Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q21/00—Antenna arrays or systems
  • H01Q21/29—Combinations of different interacting antenna units for giving a desired directional characteristic
  • H01Q21/293—Combinations of different interacting antenna units for giving a desired directional characteristic one unit or more being an array of identical aerial elements
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q1/00—Details of, or arrangements associated with, antennas
  • H01Q1/27—Adaptation for use in or on movable bodies
  • H01Q1/28—Adaptation for use in or on aircraft, missiles, satellites, or balloons
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q17/00—Devices for absorbing waves radiated from an antenna; Combinations of such devices with active antenna elements or systems
  • H01Q17/008—Devices for absorbing waves radiated from an antenna; Combinations of such devices with active antenna elements or systems with a particular shape
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q21/00—Antenna arrays or systems
  • H01Q21/06—Arrays of individually energised antenna units similarly polarised and spaced apart
  • H01Q21/061—Two dimensional planar arrays
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q9/00—Electrically-short antennas having dimensions not more than twice the operating wavelength and consisting of conductive active radiating elements
  • H01Q9/04—Resonant antennas
  • H01Q9/30—Resonant antennas with feed to end of elongated active element, e.g. unipole
  • H01Q9/40—Element having extended radiating surface

Definitions

• the invention relates to an antenna array for avionic applications, particularly to a phased array.
• Antenna arrays comprise a plurality of antennas operating together as a single antenna.
• an antenna array can achieve a higher directivity.
• a direction of a main beam of the radiation pattern can be steered electronically, without moving the array physically.
• Typical antenna arrays have a considerably large radar cross-section, RCS. This is disadvantageous, particularly in applications where it is desired to minimize visibility by radar. It is challenging to reduce the radar signature of an antenna array without substantial adverse effects on antenna gain.
• the invention provides an antenna array.
• the antenna array comprises a base and a plurality of antenna elements attached to a surface of the base.
• the antenna elements are monopole antennas that are spaced from each other and each connected with a single coaxial cable.
• Monopole antennas generally have a very simple physical structure. In particular, they can be provided in the form of a straight and/or rod-shaped conductor.
• the surface of a monopole antenna is comparatively small, which is beneficial since conductive surfaces reflect radar energy. Using monopole antennas can thus minimize the amount of energy reflected back to a radar source. Accordingly, an antenna array having a small radar cross-section (RCS) is provided.
• RCS radar cross-section
• each of the antenna elements can be connected to a separate dedicated module, particularly a transmit/receive module or a phase shift module (phase shifter).
• the antenna array can thus be provided as a phased array, for example an active electronically scanned array (AESA) or a passive electronically scanned array (PESA).
• AESA active electronically scanned array
• PESA passive electronically scanned array
• the respective materials used generally have to withstand the stresses and/or loads that occur.
• the RAM may be a radio-frequency (RF) absorbing foam.
• the RAM comprises a polymer foam, for example a polyurethane foam or a polymethacrylimide foam. Since incident RF radiation can be absorbed by the RAM, the antenna array's RCS is reduced.
• the shape of the RAM can be conducive to incoherent scattering, where radio waves reflected by the surface of the RAM are out of phase, which leads to a lower radar signal strength and thus a smaller RCS.
• the RAM may comprise an electromagnetically lossy material, particularly a material where a ratio of the real part and the imaginary part of its permittivity (i.e. the loss tangent) is larger than 0.05. More particularly, the dielectric loss is determined based on the complex permittivity at a designated operating frequency of the antenna array. For example, the designated operating frequency may be at least 5 GHz and at most 40 GHz.
• the material can absorb electromagnetic energy, particularly via dielectric relaxation effects (associated with molecular dipoles) or resonance effects (arising from rotations or vibrations of atoms, ions, or electrons). Hence, electromagnetic energy can be dissipated within the RAM.
• Radio waves that have entered the RAM can be reflected multiple times within the material.
• the number of reflections within the RAM and thus the energy dissipation can be increased.
• electrically conductive particles may be provided within the RAM.
• the particles comprise carbon and/or iron.
• the conductive particles promote destructive interference of electromagnetic waves that have entered the RAM. Hence, the RCS of the antenna array can be reduced further.
• introduction of the conductive particles can result in a composite material having a beneficial value of the loss tangent (referred to above), even in cases where the material without the particles would have unsuitable dielectric loss properties.
