更好的广播

并非所有射频标识（RFID）标签都是相等的。

As a technology that depends on broadcasting, their performance is a function of the frequency that they use, among other things. Although a number of old, proprietary frequencies are still in use in some quarters, most vendors today design their tags to operate at one of four standard frequencies: 125 kilohertz (kHz), 13.56 megahertz (MHz), 928 MHz and 2.4 gigaherts (GHz). The tags receiving the most attention these days are those that operate in the ultrahigh frequency (UHF) range between 902 and 928 MHz. Although they can broadcast as far as 60 feet, if conditions are ideal, they typically work in the 15- to 30-foot range. “They allow for high baud rates, in addition to the longer distances,” says Alex Stuebler, business manager, factory automation sensors, Siemens Energy & Automation Inc., in Norcross, Ga. For this reason, Wal-Mart Stores Inc. and other major retailers are pushing this frequency for tracking their deliveries and inventories. Tags operating at this frequency have been slow to find uses in factories, however, because RF signals become more susceptible to interference from metal and water as frequency increases. “Typically, metal and fluids don’t really work well with RFID because metal reflects the signal and liquid absorbs it,” explains Stuebler. “So embedding the tag into metal limits your reading distance to a fraction of an inch—maybe up to a half inch.” Yet, this limitation seems to be becoming less important over time. Exploiting propagative coupling, a technique used in radar, Siemens has developed what it calls metal-mountable tags. “You can mount them on metal bins, drums and other containers,” says Stuebler. Although their range is still relatively limited, they withstand industrial environments much better than tags mounted on paper labels.

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