POWER AMPLIFIERS (TUBE) Difficult-to-make harmonic measurements have been obtained on the magnetron with two different measurement techniques. Jet Propulsion Lab. measurements indicated -55 dB, -65 dB, and -68 dB for the 2nd, 3rd, and 4th harmonics, while the Raytheon measurements indicated -71 dB, -85 dB, and -86 dB. These levels are lower than those expected for klystrons but are still orders of magnitude above what would be acceptable without making special frequency allocations for these harmonics or making extensive use of filters which would badly compromise efficiency and mass of the SPS. To develop an experimental model of the SPS transmitting antenna architecture the micro wave oven magnetron has been combined with a ferrite circulator, a section of slotted waveguide radiator, and a control system to force the amplitude and the phase of the radiated output to follow phase and amplitude references. The amplitude control arrangement is shown in Figure 3. The amplitude reference is set and the amplitude of the output is maintained to within + 4% of the reference over the voltage and current operating range of the magnetron directional amplifier as indicated by the data of Figure 4. Figure 4 also shows how the amplitude control feature can be used to accommodate the tube to large variations in the characteristics of the solar cell array. In this context the amplitude control feature could replace much of the complex power conditioning associated with changing from one DC voltage to another at high power levels that would otherwise be necessary. Similarly, the phase of the radiated power as measured by a probe placed in front of the slotted waveguide radiator is controlled to within + 1 degree of the reference over the operating range of the magnetron directional amplifier. The amplitude and phase control has been achieved with solid state circuitry. The mass and cost of these devices is acceptable to the SPS but special arrangements must be made to keep them at an ambient temperature below 125°C by mounting them on the slotted waveguide radiator and using it as a heat sink as necessary. Thermal separation of the waveguide radiators from the microwave generators is accomplished by a blanket of insulation. Special problems still remain in a transition from the experimental system just reviewed and application to the SPS. The ferrite materials in the circulator are not suitable for high temperature operation in space. A ''Magic T” arrangement is an alternative but a design in which phase and amplitude control are maintained without placing solid state sensors in a high temperature environment has not been experimentally verified. Similarly the motor driven coaxial phase shifter which was used to correct for phase shift through the tube to maintain the reference phase at the output is probably not acceptable for space use. Long life is an important requirement imposed upon the generator in the SPS. Magnetrons that are expressly designed for the SPS are expected to have a very long life. Such expectations are supported by optical measurements of low cathode operating temperature in the microwave oven magnetron. At the 400 watt microwave output level these temperatures are sufficiently low to indicate lifetimes of tens of years. In scaling to the SPS requirements, tube designs with potential lifetimes of fifty or more years can be expected.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==