• the RAM may be designed in such way that for a radar wave travelling through air (with an impedance of approximately 120 ⁇ ⁇ ) towards the base of the antenna array, the wave impedance changes gradually. Hence, there are no abrupt changes of impedance and reflections of the radar wave can be reduced or avoided.
• a gradual impedance change in the RAM can be achieved for example by providing the conductive particles within the RAM in a density gradient, where the density of particles is highest near the base of the antenna array.
• the RAM is shaped such that for a radar wave travelling through air (with an impedance of approximately 120 ⁇ ⁇ ) towards the base of the antenna array, the wave impedance changes gradually. In other words, the shape of the RAM ensures that no abrupt changes of impedance occur.
• the RAM transforms the wave impedance gradually, e.g. due to its shape and/or its composition (material).
• the antenna elements may comprise a base part, wherein at least the base part is surrounded by the RAM. Reflections of incident radiation on a surface under the antenna elements, e.g. the surface of the antenna array's base, can thus be minimized. A top part of the antenna elements may remain at least partly exposed, (i.e. be free of RAM), to avoid impairing the antenna array's radiation efficiency.
• the RAM surrounding at least the base part may be shaped as a lenticular cap, an amygdaloid cap, a spherical cap, or an ellipsoidal cap.
• RF anechoic test chambers usually pyramid-shaped pieces of RAM are used. While in test chambers, RF excitations can potentially be performed in a controlled way (e.g. separated horizontal and vertical excitations), there is no prior knowledge about the angles of RF incidence in a general use case of the antenna array in a certain avionic application. Particularly, the angles of RF incidence alter. RAM having a shape differing from the traditional pyramidal shape can provide improved results when the angle of RF incidence is completely unknown. In particular, using RAM shaped as an amygdaloid cap has been found to yield an antenna array having a particularly low RCS.
• a cap is particularly to be understood as a portion of a solid cut off by a plane.
• a spherical cap (or spherical dome) is to be understood as a portion of a sphere cut off by a plane.
• Lenticular is particularly to be understood as shaped like a biconvex lens.
• Amygaloid or almond-shaped is particularly to be understood as ellipsoidal with (at least slightly) pointed ends.
• the ellipsoidal cap may relate to a triaxial ellipsioid, i.e. an ellipsoid whose three axes of symmetry have different lengths.
• the RAM surrounding at least the base part may have a prismatoid shape or a cone shape.
• a prismatoid is particularly a polyhedron whose vertices all lie in two parallel planes. Examples of prismatoids include pyramids and prisms.
• the radiation-absorbent material may have a shape of a pyramidal frustum or a conical frustum.
• the radiation-absorbent material surrounding at least the base part may have an elongated shape.
• a main axis of the radiation-absorbent material namely the structure formed by the radiation-absorbent material, deviates from being perpendicular to the surface by less than 30°.
• the main axis may deviate from being perpendicular to the surface by less than 20°, 10°, or 5°.
• the main axis of the radiation-absorbent material namely the structure formed by the radiation-absorbent material, coincides at least substantially with a main axis of the respective monopole antenna.
• a main axis of an object is particularly to be understood as an axis along a main direction of extension of the object.
• the antenna elements may be provided in a regular grid arrangement, particularly in a Cartesian grid arrangement.
• the antenna elements may be provided in a staggered arrangement, which can lead to a suppression of side lobes of the antenna elements.
• the antenna elements may be conical monopole antennas. Hence, a bandwidth of the antenna array may be increased, particularly to values of 10 to 12 GHz.
• inclusion of the cone structure on the monopole antennas may be dispensed with, for example if minimization of the RCS is the main priority, since their presence may increase the RCS.
• complementary antennas of a different type than the antenna elements may be provided.
• a radiation pattern of the antenna array may be improved.
• complementary antennas may be selected that radiate in a direction where the antenna elements have a local minimum of their far field signal strength (i.e. a null).
• the complementary antennas are directional antennas, like for example Yagi-Uda antennas or log-periodic antennas.
• the directionality can enable radiation transmission/reception by the complementary antennas in/from the direction of the nulls of the antenna elements in an efficient manner.
• the directional antennas are oriented such that a main lobe of the directional antenna's radiation pattern points in the null direction of the antenna elements.
• monopole antennas have a null in their radiation pattern at the zenith on their antenna axis.
