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	Issue TOC Print full article Print this page Download as PDF E-Mail this URL Flame the author     In this article Introduction Far-Reaching Effects Products Reviewed How We Tested How Far Can You Go? Report Card

The range of microwave systems is limited by a number of technical radio characteristics as well as by the curvature of the earth. Most high-performance systems require direct line of sight between antennas. Because designers must take into account the RF effects of a system's Fresnel zone (the pattern of electromagnetic radiation that is created by a transmitting station from its antenna to receiving antennas), bridges should be elevated sufficiently off the ground to ensure reliable operation.

The major technical variables that affect range are radio output power, radio receiver sensitivity, antenna gain and path loss. Path loss is the attenuation that occurs when radio signals pass through air, and while generally easy to calculate, it varies with frequency (the higher the frequency, the higher the path loss per meter). While somewhat of an over-simplification, at a given frequency, high radio output power combined with high antenna gain and good receiver sensitivity translates into improved range. Most professional installers also will add a fade margin to their overall range calculations. This margin will vary depending on the geographic and climatic conditions of different geographic areas, which may introduce atmospheric and multipath-related fading that must be added to the free-space path loss simulated in our tests. It's not unusual to see engineered fade margins of 20 dBm for high-reliability applications. Your vendor or reseller should be able to help you with this, and several of the vendors whose products we review here include technical information and range calculation utilities on their Web sites.

Of course, in the real world, we also have to deal with government regulations, which vary somewhat by jurisdiction. In the United States, the FCC imposes a number of restrictions on unlicensed radio operations, and vendors must gain FCC certification of compliance in order to sell their products. Key regulations involve acceptable wave forms, radio output power and EIRP (effective isotropic radiated power), which is a combination of radio output power and antenna gain. Thus, all else being equal, receiver sensitivity--a radio's ability to separate a very weak signal from lots of noise--is the most important technical design element. This is where RF engineers earn their salaries.