

Seasonal Considerations

When you initially designed your WLAN, you took the time (I hope) to minimize interference from construction materials, obstacles, and RF interference. Now that you're going to extend it, you need to take the time to do the same thing again.

If you're extending your Wi-Fi coverage outdoors, you still have to deal with obstacles and sources of RF interference. Position your access points so that your signal will pass through as few signals as possible. Vegetation, particularly trees and shrubs, can significantly reduce signal strength. This is because living plants are up to 90 percent water, and water is a resistor to 2.4 GHz RF signals. If you plan and extend a WLAN in the fall or winter, remember to take the seasonal foliage into account. You don't want to go through the trouble of relocating equipment in the winter only to have to move it again in spring. If there are deciduous trees or shrubs in the line-of-sight between your access point and clients, they will become a problem when they regain their foliage in the spring. Just how much of an obstacle trees are depends on the number and type of trees and the density of the wooded area. To be safe, just plan your connections so that they avoid obstructions including trees.

Because of their shorter wavelength, 5 GHz 802.11a signals have more trouble penetrating obstacles in the environment than 2.4 GHz signals do, so you'll have to be even more particular about placement of equipment for an 802.11a network.

Line-of-Sight and the Fresnel Zone

To limit propagation of your Wi-Fi signal all over the neighborhood and to take advantage of the increased gain of directional antennas , you can set up a line-of-sight connection between two sections of a WLAN or between two separate WLANs.

A line-of-sight connection requires an unobstructed view between transmission points that are free of obstacles in the signal path. An important consideration when establishing a line-of-sight connection between two points is the Fresnel zone.

The Fresnel zone (pronounced Fray-nel) is named after French physicist Augustin-Jean Fresnel. It refers to the pattern of RF radiation between a transmitter and receiver. Shaped like an elongated ellipse, the Fresnel zone between two antennas must be free of obstructions, or the signal will weaken dramatically
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You can demonstrate a Fresnel zone for yourself using flashlights. For the first demonstration, shine a single flashlight at a wall in a dark room. The brighter, focused circle at the center of the beam represents the Fresnel zone. Notice that, as an obstruction enters the Fresnel zone, it has more of an effect on the "signal" reaching the wall.

You also can take two flashlights and line them up so that they are pointing directly at one another in line-of-sight fashion. Turn on the lights, and you will notice that the center of the combined beam, which is slightly elliptical, is brighter. This is another illustration of the Fresnel zone concept.

Knowing the size of the Fresnel zone is important if you're setting up a link over a great distance. The size of the zone is determined by the distance of the link and the frequency of the radio signal. Antennas must be mounted high enough to allow the Fresnel zone to be clear of obstruction. Generally, 60-80 percent of the Fresnel zone must remain unobstructed.

At a mile, the Fresnel zone of a 2.4 GHz system is approximately 14 feet. At half a mile, it's roughly 10 feet. For shorter distances, you would be safe allowing 10 feet for the Fresnel zone, as long as it is clear of obstacles.

Note: Careful planning and proper setup of equipment limits the number of people that can detect your signal and possibly compromise your WLANs security.