MICROWAVE PLANAR ANTENNA

Abstract: A planar microwave antenna comprises two identical probe arrays superimposed one over the other. Each probe array comprises a sheet of insulating material provided with an array of pick-up probes (P) all aligned along a common axis: one probe array is turned through 90 DEG relative to the other probe array such that the probes of one array are oriented orthogonally to the probes of the other array.

Description: Microwave Planar Antenna This invention relates to a microwave antenna of planar construction. Various constructions of microwave antenna are known, which are able to receive satellite television transmissions of each of two orthogonal polarisations. We have now devised a microwave antenna of simple construction but which is effective in operation. In accordance with this invention, there is provided a planar microwave antenna which comprises two identical probe arrays, each probe array comprising a sheet of insulating material provided with an array of pick-up probes all aligned along a common axis, one probe array being superimposed over but turned through 90° relative to the other probe array such that the probes of one array are oriented orthogonally to the probes of the other array. In an embodiment of the invention which will be described herein, each probe array comprises a plurality of pairs of pick-up probes, the two probes of each pair being oppositely-directed and connected to a track node by paths differing in length by λ/2 (a half wavelength) . All such track nodes are connected by equal lengths of track to an output probe (or to respective ones of two oppositely-directed output probes) . The pattern of pick-up probes in each probe array is arranged in repeating blocks, wherein adjacent blocks are turned through 90° relative to each other. In each block of the pattern, there are a plurality of rows of pick-up probes, the probes of each row being oriented in a common direction. In each column of the block, however, the probes alternate their directions. It will be appreciated that because the two probe arrays of the antenna are identical and of simple construction, the antenna is relatively inexpensive and admits of mass- production. An embodiment of this invention will now be described by way of example only and with reference to the accompanying drawings, in which: FIGURE 1 is a plan view of one probe array of a planar microwave antenna in accordance with this invention; FIGURE 2 is a diagrammatic plan view of part of the probe array of Figure 1; FIGURE 3 is an enlarged view of a portion of the probe array of Figure 1; FIGURE 4 is a similar view to Figure 3, showing the relative positions of the pick-up probes of the two probe arrays of the antenna when superimposed one-over-the-other; and FIGURE 5 is an enlarged section through an antenna in accordance with this invention. A planar microwave antenna in accordance with this invention comprises two identical probe arrays one of which is shown in Figure 1. Each such array comprises a flexible sheet of low loss insulating material, with conductor tracks formed thereon, as shown. The two identical probe arrays are superimposed one-over-the-other, with one turned through 90° relative to the other, and the two arrays are disposed behind front and rear metal plates (forming ground planes) , each formed with a 24 x 24 array of circular apertures: the positions of some of these apertures in the metal plates are indicated at 10 in Figure 1. It will be seen that each probe array has a multiplicity of pick-up probes P projecting into circular areas which register with respective circular apertures in the ground planes: in each probe array there is one such probe P for each aperture 10, and all probes in that array are parallel to each other, although some are directed oppositely to the others. The pattern of probes and tracks on each probe array comprises a repeating block 20, as will be explained below with reference to Figure 2. As shown on enlarged scale in Figure 3, along a row of each block 20 of the array, the pick-up probes P alternate in direction, whereas along the perpendicular axis (i.e. along each column), the probes P are correspondingly directed. Each adjacent pair of oppositely- directed probes, e.g. probes P0, PI, are connected to a track node N by different lengths of track: the distance of probe P0 from node N is greater by λ/2 (a half wavelength) than the distance of probe PI to the corresponding node N. Thus although probes PO, PI are oppositely directed and therefore the signals induced therein are 180° out of phase, the λ/2 difference in path length to their common node N brings the two signals into phase at that node. A track extends from each node N to one or other of two oppositely-directed probes A, B at the centre of the probe array: probes A, B are parallel to all of the probes P of the array, and couple to an output waveguide. The length of track, from each node N to the respective probe A or B, is equal for every node N. Referring to Figure 1 and 2, it can be seen that there is a block 20 which is repeated around the edge of the probe array, but turned through 90° from one position to the next (e.g. from position 20 to 22) : however, in turning the block through 90° from one position to the next, the corresponding probes are reversed in direction. Each block e.g. 20 comprises an H-shaped track e.g. 21 repeated 2 x 2, the four points of each H forming respective nodes N. The cross-members of two adjacent H's are joined by a perpendicular track e.g. 23, and the mid-points of such tracks 23 are joined by another track e.g. 24 the centre of which is joined by an L-shaped track 25 to a junction J (about which block 20 is turned through its 90° into form block 22) . It will be seen that there are 8 such blocks repeated around the periphery of the array. The corners of the antenna are rounded, and the circular apertures of the ground planes are omitted from the corners: the pattern of probes is accordingly modified in each of the four corners of each probe array. A similar block can be identified in the centre of the probe array, comprising 4 H-shapes with the cross-members of two pairs of adjacent H's joined, but then the centres of these joining tracks are connected to the elbow of respective re¬ shaped tracks 25 by tracks which include folded-back portions 26, thus extending these tracks to provide the same track length, from the individual probes P of this block to the output probes A, B, as for all other probes P of the array. Again, the centre block is the same as each of its neighbouring blocks, but turned 90° relative thereto and having its probes P also turned at the same time. It will be noted that two pairs of the junctions J are interconnected by respective straight tracks 27, the centres of which are joined by tracks 28 to the respective output probes A,B. Referring to Figure 4, it will be seen how the probes P, P 1 of the two superimposed probe arrays are oriented relative to each other. Probes P of one array lie at 90° to probes P 1 of the other array. The nodes N of one array are superimposed over the corresponding nodes of the other array. It will be appreciated that the probes P of one probe array pick up one of the two orthogonal polarisations in the received satellite transmission, whilst the probes P 1 of the other array pick up the other of the two polarisations: these signals are then fed to the output probes at the centres of the respective arrays, the output probes of one array being orthogonal to the output probes of the other array. Figure 5 shows the construction of the antenna in detail, including the two probe arrays 10,12 and two metal plates 14,16 formed with the apertures. Thus, the antenna comprises a metal back plate 1, then the first probe array 10 sandwiched between foam plastics layers 2,3 acting as spacers, then the aperture plate 14 followed by the probe array 12 sandwiched between foam plastics layers 4,5 acting as spacers, then the second aperture plate 16 and a foam plastics spacer layer 6 and finally a front cover layer 7 of plastics. The probe arrays 10,12 are arranged with their conductor tracks on •their front-facing sides. Each probe array 10,12 may be made from a metal/plastics laminate sheet from which the unwanted metal is then etched away. For example the laminate may comprise ED copper (Electrodeposited copper) bonded by adhesive to a plastics (e.g. polyester) sheet: ED copper has one side smoother than the other and preferably it is adhered by its smoother side to the plastics substrate. Alternatively rolled copper is adhered to the plastics laminate, or rolled aluminium bonded to e.g. polyester may be used. Preferably, and as shown in Figure 1, much of the metal between tracks is left in place and acts as a ground plane: the main advantage of leaving as much as possible of the metal in place is that the probe array then remains flatter during processing and manufacture. Instead of etching unwanted metal from a metal/dielectric laminate, the conductor tracks may be printed or otherwise deposited onto a dielectric sheet: for example the pattern of conductor tracks may be printed using a conductive ink and this may be further plated by electrodeposition. The planar antenna which has been described is of simple and relatively inexpensive construction yet is effective in operation, having a very high signal collection efficiency and very high cross-polarisation discrimination. In particular, the geometrical arrangement of pick-up probes leads to cancellation of any cross-coupling between probes of the two probe arrays: this results because for any adjacent pair of apertures, the probes of one array are similarly directed to each other (identical phase) , whilst the probes of the other array are oppositely directed (oppositely phased) ; this can be verified in Figure 3 for each adjacent pair of apertures (whether adjacent each other in the horizontal direction or the vertical direction) . The antenna which has been shown and described comprises an array of 24 x 24 circular apertures, less one aperture in each corner and four apertures at the centre to accommodate the outlet port. However, the principles of this invention are applicable to antennae of different numbers of apertures. Also, whilst the antenna has been described for receiving two orthogonally polarised signals, it can instead be used for receiving two signals which are circularly polarised in opposite senses.

