Abstract
An antenna system for satellite applications is provide, the antenna system comprising an antenna array and a feed structure, and the antenna array is a passive antenna array configured to have a first state and a second state, the second state being a first deployed state. The feed structure is configured to provide a linearly polarized incident field for the antenna array in the first deployed state. The antenna array comprises a plurality of array elements, and the plurality of array elements forms a polarization conversion surface configured for converting the linearly polarized incident field to a reflected/transmitted circular polarized field. The antenna array is configured so that the radiation pattern of the reflected/transmitted circular polarized field corresponds to a predetermined radiation pattern and an array element geometry and an array element position of each of the plurality array elements are configured to provide the predetermined radiation pattern.
Claims
1. An antenna system for satellite applications, the antenna system comprising an antenna array and a feed structure, wherein the antenna array is a passive antenna array and configured to have a first state and a second state, the second state being a first deployed state, the feed structure being configured to provide a linearly polarized incident field for the antenna array in the first deployed state, the antenna array comprising a plurality of array elements, and wherein the plurality of array elements forms a polarization conversion surface configured for converting the linearly polarized incident field to a reflected/transmitted circular polarized field.
2. An antenna system according claim 1, wherein the antenna array is configured so that the radiation pattern of the reflected/transmitted circular polarized field corresponds to a predetermined radiation pattern and wherein an array element geometry and an array element position of each of the plurality array elements are configured to provide the predetermined radiation pattern.
3. An antenna system according to claim 2, wherein a geometry of the array element is configured to control a phase response and/or a polarisation response of the array element in the antenna array to obtain the predetermined radiation pattern.
4. An antenna system according to claim 1, wherein the feed structure comprises at least one deployable element.
5. An antenna system according to claim 1, wherein the feed structure comprises a radiating element array, including one of an array of radiating slots and an array of radiating patches.
6. An antenna system according to claim 4, wherein the feed structure comprises a primary feed element and a secondary feed element and wherein the at least one deployable element comprises the secondary feed element.
7. An antenna system according to claim 6, wherein the primary feed element is configured to excite the secondary feed element in a deployed state to provide an excited secondary feed element, and wherein the linearly polarized incident field is provided by the primary feed element and the excited secondary feed element.
8. An antenna system according to claim 7, wherein the primary feed element comprises the radiating element array, and wherein the secondary feed element comprises an electromagnetic reflecting surface, such as a passive electromagnetic reflecting surface; including a conductive surface, a reflectarray, such as a passive reflectarray.
9. An antenna system according to claim 4, wherein the at least one deployable element comprises the array of radiating elements.
10. An antenna system according to claim 1, wherein the antenna array is provided at one or more panels, such as in a single panel configuration or a multi-panel configuration, wherein the plurality of array elements are arranged at or in a support layer, and wherein the support layer comprises a composite material, such as a di-electric material; wherein the support layer comprises a metallic material, wherein the support layer is an all-metal layer, wherein the support layer comprises a membrane, wherein the support layer comprises a canopy, wherein the support layer comprises a foil, wherein the support layer comprises a honeycomb structure.
11. An antenna system according to claim 1, wherein the antenna system is configured to receiving, emitting and transmitting electromagnetic radiation in a frequency range from 6-100 GHz.
12. An antenna system according to claim 1, wherein the array elements are patch elements, such as rectangular patch elements, or wherein the array elements are cross dipole elements.
13. An antenna system according to claim 1, wherein each array element of the plurality of array elements is characterized by an array element position and an array element geometry.
14. An antenna system according to claim 1, wherein a first array element has a first array element geometry being determined in dependence of a phase of the linearly polarized incident field at the position of the first array element; and being determined in dependence of any surrounding array elements and a phase of the linearly polarized incident field at the position of each surrounding array elements.
15. An antenna system according to claim 13, wherein the array element geometry is configured to have at least one axis forming an angle of 45° with respect to the polarization of the incident field.
