WLAN Antennas

A variety of antennas suited for wireless LANs are available. Each type of antenna is designed for a specific application, either due to its ability to cover a certain area or pattern, or its capability to add gain to the signal.

Omnidirectional Antennas

Omnidirectional antennas, such as the one shown in Figure 17-18, can be made in the shape of a thin cylinder. The "rubber duckie"-style antenna that is pictured can be folded up or down, depending on how the AP is mounted. Other omnidirectional antennas include the dipole, which is also a slender cylinder, found embedded in several AP models.

Figure 17-18 An Omnidirectional Antenna and Its Coverage Pattern

Figure 17-18 An Omnidirectional Antenna and Its Coverage Pattern

An omnidirectional antenna tends to propagate a signal equally in all directions in one plane, but not along the antenna's length. The result is a donut-shaped pattern as shown on the right portion of Figure 17-18. This type of antenna is well suited for broad coverage of a large room or floor area where the AP or its antenna is located in the center.

Because the omnidirectional antenna distributes the RF energy along a broad area, it has a low gain of about 2 dBi.

TIP Sometimes an RF signal will be delayed or distorted due to reflection, refraction, or diffraction. When the affected signal arrives along with the original signal at the receiver, the two signals can tend to cancel each other. As a result, the receiver has a drop in signal level or no signal at all. When a signal competes with other copies of itself that have taken different paths, multipath interference results.

To fix this problem, the receiver's antenna can simply be moved a short distance (about a one-half wavelength) to receive a better signal. Many APs have two omnidirectional antennas by default—these are correctly spaced so that one antenna or the other will always receive the best signal. The use of two antennas simultaneously is referred to as diversity.

Semi-Directional Antennas

Semi-directional antennas cover a broad elongated area, and are well suited to cover rooms along a long hallway, where the antenna is located at one end of the area. They can also be used to cover broad outdoor areas out away from a building.

Wall-mounted patch antennas have a flat rectangular shape, so that they can be mounted on a wall at the end of a hallway. Patch antennas produce a broad egg-shaped pattern that extends away from the flat patch. These antennas have a gain of about 6-8 dBi.

The wall or mast-mounted Yagi antenna shown in Figure 17-19 is shaped like a thick cylinder, but is made up of several parallel elements. It produces a more focused egg-shaped pattern that extends out along the antenna's length. Notice that little or no signal propagation exists behind the antenna, unlike in the omnidirectional model. Yagi antennas have a gain of about 10-14 dBi.

Figure 17-19 Semi-Directional Antenna and Its Pattern

Highly Directional Antennas

In a line-of-sight wireless path, an RF signal must be propagated a long distance using a narrow beam. Highly directional antennas are tailored for that use, but focus the RF energy along one

Figure 17-19 Semi-Directional Antenna and Its Pattern

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narrow elliptical pattern. Because the target is only one receiver location, the antenna doesn't have to cover any area outside of the line of sight. This type of antenna is well suited for building-to-building or site-to-site links, where the two AP or antenna locations are separated by a long distance.

Dish antennas, such as the one shown in Figure 17-20, use a parabolic dish to focus received signals onto an antenna mounted at the center. Notice how the antenna's coverage pattern is long and narrow, extending out away from the dish. The focused pattern gives the antenna a gain of around 22 dBi—the highest gain of all the wireless LAN antennas.

Figure 17-20 A Highly Directional Dish Antenna and Its Coverage Pattern

Figure 17-20 A Highly Directional Dish Antenna and Its Coverage Pattern

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