GB2184607A - Microwave beamforming lens - Google Patents

Abstract

A microwave beam forming device which comprises a set of input ports 2 coupled to a set of output ports 3 via a planar microwave transmission path 1, in which the transmission path includes a fold 9 located so as to produce a predetermined curvature of a phase front across the path, the curvature serving in conjunction with the lengths of the individual beam paths within the device to produce a phase front having a modified curvature at the said output ports. A second fold may be added, and either set of ports may be replaced by a single linear aperture. The transmission path may be waveguide or stripline. <IMAGE>

Description

SPECIFICATION Microwave beamforming lens This invention relates to a microwave beamforming lens. It relates particularly to a beamforming lens comprising a number of input ports coupled to several output ports via a planar microwave transmission path such as a parallel plate waveguide.

One example of such a device is a Rotman beam forming lens which consists of a parallel plate wave guide structure with a curved array of input ports connected typically to the elements of an antenna array, and a curved array of output ports corresponding to different beam directions in space.

The resulting lens structure acts to combine radio frequency signals from elements of a linear antenna array so that their signals arriving from different directions in the plane of the array are focussed into different output ports of the lens. This is the arrangement in the case of a microwave receiver and, of course, in the case of a microwave transmitter, the respective signal flows would be reversed.

When a waveguide is being designed to couple to the input or output ports of a microwave communication apparatus, the necessary curvature of the array of ports causes a mechanical difficulty in the shaping of the waveguide if equal path lengths to the inputs of the lens are to be maintained. One way of simplifying the mechanical construction would be to locate one set of the ports on a straight line but such a construction would then be likely to cuase a difficulty in that the path lengths to the different ports of the set would no longer be able to be of equal length.

One object of the present invention is to provide a beamforming lens having a comparatively simple mechanical construction but in which the beam paths to the input or the output ports are able to be maintained of lengths having predetermined ratios to one another such as being of substantially equal lengths.

According to the invention, there is provided a microwave beam forming device comprising a set of input ports coupled to a set of output ports via a planar microwave transmission path, in which the transmission path includes a fold located so as to produce a predetermined curvature of a phase front across the path, the curvature serving in conjunction with the lengths of the individual beam paths within the device to produce a phase front having a modified curvature at the said output ports.

The planar microwave transmission path may be a parallel plate or a stripline waveguide structure. A Rotman lens construction may be used.

Preferably, the planar transmission path additionally includes a second fold arranged to produce a phase front having a further modified curvature at said output ports.

The curvature at the fold may be arranged such that the ports of one of said sets of ports are able to be constructed to lie on a straightline across the transmission path. Conveniently, the curvature or curvatures of the folds are arranged such that the ports of both sets of ports are able to be constructed to lie on straight lines across the transmission path.

By way of example, some embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows a parallel plate waveguide structure arranged as a Rotman lens beam former, Figure 2 shows a stripline waveguide structure formed on a copper-clad dielectric sheet, Figure 3 shows a beamforming lens in a parallel plate construction where the lens has been formed with a single fold, and, Figure 4 shows in an exploded view a beamforming lens construction where the lens includes a double fold.

As shown in the perspective view of Figure 1, one kind of lens used for beam forming has a parallel plate waveguide body 1 which, in this example, is provided with five waveguide input ports 2 and four waveguide output ports 3.

The upper surface of the body 1 has been cut away along the line 4 in order to show the junctions between the output ports 3 and the curved periphery of the body 1. At positions indicated by the wavy lines 5, the body 1 contains pieces of material intended to form an absorbing load for the radiation passing through the waveguide when in use.

The Rotman lens structure which this construction provides acts to combine radio frequency signals from the elements of a linear antenna array so that their signals arriving from different directions in the plane of the array are focussed into different output ports of the lens. The path lengths from each of the array elements into the waveguide body 1 have to be of equal length for each element so that receivers (not shown) connected to the output ports can maintain the correct relationships between the different signals reaching the array.

Similarly, the path lengths from the waveguide body 1 through the output ports 3 have to be of equal length to maintain the correct relationship.

The transmission medium in the lens of Figure 1 is formed by the material enclosed between the parallel plates of the body 1, which material is in the shape of a plane and which may be air or another dielectric.

An alternative type of beamforming lens is depicted in Figure 2. This has a stripline construction with a body 1 being formed from a suitably shaped copper conductor 6 carried on an upper surface of a dielectric sheet 7. A lower surface of the sheet 7 carries a copper ground plane 8. Input ports 2 and output ports 3 are provided by suitably shaped extension portions of the conductor 6. The transmission medium in this lens is formed by the material of the dielectric sheet 7. The wavy lines 5 indicate the positions of absorbing load material which acts to provide non-reflecting termination at the sides of the waveguide region.

