Introduction
Microwave power transfer (MPT) and energy harvesting have become increasingly important with applications ranging from Internet of Thing (IoT) devices to solar power satellites [1, 2]. These technologies facilitate the transmission of electrical energy over long distances without requiring physical wire connections. However, the electromagnetic wave is impaired in the free space between the transmitter and the receiver, mainly due to environmental factors and misalignment losses, leading to decreased overall transmission efficiency. Consequently, a critical challenge facing wireless power transfer (WPT) systems revolves around optimizing power transfer efficiency. Some approaches focus on adjusting the antenna to improve transmission efficiency.
Energy efficiency is greatly boosted by the fractal geometry technique [3]. One of the most common techniques involves a wide beamwidth antenna that increases the coverage area. For instance, in [4], broad bandwidth and large beamwidth were simultaneously achieved through a combination of a square patch antenna and four top-hat monopoles. In [5, 6], metal-column loading on a magneto-electric (ME) dipole antenna was used to achieve wideband and widebeam characteristics.
A flat-top (1 dB) radiation pattern antenna features a main lobe with a relatively constant (flat) gain over a broad angular range. This characteristic has been studied to reduce misalignment losses between the transmitter and receiver [7, 8]. For example, in [8], an aperture-coupled patch antenna using a mushroom structure was introduced to generate uniform power radiation. Thus, it is advantageous to employ a flat-top radiation pattern or a wide beamwidth to mitigate misalignment scenarios in MPT applications.
Meanwhile, the improvement of rectennas, which comprise an energy-harvesting antenna and a radio frequency (RF) to direct current (DC) power conversion rectifier, has emerged as a promising solution for MPT systems [9–11]. In one study [9], a reconfigurable rectenna demonstrated flexibility by receiving both linear and circular polarized incoming waves through the control of p-i-n diodes. Additionally, to address the polarization mismatch issue, [11] introduced a compact dual-polarization metasurface rectenna array. However, these approaches require a connected port, resulting in insertion loss and a complex structure.
To overcome the limitations of conventional rectennas, the rectifier has been integrated into the antenna to achieve more compact and efficient RF-DC conversion. In a recent study [12], a reconfigurable rectenna operated at 5.2 GHz and 5.8 GHz using a switch for convenient tuning and minimal interference. By directly mounting the diode into the receiving antenna, impedance matching between the rectifier and the antenna can easily be achieved without additional impedance-matching components [13]. In another study [14], a planar six-sector 3D spherical coverage rectenna with a fully integrated design was proposed, eliminating the need for impedance matching to reduce insertion loss. Nevertheless, most previous structures have suffered drawbacks due to their low gain and large size.
In this study, a novel uniform power radiation rectenna that aims to reduce misalignment and impedance mismatch issues is presented. The rectenna comprises a fully integrated rectifier within a receiving antenna. The receiving antenna was designed based on the magneto-electric ring dipole (MERD) antenna and incorporates eight metal columns, which utilize high gain and flat-top radiation characteristics. In addition, a class-F rectifier with high RF-DC conversion efficiency is integrated into the receiving antenna. The measured results indicate that antenna impedance matches well with rectifier impedance at 5.8 GHz. Moreover, laboratory tests of the MPT system demonstrate that the rectenna effectively receives uniform power from the power source while simultaneously achieving high RF-DC conversion efficiency.
The rest of this letter is structured as follows. Section II discusses the rectenna configuration, which gives a process of rectenna operational frequency, radiation, and its characteristics. Section III describes the fabrication and measurement of the proposed rectenna and MPT experimental verifications, and it is followed by the conclusion in Section IV.
Rectenna Design and Characteristics
1. Receiving Antenna Configuration
A uniform power radiation rectenna with a rectifier that has been fully integrated into the receiving antenna is depicted in Fig. 1. The proposed receiving antenna consists of a circular ring dipole and a parallel transmission line, which form a MERD structure. To achieve better performance, a double circular ring dipole is used in two layers of the substrate, which interconnect via two shorting lines. Positioned above the ground plane, the MERD antenna maintains a distance from the ground approximately equal to a quarter wavelength.
Additionally, for substrate reuse, the rectifier structure is situated on the back side of the parallel line. Eight metal columns were strategically arranged around the antenna structure at a distance of Lp, improving the uniform power radiation and the reflection coefficient. The overall structure employs three substrates composed of Taconic TLY materials, characterized by a relative dielectric constant of 2.2 and a loss tangent of 0.0009, with a thickness of 0.8 mm.
