# Wireless Power Transfer via Subterahertz-Wave
## 1 Introduction  
Wireless power supply is gaining attention as a new power supply source. Especially, with the miniaturization of applications, great demand has arisen for technological development to supply sufficient power to limited areas wirelessly. Wireless power transmission methods of several types exist. This paper specifically emphasizes a wireless power feeding system that can extract electromagnetic wave from outside the device as it is with DC power using a rectifier and antenna apparatus (rectenna). Long distance power feeding is possible as a point of merit of wireless power supplies using a rectenna. It has become more easily adaptable to diverse applications. Application examples include space solar power systems, power supply to UAVs such as drones, internet of things (IoT) wireless sensors, RFID (Radio Frequency Identification), etc. Using millimeter waves, sub-terahertz waves, terahertz waves, i.e., using higher frequencies for wireless power supply, one can miniaturize a rectenna and increase the DC output power per unit of rectenna area (hereinafter, rectenna power density). Results show that the larger the rectenna power density becomes, the greater the amount of DC power that can be sent to a limited area. Furthermore, by increasing the frequency, the beam directivity can be improved: The beam diffusion is suppressed. In addition, the beam spot diameter becomes small even at a long distance. However, it is suitable for long-distance wireless power supply purposes because it can reduce the power loss that the beam dissipates to the outside. It was evaluated quantitatively as the beam efficiency [1]. Rectenna RF-DC conversion efficiency for the frequency in earlier research is presented in Figure 1. The DC output power is presented in Figure 2. The rectenna power density is shown in Figure 3. In addition, the rectenna characteristics described in the research reports listed above are shown in Table 1 and comparison of this work to earlier studies are shown in Tables 2 and 3. [PERSON] et al. [2] distributed a 16-array antenna to the rectifier circuit at 2.33 GHz, wirelessly powered the drones, and let them fly. [PERSON] et al. [3,4] developed a rectenna that shares frequencies of 2.45-5.8 GHz, which strictly matches the diode impedance analysis, and recorded high rectification efficiency. Additionally, at 10 GHz and 35 GHz, high efficiency was achieved using the same method. [PERSON] et al. [5] fabricated a rectifier circuit using GaAs FET at 5.8 GHz. The results show that the rectification efficiency was 80% and that the DC power exceeds 500 mW. [PERSON] et al. [6] fabricated a circularlypolarized rectenna with high RF-DC conversion efficiency in the X-band, with a miniaturized rectenna area using a back power feeding structure. [PERSON] et al. [7] developed a rectifier equipped with class F load and recorded high RF-DC conversion efficiency of 65.5 % at 24 GHz. [PERSON] et al. [8] Recorded high RF-DC conversion efficiency of 72 % at 35 GHz. In addition, proposed a space power beaming system using a gyrotron. [PERSON] et al. [9] Fabricated a rectenna using a 4 \(\times\) 4 patch antenna array and LPF at 35 GHz, and carried out near field WPT (Wireless Power Transmission) measurement. [PERSON] et al. [10] developed a rectenna using a grid antenna at 60 GHz and reported RF-DC conversion efficiency of 32.8% and DC power of 1.22 mW. Furthermore, they proposed a WPT system combining transmission and reception. It is apparent from Figure 1 that the RF-DC conversion efficiency decreases at increasing frequency. Maximum RF-DC conversion efficiency of 80-90% is shown at 2.45 GHz, 5.8 GHz, 10 GHz in the microwave band. In contrast, in the millimeter wave band of 30 GHz to 300 GHz, the RF-DC conversion efficiency was about 50% to as low as 2%. It is apparent from Figure 2 that the microwave band DC output power can be 500-1000 mW, although the millimeter wave band can output only about 10-200 mW.