Linear-sum mode arrays and beam forming

In conventional phased arrays, antenna elements are usually simple radiators, such as dipoles, slots, dominant mode horns and microstrip patches, etc., that generate broadbeam, low gain and radiation patterns with a desired polarization property. The array beam is then formed and scanned by introducing phase shifts between the elements and modifying the phase shift values. The simplicity of array elements facilitates the design and then the fabrication. Also, because the element patterns are broad and often symmetric, with respect to the array axis, the beam scanning and its associated algorithm are easy to implement. In addition, this method of array design and beam scanning is general and can be used with different array configurations and electrical performance. However, a number of difficulties are often faced in such array implementations. Since each element, or each sub-array, needs a separate phase shifter, the array complexity and cost increases with its size and gain requirements. Also, the mutual coupling and array element pattern role off decrease the array gain for excessive scan. Furthermore, assuming array design can be achieved and implemented satisfactorily, the phase shifter insertion loss introduces an additional difficulty. Normally, this insertion loss limits the array performance in G/T, i.e. the gain to noise temperature ratio, for receive systems and the power efficiency for transmit systems. With microstrip arrays this phase shifter loss is accompanied by the feed line loss to further deteriorate the array performance.