The MERD antenna and the rectifier were separated using the High Frequency Structure Simulator and the Advanced Design System software to achieve maximum RF-DC conversion efficiency and impedance matching. The optimized parameter values of the proposed rectenna were as follows: WGND = 50, H1 = 13, Hm = 7, wp = 30, w1 = 0.7, w2 = 3, xp = 3, yp = 20, g = 11.5, R1 = 11.5, wf1 = 19, wf2 = 19.3, Lf1 = 1.1, and Lf2 = 1.1 (unit: mm).
2. Uniform Power Principle
The wide beamwidth and uniform power antenna theory were obtained by a combination of the E-fields as follows [4–6]. Fig. 2 provides a detailed visualization of the principle of generating a flat-top radiation antenna, which adds metal columns around the ME dipole antenna. The radiated field of these metal columns is produced in a manner similar to that of a monopole. According to [4], the electric field (E-field) of the metal columns can be calculated based on the array factor. Therefore, the array factor of two opposite metal columns was expressed as follows:
In each plane, the total radiated field of the structure was described as follows:
The total E-field of the antenna was primarily determined using the ratio of the E-field between the ME dipole and the metal column. The effective detail of the metal columns was optimized under two scenarios. The design details and the corresponding reflection coefficient and realized gain are depicted in Fig. 3. Initially, the antenna was simulated without metal columns, and it exhibited poor impedance matching at 5.8 GHz and low gain characteristics. However, this structure achieved a flat-top radiation pattern due to the use of a ring shape, which supported uniform energy emission in all directions. Subsequently, a metal column with a height of hc was introduced to assess its impact on beamwidth control.
Incorporating the metal columns not only achieved good impedance matching at 5.8 GHz but also enhanced the gain and flat-top radiation pattern. However, as the height Hm of the metal column increased up to 9 mm, there was degradation in the flat-top radiation because the E-field of the metal column surpassed the main E-field of the MERD dipole. To further explain the effect of the circular ring-shaped dipole in supporting a flat-top radiation pattern, the E-field distribution on the horizontal and vertical surfaces is depicted in Fig. 4. The E-field mainly radiated around the circular ring-shaped dipole. The figure for the E-field distribution on the vertical plane shows that the proposed receiving antenna obtained uniform power radiation, as illustrated in Fig. 4(a).
Fig. 5 shows the surface current distribution of the proposed rectenna at 5.8 GHz. The top view of the radiator in Fig. 5(a) shows that the current predominantly flowed through the two wings of the dipole. Furthermore, there was a concentration of current on parallel feeding and rectifiers. This is proof that the ME dipole is effective in coupling with the rectifier through parallel feeding, aligning seamlessly with the intended design objective of minimizing connection losses.
3. Fabrication and Measurement Results
The proposed receiving antenna was fabricated to confirm the simulated results, as shown in Fig. 6. The rectifier components were removed from the circuit board to test individual antenna performance. The reflection coefficient (S11) was tested using a vector network analyzer (Agilent 8719D). As shown in Fig. 7(a), the measured and simulated S11 values were lower than −16 dB at 5.8 GHz. Furthermore, the radiated performance of the proposed structure was analyzed in an anechoic chamber environment with scanning angles. As shown in Fig. 7(b), the receiving antenna achieved a maximum gain of 8.2 dBi in the measured results, which agreed well with the simulation results.
The 2D radiation pattern of the receiving antenna in the E- and H-planes is depicted in Fig. 8. The flat-top beamwidth was wider in the E-plane than in the H-plane. The flat-top (1-dB) beamwidth results for the E- and H-planes were 60° and 35°, respectively. Meanwhile, the sidelobe levels were lower than −18 dB in both planes. These results clearly show that the structure was unsymmetrical due to an additional rectifier.
The novelties of the proposed structure aim to provide a compact receiving antenna with a flat-top radiation characteristic. Furthermore, the structure not only achieves high gain but also enables the integration of the rectifier within the receiving antenna. The rectifier integration is described in detail in the next subsection.
4. Rectifier Design Principle
Rectifiers are among the most important components in WPT systems, and they are in charge of converting RF power to DC power. Therefore, power conversion efficiency (PCE), which is always expected to be as high as possible, is the most crucial performance metric. To achieve optimal PCE, the implemented rectifier structure should satisfy fundamental and harmonic matchings, such as class-C [15], class-F [16], and inverse class-F rectifiers [17]. In this study, a class-F topology with extended harmonic control beyond the third order, as proposed in [16], was implemented and integrated into an antenna to realize a complete WPT system with compactness.
The layout and functional blocks of the rectifier are shown in Fig. 9(a). It consisted of the BAT1503W Schottky diode model with detailed Simulation Program with Integrated Circuit Emphasis (SPICE) parameters, as shown in Fig. 9(b); a DC-pass filter and harmonic control network, which suppressed fundamental frequency and odd harmonics with infinite impedance; and a fundamental matching and harmonic control network, which simultaneously provided fundamental matching and short-circuit termination for the second and fourth harmonics.
