Development of a 100 mW-Class 94 GHz High-Efficiency Single-Series Rectifier Feed by Finline for Micro-UAV Application

Wireless power transfer (WPT) is one solution to realize long flight times and accommodate various missions of micro-uncrewed aerial vehicles (MAVs). Reducing the constraint of power transmission distance and realizing high beam efficiency are possible because of the high directivity of WPT using millimeter wave (MMW) methods. Nevertheless, no report of the relevant literature describes an investigation of sending power to an MAV using MMW because MMW rectennas have low efficiency. The purpose of our study is to conduct fundamental research of a high-efficiency and high-power rectenna at 94 GHz aimed at MAV application using MMW. As described herein, we developed and evaluated a 100-mW-class single-diode rectifier at 94 GHz with a finline of a waveguide (WG) to a microstrip-line (MSL) transducer. With the optimum load of 150 Ω at input power of 128 mW, the output DC power and rectifying efficiency were obtained respectively as 41.7 mW and 32.5%. By comparison to an earlier study, measurement of 94 GHz rectifiers under high power input becomes more accurate through this study.

1. Introduction
In recent years, micro-UAVs (MAVs) have been developed extensively. They are used in various applications, from commercial to military. Among them, some types have less than 100 g weight, which makes them inexpensive and easy to use. However, because of the small payload, the installed batteries are a constraint, necessitating short flight times of MAVs. One solution to resolve this constraint is to send power from a ground base to a UAV using wireless power transfer (WPT). Using magnetic resonance, WPT can send sufficient power to drive UAVs stably, but it cannot realize long-distance transmission. Moreover, it restricts the free flight of MAVs. However, WPT using RF energy can reduce the distance limit. As an example of research on UAVs, a power transmission experiment from the ground to a small airplane (SHARP) [1] has been reported in Canada. In addition, the University of Kyoto conducted experiments on power transmission to model airplanes and airships [2]. In our research group, WPT to MAVs was conducted in the microwave region [3, 4]. According to beam efficiency [5], as introduced by the Friis transmission equation, the higher the operating frequency becomes, the more efficiently WPT is realized. Shimamura et al. introduced the relation between the MAV size and the received power ratio for operating MAVs [6], as shown in the following equation:

In this equation, ηT, S, f, DT, c, and d respectively denote the beam efficiency, the MAV wing area, the operating frequency, the transmitting antenna diameter, the speed of light, and the transmission distance. Using the equation and an MAV (HS210; Holy Stone), we calculated and compared the received power under two operating frequencies: 5.8 GHz and 94 GHz. The parameters used and the calculated results are presented in Table 1. From the results, one can infer that higher frequencies can produce higher transmission efficiency, given the same transmission distance. That relation demonstrates the superiority of millimeter-wave (MMW) transmission. Among the MMWs available for use, 94 GHz is an atmospheric window. It is therefore possible to transmit power to MAVs efficiently at that frequency. Nevertheless, no study has examined the transmission of power to MAVs at 94 GHz because the antenna directivity is too high [7, 8] and because the rectifier efficiency is too low. Additionally, in MMW circuits, the conductor and dielectric loss in the oscillator and transmission line parts become greater than those related to microwaves. Moreover, conversion efficiency at the diode is low. Consequently, the conversion efficiency of the rectenna decreases. Research on MMW power in the field of wireless power transmission has remained inadequate compared to that for microwave power [9–12].