because of design improvements. At ultra high frequencies the thresholds for many pacemakers are now in the range of kilovolts per meter [Osepchuck, 92]. Also other medical electronics have to consider the interference threat. The permissible interference levels for electronic medical devices, in USA, according to MDC-E1609 [McDonnell Douglas Astronautics Company Report E-1609] are given in Figure 6.6. Other standards might be found in other countries. The given figure only gives the limit up to 1 GHz. The power beam that will be considered for a solar space power program will be in a higher frequency, but it would be possible to give a sensible extrapolation from the figure. The susceptibility of electronic devices generally decreases with increasing frequency. Interaction between microwave energy and integrated circuits can occur in various ways. An important mechanism is believed to be rectification by the various pn junctions. This rectification can inhibit or induce state changes or change the quiescent operating point of a device. An other mechanism is that leads that are connected to the device, can function as an antenna. Test of unshielded integrated circuits, 7400 Transistor transistor logic (TTL) and 4011 Complementary metal oxide semiconductor (CMOS) which have the same function, have shown CMOS to be most susceptible to microwaves at 2.45 GHz. Interference was coupled directly into input, output, and power leads of the integrated circuits [Davis & al, 1981]. Estimated interference threshold for 2.45 GHz was for 7400 14.5 mW and for 4011 6.7 mW. Small malfunctions can make changes in memory, or program errors can occur. If the interference gets much higher, even physical damage can occur. As the development goes towards more and faster transistors on the chips, die transistors gets smaller and leads thinner. Less energy is then needed to cause damage. But protection networks on the connections have become common. The coupling between a power beam and an IC is treated in a report [Ditton, 1975] which examines the coupling of microwaves into shielded and unshielded wires connected to ICs. The development also goes towards better shielding, as more and more electronic devises are introduced to our lives, and the shielding is also for reducing radiation out of the device. Figure 6.6 USA Standard for Permissible Interference Levels for Electronic Medical Devises The only place we expect the microwave beam to interfere with commercial digital circuits is within the main lobe of the power beam. Equipment that is needed for pointing the beam could be given special shielding, so the problem is if something by accident should come into the beam. If we assume that the beam direction is stable flying objects, like aircraft's are our greatest concern. The metal skin of aircraft's will function as a shield against electromagnetic waves. Some noise might, however, come in through apertures like windows. Microwaves that hit the aircraft, but do not penetrate it, will be reflected, and can cause interference other places. For these reasons air traffic should be forbidden in the beam, and a suitable security zone around. Facilities that can be very sensitive to radio noise are radio astronomy installations. Studies of faint and distant objects would be very difficult with harmful interference. Definitions of harmful
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