System Aspects of Active Phased Arrays

Abstract

Demands in satellite communication systems for large number of spot beams with high beam-to-beam isolation and the need for flexibility in steering the beams impose requirements that are easier met with active phased arrays. The advances in MMIC and digital technologies played a major role in realizing these arrays for such systems. However, the added requirements and active array characteristics introduce a set of transmission impairments in the satellite link that did not necessarily exist in the conventional reflectorbased satellite systems. The design of the antennas for multiple beam communication systems depends on the beam definition, which in turn is a function of the system capacity and projected traffic patterns. Several systems require a large number of fixed narrow spot beams to cover the service area. A single reflector with a large number of feeds may provide such coverage. However, if the scanning loss is excessive due to the large number of beams scanned in one direction, multiple reflectors may be required. Alternatively, single or multiple phased arrays may be used. The phased array may have lower scan loss and a single phased array can handle a large number of beams. The phased array solution also offers higher reliability due to the use of the beam former versus the switching arrangement in a focal-region-fed reflector antenna. For a small number of beams at a time, a microwave beam former represents a simple and attractive solution, while if the number of beams is large a digital beam former is a more viable alternative. In many applications the choice between the microwave and the digital beam formers becomes a system issue. Active phased arrays can also meet high flexibility requirements that have become the feature of several satellite communications systems. Among the flexibility requirements are the ability to form multiple beams, provide power sharing among beams through distributed amplification, and rapidly reconfigure and/or repoint the beams. The