16. An antenna system according to claim 1, wherein the array elements are asymmetric array elements.
17. An antenna system according to claim 1, wherein the linearly polarized incident field is a vertically or horizontally polarized incident field, and wherein the array elements are configured so that at least one axis of each array element is aligned in an angle of 45 degrees+/−10 degrees, with respect to the polarisation of the incident field.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0081] The above and other features and advantages will become readily apparent to those skilled in the art by the following detailed description of exemplary embodiments thereof with reference to the attached drawings, in which:
[0082] FIGS. 1a-1e illustrate schematically different perspectives of an embodiment of a deployed antenna array and a deployed feeding structure as mounted on a satellite,
[0083] FIGS. 2a and 2b illustrate schematically a satellite having an antenna array and a feed structure in the stowed state and the deployed state,
[0084] FIG. 3a shows schematically a patch array feed structure, and FIG. 3b shows schematically a slot array feed structure,
[0085] FIGS. 4a and 4b illustrate schematically the feed structure as mounted on the satellite,
[0086] FIG. 4c illustrates the surface currents from the feed structure,
[0087] FIG. 5 illustrates schematically a support layer as seen from the side,
[0088] FIG. 6 illustrates schematically a support layer with unit cells,
[0089] FIGS. 7a-7b illustrate schematically an antenna array and an expanded part of the antenna array,
[0090] FIGS. 8a-8e illustrate schematically various array element geometries,
[0091] FIG. 9 illustrates a selected array element geometry in more detail,
[0092] FIGS. 10a-10c illustrate schematically an antenna system having an antenna array and a feed structure,
[0093] FIGS. 11a and 11b illustrate the radiation pattern of an antenna system.
DETAILED DESCRIPTION OF THE DRAWINGS
[0094] FIGS. 1a-1d illustrate schematically different perspectives of an embodiment of a deployed antenna array 6 and a deployed feeding structure 4 as mounted on a satellite 8, such as e.g. a 3 U CubeSat, a 6 U CubeSat, a 12 U CubeSat, etc.
[0095] The antenna system 2 comprises an antenna array 6 and a feed structure 4. The antenna array 6 is configured to have a first state and a second state, the second state being a deployed state as shown exemplary in FIGS. 1a-1d. The feed structure 4 is being configured to provide an incident field for the antenna array 6 in the deployed state.
[0096] In FIG. 1a, the antenna array 6 is shown in the deployed state and mounted at a satellite 8. The feed structure 4 is likewise mounted at the satellite. FIG. 1a illustrates the antenna array 6 and the feed structure 4 as mounted at a first side 10 of the satellite. However, it is envisaged that the mount for the antenna array may be provided also at other sides configuring the antenna array for deployment above the first surface. A deployment mechanism 12, 14 allows the feed structure and the antenna array, respectively, to be deployed from a first state, such as a stowed state, to the deployed state as shown in FIG. 1a. It is seen that the feed structure 4 is off-set with respect to a center axis 9 of the antenna array 6. To ensure that the incident field is efficiently directed towards the antenna array, the feed structure 4 may be tilted with respect to an axis perpendicular to the center axis 9, e.g. by 3-5 degrees to reduce spill over. It is an advantage of being able to provide the feed structure off-set with respect to the center axis, as satellites, e.g. the casing of the satellite, often have a metal stabilizing bar along the center of the satellite at the first surface. Thus, by allowing the feed structure 4 to be off-set from the center axis 9, and thus a center of the satellite, no structural amendments to a casing of the satellite is needed as the feed structure 4 may be provided in a recess or cavity at a position of the first surface in which the casing of the satellite does not contain stabilizing bars.
[0097] It is seen that the feed structure is located as close as possible to the back of the satellite, that is as far as possible from the antenna array, so that if the antenna array is provided at a first end 11 of the first surface 10, the feed structure 4 is provided at a second end 13 of the first surface 10; the first end and the second end being opposite each other.
[0098] FIG. 1b shows a different view of the antenna system 2 of FIG. 1a. It is seen that the feed structure 4 comprises a primary feed element 16 and a secondary feed element 17. The primary feed element 16 may be an active element connected to a transmitter/receiver (not shown) of the satellite 8 and be configured to excite the secondary feed element 17 when the secondary feed element 17 is in the deployed state to provide an excited secondary feed element. The secondary feed element 17 may be a passive feed element. The linearly polarized incident field may be provided by both the primary feed element 16 and the excited secondary feed element 17. The primary feed element 16 is flush-mounted in a fixed position on the first side 10 of the satellite and the secondary feed element is positioned so as to receive the radiation from the primary feed element 16 so that the secondary feed element 17 act as a sub-reflector to direct radiation towards the antenna array 6. having an opening angle 21 between the deployed secondary feed element 17 and the primary feed element being mounted in the first side of the satellite.
