FR3095085A1 - Antenna comprising a printed patch and corresponding rectifier antenna - Google Patents

Abstract

Antenna comprising a printed patch and corresponding rectifier antenna The invention relates to an antenna (100) comprising at least one printed patch (100pi) on a substrate and configured to deliver a guided electromagnetic wave (100og) from a sensed electromagnetic field ( 100em). More particularly, the printed patch comprises at least one U-shaped slot (100f1, 100f2) comprising a central section (100f1c, 100f2c) and two side sections (100f1l, 100f2l) which are substantially parallel. The guided electromagnetic wave propagates substantially parallel to the side sections from the center section in a direction opposite to a direction of extension from the center section of the side sections. Abstract figure: Fig. 1

Description

Antenna comprising a printed pad and corresponding rectifier antenna

Field of the invention

The field of the invention is that of antennas of the printed type. More particularly, the invention relates to an antenna comprising a printed patch as well as to a rectifying antenna comprising such an antenna as a radiating part. In this case, the rectifying antenna delivers an electrical signal (voltage or current) rectified from an electromagnetic field picked up by the antenna of the printed type.

The invention thus has numerous applications in the fields in which an antenna of the printed type is used for transmission and/or reception of electromagnetic waves, in particular but not exclusively in the field of wireless communications or the recovery of radio frequency energy, or RF energy, when such a printed type antenna is incorporated as a radiating part of a rectifier antenna. In the latter case, these are, for example, fields in which electronic devices require a low-power autonomous power supply, such as the field of RF tags (or transponders), the field of the Internet of Things (e.g. for smart sensors, connected clothing, etc.), or else fields in which such an autonomous power supply makes it possible to recharge or increase the autonomy of electronic devices (e.g. the field of smartphones, etc.).

Prior art and its drawbacks

We focus more particularly in the remainder of this document on describing an existing problem in the field of RF energy harvesting, with which the inventors of this patent application have been confronted. The invention is of course not limited to this particular field of application, but is of interest in any field in which the use of an antenna of the printed type is of interest.

Harvesting RF energy (or equivalently harvesting electromagnetic energy, or EM energy) present in the atmosphere surrounding a given electronic device has recently become one of the most demanded technologies to power such a device. Indeed, such a power supply technology makes it possible to reduce the overall cost of a system by eliminating all or part of the need for batteries and other power resources. In this context, the rectifying antenna (or "rectenna") is a key element of an RF energy harvesting system.

A rectifying antenna conventionally comprises a radiating part and a rectifying circuit.

The radiating part (i.e. the antenna itself) plays an important role in capturing energy. More specifically, the radiating part acts as a transducer between the electromagnetic field picked up and the guided electromagnetic wave that it delivers to the rectifying circuit. In a world in which certain electronic devices tend to be more and more compact, a radiating part of small size is in great demand. Among the different types of antennas, printed type antennas present particularly sought-after solutions due to their small size, low weight, low cost and ease of manufacture.

The rectifier circuit for its part rectifies the guided electromagnetic wave in order to deliver a rectified electrical signal (voltage or current) (i.e. having a continuous, or DC, non-zero component) which can, for example, power an electronic device.

The choice of the operating frequency of the rectifier antenna is also an important factor for RF energy harvesters. Generally speaking, a low operating frequency is associated with low RF signal attenuation but simultaneously results in a large antenna size. In addition, the bandwidth is limited in this case. A choice of a high operating frequency leads to a small antenna size and a large potential bandwidth. For energy harvesting applications, it is preferable that the rectifier antennas are compact to avoid obtaining too large electronic devices in the end. The industrial, scientific and medical, or ISM, band at 2.45 GHz can be considered as a suitable option, as it offers a good compromise between the aforementioned aspects. Additionally, spectrum studies indicate that RF energy present in the 1800-2500 MHz frequency range is dominant in the ambient environment.

For example, the article by Y. Chen and C. Chiu, " Maximum Achievable Power Conversion Efficiency Obtained Through an Optimized Rectenna Structure for RF Energy Harvesting ", IEEE Trans. Antennas Propag., 2017, 65, (5), pp. 2305-2317, describes such a rectifying antenna based on the use of a printed antenna. However, the dimensions of the rectifier antenna obtained remain large for practical use.

