Double Patch Antenna Array for Communication and Out-of-band RF Energy Harvesting

This paper presents a patch antenna array topology for Simultaneous Wireless Information and Power Transfer (SWIPT) applications. The resulting Double Patch Antenna Array (DPA) is composed of two patch antennas operating at different frequencies and fabricated on a unique substrate. One of the patches operates at the 1.8 GHz mobile communication band and is used for wireless communication, while the other one operates at the 2.4 GHz ISM band, and is used for Radio Frequency Energy Harvesting (RFEH). The analyzes and measurements carried out have shown that the adopted topology has satisfactory performance for communication, with a gain of 1.5 dBi, and for out-of-band energy harvesting, with a Vout of 160 mV at 2.45 GHz. These results indicate this approach as a promising strategy for low-power wireless applications.

power to communication systems with consumption in the order of μW.
In order to reach the expected self-powered wireless devices, one of the most interesting alternatives is the combination of Wireless Information Transfer (WIT) and WPT technologies.
Research on the Simultaneous Wireless Information and Power Transfer (SWIPT), started in [7] , taking into account all the challenges and strategies involved, has become the focus of several technical publications in recent years. [5], [8], [9]. However, it can be noted that there is still a great lack of new microwave designs in this area, which can be proposed and explored by researchers and technological industries.
Among the strategies for using RFEH in the energy supply of wireless devices, the out-of-band approach has stood out, as shown in Fig. 1. The advantages of this SWIPT approach are to avoid selfinterference of dedicated sources and competition for the same RF carrier, reusing RF energy from different RF sources on the environment [10]. To use this strategy, the rectenna, which is the connection of an antenna and a RF rectifier, can use several antenna designs, and among them, the microstrip patch antenna has been widely used due to its good constructive characteristics (simplicity, small dimensions, and easy fabrication) and performance (resonance frequency, polarization, and radiation pattern) [11].
In this context, it is proposed a Double Patch Antenna Array (DPA) fabricated on a unique substrate, operating at different frequencies, with a minimum attenuation or coupling between them.
The operating bands for WIT and WPT, in this proposal, are the 1.8 GHz mobile communication band and 2.4 GHz Industrial, Scientific, and Medical (ISM) band, respectively. The adoption of this WPT band was based on the average power density available at urban indoor environments [12].
In addition, for this fully WPT implementation, an RF rectifier, also designed and fabricated, is connected at the 2.45 GHz patch antenna port, constituting a rectenna element. Both DPA and the RF rectifier was designed using an FR4 substrate (εr = 4.5, tanδ = 0.018, and h = 1.6 mm) with metallization on both sides. It was chosen due to its low cost, easy acquisition, and satisfactory characteristics at the desired frequency range [13], [14].
The paper is organized as follows: besides this introduction, the DPA design and the antenna characterization are shown in section II, the RF rectifier design, the main analysis, and results of the DPA as a rectenna are considered in section III, and finally, section IV concludes this work.

A. Antenna design
After a classical hand-calculation for patch antennas, which can be verified in [15], simulations were performed using the Keysight Advanced Design System (ADS) to optimize the performance of the DPA. The microstrip feed line, for both patches, was made as a cutout in a radiating edge, which is inset to a 50 Ω driving point [16], [17].
In this way, Fig. 2 shows the DPA layout with the designed patches having the following optimized   unnecessary increase in the prototype dimensions. Despite this fact, none of these design issues affect the input return loss or the mutual coupling between the antennas.
A current density simulation was also performed in ADS to analyze the coupling between the elements of the DPA. An RF source with a frequency equal to the central frequency of each operating band was applied in each port of the DPA, while the other port was matched with a 50 Ω load.