[0099] FIG. 1c shows an antenna system 2′, the antenna system 2′ comprising an antenna array 6 provided at three panels 7, 7′ and 7″. The center panel 7 and two outer panels 7′ and 7″. The panels 7, 7′ and 7″ may have identical sizes, and may be for example 200 mm×335 mm. It is envisaged that the panels 7, 7′, 7″ in other embodiments may have different sizes. The outer panels 7′, 7″ are slightly tilted with respect to the center panel. Hereby, the bandwidth performance may be enhanced, by reducing the spatial phase delay from the feed. The feed structure 4 is a deployable feed structure 4 and thus the illustrated feed structure 4 is the deployable element 50 and the feed structure 4 is in the deployed state configured to direct radiation from the feed structure 4 towards the antenna array 6. The feed structure is a substantially planar feed structure and forms an angle 22 greater than 90° with the first surface 10 of the satellite. The angle 22 is selected in accordance with the size of the satellite, may typically be between 100° and 120°, for example for a 6 U CubeSat. The feed structure folds in the stowed state towards the first side 10 and is deployed to form the angle with the first surface. It is seen that the feed structure 4 is provided centered around the center axis 9 of the antenna array 6 to ensure that the incident field is efficiently directed towards the antenna array.
[0100] In FIG. 1c, the field reflected from the antenna array 6 is illustrated with arrow 18 and the incident field is illustrated schematically with arrow 21. The antenna array 6 in FIG. 1c is thus a reflectarray antenna array. As illustrated in FIG. 1c, in some examples, a coordinate system may be defined such that the first side of the satellite, e.g. a top face of the satellite, defines a z, y plane, the y-axis being parallel to a longitudinal direction of the satellite, and the z-axis being parallel to a transverse direction of the satellite. The x-axis is pointing away from the z, y plane in a direction orthogonal to the z, y plane, and thus a direction pointing away from the first side of the satellite, such as in a direction away from the top face of the satellite.
[0101] The antenna array, or at least a part of the antenna array, may be configured to be parallel to the z-axis, such as substantially parallel to the z-axis, such as parallel to the z, x plane.
[0102] FIG. 1d shows a further antenna system 2″. The antenna system 2″ corresponds to the antenna system 2′ in that the antenna array comprises three panels 7, 7′, 7″ and the two outer panels 7′, 7″ are tilted with respect to the center panel. The feed structure is a substantially planar feed structure and forms an angle 22 greater than 90° with the first surface 10 of the satellite. The angle 22 is typically between 100° and 120°, for example for a 6 U CubeSat. The feed structure folds in the stowed state towards the first side 10 and is deployed to form the angle with the first surface. The illustrated deployable feed structure 4 may therefore be the deployable element 50.
[0103] In FIG. 1d, the arrow 21 illustrates schematically the incident field, and the arrow 19 illustrates the transmitted field. The antenna array 6 in FIG. 1d is thus a transmit array antenna array.
[0104] FIG. 1e shows in more detail an antenna system according to FIG. 1c. The antenna array 6 comprises three panels, a center panel 7 and two outer panels 7′, 7″ tilted with respect to the center panel 7. The feed structure 6 is shown, and the array elements 24 of the antenna array 6 are also shown. It is seen that the size of the array elements 24 varies with respect to the position of the array elements 24 on the antenna array 6.
[0105] The antenna array 6 comprises a plurality of array elements 24 arranged at or in a support layer (not shown in FIG. 1e), and the plurality of array elements 24 may form a polarization conversion surface configured for converting a linearly polarized incident field 20 to a reflected/transmitted circular polarized field 18,19. The antenna array 6 may be a passive antenna array.
