Super-direct loop antenna

Abstract

The invention relates to an antenna (1), comprising a main radiating element (2) of the loop antenna type, a first connection port (3) and a second connection port (4) arranged on either side of the main radiating element (2), a central element (5) of the electric dipole type, circumscribed in the main radiating element (2), the central element (5) being made up of two branches (6, 7) arranged symmetrically with respect to an axis of symmetry (8) passing through the first connection port (3) and through the second connection port (4), the central element (5) being fed at the level of a third connection port (9) located in the axis of symmetry (8), the main radiating element (2) and the central element (5) being coplanar.

Description

Technical field

[0001] The present invention relates to an antenna, as well as to an antenna system and to an antenna array. The invention is particularly suitable for wireless telecommunications systems, radar systems, or radiofrequency metrology, in a non-exhaustive manner.

[0002] 5G telecommunications technology is based on several frequency bands. Among these frequency bands, the FR1-5G band uses frequencies below 6 GHz. In this sub-6 GHz band, in near-field / far-field measurement, highly compact directional antennas provide focusing of radiated energy, improvement of angular resolution, and minimization of the occupied space.

[0003] For the purpose of directivity, one of the methods often used consists of producing an array of resonant elements in “End-Fire” mode, where the axis of maximum radiation is aligned with the axis of duplication of the radiating elements, thus leading to a reduced size but with little compactness in the “End-Fire” orientation of the array. This is the case, for example, of the Yagi-Uda antenna, well known to those skilled in the art.

[0004] Loop antennas are elementary antennas well defined in the literature. Their dimensions can be very small compared to the wavelength. An example of a loop antenna is illustrated by the figure 1 The loop antenna comprises an excitation point 21, and a metal loop 20, which may consist of one or more turns, or consist of a flat track, in one piece.

[0005] In particular, the passage of a radiation mode in the plane (see radiation diagram 22 on the figure 2 ) containing the loop, to an out-of-plane radiation mode when the circumference C of the loop approaches the wavelength Â, is illustrated by the figure 2 (see radiation diagrams 23 and 24.

[0006] The measures of the figure 2 are obtained with a value Ω constant, defined by: Ω = 2 . ln 2 πa b = 10 a corresponds to the radius of the loop, and b corresponds to the thickness of the loop along the z axis.

[0007] Curve 31 corresponds to the directivity along the z axis, in dBi, of the antenna described in [1], as a function of the C / λ ratio. The C / λ ratio can be modified either by a change in the size of the antenna (here the circumference) by fixing a given frequency, or for a given physical size, a variation of the frequency across the wavelength.

[0008] When the circumference C of the loop is small compared to the wavelength λ, typically when C / λ < 0.5, the maximum radiation remains in the loop plane. Indeed, the transition to the fundamental mode of the antenna is accompanied by a decrease in directivity along the axis orthogonal to the loop plane, and the radiation rather takes place in the loop plane

[0009] For networking of loop antennas in “End-Fire” mode (superposition in the propagation direction orthogonal to the loop plane), it is necessary that there is good directivity along the axis orthogonal to the loop plane.

[0010] However, the problem of maintaining the directivity outside the plane of a loop arises when miniaturizing the antenna. This problem is clearly illustrated in reference 22 of the figure 2 , through a radiation null along the Oz axis.

[0011] The miniature antenna diversity present in the literature is substantial and the design of a miniature directional radiating element represents an additional challenge. There are different approaches to meet this need.

[0012] As presented in [2], part of the solutions proposes to use Huygens sources (dipole and loop with radiation in the plane of the loop), then using the high natural directivity of this radiating element.

[0013] Other solutions are based on a joint optimization between a dipole and a reflector plane (sometimes modified for the purpose of compactness, wide matching bandwidth and directivity, as described in [3]).

[0014] In the field of loop antennas, [4] proposes to include an impedance tuning space at the loop level.

[0015] In [5], the coupling between elements is used, but without any particular constraint on the radiation pattern of the antenna.