In a plan view, the body 1 has an ellipticai shape with the junctions for the different sets of ports lying on the periphery of the ellipse. It will be clear that, particularly in the mechanical construction of the Figure 1 example, there is likely to be some difficulty in forming the necessary junctions along a line which has a curve in it. This difficulty becomes more significant if it is proposed to construct the lens by a mechanical machining operation.

Figure 3 shows the beam forming lens which was constructed according to the invention, where the parallel plate body 1 defining the transmission path has been formed with a fold 9. The dotted line 9a shows how the line of energy flow has been caused to take up a folded pathway within the lens. The fold 9 in a plan view has a parabola shape and this acts to focus a linear wave front associated with input ports 2 to a point focus at the corresponding output port 3. The line of output ports 3 follows the focal line of the parabola which is defined by the fold 9.

The presence of the fold 9 has allowed the input ports 2 to be located along a side of the waveguide body which falls on a straight line and this has thus allowed the construction of the waveguide to be considerably simplified.

In a different construction, depicted in the exploded view of Figure 4, the waveguide body is formed with a double fold and this has enabled both the sets of input and output ports to be located along straight lines.

The waveguide body 1 is designed to be made by the assembly of metal plates which are shaped by a numerically controllable milling operation.

The body comprises a first plate 10 which fits closely on to a second plate 11. The two plates together have an opening which defines an input port 2 and a surface which defines a first fold 9. In this embodiment, the input port 2 is a linear aperture instead of being composed of a line of individual narrower ports.

Underneath the second plate 11, a third plate 12 is fitted and beneath this a fourth plate 13. The third and fourth plates together include an opening which defines an output port 3 and a surface which defines a second fold 14. The output port 3 of this embodiment is again in the shape of a linear aperture.

For the mechanical construction of the waveguide of Figure 4, the first plate 10 and the fourth plate 13 are identical, and so also are the second plate 11 and the third plate 12. This provision ensures that the waveguide construction is able to be constructed in a way which avoids mechanical difficulties in the construction process.

The microwave beam forming device of the invention has been found in use to simplify the design of connecting transmission lines, particularly waveguides. There has been found to be a potential for high power capability in the beam forming device. One advantage in using the folded lens design is that the fold curvature can be changed at various stages in the design operation without these changes necessarily affecting the locations of the input and output ports in the construction.

The foregoing description of embodiments of the invention has been given by way of example only and a number of modifications may be made without departing from the scope of the invention as defined in the appended claims. For instance, although the two embodiments described relate to beamforming lenses having the parallel plate construction, it will be clearthatthe invention will also apply to the alternative form of lens having the stripline waveguide structure.

Whilst in some applications it might be useful to arrange equal path lengths to the ports of one set such that the ports could be arranged to lie on a straight line across the transmission path, it is clear that sometimes an alternative pattern of controlled path lengths might be required. Examples of alternative patterns include the provision of compensation if the aerial elements are required to lie on a curved surface, or if the aerial array includes digital phase shifters in which case a deliberate curvature of the phase front would help to randomise quantising effects in the phase shifters.

Claims (8)

1. A microwave beam forming device comprising a set of input ports coupled to a set of output ports via a planar microwave transmission path, in which the path includes a fold located so as to produce a predetermined curvature of a phase front across the path, the curvature serving in conjunction with the lengths of the individual beam paths within the device to produce a phase front having a modified curvature at the said output ports.

2. A beam forming device as claimed in Claim 1, in which the transmission path is a parallel plate waveguide structure.

3. A beam forming device as claimed in Claim 1, in which the transmission path is a stripline waveguide structure.

4. A beam forming device as claimed in any one of Claims 1 to 3, in which the transmission path additionally includes a second fold arranged to produce a phase front having a further modified curvature at said output ports.

5. A beam forming device as claimed in any one of Claims 1 to 4, in which the said curvature at the fold is arranged such that the ports of one of said sets of ports are able to be constructed to lie on a straight line across the transmission path.

6. A beam forming device as claimed in any one of Claims 1 to 5, in which the ports of both sets of ports are constructed to lie on straight lines across the transmission path.

7. A beam forming device as claimed in any one of Claims 1 to 6, in which the said set of ports of any one kind is replaced by a single port in the shape of a linear aperture.

8. A microwave beam forming device substantially as hereinbefore described with reference to Figure 3 or Figure 4 of the accompanying drawings.