Radiation Generator

Abstract

A crossed dipole antenna 100 for use in a radiofrequency (RF) directed energy weapon. The antenna has first and second dipoles 110, 120 each having two arms on a planar substrate 130. The arms are sha

Description

FIELD This specification relates to a radiation generator such as an antenna or antenna array. In particular, but not exclusively, it relates to a radio-frequency (RF) directed energy weapon (DEW). BACKGROUND Known RF DEWs comprise an antenna or antenna array to generate radiofrequency radiation. The antenna array may comprise a phased array to permit steering of the generated radiation. SUMMARY According to a first aspect of the present disclosure, there is provided a crossed dipole antenna, comprising: first and second dipoles, each dipole having two arms, wherein the arms of the first and second dipoles each define a truncated elongated ellipse, and an electrical feed arrangement for electrically feeding the arms of the first and second dipoles to cause the crossed-dipole antenna to generate radiation. The electrical feed arrangement may be configured for electrically feeding the arms of the first and second dipoles such that the crossed-dipole antenna generates radiation having a peak power of 1 kW or higher. The major axis of each ellipse may be at least 1.5 times as long as the minor axis of the ellipse. In some examples, the major axis of each ellipse may be at least twice as long as the minor axis of the ellipse. Each of the arms of the first and second dipoles may include an outer edge, wherein the distance along the major axis of the respective ellipse from an inner vertex of the ellipse to the outer edge is less than half of the distance between the inner vertex of the ellipse and an outer vertex of the ellipse. Each truncated elongated ellipse may be truncated along it major axis. An elongated ellipse may be truncated to exclude the co-vertices of the ellipse. The electrical feed arrangement may comprise an electrical connection hub. A major axis of each ellipse may extend outwardly from the electrical connection hub. The major axis of the ellipse of the first arm of the first dipole may be collinear with the major axis of the ellipse of the second arm of the first dipole. The major axis of the ellipse of the first arm of the second dipole may be collinear with the major axis of the ellipse of the second arm of the second dipole. The first and second dipoles may be arranged in a perpendicular orientation with respect to one another. Each arm may be printed on a common substrate. For each dipole, the arms of the dipole may be printed on opposite sides of the common substrate. The electrical feed arrangement may be configured for electrically feeding the arms of the first and second dipoles to cause the crossed-dipole antenna to generate circularly polarized radiation. The electrical feed arrangement may be configured to provide a 90° phase shift between a first arm of the first dipole and a first arm of the second dipole, and a 90° phase shift between a second arm of the first dipole and a second arm of the second dipole. The electrical feed arrangement may comprise a first phase shift component to provide the 90° phase shift between a first arm of the first dipole and a first arm of the second dipole, and a second phase shift component to provide the 90° phase shift between a second arm of the first dipole and a second arm of the second dipole. According to a second aspect of the present disclosure, there is provided a radiation generator comprising an antenna array having a plurality of array elements, each array element comprising a crossed-dipole antenna according to the first aspect. The radiation generator may further comprise a radiation reflector spaced from the antenna array to redirect a portion of the radiation generated by the antenna array into a desired direction. The radiation reflector may be spaced from the antenna array by a distance less than or equal to a third of a wavelength of the radiation that is generated by the array. The radiation reflector may be spaced from the antenna array by a distance less than or equal to a quarter of a wavelength of the radiation that is generated by the array. The arms of the first and second dipoles of the array elements may be printed on a common substrate. The radiation reflector may comprise a metallic sheet which is parallel to the common substrate. The gap between the radiation reflector and the antenna array may be free space only. In some examples an RF absorber component is not present between the radiation reflector and the antenna array. The radiation generator may further comprise control electronics to control the relative phase of the radiation generated by the array elements so as to generate a steerable output radiation beam. According to a third aspect of the present disclosure, there is provided a unidirectional radiation source comprising the radiation generator of the second aspect. According to a fourth aspect of the present disclosure, there is provided a radio-frequency directed energy weapon comprising the radiation generator of the second aspect, the unidirectional radiation source of the third aspect, or the crossed-dipole antenna of the first aspect. BRIEF DESCRIPTION OF THE FIGURES Embodiments of the invention will now be