[0106] FIG. 2a illustrates a satellite body 5 having an antenna array 6 and a feed structure 4 in the stowed state. It is seen that center panel 7, and the side panels 7′, 7″ are folded onto the top of the satellite body 5, i.e. onto the first surface 10. The feed structure 4, that is particularly the deployable element 50, is folded into a cavity of the first surface 10 below the antenna array panels 7. The feed structure 4 may be any suitable feed structure. It is advantageous if the feed structure may be folded into a planar package when stowed to allow for a compact first state of the feed structure. FIG. 2b illustrates the deployed antenna array 6. It is seen that the panels 7, 7′, 7″ are hinged together and mounted at the satellite body 5 via deployment mechanisms 14. The antenna array 6 is seen mounted at a first end of the satellite body 5, such as at a first end 11 of the first side 10. The satellite body 5 comprises a number of stabilizing bar 15, including a center stabilizing bar 15′. It is seen that two cavities are formed in the first surface 10. A first cavity 48 configured to accommodate the deployable element, e.g. 4, 17, of the feed structure, and a second cavity 49 configured to accommodate the feed, such as the primary feed element 16. The first cavity 48 has a first depth and the second cavity 49 has a second depth. It is seen that the first cavity 48 is shallower than the second cavity, so that the first depth is less than second depth. It is seen that the first cavity may extend over the center stabilizing bar 15′ immediately below the first surface 10, while the second cavity 49 is provided between any stabilizing bar 15, 15′ of the satellite body 5.
[0107] In FIG. 3a, a feed structure 4 is illustrated comprising a 2 by 4 microstrip patch array 25 on a single-layer dielectric substrate. The feed structure may be configured to radiate linear polarized field in any known way. Hereby, the linear polarized field may be generated with a single, two or four probes per patch 26. The number of probes per patch may influence the width of the frequency band in which an efficient feed is provided. Having a single probe per patch may result in a narrow frequency band of e.g. about 1%, however, by increasing the number of probes to 2 or 4 per patch 26, the bandwidth may be broadened. The feed structure 4 as shown in FIG. 3a needs to be mounted via a deployment mechanism, such as a rotary joint, and such a deployment mechanism will need to include also the electrical connections.
[0108] Alternatively, the feed structure 4 may include a slot antenna element which may be fed using a cavity, or a waveguide, behind the slot antenna element. It has been found that using a metallic cavity feed behind the slots increases the frequency bandwidth of the system.
[0109] The slot antenna element may for example be realized using two rows each with four slots, that is a 2 by 4 slot array, these eight slots may then be fed using a separate cavity.
[0110] In FIG. 3b a feed structure 16 is shown providing a linearly polarized incident field. A primary feed structure 16 comprising a fixed slot array 27 is shown in which four slot antenna elements 28 in a two by two setup are excited by a single cavity 29 below the slot antenna elements 28. The cavity 29 may extend below the entire fixed slot array. In some examples, two such feed structures as illustrated in FIG. 3b are combined to provide a total of eight radiating slot antenna elements 28.
[0111] This primary feed structure 16 is fixed in the first surface 10, and no deployment is necessary. Instead, the secondary feed structure 17 is mounted above the primary feed structure, to be illuminated by the field emitted from the primary feed structure 16. In combination, the primary feed structure 16 and the secondary feed structure 17 provides the incident field 20 for the antenna array 6.
[0112] It is envisaged that also a simple primary feed structure 16 having two slot antenna elements, such as a slot array having 2 slots in one row, may be used.
[0113] The dimension of the feed structure 16 may be as set out below:
TABLE-US-00001 Cavity cut-off frequency 6.9 GHz Cavity width 52.0 mm Cavity height 4.0 mm Slot length 1936 mm Slot width 6.0 mm
[0114] In FIG. 4a, a feed structure 4 as mounted on the satellite 8 is illustrated schematically. It is seen that the primary feed element 16 includes 2 by 1 slot antenna elements to illuminate the secondary feed element 17. The primary feed element is flush mounted with the first surface 10 in the second cavity 49.
[0115] The field produced by this feed structure propagates along the surface of the satellite body, such as along the first surface 10.
[0116] FIG. 4b illustrates the satellite of FIG. 4a as seen from above and including the antenna array 6. In FIG. 4b, it is seen that the feed structure 4 including the primary feed element 16 (dashed) and the secondary feed element 17, i.e. the deployable element 50, is provided off-center with respect to the center axis 9 of the satellite. Hereby, the primary feed element 16 may be provided in a cavity of the satellite body without having to cut into any of the stabilizing bars 15, 15′ (not shown). When providing the feed structure 4 off-set with respect to the center axis 9 of the satellite body, the feed structure 4 is also off-set with respect to the antenna array 6. To reduce spill-over effects of the incident field; the feed structure, including the secondary feed element 17 is provided at an angle 51 to the center axis 9 different from orthogonal. For a feed structure 4 provided centered around the center axis 9, the secondary feed element 17 will form an orthogonal angle 51 with respect to the center axis 9, such as a substantially orthogonal angle. This is particularly so when the antenna array 6 is provided centered about the center axis 9. The angle 52 between the antenna array 6 and the feed structure 4 may also be defined and will typically be 0° when the feed structure is not off-set (i.e. the feed structure and the antenna array are parallel, such as substantial parallel) and change when the feed structure 4 is off-set with respect to the center axis 9 or with respect to a center of the antenna array.
