Superdirectional 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 within the main radiating element (2), the central element (5) being composed of two branches (6, 7) arranged symmetrically with respect to an axis of symmetry (8) passing through the first connection port (3) and the second connection port (4), the central element (5) being fed at a third connection port (9) located on the axis of symmetry (8), the main radiating element (2) and the central element (5) being coplanar. Figure for the abstract: Fig. 3

Description

Title of the invention: Super directional loop antenna technical field

[0001] The present invention relates to an antenna, as well as to an antenna system and an antenna array. The invention is particularly suited to wireless telecommunications systems, radar systems, or radio frequency metrology, among other applications.

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

[0003] For the purpose of directivity, one of the methods often used consists of constructing an array of resonant elements in "End-Fire" mode, where the axis of maximum radiation is aligned with the duplication axis of the radiating elements, thus resulting in 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 in [Fig. 1]. The loop antenna comprises an excitation point 21, and a metallic loop 20, which may consist of one or more turns, or be a single, continuous flat track.

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

[0006] The measurements in Figure 2 are obtained with a constant value G, defined by:

[0007] n = 21n(^) = 10

[0008] a corresponds to the radius of the loop, and b corresponds to the thickness of the loop along the z-axis.

[0009] Curve 31 corresponds to the directivity along the z-axis, in dBi, of the antenna described in [1], as a function of the C / X ratio. The C / X ratio can be modified either by changing the size of the antenna (here the circumference) by fixing a given frequency, or, for a given physical size, by varying the frequency across the wavelength.

[0010] When the circumference C of the loop is small compared to the wavelength A, typically when C / X < 0.5, the maximum radiation remains in the plane of the loop. Indeed, the transition to the fundamental mode of the antenna is accompanied by a decrease in directivity along the axis orthogonal to the plane of the loop, and the radiation occurs primarily in the plane of the loop.

[0011] For networking loop antennas in "End-Fire" mode (superposition in the propagation direction orthogonal to the plane of the loop), it is necessary that there be good directivity along the axis orthogonal to the plane of the loop.

[0012] However, the problem of maintaining directivity outside the plane of a loop arises during antenna miniaturization. This problem is clearly illustrated in reference 22 of [Fig. 2], through a radiation zero along the Oz axis.

[0013] The diversity of miniature antennas presented in the literature is substantial, and the design of a directional and miniature radiating element represents an additional challenge. Several approaches exist to address this need.

[0014] As presented in [2], part of the solutions proposes to use Huygens sources (dipole and loop with radiation in the plane of the loop), thus using the high natural directivity of this radiating element.

[0015] Other solutions are based on a joint optimization between a dipole and a reflector plane (sometimes modified for compactness, wide matching band and directivity, as described in [3]).

[0016] In the field of loop antennas, [4] proposes to include an impedance adjustment space at the loop level.

[0017] In [5], coupling between elements is used, but without any particular constraint on the radiation pattern of the antenna.

[0018] These solutions do not meet the need for directivity and radiation control mentioned above.

[0019] In the antenna described in [6], the use of loops close to the fundamental mode C / X < 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.

[0020] In [7], loop Yagi-Uda antennas are networked in an "End-Fire" arrangement. However, the loops used are not miniature in size (C / X "1") and therefore exhibit maximum radiation orthogonal to the plane.

[0021] 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 compact arraying in an "end-fire" configuration.

[0022] Thus, the invention aims to provide a compact and directional radiating element for use alone or in a network. Summary of the invention

[0023] 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 the level of a third connection port located in the axis of symmetry, the main radiating element and the central element being coplanar.

[0024] 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.

[0025] Advantageously, each branch of the central element comprises a main strand that 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.

[0026] 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.

[0027] Advantageously, the impedance of the first connection port and the impedance of the second connection port are determined by applying a superdirectivity algorithm.

[0028] Advantageously, the superdirectivity algorithm uses a radiation diagram method.

[0029] Advantageously, the first connection port and the second connection port each include a power supply circuit.

[0030] Advantageously, the first connection port includes a load, and the second connection port includes a power supply circuit.

[0031] The invention also relates to an antenna system, comprising the aforementioned antenna, and a reflector element placed near the main radiating element and parallel to the plane of the main radiating element.

[0032] The invention also relates to an antenna array comprising at least two of the aforementioned antennas, the antennas being networked according to an "End-fire" type configuration. Description of the figures

[0033] Other features, details and advantages of the invention will become apparent from the description made with reference to the accompanying drawings given by way of example.

[0034] Fig. 1, already described, illustrates a loop antenna according to the prior art.

[0035] Figure 2, already described, illustrates radiation patterns of the loop antenna according to Figure 1, for different values ​​of C / X.

[0036] Figure 3 illustrates an antenna according to the invention.

[0037] Figure 4 illustrates radiation patterns of the loop antenna according to the invention, and according to Figure 5, for different values ​​of C / X.

[0038] Fig. 5 illustrates a loop antenna without a central element, and whose loop is excited on both sides at two connection ports.

[0039] Fig. 6 illustrates an embodiment of an antenna system, comprising an antenna and a reflector element.

