GB2205996A - Microwave lens and array antenna - Google Patents

Abstract

A microwave lens (20) may be used to steer the beam of an array antenna (5) having radiating elements connected by feed lines (9) to array ports (3) of a microwave lens. In order to provide a continuously variable range of steering angles, the microwave lens (20) is provided with a single beam port (11) that is movable along shaped tracks (10, 12) to any position along the beam contour, so producing a continuously variable phase taper at the array ports (3).

Description

MICROWAVE LENS AND ARRAY ANTENNA This invention relates generally to microwave lenses, and more particularly, but not exclusively to microwave lenses for use with array antennas.

Although the invention is not limited to any particular application, it has been conceived in the context of providing a receiving antenna for a DBS (direct broadcasting by satellite) receiver. At present, parabolic dish assemblies are used almost exclusively for this purpose. Such assemblies have a number of disadvantages. Principally, they are large in all dimensions and are unattractive; their proliferation in residential areas will seriously degrade the environment. In addition the construction, in which a feed-horn is supported on struts is inherently fragile.

A flat plate antenna system has been proposed as an alternative to the parabolic dish. A flat plate antenna comprises two basic elements: the radiating structure and the feed structure.

A radome may be used to give physical protection to the antenna and may also provide some mechanical rigidity and stability. The radome may also incorporate a polariser which converts circularly polarised signals to linearly polarised ones. The advantage of the polariser is that it simplifies design of the radiating structure, linearly polarised structures being easier to design than circularly polarised structures. The radome may be made from a thin sheet of printed circuit board material such as Kapton. As this is mounted flat on a wall it can easily be supported against the external forces placed on it.

The radiating structure is designed to satisfy a suitable specification of related gain and pattern, in other words, the specification must satisfy one of the templates laid down by international standards. The specification must be met even when the main beam is steered away from the broadside of the structure (that is, away from a direction normal to the flat plate plane).

The feed structure gathers the energy of each element to provide a single output. Obviously, it should have a low loss and this is achieved by an enclosed structure preventing radiation.

It is essential that the feed structure allows the main beam to be steered over a range of angles. Beam steering allows an antenna to be mounted facing one direction and electronically steered to receive radiation from another direction. Such an ability allows the antenna to be mounted discreetly, so minimising environmental impact.

Beam steering can be achieved by a particular combination of the signals from each element of the radiating structure. The direction of maximum directivity can be chosen by adjusting the phase of the signals before addition so that radiation from a particular direction adds up in phase. The beam is therefore steered by changing the phase of the signals from the individual elements before adding them together.

The main beam may be orientated at a fixed angle to broadside (the squint angle) by using unequal lengths of feed line to join each element to a common feed point to produce the phase shift. If the time delay produced by the extra length of line does not vary with frequency then the angle of squint remains constant.

It is relatively easy to steer a beam in one dimension but steering a beam in two dimensions (azimuth and elevation) requires a complex and expensive feed structure.

Steering a beam by phasing a conventional flat-plate array moreover requires the use of active components.

Flat plate or microwave lenses have been proposed for variably phase shifting signals. A microwave lens has a plurality of first and second ports on respective sides of the lens and separated by a lens cavity. The ports are arranged along respective contours.

A signal fed through one of the first lens ports produces an output across a number of the second ports, the output having a phase and amplitude distribution that is dependent on the position of the first port on its respective contour. Alternatively, as with any lens, operation is reciprocal and a signal with a given phase and amplitude distribution can be fed into the second ports to provide a signal at one of the first ports.

As the present invention is primarily described with relation to flat plate array antennas, it is convenient to refer to the second ports as array ports as they may be coupled through feed lines to respective columns of radiators of the array. The first ports are then referred to as beam ports, as each such port corresponds to a different direction of the antenna beam. Although microwave lenses are cheap and elegant in that they require no active components, they suffer from the disadvantage that multiple reflections tend to degrade the purity of focusing. Moreover even though a number of outputs, can be produced, each receiving energy from a particular direction, construction of a truly variable phase distribution is considerably more difficult.

Our co-pending British Patent application no. 8711270 discloses a flat plate antenna composed of radiating elements arranged in columns. A microwave lens has a plurality of array ports coupled to the columns of elements respectively and a plurality of beam ports. Selection of the beam port to be used in feeding the antenna affects coarse selection of the squint angle of the antenna.

The present invention aims to alleviate the problems mentioned in connection with microwave lenses and accordingly provides a microwave lens as defined in the claims.

The invention also provides an array antenna having a microwave lens, as defined in the claims.

A better understanding of the invention and its advantages will be gained from the following description of a preferred embodiment thereof given with reference to the accompanying drawings, wherein: Figure 1 shows an antenna array fed by a known microwave lens; Figures 2 and 3 show diagrammatically the position of beam and array ports for the Ruze type of microwave lens; Figure 4 is a diagrammatic representation of a Rotman lens; and Figure 5 is a similar representation to Figures 2 to 4 of a lens embodying the invention.