There is therefore a need for an antenna having a reduced size while having a yield which remains high at a given operating frequency, for example around 2.45 GHz, as well as for a corresponding rectifying antenna, i.e. incorporating such an antenna of reduced size in its radiating part while having a high conversion efficiency (RF to DC).

In one embodiment of the invention, an antenna is proposed comprising at least one patch printed on a substrate and configured to deliver an electromagnetic wave guided from a sensed electromagnetic field. The printed patch comprises at least one U-shaped slot comprising a central section and two substantially parallel side sections. The guided electromagnetic wave propagates substantially parallel to the side sections from the central section in a direction opposite to a direction of extension, from the central section, of the side sections.

Thus, the invention proposes a new and inventive solution for obtaining an antenna having high efficiency while having small dimensions compared to known solutions.

More particularly, the use of a slot having a U-shape on the printed pad makes it possible to reduce the dimensions of the pad in question while making it possible to obtain a resonance at the desired frequency. The performance of the antenna is thus preserved despite the reduction in size obtained. In addition, the radiation pattern obtained is symmetrical, whether in the E plane or in the H plane, which further improves the energy harvesting performance under operational conditions.

According to one embodiment, the antenna comprises a first and a second U-shaped slot each comprising a central section and two substantially parallel lateral sections. The second slot has a greater length than the first slot and is disposed outside and substantially parallel to the first slot. The propagation of the guided electromagnetic wave takes place substantially parallel to the side sections from the central section of the second slot in a direction opposite to a direction of extension, from the central sections, of the side sections.

Thus, due to its greater length, the second slot would resonate at a lower frequency than the first slot if it were alone on the patch. However, the different sections of the two slots are parallel to each other. Thus, the presence of the second U-shaped slot improves by coupling the adaptation obtained in the frequency band centered on the resonance frequency linked to the first U-shaped slot for the radiating part of the rectifier antenna.

According to one embodiment, a length of the side sections and of the central section of the first slot is respectively substantially equal to 15.8 mm and 5.2 mm. An edge-to-edge spacing between the side sections of the first and second slots is substantially equal to 0.2 mm. An edge-to-edge spacing between the central sections of the first and second slots is substantially equal to 0.7 mm. A width of the side and central sections is respectively substantially equal to 0.4 mm and 0.35 mm.

Thus, a resonance around 2.45 GHz is obtained.

According to one embodiment, the antenna further comprises a section of coplanar line printed on the substrate and guiding the electromagnetic wave substantially parallel to the lateral sections.

Thus, the adaptation of the antenna is facilitated.

In one embodiment of the invention, there is proposed a rectifying antenna delivering a rectified electrical signal from a picked up electromagnetic field. The rectifier antenna includes:
- a radiating part delivering the electromagnetic wave guided from the captured electromagnetic field; And
- a rectifier circuit configured to deliver the rectified electrical signal from the guided electromagnetic wave.
The radiating part comprises an antenna comprising at least one printed patch as described above (according to any one of the aforementioned embodiments) configured to deliver the electromagnetic wave guided from the electromagnetic field picked up.

Thus, the rectifier antenna has a high efficiency while having small dimensions when generating an electrical signal (voltage or current) from a captured electromagnetic field.

According to one embodiment, the rectifier circuit comprises a plurality of discrete components implanted at least in part in a thickness of the substrate.

Thus, an additional saving of space is obtained.

According to one embodiment, the rectifier circuit is of the voltage doubler type.

Thus, the performance of the rectifying antenna is improved. The voltage doubler rectifier circuit is for example implemented according to a Greinacher assembly.

According to one embodiment, the rectifier circuit implements at least two Schottky diodes.

Thus, the low threshold voltage of the diodes allows improved efficiency of the rectifier antenna.

According to one embodiment, the rectifier antenna further comprises an impedance matching circuit configured to match an impedance of the radiating part to an impedance of the rectifier circuit.

Thus, the performance of the rectifying antenna is improved.

According to one embodiment, the matching circuit comprises a plurality of discrete components implanted at least partly in the thickness of the substrate.

Thus, an additional saving of space is obtained.

According to one embodiment, the impedance matching circuit implements an L-shaped network comprising an inductor in series on a propagation path of the electromagnetic wave and a capacitor in parallel on said path.