B. Antenna characterization
In order to confirm the simulation results and to determine the operating frequency of the prototype, the reflection coefficient (S11) measurement of the DPA was done with the Rohde & Schwarz ZVB8 Vector Network Analyzer, with the complementary port connected to a 50 Ω load. The simulation and measurement results for DPA S11 are shown in Fig. 5.  Analyzing the measured H-field of the radiation pattern presented in Fig. 6 (the imaginary plane, orthogonal to the substrate and that cuts both radiant elements simultaneously), it is observed that there is asymmetry (around 3 dB) between the upper and lower bounds (90º and 270º, respectively) in both frequencies and that this power difference occurs in the opposite side for each antenna. This configuration suggests that there is an interaction between the active elements with respect to their radiation characteristics.
The explanation for this problem was mentioned when discussing the effect of undersizing the parameter Sant (section II.A). This fact is just a statement of the evidence presented, and does not critically interfere with the performance of the DPA. Finally, the DPA measured gains are 1.5 and 1.8 dBi, at 1.8 and 2.45 GHz, respectively. These results are in agreement with the typical gain of this antenna architecture [16].

A. RF rectifier design and characterization
A series half-wave rectifier is designed with a matching microstrip line, as shown in Fig. 7. This topology was chosen due to its simplicity and operational characteristics in the desired band [18] , [19] . The Schottky diode (D) SMS7630-079LF by Skyworks is used due to its low zerobias junction resistance and capacitance, inversely related to the diode's efficiency [20] . The output capacitance C (100 pF) and the output inductance L (10 μH) compose the output Low-Pass Filter (LPF), which is responsible for the output voltage (Vout) suppression [21].  Along with the RF rectifier circuit, the matching network (MN) represents an essential element for the maximum power transfer from the antenna to the series rectifier output. The proposed element, besides the microstrip line, is composed of an open stub, for impedance matching, and a grounded one, responsible for eliminating the direct current voltage (VDC) component at the diode anode, which can decrease the series rectifier efficiency significantly [22] , [23] . So, the RF rectifier layout with the optimized MN, presented in Fig. 7, has the following dimensions (in mm): Wm = Wos = 3.00, Wgs = 0.50, Lm = 30.30, Los = 13.00, Lgs = 19.00, and Dos = 15.00.
The S11 of the RF rectifier was used for matching the circuit at central frequency of the 2.4 GHz ISM band, through simulations, considering PIN variations from -30 to -10 dBm. According to this procedure, in Fig. 8, it can be observed that the measured S11 deeps of the prototype are at the desired frequency operation range. Fig. 8. Measured S11 parameter of the RF rectifier for PIN from -30 to -10 dBm.
The series rectifier efficiency versus the input power at the rectifier (PIN) at 2.45 GHz is presented in Fig. 9. The obtained efficiency values are satisfactory, for low power applications, reaching 10.5% for -20 dBm and 26% for -10 dBm.  From the metrics commonly associated with RFEH [18], [19], it has chosen for this work: Vout, due to be the only directly measurable parameter; the output power (Pout), due to be a parameter independent of the RF rectifier load; and, finally, the conversion efficiency (ηRF-DC).
The independent variables must be associated with the RF generator output power at 2.45 GHz (at the transmitting antenna terminal) since this is the only independent variable. In this way, through the Friis equation [26] , the SAV in the DPA region and PIN (power at the interface DPA-rectifier) can be In order to better understand the metrics used, it is assumed that ηRF-DC is related to the independent variable PIN (which appears in its equation), while the other parameters are related to SAV. This one is a more coherent independent variable in the analysis of rectenna, that is, in fact, a device that converts SAV to DC power. In this way, these analyses can also be intrinsically associated with the efficiency of the antenna.
The Fig. 11(a) shows the ηRF-DC performance of the DPA with a maximum value of 19% for PIN = -11 dBm, that matches to Vout = 160 mV and Pout = 17μW at SAV = 5 μW/cm 2 ( Fig. 11(b)), which are satisfactory results for a SWIPT system with out-of-band RFEH, using antennas in a unique substrate. However, the narrow bandwidth of the DPA represents an impedance variation around the optimum point, which, in addition to the nonlinear behavior and narrow bandwidth of the RF rectifier, can turn the system performance susceptible to the available power in the environment.