# Wireless Power Transfer via Subterahertz-Wave
## 3 Research Report on Millimeter-Wave High-Power Wireless Power Transmission  
### 28 GHz Rectenna  
A circuit pattern was fabricated using a substrate processing machine (Diclad880; Rogers Corp, Chandler, AZ, USA) and using MSL as a transmission line. The diode used a GaAs Schottky barrier diode (MA4E1317: Macom, Lowell, MA, USA). The structure of a 28 GHz antenna and a rectifier is portrayed in Figure 6. The Electromagnetic field simulators of EMPro and ADS (Keysight Technologies Co. Ltd., Santa Rosa, CA, USA) were used for designing the antenna and rectifier. Measurement results of the antenna reflection coefficient and radiation pattern are shown in Figure 7. We obtained antenna reflection coefficient -12.9 dB and gain of 9.0 dBi to achieve high gain using a four-elementarray. The rectifier is made of a single shunt in which diodes are inserted in parallel. It is equipped with a class F load which is a stub structure to reapply the fundamental wave and harmonic to the diode for enhancing RF-DC conversion efficiency [7]. Measurement results of the rectifier are shown in Figure 8. The maximum RF-DC conversion efficiency is 47.7% (295 mW input, 130 \(\Omega\) load resistance). The maximum DC output power is 171.4 mW (389 mW input, 130 \(\Omega\) load resistance). Also, the maximum rectenna power density was 0.86 kW/m\({}^{2}\). To obtain higher rectenna power density, the rectenna was arrayed 25 rectenna. the rectenna area was reduced to the greatest extent possible by assembling the antenna and the rectifier circuit with a back feeding structure. The back feed structure is shown in Figure 9. 25 array antenna was connected in 5 parallel and 5 series, shown in Figure 10. DC output power was integrated into one load resistance of 130 \(\Omega\). Figure 6: 28 GHz rectenna: (**a**) antenna and (**b**) rectifier. Figure 7: 28 GHz antenna: (**a**) S11 and (**b**) radiation pattern. The 28 GHz gyrotron used as the wireless power supply is at the Plasma Research Center of the University of Tsukuba [25]. The measurement setup is portrayed in Figure 11. The gyrotron output power was fixed at 52.9 kW. Measurements were taken at two points of 0.9 m and 1.35 m. The input power to the rectenna was altered by changing the attenuation materials. Actually, WPT using the Cassegrain antenna (CA) was also applied to increase the transmission beam directivity. The measurement result of WPT using 28 GHz gyrotron is presented in Figure 12 and Table 4. Without the Cassegrain antenna, it means that the beam is emitted directly from the corrugated waveguide which is the window of the gyrotron. The maximum DC output power and the maximum total transmission efficiency were 1.05 W and 2.30% at a distance of 900 mm. The maximum transmission efficiency of 2.30% was recorded at a DC output of 0.40 W when the attenuation was 34.7 dB. The maximum DC output of 1.05 W was recorded when the attenuation was 24.7 dB, and then the transmission efficiency was 0.29%. The transmission efficiency dropped so sharply when the distance is increased from 900 mm to 1350 mm. It is because the condensing position of the Cassegrain antenna is not a good choice. Figure 8: 28 GHz rectifier RF–DC conversion efficiency and DC output power  
Figure 7.0 28 GHz 25 array rectenna: (**a**) 25 array antenna and (**b**) 25 array rectifier  
Figure 7.1 28 GHz back feed structure of antenna and rectifier. antenna was 900 mm, therefore at 1350 mm, greatly shifted the converging position. To calculate the total transmission efficiency, considering each efficiency of Equations (1)-(5), we estimated the theoretical total transmission efficiency of 2.35% using the power ratio received by the rectenna caused by the spread of the Gaussian beam, the aperture efficiency of the antenna, and the RF-DC conversion efficiency of the rectifier. This value roughly agreed with the experimentally obtained value. Figure 7.2 Experimental system of wireless power transmission using 28 GHz gyrotron: (**a**) receiving side and (**b**) transmitting side.