## 1 Introduction  
The power supply technology used for various electronics is important, and the conventional power supply technology is provided by using cables, which can be hazardous, inconvenient, or impossible [1, 2]. The transformation of charging technology from using traditional cables to wireless power transfer (WPT) is suitable for the booming development of portable devices, consumer electronics, robots, and other fields [3, 4, 5]. The WPT technology has attracted great attention due to its promising application potential in the field of charging [6, 7]. One of the interesting and highly requested directions for wireless energy transfer is sensor applications [8, 9]. There are many types of WPT technologies including magnetic resonant coupling, microwaves, inductive coupling, radio waves, lasers, and so forth [10, 11]. Out of these, the microwave wireless power transfer (MWPT) technology can transfer energy over relatively long distances with miniaturized antennas, which has great market demand [12]. At present, the MWPT technology has been applied in various fields, such as electric vehicles and mobile phones powered by electromagnetic waves and the MWPT systems inside buildings [13, 14]. The microstrip patch antenna array has advantages of a simple process, small volume, light weight, low profile, and easy integration, which is why it is widely used in the field of WPT. A series of theoretical and experimental results about MWPT systems with the microstrip patch antenna arrays were conducted [15]. Also,many various methods have been employed to increase the power transfer efficiency (PTE), performances, and robustness of the MWPT systems by designing transmitting arrays [16; 17; 18; 19]. A MWPT system operating at the frequency of 5.8 GHz by using an 8 \(\times\) 8 transmitting antenna array and a 4 \(\times\) 4 receiving antenna array was presented, and the PTE of the system was 33.4% at the distance of 40 cm [20]. Additionally, in order to improve the efficiency of the system, an optimized MWPT system using an 8 \(\times\) 8 transmitting patch antenna array and a 8 \(\times\) 8 receiving patch antenna array at the frequency of 5.8 GHz and the distance of 100 cm. The measured results show that the PTE of this MWPT system is 46.9% [21]. However, for the above mentioned WPT systems, receivers only catch the energy at a single frequency, regardless of their demands. This may cause malfunctions and failures, and the PTE of the system may therefore be reduced. In order to resolve this issue, many methods have been adopted in the magnetic resonant coupling wireless power transfer (MRCWPT) system [22; 23; 24; 25]. Frequency reconfigurable MRCWPT systems were developed in previous works [22; 23] by adding adaptively switching resonant capacitances. In the reference [24], a new approach based on reconfigurable magnetic resonant structures is proposed to achieve high energy efficiency and low volt-amp ratings. This basic principle is to create more than one efficiency-load curve. By combining a variable coupling technique, a frequency reconfigurable MRCWPT system has been proposed by varying the distance between the transmitter and the receiver [25]. In addition, a shape-reconfigurable modularized MRCWPT system in [26] achieves shape reconfigurability by using different structures and positional combinations of resonant modules. The reconfigurability in the reconfigurable MRCWPT system in [27] is achieved by adaptively switching between different sizes of drive loops and load loops. Here, instead of a MRCWPT system, a novel MWPT system based on a frequency reconfigurable (covering the S-band and C-band) microstrip patch antenna array is proposed for the first time to power the receiver at the different frequencies. This paper is organized as follows. The frequency reconfigurable microstrip patch antenna array is described in Section 2. In Section 3, the simulation and the measurement of the proposed frequency reconfigurable MWPT system are presented.