Fiber optic signal distribution for phased array antennas

Abstract

Phased array antennas offer the attractiveness of electronic beamsteering and spatial combining of solid state amplifiers, and have been successfully implemented by TRW at S band for NASA's TDRSS spacecraft, for example. However, phased array antennas have yet to be deployed for space applications at higher frequencies such as 32 or 60 GHz. The technologies of Monolithic Microwave Integrated Circuits (MMIC) and fiber optic signal distribution to the array elements may substantially increase the feasibility and cost effectiveness of high frequency phased arrays for space application. Fiber optics can potentially provide significant benefits over RF cable such as reduced number of interconnects, smaller connectors, reduced cable weight, closer element spacing, flexible cable and EM1 resistance. However, some architectural and implementation issues arise to accommodate the fiber optic distribution system, e.g. the antenna configuration (direct radiating or with a reflector); frequency limits of the fiber optic components; developing compatible processes for monolithic integration of optical, MMIC and digital components; and phase noise / signal-to-noise ratio. An application was examined using a 32 GHz phased array transmitting antenna for a Mars Relay Satellite in the framework of the NASA Space Exploration Initiative. The application had a very demanding EIRP requirement of 86 dBW, e.g. a 5 meter antenna and a 200W transmitter. A phased array transmitter could potentially eliminate the weight, power and reliability issues associated with a TWTA and a mechanical gimbal, as well as provide a transmitter with graceful degradation. This paper examines the above architectural and implementation issues within the context of the Mars Relay Satellite transmitter.