[0016] These solutions do not meet the need for directionality and radiation control mentioned above.

[0017] In the antenna described in [6], the use of loops close to the fundamental mode C / λ < 0.5 does not take advantage of a natural compactness of the loops which is linked to a use of higher modes, in which the loop radiates orthogonally to the plane which contains it.

[0018] In [7], Yagi-Uda loop antennas are arrayed in an “End-Fire” arrangement. However, the loops used are not miniature in size (C / λ « 1) and therefore exhibit maximum radiation orthogonal to the plane.

[0019] In [8], an excitation dipole is arranged inside a loop antenna located out of the plane defined by the excitation dipole, which improves the antenna's directivity. However, the fact that the loop antenna is located out of plane prevents a compact array in an "End-fire" configuration.

[0020] Thus, the invention aims to provide a compact and directional radiating element for use alone or in a network. Summary of the invention

[0021] An object of the invention is therefore an antenna, comprising a main radiating element of the loop antenna type, a first connection port and a second connection port arranged on either side of the main radiating element, a central element of the electric dipole type, circumscribed in the main radiating element, the central element being made up of two branches arranged symmetrically with respect to an axis of symmetry passing through the first connection port and through the second connection port, the central element being fed at a third connection port located in the axis of symmetry, the main radiating element and the central element being coplanar.

[0022] Advantageously, the dimensions of the main radiating element and the central element are determined so that the impedance of the first connection port and the second connection port have a zero real part.

[0023] Advantageously, each branch of the central element comprises a main strand which extends orthogonally to the axis of symmetry, and two auxiliary strands, arranged on either side of the end of the main strand opposite the third connection port.

[0024] Advantageously, the main radiating element is composed of two semicircles connected at the first connection port and the second connection port, and the auxiliary strands have a circular arc shape, and wherein the following parameters are used to optimize the impedance of the first connection port and the second connection port: the diameter of the main radiating element, the width of the main radiating element, the spacing between each of the semicircles, the diameter of the circular arcs, the width of the circular arcs, and the length of the circular arcs.

[0025] Advantageously, the impedance of the first connection port and the impedance of the second connection port are determined by applying a superdirectivity algorithm.

[0026] Advantageously, the superdirectivity algorithm uses a radiation pattern method.

[0027] Advantageously, the first connection port and the second connection port each comprise a power supply circuit.

[0028] Advantageously, the first connection port comprises a load, and the second connection port comprises a power supply circuit.

[0029] The invention also relates to an antenna system, comprising a aforementioned antenna, and a reflector element placed close to the main radiating element and parallel to the plane of the main radiating element.

[0030] The invention also relates to an antenna network comprising at least two aforementioned antennas, the antennas being networked according to an “End-fire” type configuration. Description of figures

[0031] Other characteristics, details and advantages of the invention will emerge from reading the description given with reference to the appended drawings given by way of example. There figure 1 , already described, illustrates a loop antenna according to the state of the art. The figure 2 , already described, illustrates radiation patterns of the loop antenna according to the figure 1 , for different values ​​of C / λ. The figure 3 illustrates an antenna according to the invention. The figure 4 illustrates radiation patterns of the loop antenna according to the invention, and according to the Figure 5 , for different values ​​of C / λ. The Figure 5illustrates a loop antenna without a central element, and whose loop is excited on both sides at two connection ports. The figure 6 illustrates an embodiment of an antenna system, comprising an antenna and a reflector element. The figure 7 illustrates an embodiment of an “End-fire” type network of several antennas according to the invention. The figure 8 illustrates an embodiment of an antenna system comprising a reflector element and an “End-fire” type network of several antennas according to the invention. Detailed description of the invention

[0032] The antenna according to the invention is presented in figure 3 .

[0033] The antenna 1 comprises a main radiating element 2 of the loop antenna type. On the figure 3, the loop is circular, but other shapes can be considered for the loop, including square, rectangular, or even more complex shaped loops of the surface or volume polygonal type. The specific shape depends on the application and the desired performance characteristics. The main radiating element 2 is a loop of conductive wire or a metal track printed in the form of a loop.

