SHF Cassegrain antenna with electronic beam squint tracking for high data rate mobile satellite communication systems

The design of pointing and tracking systems for vehicular antennas required for a high data rate mobile military satellite communication system is a daunting task because of its demanding requirements. Not only does the antenna have to be satellite pointed accurately towards the satellite as the vehicle moves rapidly and changes orientation, but it must also do so under scintillations and fading of the satellite signal caused by shadowing and multipath effects from roadside obstacles. This problem becomes acutely more difficult for high gain, narrow beam antennas required for maintaining high data rate links with the satellite. It has been shown that hybrid open loop pointing combined with fast closed loop tracking offers the best performance in such mobile satcom systems [Bell and Naderi, 1986]. The open loop pointing is necessary during signal outages caused by blockage from obstacles or from prolonged deep fades; whereas fast, closed-loop tracking corrects for the drift in the low cost gyros, accelerometers or rate sensors used for open loop pointing. The paper describes the design and measurement of a prototype Cassegrain antenna with a PIN diode controlled, electronic beam squinting system which can been used for achieving fast, closed loop tracking of the satellite by sampling its beacon signal. The antenna has been designed for the Defense Satellite Communication System (DSCS), which operates from 7.25 to 7.75 GHz on the downlink and 7.9 to 8.4 GHz on the uplink, with a beacon frequency of 7.6 GHz.<>