List of Figures

Other aims, characteristics and advantages of the invention will appear more clearly on reading the following description, given by way of a simple illustrative example, and not limiting, in relation to the figures, among which:

represents an antenna comprising a printed patch according to one embodiment of the invention;

illustrates the effect of the coupling between the first and second U-shaped slots of the printed patch of the antenna of FIG. 1 on the reflection coefficient obtained for the antenna in question;

illustrates the reflection coefficient obtained for the antenna of FIG. 1 for different edge-to-edge spacings between the side sections of the first and second U-shaped slots of the antenna in question;

illustrates the reflection coefficient obtained for the antenna of FIG. 1 for different lengths of the side sections of the first and second U-shaped slots of the antenna in question;

illustrates the reflection coefficient obtained for the antenna of FIG. 1 for different edge-to-edge spacings between the central sections of the first and second U-shaped slots of the antenna in question;

represents the functional blocks of a rectifier antenna comprising the antenna of FIG. 1 as a radiating part according to one embodiment of the invention;

illustrates an implementation of the rectifier and impedance matching circuits of FIG. 6 according to one embodiment of the invention;

illustrates the power conversion efficiency of the rectifier antenna of FIG. 6 as a function of the load impedance and for different electromagnetic field power values picked up.

Detailed Description of Embodiments of the Invention

We now present, in relation to FIG. 1 an antenna 100 of the printed type according to one embodiment of the invention. The characteristics of antenna 100 are discussed further in connection with Figs. 2 , 3 , 4 , and 5 .

More particularly, the antenna 100 comprises a pad 100pi printed on a substrate and configured to deliver a guided electromagnetic wave 100og from a captured electromagnetic field 100em. The other side of the substrate (i.e. the side on which the 100pi pad is not printed) is metallized in order to act as a ground plane.

The pad 100pi comprises a first 100f1 and a second 100f2 U-shaped slot each comprising a central section (100f1c for the first slot 100f1, and 100f2c for the second slot 100f2) and two substantially parallel side sections (100f1l for the first slot 100f1, and 100f2l for the second slot 100f2). Here, by "substantially parallel", we mean that the side sections are parallel to each other within the implementation tolerances for a given implementation technology (e.g. a tolerance of a few percent (e.g. 1%, 2%, etc.) , or a tolerance as specified in DIN 7168).

The U-shaped geometry of the slots 100f1, 100f2 makes it possible to obtain an antenna 100 of small size compared to those of existing antennas while maintaining good power conversion efficiency. Moreover, the radiation pattern obtained by such a geometry is symmetrical, whether in the E plane or in the H plane, which further improves the energy harvesting performance under operational conditions.

The second slot 100f2 has a greater length than the first slot 100f1. Furthermore, the second slot 100f2 is arranged outside and substantially parallel to the first slot 100f1. Here, by "outside", it is necessary to understand outside the convex quadrilateral of which three of its four sides are the central section 100f1c and the two lateral sections 100f1l of the first slot 100f.

Furthermore, the antenna 100 is optimized to obtain adaptation in a frequency band centered on 2.45 GHz. To do this, the following dimensions are considered:
- the length of the lateral sections 100f1l and of the central section 100f1c of the first slot 100f1 is respectively substantially equal to 15.8 mm (parameter m in fig. 1) and 5.2 mm (parameter n in fig. 1 );
- The edge-to-edge spacing between the side sections 100f1l, 100f2l of the first 100f1 and second 100f2 slots is substantially equal to 0.2 mm (parameter i in FIG. 1);
- the edge-to-edge spacing between the central sections 100f1c, 100f2c of the first 100f1 and second 100f2 slot is substantially equal to 0.7 mm (parameter k in FIG. 1);
- the width of the side sections 100f1l, 100f2l of the first 100f1 and second 100f2 slots is substantially equal to 0.4 mm (=(fi)/2 with the parameter f of FIG. 1 which is equal to 1 mm);
- the width of the central sections 100f1c, 100f2c of the first 100f1 and second 100f2 slots is substantially equal to 0.35 mm (=(jk)/2 with the parameter j of FIG. 1 which is equal to 1.4 mm).