The rectifier was fabricated and measured independently for PCE verification, as shown in Fig. 9(c). The optimal load resistor was 560 Ω. The capacitors used in the circuit for the DC block and the DC pass filter were lumped elements with the C08BL242X 5UN-X0T model by Knowles Lab. Furthermore, the rectifier geometry was reshaped with an additional transmission line to connect it to the two parallel lines of the receiving antenna.
Fig. 10(a) shows the reflection coefficient S11 at several input power levels. S11 was lowest at 5.8 GHz and < −15 dB at overall input power values, indicating good fundamental matching performance over a wide range of input power. Fig. 10(b) shows the PCE and output voltage of the rectifier versus the input power. Peak PCE was achieved at 79% at 12 dBm, corresponding with an output voltage of 2.648 V. Additionally, the rectifier yielded a wide dynamic range for PCE >50% from −4 dBm to 15 dBm, indicating the suitability of the rectifier for low-power WPT applications.
Rectenna Measurements and Comparison
1. MPT Experiment and Evaluation
The MPT system was configured to demonstrate the performance of the proposed rectenna, as illustrated in Fig. 11. The transmitter setup comprised a horn antenna serving as the transmitting antenna, an RF signal generator, an amplifier, and a DC power supply. The transmitter was excited to emit a power of 30 dBm (1 W) in accordance with human safety regulations. On the receiver side, the setup included the proposed rectenna, which was designed as described in the previous section. A digital multimeter was employed to measure the output voltage of the rectifier. To optimize energy reception within the dynamic range of the rectifier, the proposed rectenna was positioned at a distance of 200 mm from the transmitter to achieve the highest RF-DC conversion efficiency.
In the proposed system, overall efficiency (ηTotal)) was calculated as follows:
where PTE is the power transfer efficiency from the transmitting source to the rectenna, and ηRec is the RF-DC conversion efficiency of the rectifier. The PTE of the MPT system was represented by the transmission coefficient (S21), which was expressed as follows [18]:
The performance of the rectifier was represented by the RF-DC conversion efficiency as follows:
where Pin and Vdc are the input power and output voltage of the rectifier, respectively, and RL is the load resistance. The load resistance of the rectifier was 560 Ω. As shown in Fig. 11, the output voltage reached 1.9 V under line-of-sight conditions, corresponding to an output DC power of 6.44 mW. To further validate the advantage of the flat-top characteristic, the rectenna’s position was kept fixed at a distance of 200 mm, while the lateral misalignment with the transmitting source was varied. Fig. 12 illustrates the stable energy reception within a diameter range of 100 mm, confirming the flat-top characteristic of the receiving antenna.
2. Comparison and Discussion
Table 1 compares the rectenna proposed in this study with those in the most recent relevant works [10, 12, 14, 19, 20]. The proposed rectenna has a smaller size than the structures in [10, 14, 15]. Furthermore, it achieved the highest performance in terms of gain and conversion efficiency. Even though the rectifier was integrated within the antenna, it still achieved the highest efficiency in converting RF to DC. These results indicate that the proposed rectenna is a compact rectenna with a distinct uniform power radiation pattern, which outperforms other works. Specifically, the output power of the proposed rectenna remained consistent even with a lateral misalignment of up to 50 mm.
In summary, the proposed rectenna exhibits several key characteristics, including high gain, compact size, and flat-top radiation pattern, with the added benefit of rectifier integration. The integration of the rectifier is expected to enhance transmission efficiency in microwave power transmission and energy harvesting applications.
Conclusion
In this letter, we introduce a novel compact uniform power radiation rectenna for minimizing losses in microwave power transmission systems. The proposed structure consists of a rectifier that is directly integrated into the receiving antenna where the rectifier’s input impedance is conjugated and matched with the two-parallel feeding of the antenna. Additionally, eight metal columns were strategically placed around the rectenna to enhance the flat-top radiation pattern. The results showed that the receiving antenna yielded a maximum gain of 8 dBi with a uniform beamwidth (1-dB) of 60°, and the rectifier obtained the highest conversion efficiency of 79% at 5.8 GHz. Furthermore, an MPT system was built to demonstrate the rectenna’s performance. The results showed that the proposed rectenna can maintain a uniform voltage of 1.9 V within a lateral misalignment range of 50 mm. With its compact size, high conversion efficiency, and ability to receive uniform power, the proposed rectenna holds promise for minimizing losses in MPT and energy harvesting applications.
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