Claims: Claims 1) A planar microwave antenna which comprises two identical probe arrays, each probe array comprising a sheet of insulating material provided with an array of pick-up probes all aligned along a common axis, one probe array being superimposed over but turned through 90° relative to the other probe array such that the probes of one array are oriented orthogonally to the probes of the other array. 2) An antenna as claimed in claim 1, in which each probe array comprises a plurality of pairs of said pick-up probes, the two probes of each pair being oppositely-directed and connected to a track node by paths differing in length by a half wavelength. 3) An antenna as claimed in claim 1 or 2, in which the pattern of pick-up probes in each probe array is arranged in repeating blocks, in which adjacent blocks are turned through 90° relative to each other. 4) An antenna as claimed in claim 3, in which each block of said pattern comprises a plurality of rows of pick-up probes, the probes of each row being oriented in a common direction. 5) An antenna as claimed in claim 4, in which the pick-up probes of adjacent rows, in each said block, are oppositely directed. 6) An antenna as claimed in any preceding claim, further comprising at least one electrically conductive plate which is formed with an array of apertures, each aperture being registered with one pick-up probe of each probe array.

Inventors: DUNWELL GEOFFREY IAN, DUNWELL, GEOFFREY, IAN
Applicants: CONTINENTAL MICROWAVE TECHNOLO, DUNWELL GEOFFREY IAN, CONTINENTAL MICROWAVE TECHNOLOGY LIMITED, DUNWELL, GEOFFREY, IAN
Application year: 1995
Main classification: H01Q21
IPC codes: H01Q21