Here, by "substantially equal to", we mean that the targeted dimension is reached within implementation tolerances for a given implementation technology (for example a tolerance of a few percent (e.g. 1%, 2%, etc.), or a tolerance as specified according to the DIN 7168 standard). Furthermore, the substrate, here an FR-4 type epoxy glass fabric (presenting a dielectric permittivity of 4.4 +/- 5% and a loss tangent of 0.02), has a thickness substantially equal to 1, 6mm. Similarly, the width (parameter a in Fig. 1) and the length (parameter b in Fig. 1) of the antenna 100 are 8.6 and 24.9 mm respectively. In other embodiments, other types of substrates are used.

Back to fig. 1, the first 100f1 and second 100f2 U-shaped slots cause two different resonances. Indeed, if we refer to FIG. 2, it appears that:
- in the theoretical case in which no slit is present on the printed patch 100pi, no resonance appears in the analysis frequency band (curve 200_0 in FIG. 2);
- in the theoretical case in which only the first U-shaped slot 100f1 is present on the printed patch 100pi, a single resonance at 2.45 GHz exists (curve 200_1 in FIG. 2); And
- in the theoretical case in which only the second U-shaped slot 100f2 is present on the patch 100pi, a single resonance at 2.1 GHz exists (curve 200_1 in FIG. 2).

However, when the two slots 100f1, 100f2 are present simultaneously, only the resonance at 2.45 GHz remains present (curve 200_3 in FIG. 2), but with an improved adaptation in the frequency band of interest due to the coupling effect existing between the two slots 100f1, 100f2. In this way, the conversion efficiency of the rectifier antenna is improved by the presence of the second slot 100f2 in U.

It is thus understood that in certain embodiments not illustrated, a single U-shaped slot can be used in order to produce the antenna, allowing a simpler realization even though the adaptation is slightly degraded. Conversely, in other embodiments, more than two U-shaped slots are used in order to further improve adaptation.

In other embodiments, other values of the aforementioned parameters of FIG. 1 are considered in order to optimize the resonance of the slots 100f1, 100f2 and therefore the adaptation of the antenna 100 in the end, or alternatively to obtain an adaptation of the antenna 100 in another frequency band than around 2 .45GHz. For example :
- fig. 3 illustrates the reflection coefficient obtained for the antenna 100 for different spacings from edge to edge between the side sections 100f1l, 100f2l of the first 100f1 and second 100f2 U-shaped slots (parameter i of FIG. 1);
- fig. 4 illustrates the reflection coefficient obtained for the antenna 100 for different lengths of the side sections 100f1l, 100f2l of the first 100f1 and second 100f2 U-shaped slots (parameter m of FIG. 1); And
- fig. 5 illustrates the reflection coefficient obtained for the antenna 100 for different edge-to-edge spacings between the central sections 100f1c, 100f2c of the first 100f1 and second 100f2 U-shaped slots (parameter k of FIG. 1).

Back to fig. 1, the antenna 100 also includes a section of coplanar line printed on the substrate. The line section guides the electromagnetic wave 100og substantially parallel (the terminology “substantially parallel” being interpreted according to the meaning discussed above) to the side sections 100f1l, 100f2l. More particularly, the electromagnetic wave 100og is guided from the central section 100f2c of the second slot 100f2 in a direction opposite to the direction of extension, from the central sections 100f1c, 100f2c, of the side sections 100f1l, 100f2l.

Back to fig. 1, such a section of line makes it possible to modify the input impedance of the antenna 100, for example to facilitate its adaptation, for example with a rectifier circuit which will process the electromagnetic wave 100og to rectify it in the case where the antenna 100 is incorporated in a rectifier antenna. For example, the following dimensions are considered:
- the width of the line is substantially equal to 2 mm (parameter c in Fig. 1);
- the interval between the line and the ground plane present on either side of the line is substantially equal to 0.45 mm (parameter e in fig. 1); And
- the gap between the 100pi patch and the line is substantially equal to 0.15 mm (parameter g in fig. 1).

In this way an input impedance of 50 Ohm of the antenna 100 is obtained.

In the aforementioned embodiments in which a single U-shaped slot, the coplanar line section extends from the central section of the slot in question. In the event that more than two U-shaped slots are used, the coplanar line stub extends from the center section of the outermost slot of the set of slots.

