GPS Spaceborne High Power/High Efficiency L-Band Sources

The microwave power module (MPM) obtains the best features of solid state and vacuum electronics technology by combining a TWT, a solid state driver and their required power supplies in a single package. The leader in the manufacture of the light weightMPM is Northrop Grumman, which has produced a 220 Watt, C-Band MPM that weighs only 4.2 pounds and has a volume of 48 cubic inches. For operation at the GPS L-Band frequency, existing technology may be able to provide approximately the same power andefficiency that has been demonstrated at C-Band, but the weight of the TWT would increase significantly. The innovation proposed here is to produce an L-Band MPM with that exceeds the performance requirements of the GPS application, but containing a TWTwith significantly reduced length. This would be accomplished by replacing the conventional thermionic cathode in the electron gun with a modulated field emission cathode. By launching an electron beam that is already modulated at the cathode, the lengthof the slow wave circuit can be significantly reduced. Furthermore, by eliminating the heater of the thermionic cathode, the TWT efficiency will be increased The primary cost factors in communications satellite payloads are the mass and efficiency of thepower amplifiers. Typically spacecraft have a limited availability of power, and typically the power amplifiers are the largest consumer of power on board. For a commercial satellite, a seemingly small increase in efficiency may be worth millions ofdollars to the system operator if it allows the spacecraft to carry even one additional revenue generating transmitter. Improvements in efficiency can translate directly into reductions in the mass of solar cells and thermal control systems. The massand efficiency of the power amplifiers control the size of the spacecraft, and, therefore, the choice of launch vehicle.