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Deploying Point-to-Point Wireless Links: Page 5 of 8

Loss also occurs in other areas, most notably in the cabling that connects the radio transmitter to the antenna. By integrating the antenna and radio into a single weatherproof outdoor unit, some vendors eliminate this loss.

As noted earlier, antennas provide gain in a radio system, but the way they do this is different from the way radios deliver gain. Radio-system engineers often refer to an isotropic antenna, a theoretical concept that shows the spherical nature of how signals would radiate from a single point in space. You can't build an isotropic antenna, but some designs come close. Omnidirectional dipole antennas (rabbit ears), for example, radiate in a pattern that looks like a doughnut with a very small hole at the center. Omnidirectional antennas generally provide modest gain and are inappropriate for point-to-point links.

Directional antennas provide additional gain by focusing the radio energy in a directional beam. They don't add any power to the radio signal, but they concentrate it, thereby increasing range relative to the theoretical isotropic antenna. The gain of an antenna relative to isotropic is expressed in dBi. There are many different directional antenna designs, but the most common alternatives used in point-to-point links are patch antennas, multi-element Yagi antennas and parabolic dish antennas.

Two examples: A 2.4-GHz, 15-element Cushcraft Corp. Yagi antenna has a beam width of 30 degrees and a gain of 14 dBi, while a 3-foot Cushcraft parabolic antenna has a beam width of 10 degrees and a gain of approximately 24 dBi.

Now the key measures are in place. You start with output power, say 20 dBm, add antenna gain and then subtract loss from cables and free space. If the resulting number still exceeds the minimum for receive sensitivity, the signal gets through. To provide some margin for error, installers will typically define a fade margin of perhaps 20 dB.

While point-to-point wireless systems require line of sight between antennas, getting it is not quite as simple as aiming the antennas by using a pair of binoculars. In some cases, you might have direct visual line of sight, but not true RF line of sight. Likewise, minor foliage might inhibit visual line of sight, but under some circumstances, a point-to-point wireless link may be able to tolerate leaves and tree branches.