Wideband antenna array

Abstract

An antenna array (10) for detecting an incoming radio wave (52) having an operating wavelength, comprising: a plurality of antenna elements (12) arranged in an array with a periodic repetition of the antenna elements (12). Each antenna element (12) comprises a slot (32) being shaped such that the polarisation of the corresponding antenna element (12) is non-linear, and having a first axis (A1) and a second axis (A2) orthogonal to the first axis. Each of the first and second axes (A1; A2) has a length in the range of about 0.05-0.2 times the operating wavelength of the incoming radio wave (52) and the ratio of the length of the first axis A.sub.1 to the length of the second axis A.sub.2 is between about 1-2.5. There is also a method of configuring an antenna array 10 for detecting an incoming radio wave (52), and a method of determining the angle of arrival of a radio wave (52) impinging on such an antenna array (10).

Claims

1. An antenna array for detecting an incoming radio wave having an operating wavelength, comprising: a plurality of antenna elements, the antenna elements arranged in an array with a periodic repetition of the antenna elements; wherein each antenna element comprises a slot, the slot being shaped such that the polarization of the corresponding antenna element is non-linear and having a first axis and a second axis orthogonal to the first axis; and wherein each of the first and second axes of the slot has a length in the range of about 0.05-0.2 times the operating wavelength of the incoming radio wave and the ratio of the length of the first axis to the length of the second axis is between about 1-2.5.

2. The antenna array of claim 1, wherein the periodic repetition of the antenna elements is at a minimum distance in the range of about 0.25-0.75 times an operating wavelength of an incoming radio wave or integer multiples of the selected fraction of the operating wavelength.

3. The antenna array of claim 1, wherein the shape of the slot is one of: a polygon, optionally a diamond; and a circle.

4. The antenna array of claim 1, wherein the shape of one or more of the plurality of antenna elements is one of: a polygon; and a circle.

5. The antenna array of claim 1, wherein the antenna array is linear.

6. The antenna array of claim 1, wherein the antenna array is two dimensional.

7. The antenna array of claim 6, wherein the plurality of antenna elements are arranged in a grid, optionally wherein the grid is square, optionally wherein the grid is rectangular.

8. The antenna array of claim 1, comprising exactly or at least two antenna elements.

9. The antenna array of claim 1 comprising exactly or at least three antenna elements, optionally exactly or at least four antenna elements, optionally exactly or at least five antenna elements, optionally exactly or at least six antenna elements.

10. The antenna array of claim 1, wherein the plurality of antenna elements comprises two or more patch antenna elements.

11. The antenna array of claim 1, wherein the antenna arrays are formed as or on printed circuit boards.

12. The antenna array of claim 1, wherein the slot comprises a conducting member inserted therein, optionally wherein the conducting member is metallized.

13. The antenna array of claim 1, wherein the antenna array receives electrical signals by one or more of: one or more co-axial cables; one or more vertical interconnect accesses (VIAs) and one or more co-planar waveguide (CPW) tracks; and one or more VIAs and one or more microstrips.

14. The antenna array of claim 1, wherein the antenna array is an ultrawide band (UWB) array.

15. The antenna array of claim 1, the antenna array having a fractional bandwidth of at least about 10%.

16. The antenna array of claim 1, the slot being shaped such that the corresponding antenna element is dual polarized.

17. An antenna system comprising two or more of the antenna arrays of claim 1.

18. The antenna system of claim 17, wherein a first of the two or more antenna arrays lies in a first plane, and a second of the two or more antenna arrays lies in a second plane, and wherein the first plane is parallel to the second plane.

19. The antenna system of claim 17, wherein the two or more antenna arrays are arranged back to back, optionally in opposite orientations.

20. The antenna system of claim 17, wherein a first antenna element of a first of the two or more antenna arrays has a common axis with a second antenna element of a second of the two or more antenna arrays, optionally wherein the first and second antenna elements receive electrical signals along this axis.

21. A method of determining the Angle of Arrival (AoA) of a radio wave impinging on the antenna array of claim 1, optionally wherein the antenna array is in the antenna system of claim 17, comprising: detecting a radio wave impinging on the antenna array; measuring the Phase Difference of Arrival (PDoA) at outputs of two or more of the antenna elements; and determining the AoA of the impinging radio wave based on the measured PDoA.

22. A method of configuring an antenna array for detecting an incoming radio wave having an operating wavelength, comprising: arranging a first antenna element; arranging a second antenna element, the second antenna element spaced apart from the first antenna element; wherein each antenna element comprises a slot and the method further comprises: shaping the slot such that the polarization of the corresponding antenna element is non-linear and has a first axis and a second axis orthogonal to the first axis; and shaping the slot such that each of the first and second axes of the slot has a length in the range of about 0.05-0.2 times the operating wavelength of the incoming radio wave and the ratio of the length of the first axis to the length of the second axis is between about 1-2.5.