[0117] It is envisaged that the position of the feed structure 4, 16, 17 should be determined with respect to the position of the antenna array 6, such as the position of the antenna array in the deployed state, so as to reduce or minimize any spill-over of the field emitted from the feed structure.
[0118] FIG. 4c illustrates an RF model of the feed and shows the electric surface current density at 8.4 GHz. It is seen that the strongest currents are localized on the primary feed element 16 and the secondary feed element 17, which in this case is the same as the deployable element 50. It is also seen that there is a contribution from the first surface 10 of the satellite 8. Thus, in this embodiment, the resulting incident field 20 results from the primary feed element 16, the secondary feed element 17 and the first surface 10.
[0119] It should be noted that typically, the feed structure 4, 16, 17 includes a feed to excite the feed structure 4, 16, 17 in any know way, thus, also when reference is made to the feed structure 4, such feed structure includes a feed to excite the feed structure 4 in any know way, e.g. via a microstrip, via a coax cable, etc.
[0120] FIG. 5 illustrates schematically a support layer as seen from the side, The array elements 24 are arranged at or in a support layer 30. The array elements 24 are for example provided in a conducting layer 31, such as copper, provided on a dielectric substrate 32, such as on a Rogers substrate. The array elements 24 may be printed on the surface of the dielectric substrate using any known processes. The other side of the dielectric substrate, i.e. the backside of the dielectric substrate 32 is provided with a conducting layer 33, such as a copper layer, to provide a ground layer for the array elements. Typically, a stability core 34 is provided to increase the mechanical stability of the antenna array 6, and of any antenna array panels 7, 7′, 7″. The dielectric substrate 32 may be positioned on a stability core 33, such as a glass fibre core. The stability core 34 may be treated with one or more layers of prepreg 35 for assembly with the metal coated dielectric layer.
[0121] To ensure a symmetric antenna array 6, a further dielectric substrate 32′ is provided at the back side of the stability core 34, the further dielectric substrate 32′ having a metal layer 36, such as a copper layer, on both sides. The total layer of such an antenna array may be kept at less than 3 mm, such as at 2.178 mm+/−10%.
[0122] In a specific example, the electrical properties of the support layer is as follows:
TABLE-US-00002 Thickness Material ϵ.sub.r tan δ [mm] Rogers RO4003C 3.55 0.0027 0.813 Ventec VT-901 glass fibre 4.05 0.012 0.152 Ventec VT-901 prepreg 4.05 0.012 0.065 Copper σ = 58 MS/m 0.035 Total thickness N/A 2.178
[0123] In FIG. 6 it is illustrated schematically that an antenna array 6 has a plurality of unit cells 38 distributed regularly over the antenna array 6. Such a unit cell 38 may measure between 9-11 mm in both height and width. Each unit cell 38 comprises an array element 24. FIG. 7a illustrates a specific cross-shaped array element 24, the array elements 24 being distributed in the different unit cells. As is seen, the geometry of the array element 24 is dependent on the position of the unit cell 38 on the antenna array 6. FIG. 7b shows an expanded part of the antenna array 6. In the present example, the unit cells are shown as square or rectangular. The unit cell may also be polygonal, such as hexagonal, etc.
[0124] The feeding structure of e.g. FIG. 3b provides a linearly polarized incident field illuminating the antenna array, the antenna array focuses the reflected/transmitted signal and simultaneously converts the incident linearly polarized field to a circularly polarized reflected/transmitted field. This is achieved by e.g. utilizing array elements that provide a relative phase shift of ±90° between the two orthogonal linear components of the reflected field.
[0125] The phase behaviour of the reflected field depends on the geometry of the array elements as well as the thickness and material properties of the support layer and the incident field at the specific array element.
