The intersection with the A, line determines the area of the solar array at about 50 km2 read, as shown by the arrow, on the left side of the figure. The power per module line, labelled Pm, intersects the 5000 MW line above the figure. However, due to the cyclic nature of the corrdinates, it may be read from the 500 MW system line by multiplying abscissa by 10 and ordinate by 100. The power per unit module is seen to be close to 400 W. Finally, the substrate temperature is seen to be somewhat over 150°C. Perhaps, the most significant feature shown by this figure is the inverse relationship between the transmitter and receiver antenna diameters which is imposed by the Goubau condition of the constancy of their product. High power designs require larger receiving antennas in order to keep the received power density constant to avoid ionospheric heating effects. This requires smaller transmitting antennas to keep the product DrDt constant and, consequently, higher power densities. The klystron satisfies the power density requirement but requires liquid cooling for its collector and a magnetic field for electron beam focussing. In addition, a high voltage (40 kV) is required for beam acceleration and a heater supply must be provided for the cathode. There is also some question about the possibility of the klystrons meeting the desired 30-year life requirement. To circumvent some of these drawbacks, consideration has been given to the replacement of the klystron by solid state devices. Inherently, solid state devices have long life and require neither high voltage nor heater power. Of particular interest for purposes of solid state transmitter design is the power per module. For convenience, a module is taken as occupying an area of one wavelength by one wavelength. The quantity shown in Figure 1 as Pm is the peak module power
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