[0034] The printed metal track, typically comprising copper, can be produced on a dielectric substrate using PCB (Printed Circuit Board) technology.

[0035] The substrate can be made from materials such as epoxy fiberglass (FR-4) or polytetrafluoroethylene (PTFE), although other materials can be used depending on the specific requirements of the antenna.

[0036] The main radiating element 2 is located in the xy plane, which means that its thickness, along the z axis, is negligible compared to the width of the loop, in the xy plane.

[0037] A central element 5 of the electric dipole type is circumscribed in the main radiating element 2. The central element 5 consists of a first branch 6 and a second branch 7. The two branches are arranged symmetrically with respect to an axis of symmetry 8 which passes through a first connection port 3 and through a second connection port 4.

[0038] The central element 5 of the electric dipole type is supplied by a balanced two-wire line represented by a connection point at the center of the dipole. One of the poles is excited in phase opposition with respect to the second pole, which can be carried out by a balanced excitation at the output of a symmetrizing circuit (device called "balun".

[0039] The first connection port 3 and the second connection port 4 are arranged on either side of the main radiating element 2, that is to say diametrically opposite relative to the center of the loop. The arrangement on either side of the main radiating element 2 (relative to the center of symmetry) can be applied both to a circular loop antenna and to the other forms of loop antennas mentioned above.

[0040] The number of connection ports can be greater than two, provided that symmetry with respect to the 8 axis is respected.

[0041] According to a first embodiment, one of the connection ports (for example the first connection port 3) comprises a load, and the other connection port (for example the second connection port 4) comprises a power supply circuit.

[0042] The feed circuit serves as an electrical connection between the main radiating element 2 and the transmitting / receiving electronics of the antenna. The electrical signals generated by the antenna (in the case of reception) or the electrical signals intended to be transmitted by the antenna (in the case of transmission) are transmitted through the feed circuit.

[0043] In this embodiment, the load is a so-called passive load, and may include a resistor. But active loads can also be considered, making it possible to bring impedances whose real parts are negative.

[0044] Loads can be designed with series or parallel RLC equivalent circuits.

[0045] According to a particularly advantageous embodiment, each of the connection ports comprises a feed circuit. The symmetry of the dipole with respect to the axis of symmetry 8 allows balanced coupling on the loop (two excitation points per coupling). This makes it possible to feed the dipole at the center and to weight, with the same set of loads, the two ports of the loop. The application of two symmetrical loads at the level of the main radiating element makes it possible to maintain the radiation orthogonal to the xy plane containing the loop.

[0046] The antenna 1 according to the invention comprises a central element 5 of the electric dipole type, circumscribed in the main radiating element 2. By "circumscribed", it is meant that the central element 5 is contained within specific limits defined by the interior of the loop of the main radiating element 2.

[0047] The central element 5 is powered at a third connection port 9 located on the axis of symmetry 8. The third connection port 9 comprises a power supply circuit. Furthermore, the third connection port 9 may comprise an impedance matching circuit including loads in series and / or in parallel.

[0048] The central element 5 is located in the same plane as the main radiating element 2. The fact that they are located in the same plane makes it possible to reduce the inter-element space, compared to a configuration in which one of the two elements would be out of the plane. The arrangement, in the same plane, of the main radiating element 2 and the central element 5 simplifies the installation of the first connection port 3 and the second connection port 4.

[0049] Due to the coplanar nature of the central element 5 and the main radiating element 2, it is possible to connect a complex coupling circuit 32 between the central element 5 and the main radiating element 2 (for example a series or parallel RLC circuit). Such a more complex coupling would be difficult or even impossible to integrate if the main radiating element 2 were out of the plane of the central element 5 (in this case, the coupling is done solely by capacitive coupling via the distributed capacitance between the main radiating element 2 and the central element 5). Thus, by varying the loads of the complex coupling circuit, it is possible to obtain different radiations without modifying the initially planned radiating structure.