• Complementary antennas may be located within an area, wherein the antenna elements encircle the complementary antennas.
• the complementary antennas are not located at an edge of the surface of the base.
• RAM provided at the antenna elements decreases the radar visibility of the complementary antennas, particularly from lateral directions as antenna elements are located at the edges of the surface of the base.
• the complementary antennas can thus be included in the antenna array whilst maintaining a low RCS.
• the area is a central area of the base. Therefore, the antenna elements may be evenly distributed around the complementary antennas. RAM provided at the antenna elements can thus reduce radar visibility of the complementary antennas from lateral directions in a uniform manner.
• the complementary antennas may be located in a recess in the base such that the complementary antennas are located in a plane different to the plane at which the antenna elements are located, e.g. placed on the surface.
• the radar visibility of the complementary antennas can thus be reduced further, particularly also the visibility from acute angles relative to the base.
• the complementary antennas may be planar antennas. Due to their minimal extension in a direction perpendicular to the base, radar visibility of the planar antennas from lateral directions (or even acute angles relative to the base) can be reduced in a particularly effective manner.
• At least the antenna elements may be attached to an antenna ground plane.
• the antenna ground plane is provided by the base. More particularly, the antenna ground plane acts as a simulated electrical ground.
• electrical mirror-images (image antennas) of the antenna elements can be formed even in cases where the array is operated remote from earth ground, for example when it is installed on an aircraft.
• the monopole antennas may thus provide a radiation pattern at least similar to a top half of a radiation pattern of a dipole antenna having the same orientation. However, the directivity (and thus gain) of the monopole antennas is higher, since they radiate only into the space above the ground plane.
• the RAM provided at the monopole antennas can ensure that incident radio waves are dampened in such a way that a reflection of the dampened radiation at the ground plane provides only a weak signal or is even undetectable by a radar device.
• the antenna ground plane may be connected with all coaxial cables, e.g. an outer conductor of the coaxial cables.
• a distance between neighboring antenna elements may be between 0.3 and 0.5 of a designated operating wavelength of the antenna array.
• the operating frequency of the antenna array may be for example from 5 to 40 GHz.
• a total surface of the antenna array may be curved.
• the antenna elements may be attached to the curved surface in an angled orientation such that their main directions of extension (i.e. the antenna axes) are parallel to each other.
• the antenna elements can provide a radiation pattern corresponding at least substantially to the radiation pattern of antenna elements attached to a flat surface.
• the phrase "at least one of A, B, and C", for example, means (A), (B), (C), (A and B), (A and C), (B and C), or (A, B, and C), including all further possible permutations when more than three elements are listed.
• the term “at least one of A and B” generally means “A and/or B", namely "A” alone, “B” alone or "A and B”.
• FIG 1 schematically shows an antenna array 10 for avionic applications according to an embodiment of the present disclosure.
• the antenna array 10 comprises a base 12 and a plurality of antenna elements 14 attached to a surface 15 of the base 12.
• the surface 15 may be a planar surface.
• the antenna elements 14 are monopole antennas that are spaced from each other.
• a main axis of antenna elements 14 i.e. the antenna axis
• the antenna axes may deviate from being perpendicular to the surface 15 by less than for example 30°, 20° or 10°.
• the antenna array 10 consists of 8 times 8 antenna elements 14.
• the depicted antenna array 10 comprises 64 antenna elements 14 in total.
• the antenna array 10 may comprise for example at least 12, at least 16, or at least 32 antenna elements 14.
• the antenna array 10 may be for example an active electronically scanned array (AESA).
• AESA active electronically scanned array
• each of the antenna elements 14 is connected with a single coaxial cable 13 as depicted schematically for three antenna elements 14 in Figure 1 .
• each of the antenna elements 14 can be connected to a separate dedicated transmit/receive module.
• radiation-absorbent material (RAM) 16 may be provided at the antenna elements 14.
• the RAM 16 may be a radio-frequency (RF) absorbing foam, particularly a polymer foam.
• RF radio-frequency
• the RAM 16 may comprise Laird Eccosorb AN 79 or an Evonik ROHACELL polymethacrylimide (PMI) foam.
• the antenna elements 14 may comprise a base part, wherein at least the base part is surrounded by the RAM 16.