described by way of example only with reference to the figures, in which: Figure 1 shows a crossed-dipole antenna in accordance with a first example; Figure 2 shows the electrical connection hub of the crossed-dipole antenna; Figure 3 shows a radiation generator comprising an antenna array. DETAILED DESCRIPTION Figure 1 schematically illustrates a crossed-dipole antenna 100 in accordance with a first example. The crossed-dipole antenna 100 comprises a first dipole 110, a second dipole 120, a planar substrate 130, and an electrical feed arrangement comprising an electrical connection hub 140. As shown, the first and second dipoles 110, 120 are provided on the planar substrate 130 in a generally perpendicular orientation with respect to one another. As explained below, the crossed-dipole antenna is adapted for generating high power RF output. Referring again to Figure 1, the first dipole 110 comprises a first arm 112 (shown as a solid line) formed on a top side of the substrate 130. The first dipole 110 further comprises a second arm 114, which is shown as a dotted line to indicate that it is formed on the underside of the substrate 130. It will be understood that the terms “top side” and “underside” are used herein relative to the perspective of Figure 1 and do not imply any preferred orientation of the antenna 100. Thus, the first and second arms 112,114 are disposed on opposite sides of the substrate 130. The first and second arms may comprise a suitable conductive material, and may be printed on the respective side of the substrate 130, for example using known etching techniques. The substrate 130 may comprise a suitable insulating material such as FR4. Similarly, the second dipole 120 comprises a first arm 122 printed or otherwise formed on the top side of the substrate 130, and a second arm 124 printed or otherwise formed on the underside of the substrate 130. Thus, the first arm 112 of the first dipole 110 and the first arm 122 of the second dipole 120 define a layer on the top surface of the substrate 130, and may be referred to herein as the “top layer” arms. Similarly, the second arm 114 of the first dipole 110 and the second arm 124 of the second dipole 120 define a layer on the bottom surface of the substrate 130 and may be referred to herein as the “bottom layer” arms. The electrical connection hub 140 is configured to electrically drive the arms 112,114, 122, 124 of the first and second dipoles to generate radiation. In one example, the electrical connection hub may be connected to a single coaxial cable (not shown) having an inner pin and an outer conductor. The top layer arms may be connected to the inner pin, while the bottom layer arms may be connected to the outer conductor. As shown in more detail in Figure 2, the electrical connection hub 140 includes a phase shift line 150 in the form of a % ring, which is disposed on the top side of the substrate 130 and provides a 90° phase shift between the signals delivered to the top layer arms 112,122. The electrical connection hub 140 further comprises a similar phase shift line (not shown) disposed on the underside of the substrate; this provides a 90° phase shift between the signals delivered to the bottom layer arms 114,124. In this way, pairs of the crossed dipole arms are fed in quadrature so as to generate circularly polarized radiation. Advantageously, the arrangement of Figure 2 allows this to be achieved with a signal coaxial cable. As shown in Figure 1, the general shape of each arm 112, 114, 122, 124 is that of a truncated elongated ellipse, with the major axis 113, 115, 123,125 of each respective ellipse extending outwardly from the electrical connection hub 140. This allows a configuration in which the gaps between the dipole arms are large enough for high power operation, whilst also allowing for strong mutual coupling and thus a wide bandwidth. If the ellipse were not elongated (i.e. if the ratio between the major and minor axes were close to 1), then this would result in “fat” arms with small gaps between them; such a configuration is unsuitable for high power operation due to the risk of breakdown (i.e. arcing). In various examples, the major axis of the ellipse may be at least 1.5 times as long as the minor axis of the ellipse, for example twice as long as the minor axis of the ellipse, for example three times as long as the minor axis of the ellipse, for example four times as long as the minor axis of the ellipse. It will be understood that that the terms “major axis” and “minor axis” as used herein in relation to an ellipse, refer to the larger and smaller diameters of the ellipse, respectively. As shown in Figure 1, in this example, the major axis of the ellipse of the first arm 112 of the first dipole 110 is collinear with the major axis of the ellipse of the second arm 114 of the first dipole 110. Similarly, the major axis of the ellipse of the first arm 122 of the second dipole 120 is collinear with the major axis of the ellipse of the second arm 124 of the second dipole 120. In the illustrated example, the major axes of the ellipses of the first dipole 110 are perpendicular to the major axes of the ellipses of the second dipole 120. As described above, each arm in Figure 1 is formed as a layer on the substrate, and thus forms a truncated elongated ellipse in a plane parallel