The beam center frequency choice of 2.45 GHz is a reasonable one from many viewpoints. Nonetheless, no international frequency assignment has been made, and this choice cannot yet be considered final. B. SATELLITE HARMONIC RADIATION Any microwave power generator can be expected to produce some power at frequencies harmonically related to the main frequency generated. For the klystron tubes of the Reference System, the fraction of output power appearing as harmonics is expected to be small, but no values are given. The radiation pattern of the transmitting phased array when driven at harmonic frequencies is very dependent on details of feed design, and again no information is given. Nonetheless, the large powers involved mean that even a fractionally small diversion of power into harmonics could be significant, and persons evaluating potential effects should attempt to determine effect thresholds at the first few integer multiples of 2.45 GHz. C. SATELLITE NOISE RADIATION At frequencies far removed from the beam center frequency, the dominant microwave radiation source will be thermal emission from the hot solar cell blanket. A 330-Kelvin blackbody radiator (case of Si cells, microwave emissivity = 1) in the Rayleigh-Jeans limit has a surface brightness of 7.3 x 10 W/m Hz Rad at 2.45 GHz. Since the solar cell blanket subtends 4.32 x 10 Rad , this surface brightness results in a noise power density at the Earth's surface of 3.16 x 10”^ W/m^ Hz, or -255 dB W/m^ Hz. This density should be multiplied by the actual microwave emissivity of the cell blanket, a number less than unity. Unless the emissivity is very small, it will not affect the qualitative conclusions below. Figure 26 of the Reference System Report shows an actively radiated noise power density equal to -255 dB W/m Hz at 70 MHz away from the beam center frequency, rising rapidly as the center frequency is approached. The central plateau region is a factor of 10 (60 dB) brighter. Since the transmitting antenna is a much smaller object than the blanket, this brightness comparison is valid only for an antenna whose main beam is at
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