[0050] There figure 4illustrates radiation patterns obtained for different values ​​of C / λ (C corresponds to the circumference of the main radiating element 2 and λ corresponds to the carrier wavelength), with the central element 5 (diagrams 20, 21 and 22) and without the central element (diagrams 23, 24 and 25). Each of the radiation patterns allows the directivity of the antenna to be visualized, i.e. in which directions the antenna radiates or picks up electromagnetic signals best.

[0051] There figure 4 also illustrates the directivity of the antenna along the z axis of the figure 3 , that is to say the axis orthogonal to the plane of the loop (see curve 26 with the central element and curve 27 without the central element). The Figure 5illustrates the loop antenna 31 without a central element, used for plotting curve 27, and whose loop is excited on both sides at the level of the first connection port 3 and the second connection port 4 with the correct phase distribution.

[0052] It appears that for C / λ < 1 and C / λ > 1.25, and in particular C / λ = 0.25, the antenna equipped with the central element radiates in the plane orthogonal to the plane of the antenna (so-called "broadside" radiation). The radiation pattern 23 with the central element, for C / λ = 0.25, shows that there is maximum directivity in the z axis and good symmetry of the pattern.

[0053] This radiation characteristic, even for low values ​​of C / λ, makes it possible to envisage an “End-fire” type network of several antennas according to the invention. Thus, at constant wavelength, the circumference of the antenna can be reduced, without the directivity being affected outside the plane.

[0054] End-fire arraying involves aligning a plurality of antennas along the axis orthogonal to the plane of each antenna. By concentrating the energy in a specific direction (which corresponds to the orthogonal axis), end-fire antennas provide high gain in that direction, which improves the effective power of the signal transmitted or received in that direction.

[0055] An embodiment of the antenna according to the invention is described in connection with the figure 3The branch 6 of the central element 5 comprises a main strand 10 which extends orthogonally to the plane (or axis) of symmetry 8, and two auxiliary strands (12 and 13), arranged on either side of the end 17 of the main strand 10 opposite the third connection port 9.

[0056] By "strand" (main strand or auxiliary strand) we mean an elongated, relatively thin part.

[0057] Thus, each auxiliary strand forms an arc of a circle which extends from the end 17 of the main strand, at an angle αd of between 0° and 90° (excluding these values), and the main radiating element 2 is composed of two semicircles. If the main radiating element 2 has another shape, each auxiliary strand has a shape such that there is a free space (devoid of metal track) between the auxiliary strand and the interior of the main radiating element 2, so as to reveal coupling zones between the main radiating element 2 and the central element 5. For example, if the main radiating element 2 has a square shape, the auxiliary strands have a rectilinear shape, so as to be parallel to the main radiating element 2.

[0058] By symmetry, the branch 7 of the central element 5 comprises a main strand 11 which extends orthogonally to the plane of symmetry 8, and two auxiliary strands (14 and 15), arranged on either side of the end 16 of the main strand 11 opposite the third connection port 9.

[0059] The dimensions of the main radiating element 2 and of the central element 5 are determined so that the impedance of the first connection port 3 and of the second connection port 4 has the lowest possible real part, in particular a zero or quasi-zero real part. By "quasi-zero" is meant a real part much lower than the radiation resistance of the antenna (of the order of at least a hundred times lower).

[0060] This makes it possible to power the central element 5 in the center and to weight the two connection ports (3, 4) of the main radiating element 2 with the same load set. Thus, the phase planes of the main radiating element 2 and the central element 5 are co-located.

[0061] Furthermore, adding a central element 5 to the center of the main radiating element 2 makes it possible to optimize the impedance of the first connection port 3 and of the second connection port 4, by acting on a plurality of parameters, and not only on the track width w of the main radiating element 2, as is the case in loop antennas of the state of the art.