• the base part of the antenna elements 14 is surrounded by the RAM 16 and a top part 17 of the antenna elements 14 is exposed, i.e. is free of or not surrounded by the RAM 16.
• the RAM 16 surrounding at least the base part may have a prismatoid shape, for example a pyramidal shape, particularly a shape of a pyramidal frustum as shown in the depicted example of Figure 1 .
• the RAM 16 surrounding at least the base part may have a cone shape.
• the RAM 16 surrounding at least the base part may have an elongated shape.
• a main axis of the RAM 16 with the elongated shape is perpendicular to the surface 15 of the base 12.
• the main axis of the RAM 16 with the elongated shape may deviate from being perpendicular to the surface 15 by less than for example 30°, 20° or 10°.
• the RAM 16 provides RAM structures for the antenna elements 14. Particularly, each of the antenna elements 14 has a dedicated RAM structure which is shaped as discussed above.
• the antenna elements 14 are provided in a regular grid arrangement, particularly in a Cartesian grid arrangement.
• the antenna elements 14 may be provided in a staggered arrangement.
• At least the antenna elements 14 may be attached to an antenna ground plane 18.
• the antenna ground plane 18 is provided by the base 12.
• the antenna ground plane 18 acts as a simulated electrical ground.
• the antenna ground plane 18 may be connected with all coaxial cables 13 connected with the antenna elements 14, e.g. an outer conductor of the coaxial cables 13.
• a distance between neighboring antenna elements 14 may be between 0.3 and 0.5 of a designated operating wavelength of the antenna array 10.
• the operating frequency of the antenna array 10 may be for example from 5 to 40 GHz.
• a total surface of the antenna array 10 may be curved in alternative embodiments. Then, the antenna elements 14 may be attached to the curved surface in an angled orientation such that their main directions of extension (i.e. the antenna axes) are parallel to each other or the antenna elements 14 may be attached to the curved surface such that they are perpendicular to the curved surface.
• FIG. 2 schematically shows an antenna array 10 according to another embodiment of the present disclosure.
• the description above regarding Figure 1 is referred to.
• the RAM 16 surrounding at least the base part of the antenna elements 14 may be shaped as an amygdaloid cap (i.e. a cap of an almond shape). Using RAM 16 of this shape has been found to yield an antenna array 10 having a particularly low RCS.
• the RAM 16 surrounding at least the base part of the antenna elements 14 may also be shaped as a lenticular cap, a spherical cap, or an ellipsoidal cap. Moreover, two or more different shapes described herein may be used together on a single antenna array 10. In particular, for each of the antenna elements 14, the respective shape of the RAM 16 may be selected from lenticular cap, spherical cap, ellipsoidal cap, prismatoid shape, and cone shape.
• the RAM 16 namely the respective RAM structures (e.g. the amygdaloid caps), may be alternately rotated by 90°.
• the axis of rotation is parallel to a main direction of extension of the respective structure and/or parallel to a normal of the surface 15 of the base 12.
• the axis of rotation relates to an axis of symmetry of the RAM structure. This can yield increased scattering of incident radar waves and thus improved absorption of the radar energy by the RAM 16.
• each of the RAM structures may be rotated by an angle having a random or pseudorandom value.
• the axis of rotation is in particular parallel to a main direction of extension of the respective structure and/or parallel to a normal of the surface 15 of the base 12.
• the RAM structures e.g. the amygdaloid caps
• the RAM structures may also be provided all in a uniform orientation, for example to limit production effort of the antenna array 10.
• the RAM structures may be symmetrical with respect to an axis of symmetry in general.
• FIG 3 schematically shows an antenna array 10 according to another embodiment of the present disclosure. Henceforth, only the differences with respect to the antenna array 10 shown in Figure 2 are described.
• Complementary antennas 20 of a different type than the antenna elements 14 may be provided.
• the complementary antennas 20 may be planar antennas.
• the antenna array 10 may comprise for example at least 4, at least 9, or at least 16 complementary antennas 20.
• the set of complementary antennas 20 may have an antenna gain similar to that of the antenna elements 14, namely the sub-array of monopole antennas. This can be enabled by selecting the physical dimensions of the complementary antennas 20 accordingly.
• the complementary antennas 20 are directional antennas, like for example Yagi-Uda antennas or log-periodic antennas. Hence, a radiation pattern of the antenna array 10 can be improved, as explained in the following.