23. The method of claim 22, wherein the second antenna element is spaced apart from the first antenna element by a minimum distance in the range of about 0.25-0.75 times an operating wavelength of an incoming radio wave or integer multiples of the selected fraction of the operating wavelength.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:

(2) FIG. 1 illustrates a radio wave from a source impinging on an array of antenna elements, which array can be an antenna array in accordance with embodiments of the present invention;

(3) FIG. 2 illustrates a linear antenna array comprising five antenna elements in accordance with embodiments of the present invention;

(4) FIG. 3 illustrates the impact of ground plane truncation and mutual coupling on inter-element phase coherence in accordance with embodiments of the present invention;

(5) FIG. 4 illustrates a five element array of diamond-slotted broadband patch antennas in accordance with embodiments of the present invention;

(6) FIG. 5 illustrates a five element array of broadband circular slot antennas with diamond-shaped metallic insertion in accordance with embodiments of the present invention;

(7) FIG. 6 illustrates microstrips on the back of an array to feed patch antennas through feeding vias in accordance with embodiments of the present invention, with transparent substrate for ease of reference;

(8) FIG. 7 illustrates the array of FIG. 6 with non-transparent substrate;

(9) FIG. 8 is a graph illustrating the effectiveness of an embodiment of the present invention over the whole front half-hemisphere of an array; and

(10) FIG. 9 illustrates a two-by-two array arrangement in accordance with embodiments of the present invention.

(11) In the drawings, similar elements will be similarly numbered whenever possible. However, this practice is simply for convenience of reference and to avoid unnecessary proliferation of numbers, and is not intended to imply or suggest that the invention requires identity in either function or structure in the embodiments.

DETAILED DESCRIPTION OF THE INVENTION

(12) As shown in FIG. 1, an antenna array 10, which comprises a plurality of antennas or elements 12, has an array plane 14 that defines a front hemisphere 16 and a back hemisphere 18 of the array 10. Radio waves 52 from a source 50 impinge on the elements 12 of the array 10 at an Angle of Arrival (AoA). Determining the AoA provides a measure of the direction of propagation of the radio wave impinging on the elements 12 of the array 10. The AoA is determined by measuring the Phase Difference of Arrival (PDoA) at two or more of the elements 12 of the array 10.

(13) FIG. 2 illustrates a linear antenna array 10 comprising five antenna elements 12, which are broadband antennas. Of course, embodiments of the invention are not limited to having five antenna elements and fewer (two, three or four elements 12) or more (six elements 12 or more) may be provided in accordance with present invention. Additionally or alternatively, the array 10 is not limited to being a linear array and may have other configurations, such as a grid of elements 12 or other suitable arrangement. Each of the elements 12 in the linear array 10 is a dual-polarised element 12. The vertical 22 and horizontal 24 electric field components and the resulting electric field component 26 are illustrated for each element 12.

(14) As further illustrated in FIG. 3, the impact of ground plane truncation and mutual coupling on inter-element phase coherence is limited by the arrangement of the elements 12 of the linear array 10. As discussed above, each element 12 of the array 10 is dual-polarised. This enables the array 10 to be sensitive to the incident signal 52 with arbitrary polarisation. The electric field polarisations 22, 24, 26 are coherent in phase for any polarisation of the impinging wave 52, as shown in FIG. 3. The impact of the diffraction from the ground plane edges and of the mutual coupling between elements 12 of the array 10 on the phase relation between the array elements 12 is limited. This behaviour holds across the broad frequency band that the system is required to accurately estimate the AoA of the source 50 of the impinging signal 52.

(15) The spacing between the elements 12 (the inter-element spacing) is optimised for at least two reasons. Firstly the optimised spacing mitigates the influence of the mutual coupling that may affect the PDoA. Additionally or alternatively the optimised spacing avoids ambiguity in the estimated AoA with respect to the measured PDoA. Phase linearity and group delay angular variation of each element 12 of the array 10 is controlled across the operating bandwidth of the system. These characteristics prevent distortions of the broadband signal 52 as it travels through the antennas 12 to the processing unit.