[0062] Thus, when designing the antenna, the designer can adjust at least one of the following parameters, so as to weight the two connection ports with the same load set: the diameter r of the main radiating element 2, the width w of the main radiating element 2, the spacing δ between each of the semicircles (18, 19) which constitute the main radiating element, the diameter rd of the circular arcs (12, 13, 14, 15), the width wd of the circular arcs (12, 13, 14, 15), and the length αd of the circular arcs (12, 13, 14, 15).

[0063] For example, an antenna according to the invention can be implemented with the values ​​indicated in the table below (units indicated in parentheses): λ (m) r (m) w (m) δ (m) rd (m) wd (m) αd (°) 0,3 0.25.λ / 2π λ / 100 λ / 400 λ.(r-2.5w) λ / 300 70

[0064] For these values, we obtain the following impedances (in Ω), respectively at the first connection point 3 and the second connection point 4: Z3 = 0 + 1j*535 Z4 = 0 + 1j*535

[0065] The impedances at the connection points are identical, which guarantees balanced coupling at the main radiating element 2, and radiation that remains orthogonal to the loop plane, even with a low value of C / λ (C / λ = 0.25).

[0066] Load optimization can be implemented by applying a superdirectivity algorithm, which makes it possible to obtain analytically (i.e. by solving a linear system of equations) the complex amplitudes at the antenna connections, i.e. directly at the loads and / or feed points.

[0067] An example of a superdirectivity algorithm is described in [9].

[0068] Preferably, the superdirectivity algorithm uses a radiation pattern method, which is easily implementable. Indeed, it does not require iteration, or even decomposition in a particular basis.

[0069] Alternatively, load optimization can be implemented by other methods, in particular by the spherical mode method, or by the characteristic mode method.

[0070] There figure 6 illustrates an antenna system, comprising an antenna 1 as defined previously, and a reflector element 28 placed close to the main radiating element and parallel to the plane of the main radiating element.

[0071] This configuration provides an increase in directivity in the plane orthogonal to the antenna. In particular, the increase in directivity can be by a factor of 2 if the reflector is a good metallic conductor and has a diameter greater than the wavelength.

[0072] The reflector is flat and may have a diameter of the order of the wavelength of the transmit / receive signal. The effects on directivity are significant as the antenna gets closer to the reflector element 28 (e.g., a distance equal to 0.1λ). However, the closer the antenna 1 gets to the reflector element 28, the more difficult the impedance matching of the reflector element 28 becomes. A compromise between improving directivity and impedance matching must therefore be made.

[0073] There figure 7 illustrates an antenna array 29 according to the invention. It comprises at least two antennas described previously, arrayed in an “End-fire” type configuration. It is not necessary for the central elements of the different antennas to be aligned. The antennas have a radiation along the axis orthogonal to the plane of the loop, which is particularly compatible with arraying.

[0074] Indeed, the axis of maximum radiation is aligned with the axis of duplication of the radiating elements, and the space between the antennas can be greatly reduced due to the planar nature of the antenna and the central element. This makes it possible to obtain a very compact and very directional network.

[0075] The embodiments of the system with reflector and the networking of antennas according to the "End-fire" type configuration can be combined, as illustrated in figure 8 The antenna array 30 comprises a plurality of arrayed antennas 1, placed facing a reflector 28. References cited :