to the plane of the substrate 130. It will be understood that in some examples, the arm may also include other elements. Even so, it can generally be said that each arm 5 generally defines a truncated elongated ellipse, for example in a plane and / or printed on the substrate 130. It is convenient herein to describe the shape of the arms 112, 114, 122, 124 in relation to the ellipse on which the shape is based, for example by referring to the axes of the ellipse, and / or its vertices and co-vertices. However, it will be 10 understood that the shape that each respective arm 112, 114, 122, 124 defines may not necessarily include every part of the respective ellipse on which it is based. This is because the shape that each arm defines is generally that of a truncated ellipse. For example, in Figure 1, the ellipse on which each arm 112, 114, 122, 15 124 is based is truncated in both directions along its major axis. In particular, each ellipse is truncated inwardly (i.e. towards the connection hub 140) to form an integral connection between the respective arm and the connection hub 140. Each ellipse is also truncated outwardly to form an outer edge 112a, 114a, 122a, 124a of the respective arm. Thus, in the example of Figure 1, each arm 112,114, 20 122, 124 does not include either vertex of the ellipse on which it is based. Moreover, since the ellipse is truncated prior to its co-vertices, the arm does not include the co-vertices of the ellipse either. For each arm 112, 114, 122, 124, the distance along the major axis between the outer edge 112a, 114a, 122a, 124a and the closest (i.e. inner) vertex 25 of the respective ellipse may be less than half of the distance between the vertices of the ellipse, for example less than a third of the distance between the vertices of the ellipse, for example less than a quarter of the distance between the vertices of the ellipse, for example less than a fifth of the distance between the vertices of the ellipse, for example less than a sixth of the distance between 30 the vertices of the ellipse. The antenna 100 may be of any suitable size dependent on the wavelength of the radiation to be generated. For RF radiation, the distance between the edge 122a and the edge 124a may for example be between 100 mm and 200 mm. The distance between the edge 112a and the edge 114a may be the same. The electrical feed arrangement may further comprise appropriate electrical signal generation equipment to drive the crossed-dipole antenna to generate radiation having a peak power of 1 kW or higher, such as 2kW or higher. Suitable electrical signal generation equipment is well known to those skilled in the art and will not be described here. Figure 3 shows a radiation generator 300 in accordance with a second example. As shown, the radiation generator comprises an antenna array having a plurality of array elements 310. In some cases, appropriate control electronics may be provided to control the relative phase of the radiation generated by the array elements so as to generate a steerable output radiation beam. That is, the radiation generator may comprise a phased array. Each array element 310 comprises a crossed-dipole antenna 100 according to the first example. In Figure 3, a 2x2 array of elements 310 is shown; however it will be appreciated that more generally, the array may be formed of any N x M array of array elements, where N and M are appropriately selected numbers. The radiation generator 300 of Figure 3 is configured to operate over a 30% bandwidth (S11 <-10dB, axial ratio <3dB) with peak powers of 2-3kW per crossed dipole antenna 100. As shown in Figure 3, the arms of the first and second dipoles of each of the array elements are printed on a common substrate 330, which may be formed of any suitable insulating material such as FR4. The radiation generator is made unidirectional by way of a radiation reflector (ground plane) 340, which is positioned behind the array, parallel to the substrate. The radiation reflector may comprise a metal plate. Advantageously, to increase efficiency of the radiation generator, no absorber is present between the substrate and the radiation reflector. That is, the gap between the substrate and the radiation reflector may be free-space only. The radiation reflector 340 may be positioned at a distance less than or equal to 0.8 wavelengths from the substrate 330, for example at a distance less than or equal to 0.5 wavelengths from the substrate 330, for example at a distance less than or equal to a third of a wavelength from the substrate. In one particular example, the radiation reflector may be positioned 0.25 wavelengths behind one face of the array. The array of the radiation generator may be of any suitable size. For example, the substrate 330 of Figure 3 may be 300 mm square. It will be 5 appreciated that larger arrays may be provided for increased power. Radiation generators according to exemplary implementations of this disclosure may, for example, be applied in ground-based air defence, force protection, check point protection systems, counter-UAS (unmanned aerial vehicle), counter-mobility, or e-bike, e-scooter or electric car stopping. 10 Although several embodiments have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles of this disclosure, the scope of which is defined in the claims.

