Many fixed-link wirless communication systems require the accurance pointing of high-gain antennas. In a mobile wireless situation the ability to perform this pointing action automatically using adaptive antenna techniques has been previously demonstrated and has lead to new possibilities for frequency reuse and increased traffic capacity for a given bandwidth usage. A self-phased, or retrodirective, array-performs steering action automatically by virtue of the array architecture used. In a retrodirective antenna array each array element is independently phased; this phasing is derived directly from the signals each array element receives. Thus the antenna array continuously adapts its phase response to track an incoming signal without a prior knowledge of the spatial position of this incoming signal and, in general, the self-phasing behaviour of the antenna array can compensate for inhomogeneities in thepropagation characteristics of the envronment through which the signal has to travel. In principle a self-phasing antenna array can acquirean incoming signal in a very short time period, automatically compensate for propagation path variation and antenna array misalignment and return the incoming signal back in thedirection of the incoming wavefront.

The basic principle on which a retrodirective antenna array operates relies on the self conjugation of an incoming wavefront, i.e. the phase of the signal retransmitted from each element in the array is advanced as much as the phase received by that element was delayed and vice versa.

A well known example of a passive retrodirective antenna, used mostly in the marine environment, is the dihedral corner reflector. Here its purpose is to enhance the radar cross-section of the boat on which it is mounted in order to enhance radar visibility to other shipping.

CORNER REFLECTOR

Fig. 1 shows a retrodirective corner reflector. This reflector consists of two flat conducting plates set at 900 to each other. An incident signal arriving at an angle 0 with respect to plate 'Y' will be reflected back at its Snell angle as shown in Fig.1. The reflected signal from the plate 'Y' will arrive at an angle 900-0 with respectto the plate 'X' and will again be reflected at its Snell angle. Here the angle of this reflected signal with respect to axis a-a', which is parallel to plate 'Y', is the same as the angle of the incident wave with respect to plate 'Y'. This indicates that the outgoing signal is returned in parallel to be incoming signal, i.e. the incoming wave returns back in the derection from which it came. A signalariving at plate 'X' will be reflected back from plate 'Y' in a similar manner. Other configurations, such as the triangular trihedral reflector, exist which can provide three-dimensional retrodirective responses as compared to the two-dimensional coverage of a simple dihedral corne reflector.

VAN ATTA ARRAY

In 1959 L.C. Van Atta patented an array variant of the corner reflector. This apparatus, known as the Van Atta array, is shown in its simples embodiment in Fig.2.

In this arrangement samples of the incoming wave front received by the antenna array elements to the right of the centre of the array are retransmitted from the mirror image element on the left side of the array. Each wavefront sample experiences the same timedelay, achieved by interconnecting the elements of each mirror image pair by a transmission line, the lengths of the transmission lines being equal.