Rectenna device using loop antenna

Abstract

To provide a rectenna device using minute loop antenna with a small shape applicable to received power larger than before.SOLUTION: A rectenna device includes a loop antenna and an impedance transformation capacitor connected in series to an output part of the loop antenna, and is connected with a rectifier through the impedance transformation capacitor.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a rectenna device using a loop antenna, and more particularly to miniaturization of the loop antenna. [Background technology]

[0002] A rectenna receives high-frequency radio waves with an antenna and converts them into DC power with a rectifier. For example, Non-Patent Document 1 discloses a rectenna that resonates with a small inductive loop antenna with high impedance and a rectifier with capacitive impedance. However, this technology increased the RF voltage applied to the rectifier, aiming to achieve high efficiency at micro-watt level radio waves. In recent years, when trying to obtain high power at sub-mW level at a receiving distance of several meters with a 920 MHz band, 1 W transmitter, which has been considered in the IoT field, the loop antenna becomes larger, leaving room for further improvement. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] Shinichiro Tsujita, Naoki Sakai, Kenji Itoh, et.al. ”920MHz band high sensitive rectenna with a small loop antenna” 2019 Asia Pacific Microwave Conference Summary of the Invention [Problem to be solved by the invention]

[0004] An object of the present invention is to provide a luxena device using a small loop antenna having a small shape that can be applied to receiving power greater than that of conventional devices. [Means for solving the problem]

[0005] The rectenna device according to the present invention is characterized in that it comprises a loop antenna and an impedance transformation capacitor connected in series to the output portion of the loop antenna, and is connected to a rectifier via the impedance transformation capacitor. Here, the term "impedance transformation capacitor" is used to mean that the capacitance is smaller than that of the rectifier and is different from conventional DC cut capacitors. In the present invention, a loop antenna refers to an antenna in a loop shape, and the loop shape is not limited to a circular shape, but may be an elliptical shape, a square shape (rectangle, polygonal shape), or the like, as long as a loop is formed. [Effects of the Invention]

[0006] The rectenna of the present invention is a small, high-impedance antenna, but by connecting a capacitor (impedance transformation capacitor) in series to the output section, it can be made to operate as a low-impedance antenna from the rectifier's perspective, and can obtain DC power of 0.1 to 10 mW for a 920 MHz band, 1 W transmitter, for example. This makes it possible to apply it to power sources for sensors used in the IoT field and charging power sources for operating transmitters such as Bluetooth (registered trademark). Furthermore, since it can be made into a small loop-shaped antenna, it can follow deformations in shape. [Brief explanation of the drawings]

[0007] [Figure 1] 1 shows an example of the configuration of a rectenna according to the present invention. [Figure 2] 1A and 1B are diagrams illustrating the operation of a rectenna according to the present invention. [Figure 3] 1 shows a first embodiment of a rectenna according to the present invention. [Figure 4] 1 shows a comparison between Example 1 and the comparative example, where (a) shows the rectification efficiency and (b) shows the output voltage. [Figure 5] An example of the configuration of a loop antenna is shown, where (a) is a perspective view of the front substrate and (b) is a perspective view of the back substrate. [Figure 6](a) and (b) show examples of rectifiers. [Figure 7] An example of using a rectenna is shown below. [Figure 8] 1 shows a configuration example of a comparative example. [Figure 9] The operating principle of the rectenna is shown. [Figure 10] 1 shows a loop antenna as a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0008] Currently, practical use of wireless power transmission systems using the 920 MHz, 2.4 GHz, and 5.7 GHz bands is being considered. The 920 MHz band has a longer communication distance than the 2.4 GHz band used for Wi-Fi and other applications, so rectennas are being considered as a means of making various sensors such as smart meters and monitors used in energy management systems such as HEMS (Home Energy Management Systems) battery-free. For example, the conceptual diagram is shown in FIG. The 920 MHz band radio waves emitted from a 920 MHz, 1 W transmitter are converted into DC power by a rectenna, which can be used to power sensors or as a charging power source for Bluetooth batteries used to transmit information over the Internet.

[0009] The aforementioned Non-Patent Document 1 discloses a rectenna that is a combination of a minute loop antenna having high inductive impedance and a capacitive rectifier made of 40 nm SOI-CMOS with a low threshold. The equivalent circuit and circuit diagram are shown in Figure 8. Here, the small loop antenna has a width W of 5 mm and an outer diameter φ of 20 mm, the antenna impedance R0 is set to 27 kΩ, and the input power is operated at −25 dBm (3.2 μW).