Claims

1. A crossed-dipole antenna, comprising:first and second dipoles, each dipole having two arms, wherein the arms of the first and second dipoles each define a truncated elongated ellipse, andan electrical feed arrangement for electrically feeding the arms of the first and second dipoles to cause the crossed-dipole antenna to generate radiation.

2. The crossed-dipole antenna of claim 1, wherein the electrical feed arrangement is configured for electrically feeding the arms of the first and second dipoles such that the crossed-dipole antenna generates radiation having a peak power of 1 kW or higher.

3. The crossed-dipole antenna of claim 1 or claim 2, wherein the major axis of each ellipse is at least 1.5 times as long as the minor axis of the ellipse.

4. The crossed-dipole antenna of claim 3, wherein the major axis of each ellipse is at least twice as long as the minor axis of the ellipse.

5. The crossed-dipole antenna of any one of the preceding claim, wherein each of the arms of the first and second dipoles includes an outer edge, and wherein the distance along the major axis of the respective ellipse from an inner vertex of the ellipse to the outer edge is less than half of the distance between the inner vertex of the ellipse and an outer vertex of the ellipse.

6. The crossed-dipole antenna of any one of the preceding claims, wherein each truncated elongated ellipse is truncated along it major axis, wherein the elongated ellipse is truncated to exclude the co-vertices of the ellipse.

7. The crossed-dipole antenna of any one of the preceding claims, wherein the electrical feed arrangement comprises an electrical connection hub,wherein a major axis of each ellipse extends outwardly from the electrical connection hub.

8. The crossed-dipole antenna of any one of the preceding claims, wherein the major axis of the ellipse of the first arm of the first dipole is collinear with the major axis of the ellipse of the second arm of the first dipole, andthe major axis of the ellipse of the first arm of the second dipole is collinear with the major axis of the ellipse of the second arm of the second dipole.

9. The crossed-dipole antenna of any one of the preceding claims, wherein the first and second dipoles are arranged in a perpendicular orientation with respect to one another.

10. The crossed-dipole antenna of any one of the preceding claims, wherein each arm is printed on a common substrate.

11. The crossed-dipole antenna of claim 10, wherein for each dipole, the arms of the dipole are printed on opposite sides of the common substrate.

12. The crossed-dipole antenna of any one of the preceding claims, wherein the electrical feed arrangement is configured for electrically feeding the arms of the first and second dipoles to cause the crossed-dipole antenna to generate circularly polarized radiation.

13. The crossed-dipole antenna of any one of the preceding claims, wherein the electrical feed arrangement is configured to provide a 90° phase shift between a first arm of the first dipole and a first arm of the second dipole, and a 90° phase shift between a second arm of the first dipole and a second arm of the second dipole.

14. The crossed-dipole antenna of claim 13, wherein the electrical feed arrangement comprises a first phase shift component to provide the 90°phase shift between a first arm of the first dipole and a first arm of the second dipole, and a second phase shift component to provide the 90° phase shift between a second arm of the first dipole and a second arm of the second dipole.

15. A radiation generator comprising an antenna array having a plurality of array elements, each array element comprising a crossed-dipole antenna according to any one of the preceding claims.

16. The radiation generator of claim 15, further comprising a radiation reflectorspaced from the antenna array to redirect a portion of the radiation generated by the antenna array into a desired direction.

17. The radiation generator of claim 16, wherein the radiation reflector is spaced from the antenna array by a distance less than or equal to a third of a wavelength of the radiation that is generated by the array.

18. The radiation generator of claim 17, wherein the radiation reflector is spaced from the antenna array by a distance less than or equal to a quarter of a wavelength of the radiation that is generated by the array.

19. The radiation generator of any one of claims 15 to 18, wherein the arms of the first and second dipoles of the array elements are printed on a common substrate.

20. The radiation generator of claim 19 when dependent on claim 16, 17 or 18, wherein the radiation reflector comprises a metallic sheet which is parallel to the common substrate.

21. The radiation generator of any one of claim 16 to 20, wherein the gap between the radiation reflector and the antenna array is free space only.

22. The radiation generator of any one of claims 16 to 21, therein an RF absorber component is not present between the radiation reflector and the antenna array.5 23. The radiation generator of any one of claims 15 to 22, further comprising control electronics to control the relative phase of the radiation generated by the array elements so as to generate a steerable output radiation beam.

24. A unidirectional radiation source comprising the radiation generator of any 10 one of claims 15 to 23.25 A radio-frequency directed energy weapon comprising the radiation generator of any one of claims 15 to 24, or the crossed-dipole antenna of any one of claims 1 to 14.15

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