# Dual-Band RF Wireless Power Transfer System with a Shared-Aperture Dual-Band Tx Array Antenna
## 1 Introduction  
Recently, with the advancement of Internet of Things (IoT) technology and the expansion of the corresponding market, the issue of supplying energy to wireless sensors for the IoT has become a hot topic. Most IoT devices must be supplied constantly but many consume relatively less power, making them suitable for charging by wireless power transfer (WPT) through radio frequency (RF) technology. Various studies related to this have been conducted [1, 2, 3, 4, 5].  
Most RF WPT systems use a single-band frequency within the industrial scientific medical (ISM) band, such as 915 MHz, 2.4 GHz, or 5.8 GHz [6, 7, 8, 9]. These ISM bands are highly utilized and are already occupied by various applications and devices [10, 11]. Therefore, a RF WPT system using only a single frequency on the ISM band is not only exposed to a complex frequency environment but can also cause disruptions to nearby communication devices. If a dual-band WPT system with a dual-band power transmitter (Tx) system is implemented, as shown in Figure 1, the operating frequency (in this case, either \(f_{1}\) or \(f_{2}\)) of the WPT system can be selected appropriately according to the givenfrequency environment and wireless power can be transmitted to a receiver (Rx). Examples include (a) transmitting \(f_{1}\) wireless power to an Rx for \(f_{1}\), (b) transmitting \(f_{2}\) wireless power to an Rx for \(f_{2}\), (c) transmitting dual-band wireless power of \(f_{1}\) and \(f_{2}\) to an Rx for the dual band, and (d) transmitting \(f_{1}\) wireless power and \(f_{2}\) wireless power to an Rx for \(f_{1}\) and to another Rx for \(f_{2}\), respectively. This facilitates diversity of the operating frequency and therefore allows flexible operation of the WPT system in various situations and scenarios. However, most research on dual-band WPT systems focuses on inductive coupling and magnetic resonant coupling WPT methods [12; 13; 14; 15]. These WPT systems have a disadvantage in that the power transmission distance is short. On the other hand, for dual-band RF WPT, a printed dual-band antenna of 2.5 and 4.5 GHz was proposed [16] and a dual-band antenna operating at 2.45 and 5.8 GHz was designed based on a split-ring resonator and fractal geometry [17], respectively. Another study presents the design of a dual-band power amplifier operating at 2.4 and 5.8 GHz based on the concept of composite right/left-handed (CRLH) unit cells and substrate-integrated waveguide (SIW) technology [18]. A dual-band rectenna optimized to convert far-field RF energy to DC voltage at very low received power levels at 915 MHz and 2.44 GHz was also developed [19]. Like this, most works related to dual-band RF WPT have concentrated on dual-band components such as antennas, amplifiers, and circuits, but studies of the design and testing of dual-band RF WPT systems have rarely been carried out. Recently, a dual-band RF WPT system based on a frequency-reconfigurable antenna array (covering the S-band and C-band) was proposed [20], but the frequency reconfigurable antenna required a change of the connection method between the main patch and smaller patches in order to adjust the operating frequency. Furthermore, in this case, the power transmission distance was relatively short, from 20 mm to 80 mm, and beam-forming was not considered.  
In this paper, a dual-band WPT system with a shared-aperture dual-band employing 2.4 GHz radiating elements and 5.8 GHz radiating elements in a single aperture is proposed. In Section 2, a dual-band aperture-sharing array as a Tx antenna is optimally designed to achieve high transmission efficiency in various WPT scenarios. Based on this optimized Tx array antenna, the dual-band RF Tx system is implemented. Rx antennas are also designed. In Section 3, the measured results from WPT experiments according to various WPT scenarios, as depicted in Figure 1, are detailed. Finally, Section 4 presents the conclusions of this study.