[0040] Figure 7 illustrates an embodiment of an "End-fire" type networking of several antennas according to the invention.

[0041] Figure 8 illustrates an embodiment of an antenna system comprising a reflector element and an "End-fire" type networking of several antennas according to the invention. Detailed description of the invention

[0042] The antenna according to the invention is shown in [Fig.3].

[0043] The antenna 1 comprises a main radiating element 2 of the loop antenna type. In [Fig. 3], the loop is circular, but other shapes can be considered for the loop, including square, rectangular, or even more complex loops of the polygonal surface or volume 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 metallic track printed in the form of a loop.

[0044] The printed metal track, typically comprising copper, can be made on a dielectric substrate according to a PCB (for "Printed Circuit Board") type technology.

[0045] The substrate can be made from materials such as epoxy glass fiber (FR-4) or polytetrafluoroethylene (PTFE), although other materials may be used depending on the specific requirements of the antenna.

[0046] 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.

[0047] A central element 5 of the electric dipole type is circumscribed within 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 a second connection port 4.

[0048] The central element 5, of the electrical 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 opposite phase with respect to the second pole, which can be achieved by a balanced excitation at the output of a symmetrizing circuit (a device called a "balun").

[0049] The first connection port 3 and the second connection port 4 are arranged on either side of the main radiating element 2, that is, diametrically opposite each other with respect to the center of the loop. This arrangement on either side of the main radiating element 2 (with respect to the center of symmetry) can be applied to both a circular loop antenna and the other aforementioned loop antenna shapes.

[0050] The number of connection ports may be greater than two, provided that symmetry with respect to axis 8 is respected.

[0051] According to a first embodiment, one of the connection ports (for example the first connection port 3) includes a load, and the other connection port (for example the second connection port 4) includes a power supply circuit.

[0052] The power supply circuit serves as an electrical link between the main radiating element 2 and the antenna's transmitting / receiving electronics. 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 power supply circuit.

[0053] In this embodiment, the load is a so-called passive load, and may include a resistor. However, active loads can also be considered, allowing for the introduction of impedances whose real parts are negative.

[0054] The loads can be designed with equivalent circuits of series or parallel RLC type.

[0055] According to a particularly advantageous embodiment, each of the connection ports includes a power supply circuit. The symmetry of the dipole with respect to the axis of symmetry 8 allows for balanced coupling on the loop (two excitation points per coupling). This makes it possible to supply the dipole at the center and to weight, with the same load set, the two ports of the loop. The application of two loads symmetrical at the level of the main radiating element allows the radiation to remain orthogonal to the xy plane containing the loop.

[0056] The antenna 1 according to the invention comprises a central element 5 of the electric dipole type, circumscribed within the main radiating element 2. By "circumscribed", it is understood that the central element 5 is contained within specific limits defined by the inside of the loop of the main radiating element 2.

[0057] The central element 5 is powered at a third connection port 9 located on the axis of symmetry 8. The third connection port 9 includes a power supply circuit. Furthermore, the third connection port 9 may include an impedance matching circuit including series and / or parallel loads.

[0058] The central element 5 is located in the same plane as the main radiating element 2. Their being located in the same plane reduces the inter-element spacing compared to a configuration in which one of the two elements would be outside the plane. The arrangement of the main radiating element 2 and the central element 5 in the same plane simplifies the placement of the first connection port 3 and the second connection port 4.

[0059] Figure 4 illustrates radiation patterns obtained for different values ​​of C / X (C corresponds to the circumference of the main radiating element 2 and X corresponds to the wavelength of the carrier), 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 visualization of the antenna's directivity, that is, in which directions the antenna radiates or receives electromagnetic signals best.

[0060] Figure 4 also illustrates the antenna's directivity along the z-axis of Figure 3, that is, the axis orthogonal to the plane of the loop (see curve 26 with the central element and curve 27 without the central element). Figure 5 illustrates the loop antenna 31 without a central element, used for plotting curve 27, and whose loop is excited on both sides at the first connection port 3 and the second connection port 4 with the correct phase distribution.

[0061] It appears that for C / X < 1 and C / X > 1.25, and in particular C / X = 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 / X = 0.25, shows that there is maximum directivity along the z-axis and good symmetry in the pattern.

[0062] This radiation characteristic, even for low C / X values, makes it possible to consider an "end-fire" type network of several antennas according to the invention. Thus, at a constant wavelength, the circumference of the antenna can be reduced without affecting the off-plane directivity.

[0063] End-fire arraying consists of aligning a plurality of antennas along an axis orthogonal to the plane of each antenna. By concentrating energy in a specific direction (corresponding to the orthogonal axis), end-fire antennas offer high gain in that direction, thereby improving the effective power of the signal transmitted or received in that direction.

[0064] An embodiment of the antenna according to the invention is described in relation to [Fig.3]. The branch 6 of the central element 5 comprises a main element 10 which extends orthogonally to the plane (or axis) of symmetry 8, and two auxiliary elements (12 and 13), arranged on either side of the end 17 of the main element 10 opposite the third connection port 9.

[0065] By "strand" (main strand or auxiliary strand), we mean an elongated and relatively thin part.

