The growing demand for modern communication systems has increased the complexity
of systems for communication and surveillance. The air interfacing is a major part of
the communication systems which needs efficient and compact antenna systems.
For radar and communication purposes, especially mobile and cellular communication, it is
required to generate multiple beams using an antenna array. To have multiple
beams, a multiple beam forming network is required to control the amplitude and phase at each element
of the antenna array. Several beam forming
technologies, such as Radio Frequency (RF), Intermediate Frequency (IF), digital and optical beam
forming, are available to meet this need. The RF or microwave beam
forming has advantages over other multiple beam
forming networks due to their simplicity and good capability for required applications. The
bootlace lens is one of the microwave lenses used for multiple beam
forming. Wide-angle scanning capabilities of Rotman lens
[1, 2] are well-established [3-6]. Design approach of the
Rotman lens is based on geometrical ray optics tracing technique. The
Rotman, which was obtained by conventional design approach
[1], has unequal height of array contour and feed
contour. Feed contour and array contour are expected to have equal heights, as far as
possible, to couple the maximum power from the feed contour to array contour. A design approach
in [7] is reported to equalize the height of array and feed contour to achieve better
performance. Electronic scanning antennas have numerous applications in communication and
collision avoidance system. An equal height circular lens with tapered line section [7] is
an attractive choice for wide-angle coverage because of its simple design and compact size. The
multiple detector or sources mounted on the focal arc of the lens provide a convenient way
for either the detection of spatially separated multiple targets or the generation of
multiple beams. In this paper, equal height circular lens with tapered line section has been
designed in microstrip configuration, fabricated and
tested. The tested design is an optimized design of
the idea proposed in [7].
Figure 1 shows the cross section of a trifocal Bootlace microwave lens. One focal point F0 is located on the central axis and the two others F1 and F2 are symmetrically located on
either side of a circular focal arc (also called feed contour). Contour I2 is a straight line and
defines the position of the radiating elements. I1 is the inner contour of the lens (also called
the array contour). Two off axis focal points, F1 and F2, are located on the focal arc at angles
+β and -β. It is required that the lens be designed in such a way that outgoing beams
make angles -α, 0 and +α with the x axis when feeds is placed at F1, F0 and F2 respectively. It may be noted that in this proposed approach the two off axis focal points are located at
angles ±β, whereas in the approach suggested in [4] these were located at angles
±α. The array contour I1 is defined by coordinates
(X, Y). The position of the radiating elements on
the straight-line I2 is determined by
a single coordinate N, measured relative to point O1. Point's O1 and O lie on the contours I2 and I1 respectively and connected by
transmission line of length W(0). A general point P(X, Y) on the array contour is connected to
element Q(N), which lies on I2, by the transmission line of length W(N).
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