[0010] The principle of this operation will be explained with reference to FIG. The inductive minute loop antenna and the capacitive rectifier are resonated at angular frequency ω0, and the RF voltage V applied to the rectifier rf By increasing this, we aim to achieve highly efficient operation even with weak radio waves. In the design shown in FIG. 8, the rectification efficiency is 25% and the output voltage is 0.35V when the received power is -30dBm. where the RF voltage V rf is set to be sufficiently higher than the diode threshold voltage and below the breakdown voltage, and the received power of the antenna is P rf Then, V rf The relationship is shown in the following formula. TIFF0007742633000001.tif31166V rf : RF voltage R0: Impedance at resonance P rf : Receiving power ω0: Resonance angular frequency L a :Antenna inductance R a :Antenna resistance C a : Parasitic capacitance C r : rectifier input capacitance R r : Rectifier resistance Appropriate V rf is determined by the diode used in the rectifier, so P rf To increase the power, it is necessary to reduce the impedance R0 at resonance according to the above formula (1). To achieve this, the antenna inductance L a needs to be made smaller. Also, the antenna reactance X a is C r and is constant regardless of R0. TIFF0007742633000002.tif16166Therefore, to achieve a low antenna inductance La, the parasitic capacitance Ca must be increased.

[0011] From the above viewpoint, the rectenna according to the present invention is configured such that an impedance transformation capacitor C is connected in series to a minute loop antenna 11 as shown in FIG. s is connected. Here, in the impedance transformation capacitor, two C s The combined capacitance of the rectifier is C r is smaller than. In addition, the capacitance of the DC cut capacitor used in conventional rectennas is C s is sufficiently larger than C r is greater than. The impedance transforming capacitor in the present invention functions differently from a DC blocking capacitor.

[0012] Impedance transformation capacitor C in the present invention s The function of will be explained with reference to FIG. Output RF voltage V of the minute loop antenna c is the impedance transformation capacitor C s The rectifier is divided by the RF voltage V r is applied. If the voltage division ratio is n, then C s =2C r / (n-1),|V r |=|V c | / n, so increasing the voltage division ratio n results in low C s This becomes: As shown in Figure 2, in the equivalent circuit at resonance, the antenna inductance L a And, C s and C r The combined capacitance of these two components cancels out, resulting in an equivalent circuit configuration via an ideal impedance transformer (transformer). Therefore, the resonant impedance R0 of the antenna seen from the rectifier is 1 / n 2 This means that the impedance can be made low.

[0013] As an example 1, a loop antenna 11 is connected to C sThis shows an example where a resonant impedance R0 is designed to be 4 kΩ by connecting a capacitor of R0 = 0.3 pF in series. The width W of the loop antenna was 2.0 mm and the outer diameter φ was 20.5 mm. In contrast, when the design was made based on Non-Patent Document 1 so that the resonant impedance would be equivalent, the comparative example had a large antenna shape with an antenna width W of 20.0 mm and an outer diameter φ of 30.0 mm, as shown in FIG. A graph comparing these rectification efficiencies and output voltages is shown in Figure 4. When comparing the rectification efficiency at an input power of -10 dBm, the present invention (Example 1) is 34% and the comparative example is 37%, which is almost the same, but the outer diameter of the loop antenna is smaller than that of the comparative example, from 30.0 mm to 20.5 mm, and the antenna width is about 1 / 10, from 20.0 mm to 2.0 mm.

[0014] In the loop antenna according to the present invention, a capacitor may be introduced into the output section of the antenna by patterning a loop antenna 11 on the surface (a) of the substrate as shown in FIG. 5 and providing antenna feeding points 11b, 11b on the back surface of the substrate corresponding to the output sections 11a, 11a. Furthermore, the configuration of the rectifier is not limited, and examples include the CMOS rectifier circuit described in FIG. 8 as a comparative example, a balanced type using a voltage doubler circuit shown in FIG. 6(a), and a bridge type shown in (b). [Explanation of symbols]

[0015] 11 Loop antenna 12a Impedance transforming capacitor

Claims

[Claim 1] A loop antenna and two impedance transforming capacitors connected in series to two output terminals of the loop antenna, the two impedance-transforming capacitors have equal capacitance; connected to a balanced rectifier via the two impedance transforming capacitors; A rectenna device characterized in that the capacitance value of the series combination of the two impedance transformation capacitors is smaller than the input capacitance of the balanced rectifier.

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