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Output power level is generally given in dBm, defined by the IEEE as a unit for expression of power level in decibels with reference to a power of one milliwatt. Most units we tested transmit at 20 dBm. While both 5.8 GHz and 2.4 GHz are unlicensed bands, FCC regulations for the two differ slightly. Some countries impose EIRP (effective isotropic radiated power) limits; Canada, for example, limits output power at the antenna to 36 dBm.

By using dBm as a measure, it is relatively easy to calculate the cumulative effect of all passive and active system components. A unit's receiver sensitivity provides a measure of how weak a received signal can be while still providing error-free communications. GWM's Lynx.sc has a receiver sensitivity of -93 dBm. Thus, Lynx.sc's signal can be attenuated by up to 113 dB while maintaining an acceptable signal level.

Antennas provide signal gain, which increases the possible distance of a link. Although low-cost Yagi-style antennas can be used with these products, most vendors recommend highly directional parabolic antennas. These antennas are available from a number of manufacturers and must be selected based on the transmission frequency of the radio. At 5.8 GHz, a two-foot antenna will provide about 29 dB of gain, while an eight-foot antenna provides about 41 dB of gain.

Cable loss between the radio and the antenna and over-the-air path loss both contribute to the total attenuation of a signal. Cable loss varies according to the specific cabling used and the radio's transmission frequency. According to GWM, which offers both 2.4-GHz and 5.8-GHz systems, most of its 2.4-GHz radio customers use half-inch coax cable, which has an attenuation of 3.8 dB/100 foot at 2.4 GHz (6.6 dB/100 foot at 5.8 GHz). Their 5.8-GHz radio customers usually choose five-eighth-inch heliax cable, which has an attenuation of 4.7 dB/100 foot at 5.8 GHz. Some vendors offer mast-mounted RF units that largely eliminate cable loss.

You can calculate the over-the-air path loss using a mathematical equation that factors the radio's transmission frequency and the distance between sites. For example, a 10-mile link using 5.8-GHz radios would experience a total path loss of approximately 132 dB.

The final factor to consider in engineering a link is the fade margin. This provides a cushion to accommodate environmental conditions that can increase path loss. Using all these factors, vendors can provide an estimated system availability based on the geographic area where the system is installed.