[0066] Thus, each auxiliary strand forms an arc of a circle extending from the end 17 of the main strand, at an angle ad 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 a different shape, each auxiliary strand has a shape such that there is a free space (without a metallic track) between the auxiliary strand and the interior of the main radiating element 2, so as to create 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 straight shape, so as to be parallel to the main radiating element 2.

[0067] 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.

[0068] The dimensions of the main radiating element 2 and the central element 5 are determined so that the impedance of the first connection port 3 and the second connection port 4 has the smallest possible real part, in particular a zero or near-zero real part. By "near-zero," we mean a real part much smaller than the radiation resistance of the antenna (on the order of at least one hundred times smaller).

[0069] This allows the central element 5 to be supplied at the center and the two connection ports (3, 4) of the main radiating element 2 to be weighted with the same load set. Thus, the phase planes of the main radiating element 2 and the central element 5 are co-located.

[0070] Furthermore, adding a central element 5 to the center of the main radiating element 2 allows for optimization of the impedance of the first connection port 3 and the second connection port 4, by manipulating 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 prior art.

[0071] 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 ô 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 ad of the circular arcs (12, 13, 14, 15).

[0072] For example, an antenna according to the invention can be implemented with the values ​​indicated in the table below (units shown in parentheses): X (m) r (m) w (m) ô (m) rd (m) wd (m) ad (°) 0.3 0.25.X / 2ir

[0073] For these values, the following impedances (in Q) are obtained, respectively at the first connection point 3 and the second connection point 4:

[0074] Z3 = 0+lj*535

[0075] Z4 = 0+lj*535

[0076] The impedances at the connection points are identical, which guarantees balanced coupling at the main radiating element 2, and radiation which remains orthogonal to the plane of the loop, even with a low value of C / X (C / X = 0.25).

[0077] Load optimization can be implemented by applying a superdirectivity algorithm, which allows the complex amplitudes to be obtained analytically (i.e. by solving a linear system of equations) at the antenna connections, i.e. directly at the level of the loads and / or feed points.

[0078] An example of a superdirectivity algorithm is described in [9].

[0079] Preferably, the superdirectivity algorithm uses a radiation diagram method, which is easily implementable. Indeed, it does not require iteration or decomposition in a particular basis.

[0080] Alternatively, load optimization can be implemented by other methods, in particular by the spherical mode method, or by the characteristic mode method.

[0081] Fig. 6 illustrates an antenna system, comprising an antenna 1 as defined above, and a reflector element 28 placed near the main radiating element and parallel to the plane of the main radiating element.

[0082] This configuration provides an increase in directivity in the plane orthogonal to the antenna. In particular, the increase in directivity can be of a factor 2 if the reflector is a good metallic conductor and has a diameter greater than the wavelength.

[0083] The reflector is flat and can have a diameter on the order of the wavelength of the transmit / receive signal. The effects on directivity are significant as the antenna approaches the reflector element 28 (for example, a distance of 0.1 kΩ). However, the closer the antenna 1 gets to the reflector element 28, the more difficult impedance matching of the reflector element 28 becomes. A compromise between improving directivity and impedance matching must therefore be made.

[0084] Figure 7 illustrates an antenna array 29 according to the invention. It comprises at least Two antennas, as described previously, are networked in an end-fire configuration. The central elements of the different antennas do not need to be aligned. The antennas radiate along an axis perpendicular to the loop plane, which is particularly well-suited to networking.

[0085] Indeed, the axis of maximum radiation is aligned with the duplication axis 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 highly directional array.

[0086] Embodiments of the system with reflector and the antenna networking according to the "End-fire" configuration can be combined, as illustrated in [Fig. 8]. The antenna network 30 comprises a plurality of networked antennas 1, positioned facing a reflector 28.

[0087] References cited:

[0088] [1] Balanis, C., 2005. Antenna theory. New York: Wiley-Interscience.

[0089] [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

[0090] [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

[0091] [4] EP2178166A1, « Antenne à boucle incluant un espace de réglage d’impédance et procédés associés »

[0092] [5] US7298343B2, « RFID tag with enhanced readability »

[0093] [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

[0094] [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

[0095] [8] F. Munoz et al. "Compact Superdirective Electric Dipole Array for Far-field Mea- surement 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.

[0096] [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

Demands

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 the level of 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 the central element (5) are determined so that the impedance of the first connection port (3) and the second connection port (4) have a zero real part.

3. 3. Antenna according to any one of the preceding claims, wherein each branch (6, 7) of the central element (5) comprises a main strand (10, 11) extending 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. 4. 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 elements (12, 13, 14, 15) have a circular arc shape, and wherein the following parameters are used to optimize the impedance of the first and second connection ports: the diameter (r) of the main radiating element (2), the width (w) of the main radiating element (2), the spacing (ô) 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 (ad) of the circular arcs (12, 13, 14, 15).

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

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

7. 7. Antenna according to any one of the preceding claims, wherein the first connection port (3) and the second connection port (4) each comprise a power supply circuit.

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

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

10. 10. Antenna array (29), characterized in that it comprises at least two antennas according to any one of claims 1 to 8, the antennas being networked in an "End-fire" type configuration.

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