• the radiation pattern of the antenna array 10, particularly the one of the monopole sub-array, is optimized for angles between 10° and 60° relative to a normal of the surface 15 of the base 12.
• Monopole antennas have a null in their radiation pattern at the zenith on their antenna axis. Accordingly, an array of monopole antennas with their respective antenna axes being parallel to a normal of the surface 15 of the base 12 cannot radiate in a direction of the surface normal (i.e. broadside direction).
• This limitation can be overcome by providing directional antennas oriented such that they can radiate (and receive) in broadside direction. During operation of the antenna array 10 it is then possible to switch transmission/reception to the directional antennas if required.
• the complementary antennas 20 may be located within an area 22 where the antenna elements 14 encircle the complementary antennas 20.
• the area 22 is a central area of the base 12.
• RAM 16 provided at the antenna elements 14 decreases the radar visibility of the complementary antennas 20, particularly from lateral directions.
• the complementary antennas 20 may be located in a recess 24 in the base 12 such that the complementary antennas 20 are located in a plane different to the plane at which the antenna elements 14 are located.
• FIG 4 schematically shows an arrangement of antenna elements 14 formed as conical monopole antennas for the antenna array 10.
• conical monopole antennas may be used in the antenna arrays 10 described herein (e.g. in the arrays described above with regard to Figures 1 - 3 ).
• four antenna elements 14 formed as conical monopoles are depicted in Figure 4 .
• Conical monopole antennas comprise a cone 26 (a cone-shaped structure) at its top, wherein the base of the cone 26 points upwards (to the zenith of the antenna axis).
• the cone 26 may be filled with RAM 16 to decrease the RCS of the antenna array.
• the upper surface of the RAM 16 located within the cone 26 may be flat or form a spherical cap at the base of the cone 26.
• the volume between the cones 26 of the monopole antennas may be filled completely with a polymer foam.
• a polymer foam For example, Evonik ROHACELL polymethacrylimide (PMI) foam may be used, which has a dielectric permittivity near 1 and can provide good stability.
• PMI Evonik ROHACELL polymethacrylimide
• An increased mechanical stability of the antenna arrangement is particularly beneficial if the antenna array 10 is to be used in an aircraft.
• circuitry e.g., one or more circuits
• circuitry operably couple two or more components, generate information, process information, analyze information, generate signals, encode/decode signals, convert signals, transmit and/or receive signals, control other devices, etc.
• Circuitry of any type can be used.
• circuitry includes, among other things, one or more computing devices such as a processor (e.g., a microprocessor), a central processing unit (CPU), a digital signal processor (DSP), an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), a system on a chip (SoC), or the like, or any combinations thereof, and can include discrete digital or analog circuit elements or electronics, or combinations thereof.
• a processor e.g., a microprocessor
• CPU central processing unit
• DSP digital signal processor
• ASIC application-specific integrated circuit
• FPGA field-programmable gate array
• SoC system on a chip
• circuitry includes hardware circuit implementations (e.g., implementations in analog circuitry, implementations in digital circuitry, and the like, and combinations thereof).
• circuitry includes combinations of circuits and computer program products having software or firmware instructions stored on one or more computer readable memories that work together to cause a device to perform one or more protocols, methodologies or technologies described herein.
• circuitry includes circuits, such as, for example, microprocessors or portions of microprocessor, that require software, firmware, and the like for operation.
• circuitry includes one or more processors or portions thereof and accompanying software, firmware, hardware, and the like.
• the present application may reference quantities and numbers. Unless specifically stated, such quantities and numbers are not to be considered restrictive, but exemplary of the possible quantities or numbers associated with the present application. Also in this regard, the present application may use the term “plurality” to reference a quantity or number. In this regard, the term “plurality” is meant to be any number that is more than one, for example, two, three, four, five, etc. The terms “about”, “approximately”, “near” etc., mean plus or minus 5% of the stated value.

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• Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• Astronomy & Astrophysics (AREA)
• Aviation & Aerospace Engineering (AREA)
• General Physics & Mathematics (AREA)
• Remote Sensing (AREA)
• Variable-Direction Aerials And Aerial Arrays (AREA)

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Citations (7)

• 2024
  • 2024-05-03 EP EP24174139.6A patent/EP4645593A1/de active Pending

Patent Citations (7)

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