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Phasing Antenna Array
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.
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.
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.
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