To aid understanding of the invention, two conventional microwave lenses will be described with reference to Figures 1 to 4.

A microwave lens (20) is a device wherein a signal fed into one of several first or beam ports 1 (Figure 1) produces a phase and amplitude distribution across a set of second ports 3. The phase taper may be used to excite an array of antenna elements 5 to produce a beam in a particular direction dependent on the position of the selected beam port on the beam contour 6. The beam ports 1 transmit or receive the signals for each beam direction. Focusing occurs in the lens cavity region 7 and the array ports connect to individual array elements of the antenna 5 through coaxial lines 9.

The array ports are arranged on an array contour 8.

The lens may be conveniently made using a triplate structure.

Alternatives include micros trip or parallel plates with either probes or flared waveguides as lens ports. A triplate structure may be etched on to a copper-on-Kapton sheet sandwiched between two layers of foam and two ground planes. Triplate launchers form the interface between the coaxial lines 9 and the triplate lines inside the lens. The beam and array ports consist of transmission lines that taper from the width corresponding to the impedance of the coaxial lines to the required aperture of the port. The taper may be exponential or linear so long as the change in impedance is gradual along the port. For a straight taper, a 12" flare angle has proved to be acceptable.The lens cavity 7 has extra copper area 7a between the contours to carry spillover energy into an absorber so that it does not reflect back into the lens and cause phase errors. Microwave absorbing material can be used to absorb any energy that is not incident on the array port contour.

If, in a flat plate antenna, the feedlines to each element are of variable length, the antenna beam will be squinted or slewed.

Feeding each antenna element with a relative phase shift has the same effect. The degree of phase taper determines the squint or slew angle of the beam.

The microwave lens can be thought of as replacing lines of varying length to each antenna element. However, the lens provides the correct line length from all the inputs to each antenna array element. The result of this is that it can form as many beams as there are beam ports. Thus in the example shown in Figure 1, three beams can be formed. The beam directions are stationary with frequency, as the microwave lens uses differences in path length to produce the phase taper that deflects the beam.

The beam ports are arranged on the beam contour 6, which is the focal arc of the lens 20 and the lens contours are lines drawn through the centres of the beam and array ports and are specified by the path lengths required to provide the phase taper to squint the beam.

In an ideal situation, a signal fed into the lens through one particular beam port will produce a set of path lengths to the array elements through the array ports, which will excite the array to produce a beam in a particular direction. For a linear array, a linear phase variation is required across the elements to squint the beam. The input ports are ideally located at perfect focus points FP (Figures 2, 3), that is points on the input contour that provide exactly linear phase variations across the antenna array. There are a number of lens designs that have been studied that have two or three perfect focal points. The lens shown in Figures 1 to 3 has two such points and is known as a Ruze lens.A beam port that is not at a perfect focus will have path length errors to the array so that the phase taper will not be linear and ensuing phase aberrations may limit performance of the lens. Phase and amplitude aberrations generally increase the level of side lobes that appear in the beam from the array.

The Ruze lens is illustrated schematically in Figures 2 and 3. The lens has an elliptical array port contour 8 and the beam port contour 6 is a circular arc through the two focal points which are symmetrically positioned around the central axis. The simplest lens of this type has the beam port contour centred on the midpoint of the array port contour (Figures 1 and 2) although this is not usually the case. An f parameter is introduced to describe the arrangement, f being equal to F/G where F length to the focal point G = length between contours as shown in Figure 3.

From Figure 3 it will be apparent that f = cos a, where a is the angle between lines F and G.

One feature of the Ruze lens is that the lengths of each of the coaxial lines leading from the array ports to the array elements of the antenna is equal.

Lenses with smaller phase aberrations than those obtained with the Ruze lens can be made and one example of this is the Rotman lens.

The Rotman lens has three focal points, the third lying on the central axis.

Whereas the Ruze lens has an elliptical array port contour, a circular beam port contour and equal line lengths from the output contour to the array, the Rotman lens allows the line lengths from the array ports to the array elements to vary. This achieves the on-axis focal point. A diagrammatic example of a Rotman lens is shown in Figure 4. The ratio of the on-axis focal length to the off-axis focal length is C/F = g which can be altered to vary the shape of the lens contours. As g is increased the array port contour (the inner contour) becomes less curved, and the beam port contour more curved.

A value for g may be obtained by phase error analysis to give minimum phase aberrations for any point on the beam port contour. A value of g = 1/cos a gives a reasonable approximation to this minimum.

Where a = 270, g = 1.111.

Typically the width of the individual array ports is half a wavelength, that is, in the order of 1.25 cm for a 12 CHz signal.