In other embodiments, such a section of line has a different length, or even zero, and an impedance match on 50 Ohm of the radiating part is not obtained. The adaptation of the radiating part is done in this case directly to that of the rectifier circuit via for example a dedicated adaptation circuit (for example a power adaptation on the conjugate complex of the impedance presented by the rectifier circuit at the frequency of interest), or via a specific dimensioning of the antenna 100 (for example so that the input impedance of the antenna 100 is directly the complex conjugate, at the frequency of interest, of the impedance presented by the circuit which charges the antenna 100 (for example a rectifier circuit) when possible).

We now present, in relation to FIGS. 6 and 7 the functional blocks of a rectifier antenna 600 according to one embodiment of the invention.

More particularly, the rectifier antenna 600 comprises (FIG. 6):
- a radiating part 630 comprising the antenna 100 (according to any one of the aforementioned embodiments) configured to deliver the guided electromagnetic wave 100og from the captured electromagnetic field 100em;
- A rectifier circuit 620 configured to deliver a rectified electrical signal (voltage or current) (ie having a non-zero DC component) from the guided electromagnetic wave 100og; And
- an impedance matching circuit 610 configured to match the impedance of the radiating part 100 to the impedance of the rectifier circuit 620.

In certain embodiments not illustrated, the rectifying antenna 600 does not include an impedance matching circuit 610. For example, when the antenna 100 of the radiating part 630 is sized to present an impedance approaching the complex conjugate of the impedance of the rectifier circuit 620 at a given frequency (power matching) as discussed above.

Back to fig. 6, a load impedance RL is shown. The RL impedance in question is not part of the rectifier antenna 600 itself, but simulates the effect of a load circuit which the rectifier antenna 600 might be required to supply with electrical power. The load impedance RL is thus used to evaluate the performance obtained for the rectifier antenna 600 under operational conditions. For example, a pressure sensor of the SXL series marketed by the company Honeywell has such an input impedance of 4.7 kOhm.

The rectifier circuit 620 is passive and is of the voltage doubler type. In this way, the performance of the rectifier antenna 600 is improved. In other embodiments, the rectifier circuit used is not a voltage doubler (e.g. a half-wave rectifier) in order to simplify the design.

Back to fig. 6, the rectifier circuit 620 is implemented according to a Greinacher circuit. However, in other embodiments, other types of voltage doubler circuits are used, e.g. a Dickson circuit.

Back to fig. 6, the 620 rectifier circuit includes a source capacitance C2 of 100 pF and a load capacitance C1 of 100 pF to obtain the desired performance at 2.45 GHz with a load impedance of 4.7 kOhm. Furthermore, the rectifier circuit 620 includes two Schottky diodes HSMS-2852 (Vbr = 3.8 V, Vfwd = 150 mV and Rs = 25 Ohm). The diodes in question were chosen for the rectification because of their relatively low forward bias. However, in other embodiments, other types of diodes are implemented. In this case, other values of components C1 and C2 can be considered. The same is true for other parameters of the antenna 100 of the radiating part 100 and/or another adaptation frequency value.

Back to fig. 6, the impedance matching circuit 610 is based on an L topology. More particularly, the matching circuit 610 comprises an inductance L1 in series on the propagation path of the electromagnetic wave 100og, as well as an capacitor C3 in parallel on the path in question. A power matching of the radiating part 100 with the rectifier circuit 620 is obtained at 2.45 GHz with the following component values: C3=1.6 pF and L1=9 nH. For other parameters of the radiating part 100 and/or other parameters of the rectifier circuit 620 and/or another adaptation frequency value, other values of the components C3 and L1 can be considered, or even other matching circuit topologies 610 (e.g. a pi topology).

Back to fig. 6, impedance matching circuit 610 as well as rectifier circuit 620 include various discrete components as discussed above. In other embodiments, the circuits in question include all or part of their constituent elements in a distributed form.

Back to fig. 6, the discrete components of the rectifier 620 and impedance matching 610 circuits are implemented in the thickness of the substrate of the antenna 100 of the radiating part 100 as illustrated in FIG. 7. Thus, an additional space saving is obtained for the rectifier antenna 150. However, in other embodiments, none or only part of the discrete components of the rectifier circuit 620 and the transimpedance rectifier circuit 610 are implemented in the thickness of the substrate for reasons of manufacturing simplicity.