[0126] A number of different array elements may be used, including a rotated rectangular patch, as shown in FIG. 8a, a multi-resonance element as shown in FIG. 8b, a strip cross of different strip length to thickness ratios, as seen in FIGS. 8c and 8d. It is envisaged that many geometries can be used as the array element. It is advantageous that the element has at least one axis having an angle of 45 degrees, such as at 45 degrees+/−2 degrees, such as at 45 degrees+/−5 degrees with respect to at least one other axis of the array element, as is illustrated in FIG. 8e. The array element 26 may in some examples comprise a loop, the loop may be of any shape including, elliptical, rectangular, etc.
[0127] As is seen, the array elements in FIGS. 8a, 8c, 8d and 8e are asymmetric array elements. The array element in FIG. 8 is symmetric, but may be rendered asymmetric should this be needed.
[0128] The array elements 24 may provide a controlled reflection/transmission phase of the incident field. The geometrical dimensions of the array elements may be varied from unit cell to unit cell. The thickness, the length, the rotation, the symmetry of an array element may be adapted to ensure that a predetermined radiation pattern is obtained by the antenna array 6. By varying the geometry of the array elements across the antenna array, in dependence on the properties of the incident field at a specific array element, an electrically controlled array element is not needed for controlling the array element properties.
[0129] The antenna array is a passive polarising and focusing antenna array configured for polarising and focusing an incoming RF signal, i.e. the incident field.
[0130] In FIG. 9, an array element having two crossed dipoles 41, 42 at an angle 43 of 45 degrees with respect to the polarization of the incident field is shown. It should be noted that the angle 43 may be substantially 45 degrees, such as an angle of 45 degrees+/−2 degrees, such as an angle of 45 degrees+/−5 degrees. Two coordinate systems are introduced, an xy-coordinate system that is aligned with the unit-cell boundaries and the rotated coordinates x′ y′ that are aligned with the strips, i.e. with the at least one axis of the array element. A square unit-cell with period Px=Py=11.10 mm is used and the width of the strips, w=3.5 mm, is maximised for the given unit-cell size in order to achieve a less steep reflection phase response as a function of the dipole length parameters I.sub.x and I.sub.y. By illuminating the array elements with a vertically (or horizontally) polarized field (or wave), and adjusting the strip lengths I.sub.x and I.sub.y such that the ±90 degree phase shift is achieved, the reflected wave will be right-hand or left-hand circularly polarized. Typically, the strip lengths I.sub.x and I.sub.y are different.
[0131] The bandwidth of an antenna array may be dependent on the thickness of the substrate 32, or the support layer 30. In FIG. 9, the thickness of the dielectric substrate is denoted h. The support layer typically has a thickness below 1 mm, such as about 0.8 mm in order to fulfil the requirements of the total reflectarray thickness being less that 10 mm in the first state, such as a stowed state. This will affect the achievable bandwidth and phase range of the reflectarray.
[0132] In some examples, the support layer may be an all-metallic support layer so that the support layer does not comprise a dielectric substrate. In some examples, the array elements are formed in a metal layer, such as in a conducting layer and formed using indentations and/or protrusions in the material.
[0133] FIGS. 10a-10c illustrate schematically an antenna system 2 having an antenna array 6 and a feed structure 4, the feed structure 4 having at least one deployable element 50. The antenna array 6 is deployed and it is seen that the antenna array is provided at 3 panels 7, 7′, 7″ which are not tilted with respect to one another. The panels 7, 7′, 7″ may be folded over the satellite for stowing. It is seen that the feed structure 4 is off-set with respect to the center of the antenna array 6, and thus with respect to the center axis 9 of the satellite 8, such as the center axis 9 of the first side 10 of the satellite 8. The array elements are shaped as crosses, e.g. as shown in FIGS. 7a-b, FIG. 8d and in FIG. 9.
[0134] In FIG. 10b, the antenna system 2 is shown from a different perspective, and the feed structure 4 may be tilted to form an angle different from 0 degrees with respect to the antenna array panels 7, 7′, 7″. The feed structure 4 comprises a primary feed element 16 comprising a slot antenna 46 in the form of two slots 47, 47′ fed by a cavity feed (not shown in FIG. 10b). The secondary feed element 17 comprises a rectangular conductive element, such as sub reflector 17.
[0135] FIG. 10c shows a transmit antenna array 6, as illustrated by the transparency of the antenna array 6.
[0136] FIGS. 11a and 11b illustrate the radiation pattern of the antenna system disclosed.