The overriding problem with using microwave lenses such as Ruze and Rotman lenses with multiple beam ports is that they have a finite set of beam directions each providing the antenna with a different angle of reception. The separation angle is a design parameter which, for a DBS lens may be about 5 . For the antenna to be pointed accurately in any direction in order to pick up a broadcast, some sort of mechanical adjustment of the antenna is necessary in order to effect fine adjustment of the squint angle, coarse adjustment only being effected by selection of the beam port 1. This adjustment may be provided by small adjusting screws.

The present invention proposes to modify the structure of the microwave lenses previously described, so that, rather than having a number of fixed beam ports, a single beam port 11 (Figure 5) is used, that is similar in design to conventional ports, but mechanically adjustable so that it may be moved along the beam port contour to provide beam steering over a continuous range of angles.

This is because the phase taper across the array ports changes with movement of the beam port 11 and thus changes the slew angle of the antenna beam.

The beam slew obtainable with the antenna defines a twodimensional arc. In order to obtain beam steering in three dimensions it is therefore necessary for the whole antenna and feed structure to be rotatable about the central axis of the antenna, (shown by a dashed line in Fig. 5). By this means the antenna beam can be pointed in any direction within a cone of space centred on the dashed line in Fig.5 and bounded by the maximum slew angle of the lens.

There are two requirements that a beam port for such an antennae must conform to. Firstly, its movement must be confined to the conventional beam port contour 6. If this were not adhered to the energy from the array ports would have incorrect phase delays at the beam port. Secondly, the beam port 11 should be pointed in the direction of the centre of the array port, or close to this direction. This centre is equivalent to point 0 in Figure 4.

The earlier description of the fixed beam port was given with reference to a triplate stripline structure. It will be appreciated that other constructions such as microstrip or parallel plate may be used. With microstrip or trip late structures it is essential that there is a close juxtaposition of the conductors of the lens and the moving port. Furthermore, there must be no perturbation in the dielectric across the lens/port boundary. Any perturbation could produce focusing errors and vary the phase taper. In the case of a parallel plate lens, a flared waveguide beam port could be used.

The beam port 11 is guided around the beam contour by a guide device. This device may comprise a pair of tracks 10, 12 into which locate a pair of pegs or bearings formed on the beam port. As the beam contour is a circular arc, it will be appreciated that the tracks should be shaped so as to ensure that movement of the port is confined to the beam contour and that the port is pointed towards the centre of the array contour. In one embodiment the peg or bearing spacing is equal to the track spacing and, by locating one of the tracks actually on the beam contour, the port can be guided around the beam contour and correctly positioned to point at the centre of the array port contour.

A final point that must be observed is that energy not picked up by the beam port must be absorbed in some way to prevent multiple reflections occurring in the lens cavity. For example, this could be done by using dummy ports, each having a load on, or microwave absorbing material could be placed around the edge of the lens.

The lens arrangement has been described with reference to beam steering for a flat plate antenna. However, although this is one facet of the invention, the modified lens may find application in many other fields such as radar where considerable use is made of phase shifters.

Claims (15)

CLAIMS:

1. A microwave lens, comprising a lens cavity, first and second contours, the second contour having a plurality of second ports arranged thereon and the first contour having a first port movable to any position along the first contour, whereby a signal introduced into the lens through the first port produces a phase taper at the second ports whose magnitude is dependent on the position of the first port on the first contour.

2. A microwave lens according to claim 1, wherein the first port is arranged to point substantially towards the centre of the second contour.

3. A microwave lens according to claim 1 or 2, wherein the first port is guided along the first contour by a guide means.

4. A microwave lens according to claim 3, wherein the guide means comprises a pair of tracks and the first port has a pair of locating elements each of which follow a respective track.

5. A microwave lens according to claim 4, wherein one of the tracks is coincident with the first contour.

6. A microwave lens according to any one of claims 1 to 5, wherein the lens is of micros trip construction.

7. A microwave lens according to any one of claims 1 to 5, wherein the lens is of triplate construction.

8. A microwave lens according to any one of claims 1 to 5, wherein the lens is of parallel plate construction.

9. An array antenna comprising a plurality of radiating elements and a feed structure, the feed structure including a microwave lens according to any one of claims 1 to 8, each second port being connected to a respective radiation element.

10. An array antenna according to claim 9, wherein the radiating elements form a two-dimensional array arranged in columns of elements, and the second ports of the microwave lens are connected to the columns respectively of radiating elements.

11. An array antenna according to claim 9 or 10 wherein the second ports are connected to respective radiating elements by feed lines of different length.

12. An array structure according to claim 9 or 10 wherein the second ports are connected to respective radiating elements by feed lines, the length of an individual feed line with respect to the length of other feed lines being dependent on the position of the second port on the second contour.

13. A method of aiming an array antenna according to any of claims 9 to 12 at a predetermined point in space, the method comprising the steps of adjusting the position of the first port to select the squint angle and rotating the antenna within its own plane.

14. A microwave lens substantially as herein described with reference to Figure 5 of the accompanying drawings.

15. An array antenna substantially as herein described with reference to Figure 5 of the accompanying drawings.