We now present, in relation to FIG. 8 the power conversion efficiency (PCE) obtained for the rectifier antenna 600 as a function of the load impedance RL and for different power values Pin of electromagnetic field captured 100em.

As observed, for Pin = -20 dBm, the PCE reaches its maximum (i.e. 19.1%) for an RL load value of 8.5 kΩ. It can also be seen that the PCE curve is relatively flat around the optimum resistance. Recall that, as discussed above, the load RL can take any other value, provided that the elements of the matching circuit 620 (i.e. C3 and L1) are readjusted.

Thus, the rectifier antenna 600 provides PCE results comparable to those presented for example in the aforementioned article by Cheng for the same ranges of Pin power levels, but with a much smaller size. For example in the article by Cheng in question, for Pin = -15 dBm, the PCE values obtained are respectively 31.8 and 30% with loads of 10 and 2.2 kΩ. For the 600 rectifier antenna, the PCE values with these two loads are respectively 29.66 and 24.05% as can be read in fig. 8. By way of comparison, the area of the rectifier antenna presented in Cheng's article is 2000 mm2 while the area of the rectifier antenna 600 optimized for operation at 2.45 GHz according to the parameters described above is 214mm2.

Claims (11)

Antenna (100) comprising at least one printed patch (100pi) on a substrate and configured to deliver a guided electromagnetic wave (100og) from a captured electromagnetic field (100em),
characterized in that said printed patch comprises at least one U-shaped slot (100f1, 100f2) comprising a central section (100f1c, 100f2c) and two substantially parallel lateral sections (100f1l, 100f2l),
said guided electromagnetic wave propagating substantially parallel to said side sections from said central section in a direction opposite to a direction of extension, from said central section, of said side sections.
Antenna according to claim 1 comprising a first (100f1) and a second (100f2) U-shaped slot each comprising a central section and two substantially parallel lateral sections,
said second slot having a greater length than said first slot and being disposed outside and substantially parallel to said first slot,
said propagation of said guided electromagnetic wave taking place substantially parallel to said side sections from said central section of said second slot in a direction opposite to a direction of extension, from said central sections, of said side sections.
Antenna according to claim 2 wherein a length of said side sections and of said central section of said first slot is respectively substantially equal to 15.8 mm and 5.2 mm,
wherein an edge-to-edge spacing between said side sections of said first and second slots is substantially equal to 0.2 mm;
wherein an edge-to-edge spacing between said central sections of said first and second slots is substantially equal to 0.7 mm, and
wherein a width of said side and center sections is substantially 0.4mm and 0.35mm, respectively.
Antenna according to any one of Claims 1 to 3, further comprising a section of coplanar line printed on said substrate and guiding said electromagnetic wave substantially parallel to said lateral sections.
Rectifier antenna (600) delivering a rectified electrical signal from a sensed electromagnetic field, said rectifier antenna comprising:
- a radiating part (630) delivering said guided electromagnetic wave from said captured electromagnetic field; And
- a rectifier circuit (620) configured to deliver said rectified electrical signal from said guided electromagnetic wave,
characterized in that said radiating part comprises an antenna (100) according to any one of claims 1 to 4 configured to deliver said guided electromagnetic wave from said picked up electromagnetic field.
Rectifier antenna according to Claim 5, in which the said rectifier circuit comprises a plurality of discrete components implanted at least partly in a thickness of the said substrate.
Rectifier antenna according to Claim 5 or 6, in which the said rectifier circuit is of the voltage doubler type.
Rectifier antenna according to any one of Claims 5 to 7, in which the said rectifier circuit implements at least two Schottky diodes.
A rectifier antenna according to any of claims 5 to 8 further comprising an impedance matching circuit (610) configured to match an impedance of said radiating portion to an impedance of said rectifier circuit.
Rectifier antenna according to Claim 9, in which the said matching circuit comprises a plurality of discrete components implanted at least partly in the said thickness of the said substrate.
Rectifier antenna according to Claim 9 or 10, in which the said impedance matching circuit implements an L-network comprising an inductance in series on a propagation path of said electromagnetic wave and a capacitance in parallel on said path.