or in power conditioning related to it was a prime consideration. As Fig. 2 indicates the heat must be radiated into space on the back side of the transmitter, while the front side of the transmitter is forming the microwave beam and serving as a mounting for many solid-state devices operating at a sufficiently low temperature level to guarantee their long-term survival. A difference of 200° Kelvin between the front and back surfaces makes a difference of a factor of 5.5 in their ability to radiate heat. Fig. 2. The SPS transmitting antenna, which is approximately a kilometre in diameter has a high temperature back side to radiate heat to space. The front is insulated from back side to allow low-level solid-state amplifiers and solid-state control circuits to be mounted and cooled on the front side. Very high efficiency is an important factor in reducing the amount of heat generated for a given power output. Fig. 3 shows the amount of microwave power that can be radiated per square metre as a function of the efficiency and operating temperature of the generator. Solid-state devices are distinctly inferior to electron tubes in their ability to operate at high temperatures and thus to dispose of their heat as efficiently. Table I. Requirements imposed upon the micro wave generator (amplifier) for the satellite transmitter.
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