# Wireless Power Transfer via Subterahertz-Wave
## 1 Introduction
Because the rectenna is smaller even if the power decreases, it is apparent that the rectenna power density of Figure 3 tends to improve along with increased frequency. The greatest cause is circuit loss: diode loss increases as the frequency becomes higher. From Table 1, the millimeter wave rectenna mainly used the Schottky barrier diode, which has low junction capacitance and low forward voltage; alternatively, it used CMOS (Complementary metal oxide semiconductor) technology, which is an easily integrated substrate and circuit component. The mainstream of current millimeter-wave rectenna research includes energy-harvesting applications, which achieve high efficiency with low power operation. Such rectenna development is being undertaken for RFID, with wireless sensors for IoT, etc. [PERSON] et al. proposed a method of implementing an energy-harvesting system applied to IoT from the perspective of rectenna performance, power system, lifetime, and cost [11]. Moreover, [PERSON] et al. have proposed energy-efficient power control in wireless powered communication networks [12]. Research examples of millimeter wave rectennas are the following: rectenna for energy harvester of temperature sensing tag using CMOS 65 nm [13], ultra-small rectenna for microrobot [14], high-efficiency dual-frequency rectenna using CMOS 0.13 \(\upmu\)m [15], rectenna as first fully on-chip energy harvester using CMOS 65 nm [16], RF-DC conversion efficiency improvement achieved using high-precision fabrication of MMIC (Monolithic Microwave Integrated Circuit) and MEMS (Micro Electro Mechanical Systems) [17; 18]. Furthermore, as another major factor with which the high-power wireless power supply experiment in the millimeter wave band is not actively conducted, the maximum output power of the oscillation source greatly decreases as the frequency increases [19]. As might be apparent from Figure 4, there is a large power oscillation source that is readily available in the microwave band. However, in the millimeter wave band, only TWT (Traveling Wave Tube), a klystron, and a gyrotron can output power beyond kilowatt order. Furthermore, for frequencies higher than 100 GHz, only a gyrotron has a sufficiently high power oscillation source. Gyrotrons are gaining attention as a source of future high-power wireless power supply for space solar power systems and stratospheric aircraft [20]. Output power and output frequency have improved each year [21]. Nevertheless, few earlier studies have used a gyrotron for wireless power supply: Ariel University in Israel has conducted wireless power supply experiments using a gyrotron with 5 kW output at 94 GHz [22]. The University of Tsukuba in Japan has 25 array rectennas using a 28 GHz gyrotron [23], achieving a submillimeter-wave wireless power supply using a 303 GHz gyrotron [24]. It is therefore considered that experiment of high-power wireless power supplies using gyrotrons will have great influence. To summarize, it is possible to carry out long-distance and high-power wireless power supply with high directivity, which raises the rectenna power density using millimeter waves. However, the RF-DC conversion efficiency and DC output power decrease because circuit and diode losses increase. It is necessary to conduct a wireless power supply using a high-powered oscillation source such as a gyrotron. We focused on the superiority of the sub-terahertz and conducted high power wireless power transmission at 28 GHz, 94 GHz and 303 GHz [18; 23; 24]. Those works are the cutting edge of high power and high-frequency wireless power transmission. We succeeded in improving RF-DC conversion efficiency and rectenna power density of sub terahertz rectenna and demonstrated feasibility of wireless power transmission using gyrotron. These results conclude that sub-terahertz is suitable for long distance high power wireless power transmission. Figure 1: Earlier reports of studies examining rectenna RF–DC conversion efficiency for frequency [2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 22; 23; 24].