that manufactures several tens of millions of these tube per year to improve their efficiency. Rather, the effort has been devoted to reduce their cost. Ironically, the corrections to increase the operating efficiency would probably not increase the cost of the tube. It will be noted from Figures 3 and 4 that increasing the magnetic field will increase the efficiency; recently', one of the producers has increased the magnetic field by 20%, and increased the efficiency by 5%. This is a worthwhile increase in tube efficiency but the supplier concludes that the increased cost of the higher strength magnets is too much to introduce the tube to the market place. Although the microwave oven market may place some restrictions upon increasing the efficiency of the tube because of a cost increase, that cost increase is of little consequence in a power beaming application. The economies reflected in higher overall system efficiency would pay for the increased cost of the tube many times. The conclusion is that suppliers have largely defaulted on following up a good opportunity to improve the efficiency of magnetrons. Hence, the opportunity still exists to improve the efficiency and the improvement may well be coupled with the next topic, that of noise in the magnetron. Noise and Harmonic Radiation Considerations The compatibility of wireless power transmission with communication uses of the frequency spectrum is an inportant issue with respect to the SPS. An intuitive feeling that a five gigawatt microwave beam system would cause interference is to be expected. It is therefore surprising, and indeed startling, to know that if the SPS system has the same signal to noise ratio as a microwave magnetron that has been measured, there would be no interference from broadband noise generated by the magnetron outside of the 2.4 to 2.5 MHz ISM band. Broadband or White Noise Measurements During the DOE/NASA study of the SPS system, there was an opportunity to make noise measurements on the microwave magnetron with very sensitive noise measuring equipment. [2] Without any modification of the microwave oven magnetron, the noise level in a 1 Hertz band that was ten megahertz or more from the carrier was 170 db below the carrier. With an external modification of the leads to the filament of the magnetron, a noise level of 195 db below the carrier was measured. If an SPS transmitter radiating 7 gigawatts of power had an equally low level of noise, then the noise emitted in a 4 KHz bandwidth would be only one microwatt. Furthermore, unlike the carrier signal whose gain is proportional to the area of the transmitter, the noise gain from each small section of slotted waveguide array is uncorrelated with that from the other small sections so at Earth distance from the SPS satellite, the noise is spread over an area the diameter of the Earth and the microwave power density at the Earth’s surface is actually less than that in the sun’s spectrum Figure 5 shows the noise level measured in the microwave oven magnetron relative to that collected from klystrons as a function of distance from the carrier on a logarithmic scale. Table I shows that even if all the noise radiated from the narrow band slotted waveguide array module was in phase, still there would be an ample safety factor to meet the CCIR requirement for noise radiated from a satellite.
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