(16) As illustrated in FIG. 4, the elements 12 of the array 10 in this exemplary arrangement are printed patch antennas 12. Each element 12 has a slot 32 cut out from the radiating element 12. The patch antennas 12 consists of a ground plane and a radiating element 12 which may be suspended or printed on dielectric material. The radiating element 12 may have circular or polygonal shape; in this Figure the radiating element 12 is circular. The slot 32 may have rectangular or arbitrary geometry with two main or dominant axes, which are substantially orthogonal to each other (within operational tolerances). The slot 32 comprises two dominant axes (A.sub.1, A.sub.2), and whilst the slot shapes mentioned herein work well, some particularly well, the slots 32 of the present invention are not intended to be restricted to any specific shape. Other shapes not mentioned herein may not significantly affect the function of the array, and so the present invention is intended to cover such other shapes. Where a polygonal shape is employed, for instance a substantially diamond shape, one or more of the corners and/or edges of the polygon being chamfered and/or irregular and/or non-linear would still allow the slot to function as required, due to the two dominant axes and minor variations in shape should not significantly affect the performance. It is noted that, according to Babinet's Principle, a unit element of an array may be a printed slot antenna with a metallised member inserted in the radiating aperture. This is within the scope of embodiments of the present invention. As discussed above, the slot antenna 12 of the array 10 of FIG. 3 consists of a ground plane and a radiating aperture which may be suspended or printed on dielectric material. The radiating aperture may have circular or polygonal shape with two main orthogonal axes (A.sub.1, A.sub.2). The length of each axis (A.sub.1, A.sub.2) may vary between about 0.05 and about 0.2 times the wavelength corresponding to the centre frequency of the operating bandwidth of the radio wave 52. The ratio between the longer axis (A.sub.1) and the shorter axis (A.sub.2) may vary between about 2.5 and about 1. The array 10 is obtained by a periodic repetition of the unit element 12 with a distance (D) between about 0.25 and about 0.5 times the wavelength corresponding to the centre frequency of the operating bandwidth of the radio wave 52. The distance (D) may be larger than this, which may give multiple PDoA solutions that may be resolved using various methods. FIG. 4 is an example according to an embodiment of the present invention and illustrates a five-element 12 array of diamond-slotted 32 broadband patch antennas 12. The slots 32 may take other shapes. FIG. 5 is an example according to another embodiment of the present invention and illustrates a five-element 12 array of circular-slotted 32 broadband antennas 12, having diamond-shaped metallic members inserted therein. The array 10 is made with Printed Circuit Board (PCB) technology to enable inexpensive manufacturability and compactness.

(17) The slots in the patches are optimised to have nearly constant group delay for AoAs in ±90 degrees range, i.e. in the whole front half-hemisphere of the array.

(18) Due to the above-described mechanisms, an array 10 according to the invention has a PDoA on its output that varies little with the polarisation of the impinging wave 52 for AoAs in ±90 degrees range, i.e. in the whole front half-hemisphere 16 of the array 10. Due to the optimised geometry of the array elements 12, an array 10 according to the invention has nearly constant group delay for AoAs in ±90 degrees range, i.e. in the whole front half-hemisphere 16 of the array 10, which allows precise ranging, regardless of the AoA. For the patch antennas 12 with slots 32, the shape of the slots 32 in the patch antennas 12 is used to alter the otherwise strongly linear polarisation of the antennas 12. The slots 32 of the patches 12 are optimised to achieve a large operating band of the antennas 12 (about 10% fractional bandwidth). As previously discussed, the slots 32 of the patches 12 are optimised to make the antennas 12 sensitive for any polarisation of the impinging wave 52 for AoAs in ±90 degrees range, i.e. in the whole front half-hemisphere 16 of the array 10. Therefore the illustrated arrays 10 in accordance with the invention are advantageous compared with known arrays.

(19) The antennas 12 of the arrays 10 discussed above may be fed by any suitable means, for example by coaxial cables, or with vias and co-planar waveguide (CPW) tracks, or, as illustrated in FIGS. 6 and 7, with vias 40 and microstrips 42. FIG. 6 has transparent substrate so that the vias 40 are visible, whereas FIG. 7 has non-transparent substrate so the vias 40 cannot be seen. The microstrips 42 at the back of the anchor point of each element 12 feeds the patches 12 through the feeding vias 40 as illustrated in FIG. 6.

(20) FIG. 8 is a graph showing experimental results from an embodiment of the present invention, and illustrates the effectiveness of the embodiment over the whole front half-hemisphere 16 of the array 10. The Y-axis shows the measured PDoA and the X-axis shows the AoA from −90 to +90 degrees. As illustrated in FIG. 8, embodiments of the invention have a small dependence of the measured PDoA on the polarization of the impinging wave 52, whether the polarisation is vertical, horizontal, or circular, compared with the theoretical PDoA.

(21) Different arrays 10 are discussed above and various embodiments are disclosed. It is also within the scope of the present invention to combine two or more arrays 10 according to the present invention. For example, multiple arrays may be positioned in different geometries in order to provide for better angular coverage. One example is illustrated in FIG. 9, in which a two-by-two array arrangement is shown (the top and bottom layer), each array 10 comprising two elements 12 that are diamond-slotted 32 patch antennas 12, and illustrating the microstrips 42 to feed the elements 12 of the opposite layer. Other configurations are of course possible.

(22) Although the present invention is described above in the context of particular embodiments, one of ordinary skill in the art will readily realise that many modifications may be made in such embodiments to adapt to specific implementations. The scope of the invention is defined by the appended claims.