Significantly more information is available on the susceptibility of pacemakers at microwave frequencies. Generally, experiments have shown that pacemakers are safe in fields of 75 volts/meter (1.5 mw/cm2 ) or less. The only indication of failures in fields below 75 volts/meter is from a study by Georgia Institute of Technology where failures in fields as low as 40 volts/meter (0.5 mw/cm2 ) were observed at 3.1 to 3.5 GHz in 10% of a population sample. One must note, however, that the bulk of the experimental data to date on the susceptibility of pacemakers has been collected in either pulsed or amplitude modulated microwave fields. In any event, there is a growing body of evidence that suggests that pacemakers are not likely to be affected by the much lower microwave fields found outside of the immediate rectenna area. Studies of the performance of other types of medical electronic devices in interference are generally inconclusive. Current guidelines are that medical electronic devices should be able to operate in fields of 7 volts/meter (0.01 mw/cm2 ) at frequencies of 1.0 GHz or higher without undue interference. Laboratory confirmation of whether or not existing equipments meet these guidelines is not available. The study of the performance of three integrated circuits in microwave fields provided a valuable insight into the mechanisms that produce interference in these types of devices. Basically the interaction of integrated circuits and microwave fields in the GHz region is due to the rectification of microwave energy in the numerous pn junctions that are found on these devices. These rectification currents change the normal quiescent operating points of the junctions which reduces noise immunity or causes a state change. Estimates were made as to the 2.45 GHz field intensities that would produce interference in 7400 bipolar NAND gates, 4011 CMOS NAND gates, and 5474 flip flops. These estimates are based on data taken at frequencies of 910 MHz and 3.0 GHz. The conclusions are that the devices would generally not be affected by SPS fields of 0.1 mw/cm2 or below. The estimates showed that all these devices potentially could be affected in SPS field intensities of 1.0 mw/cm2 . It is interesting to compare the integrated circuit interference estimates with actual tests of an electronic calculator that uses similar integrated circuits. The electronic calculator had a measured interference threshold at 3.1 to 3.5 GHz of 0.1 mw/cm2 , which is identical to the projections for the individual circuits. All of the site security devices examined had an interference threshold of 10 -1 mw/cm or greater. This included radar intrusion detectors, microwave motion detectors, proximity detectors, security detectors, infrared detectors, and seismic detectors. None of these devices should be affected by SPS signal levels unless they are within the exclusion fence at a rectenna site.
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