[0076] [1] Balanis, C., 2005. Antenna theory. New York: Wiley-lnterscience. [2] Pigeon, M., Delaveaud, C., Rudant, L., & Belmkaddem, K. (2014). Miniature directive antennas. International Journal of Microwave and Wireless Technologies, 6(1), 45-50. doi:10.1017 / S1759078713001098 [3] R. W. Ziolkowski, M. -C. Tang and N. Zhu, "An efficient, electrically small antenna with large impédance bandwidth simultaneously with high directivity and large front-to-back ratio," 2013 International Symposium on Electromagnetic Theory, Hiroshima, Japan, 2013, pp. 885-887 [4] EP2178166A1, « Antenne à boucle incluant un espace de réglage d'impédance et procédés associés » [5] US7298343B2, « RFID tag with enhanced readability » [6] O. S. Kim, S. Pivnenko and O. Breinbjerg, "Superdirective Magnetic Dipole Array as a First-Order Probe for Spherical Near-Field Antenna Measurements," in IEEE Transactions on Antennas and Propagation, vol. 60, no. 10, pp. 4670-4676, Oct. 2012, doi: 10.1109 / TAP.2012.2207363 [7] S.Ito, N. Inagaki and T. Sekiguchi, "An investigation of the array of circular-loop antennas," in IEEE Transactions on Antennas and Propagation, vol. 19, no. 4, pp. 469-476, July 1971, doi: 10.1109 / TAP.1971.1139954 [8] F. Munoz et al. "Compact Superdirective Electric Dipole Array for Far-field Measurement in a Semi-Anechoic Environment," 2023 17th European Conférence on Antennas and Propagation (EuCAP), Florence, Italy, 2023, pp. 1-5, doi: 10.23919 / EuCAP57121.2023.10133657. [9] E. E. Altshuler, T. H. O'Donnell, A. D. Yaghjian, and S. R. Best, "A Monopole Superdirective Array", IEEE Transactions on Antennas and Propagation, vol. 53, no. 8, August 2005.

Claims

1. Antenna (1), comprising a main radiating element (2) of the loop antenna type, a first connection port (3) and a second connection port (4) arranged on either side of the main radiating element (2), a central element (5) of the electric dipole type, circumscribed in the main radiating element (2), the central element (5) being made up of two branches (6, 7) arranged symmetrically with respect to an axis of symmetry (8) passing through the first connection port (3) and through the second connection port (4), the central element (5) being fed at a third connection port (9) located in the axis of symmetry (8), characterized in that the main radiating element (2) and the central element (5) are coplanar.

2. Antenna according to claim 1, wherein the dimensions of the main radiating element (2) and of the central element (5) are determined so that the impedance of the first connection port (3) and of the second connection port (4) have a zero real part.

3. Antenna according to one of the preceding claims, in which each branch (6, 7) of the central element (5) comprises a main strand (10, 11) which extends orthogonally to the axis of symmetry (8), and two auxiliary strands (12, 13, 14, 15), arranged on either side of the end (16, 17) of the main strand (10, 11) opposite the third connection port (9).

4. An antenna according to claim 3, wherein the main radiating element (2) is composed of two semicircles (18, 19) connected at the first connection port (3) and the second connection port (4), and the auxiliary strands (12, 13, 14, 15) have a circular arc shape, and wherein the following parameters are used to optimize the impedance of the first connection port and the second connection port: the diameter (r) of the main radiating element (2), the width (w) of the main radiating element (2), the spacing (δ) between each of the semicircles (18, 19), the diameter (rd) of the circular arcs (12, 13, 14, 15), the width (wd) of the circular arcs (12, 13, 14, 15), and the length (αd) of the circular arcs (12, 13, 14, 15).

5. Antenna according to one of the preceding claims, in which the impedance of the first connection port (3) and the impedance of the second connection port (4) are determined by applying a superdirectivity algorithm.

6. Antenna according to claim 5, in which the superdirectivity algorithm uses a radiation pattern method.

7. Antenna according to one of the preceding claims, in which the first connection port (3) and the second connection port (4) each comprise a feed circuit.

8. Antenna according to one of claims 1 to 6, in which the first connection port (3) comprises a load, and the second connection port (4) comprises a power supply circuit.

9. Antenna according to one of claims 1 to 8, in which a coupling circuit is connected between the central element (5) and the main radiating element (2).

10. Antenna according to claim 9, in which the coupling circuit is a series or parallel RLC circuit.

11. Antenna system, comprising an antenna (1) according to one of the preceding claims, and a reflector element (28) placed close to the main radiating element and parallel to the plane of the main radiating element.

12. Antenna network (29), characterized in that it comprises at least two antennas according to one of claims 1 to 10, the antennas being networked according to an “End-fire” type configuration.

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