Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q25/00—Antennas or antenna systems providing at least two radiating patterns
  • H01Q25/007—Antennas or antenna systems providing at least two radiating patterns using two or more primary active elements in the focal region of a focusing device
  • H01Q25/008—Antennas or antenna systems providing at least two radiating patterns using two or more primary active elements in the focal region of a focusing device lens fed multibeam arrays
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q21/00—Antenna arrays or systems
  • H01Q21/0006—Particular feeding systems
  • H01Q21/0031—Parallel-plate fed arrays; Lens-fed arrays
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q3/00—Arrangements for changing or varying the orientation or the shape of the directional pattern of the waves radiated from an antenna or antenna system
  • H01Q3/12—Arrangements for changing or varying the orientation or the shape of the directional pattern of the waves radiated from an antenna or antenna system using mechanical relative movement between primary active elements and secondary devices of antennas or antenna systems
  • H01Q3/14—Arrangements for changing or varying the orientation or the shape of the directional pattern of the waves radiated from an antenna or antenna system using mechanical relative movement between primary active elements and secondary devices of antennas or antenna systems for varying the relative position of primary active element and a refracting or diffracting device

Definitions

• This invention relates generally to microwave lenses, and more particularly, but not exclusively to microwave lenses for use with array antennas.
• 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.
• 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 oust 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• Figure 1 shows an antenna array fed by a known microwave lens
• FIGS 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
• Figure 5 is a similar representation to Figures 2 to 4 of a lens embodying the invention.
• 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 microstrip 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.
• 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.
• 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.
• 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.
• FP perfect focus points
• 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.
• 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.
• Rotman lens has three focal points, the third lying on the central axis.
• 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.
• a value for g may be obtained by phase error analysis to give minimum phase aberrations for any point on the beam port contour.
• the width of the individual array ports is half a wavelength, that is, in the order of 1.25 cm for a 12 GHz 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°.
• 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 two- dimensional arc.
• the whole antenna and feed structure 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).
• 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.
• 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 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.
• 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.
• 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.

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• Aerials With Secondary Devices (AREA)
• Variable-Direction Aerials And Aerial Arrays (AREA)

Applications Claiming Priority (2)

Publications (1)

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ID=10617246

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• 1987
  • 1987-05-13 GB GB878711271A patent/GB8711271D0/en active Pending
• 1988
  • 1988-05-13 JP JP50411088A patent/JPH01503429A/ja active Pending
  • 1988-05-13 EP EP19880904246 patent/EP0313623A1/de not_active Withdrawn
  • 1988-05-13 WO PCT/GB1988/000374 patent/WO1988009066A1/en not_active Application Discontinuation
  • 1988-05-13 GB GB08811407A patent/GB2205996A/en not_active Withdrawn

Non-Patent Citations (1)

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