Radios for communication, interconnect, synchronization, radar, and orientation
As a new service, server sky will likely be allocated EHF frequencies for the downlink - in the 30GHz to 300GHz range. For now, let's assume a frequency of 38GHz and a wavelength of 8 millimeters. This wavelength is smaller than the size of a server-sat, so directional beams can be made with server-sat scale antennas. Each server-sat can direct radio energy into an angle of perhaps sin-1(0.1) or 6 degrees, for a ground spot of perhaps 600km.
However, the directionality of sever-sats comes from their ability to act as a phased array. Constructive and destructive interference between phase locked arrays of server-sats permits ground spots of a few tens of meters - better than cellular service and wimax. The wider the array, the smaller the ground spot, so for downlink at least, adding server-sats will improve spatial multiplexing bandwidth, with no practical limits on download bandwidth to billions of customers on earth.
A phased array works by adjusting the time delay of each server-sat radio so that the signals from each radio, located at a different distance from the receiver, all arrive at the receiver at same time. If each transmitter is emitting a pure sine wave, this can be accomplished by shifting the phase of the outgoing signal.
However, traditional phased arrays have a problem called grating lobes. If the spacing of the transmitter nodes is wider than the wavelength of the sine wave source, then there are many ground spots and many angles that show a constructive interference maximum. These spatial lobes are called grating lobes, and resemble the off-axis lobes in an x-ray crystallography pattern. If the precise spacing of server-sats is 10 meters, there will be grating lobes at .46 degree spacings. Although the main ground spot of a large array of transmitters will be small, there will be many more than one.
||||The grating lobes near the main ground spot for a 10 meter spacing and a 8mm wavelength. The peaks shown are actually the maximum of many peaks - the lobes have a fine structure.
full size plot||
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