[0137] The reflection coefficient of array elements has been simulated as a function of a design parameters of the array element and the support layer, including geometrical parameters of the array elements, including shape, size and thickness of the array elements, and including e.g. a thickness of the support layer to arrive at a predetermined radiation pattern. The performance of the antenna array, in this case a reflectarray is evaluated using a feed structure as herein disclosed. The array elements are optimized for operation in the Tx-band between 8.025-8.40 GHz for an Earth Observation (EO) mission, or at both Rx-band between 7.145-7.19 GHz and the Tx-band between 8.40-8.45 GHz for a Deep Space (DS) mission. The calculations are carried out using a higher-order spectral domain method of moment (SDMoM) solver. In the solver the periodic problem is formulated in terms of an IE (Integral Equation) and solved in the spectral domain. The Green's function on the IE consists of a double summation of Floquet harmonics. The SDMoM is configured to handle a plurality of dielectric layers, but the metallization layers must be confined to the interfaces between the dielectric layers.
[0138] In the array element calculations the SDMoM solver (by TICRA) was used locally for each reflectarray element to determine the equivalent currents associated with the given element. These equivalent currents were then used to calculate the far-field of the reflectarray. The geometry of all the array elements was optimised simultaneously on a global level, using a non-linear minmax optimization algorithm. The illumination of the antenna array was modelled using a feed structure as herein disclosed which was simulated using higher-order method of moments (MoM). In all antenna array simulations the far-field is evaluated with respect to an output coordinate system.
[0139] This coordinate system is defined such that the z-axis is aligned with the direction of specular reflection of the feed illumination by the antenna array, at an elevation of 21° from the normal vector of the reflectarray panels. The y-axis is directed parallel to the CubeSat body, and the x-axis is pointing away from the CubeSat in a direction orthogonal to the y and z-axes to achieve a right-handed orthogonal coordinate system.
[0140] FIG. 11a shows Tx band simulation results of a reflectarray as disclosed, and FIG. 11b shows Rx-band simulation results of the same reflectarray. The simulations are performed using a y-polarized cavity feed antenna.
[0141] A peak gain of 29.25 dB in the EO mission Tx-band, an XPD better than 24.8 dB and side lobe levels of 15.6 dB below the on-axis gain have been achieved. The antenna array, including the plurality of array elements, was then re-optimised for the DS mission and the results are presented in FIGS. 11a and 11b. In the Rx-band (FIG. 11a) the peak gain is 24.62 dB, the XPD is better than 17.6 dB and the side lobe levels are 13.1 dB below the on-axis gain. In the Tx-band a peak gain of 29.63 dB was achieved, the XPD is better than 25.1 dB and the side lobe levels are 15.3 dB below the on-axis gain. Thus, it is seen that the present antenna system provides a better aperture efficiency and an increased wideband frequency performance.
REFERENCE NUMBERS
[0142] 2 antenna system [0143] 4 feed structure [0144] 5 satellite body [0145] 6 antenna array [0146] 7 center panel [0147] 7′ 7″ outer panels, side panels [0148] 8 satellite [0149] 9 center axis [0150] 10 first side of the satellite [0151] 11 first end of first side [0152] 13 second end of first side [0153] 12, 14 deployment mechanism [0154] 15 stabilizing bars [0155] 16 primary feed element [0156] 17 secondary feed element [0157] 18 reflected field [0158] 19 transmitted field [0159] 20 incident field [0160] 21 opening angle [0161] 22 feed structure angle [0162] 24 array elements [0163] 25 microstrip patch array [0164] 26 patch [0165] 27 fixed slot array [0166] 28 slot antenna elements [0167] 29 excitation cavity [0168] 30 support layer [0169] 31 conducting layer [0170] 32, 32′ dielectric substrate [0171] 33 ground layer [0172] 34 stability core [0173] 35 prepreg [0174] 36 conducting layer [0175] 38 unit cell [0176] 41, 42 crossed strips [0177] 43 angle between strips [0178] 46 slot antenna [0179] 47, 47′ slots [0180] 48 first cavity [0181] 49 second cavity [0182] 50 deployable element [0183] 51 first off-set angle [0184] 52 second off-set angle
[0185] Although particular features have been shown and described, it will be understood that they are not intended to limit the claimed invention, and it will be made obvious to those skilled in the art that various changes and modifications may be made without departing from the scope of the claimed invention. The specification and drawings are, accordingly to be regarded in an illustrative rather than restrictive sense. The claimed invention is intended to cover all alternatives, modifications and equivalents
