# Wireless Power Transfer via Subterahertz-Wave
## 3 Research Report on Millimeter-Wave High-Power Wireless Power Transmission
### 303 GHz Rectenna  
The 303 GHz rectenna structure is shown in Figure 20. The 303 GHz rectenna made of MSL and used two filter types of Rectenna A (with a notch filter) and Rectenna B (with a low-pass filter). Rectennas comprise a patch antenna and the single series structure with the diodes connected in series. The Electromagnetic field simulators of EMPro and ADS (Keysight Technologies Co. Ltd., Santa Rosa, CA, USA) were used for designing the antenna and rectifier circuit. The antenna and the rectifier were designed with matching impedance respectively and integrated as a rectenna. The PEFE substrate (NPC-F220A; Nippon Pillar Packing Co. Ltd., Osaka, Japan) was used. The diode (MA4E1310; Macom, Lowell, MA, USA) has the following parameters: slope resistance \(R_{d}=7\)\(\Omega\), junction capacitance \(C_{j}=0.010\) pF, forward voltage \(V_{f}=0.70\) V, and reverse breakdown voltage \(V_{br}=7\) V. The diode was attached with high accuracy using a conductive paste and flip chip bonder. On the input side, both Rectenna A and Rectenna B have short stubs to create DC conduction and to facilitate impedance matching. On the output side, Rectenna A has a notch filter that prevents fundamental waves from flowing out to the load resistance. Rectenna B has a low pass filter that prevents the fundamental wave and higher harmonic waves from flowing out to the load resistance. For measurement of the antenna, inline and fixture were used to connect the network analyzer. The antenna gain was measured using a three-antenna method with three inline antennas of identical shape. Results show that the reflection  
Figure 19: 94 GHz rectifier RF–DC conversion efficiency.  
coefficient was \(-18.7\) dB; the antenna gain was 8.32 dBi at 303 GHz. This value was approximately equal to the calculated antenna gain of 7.89 dBi using the physical dimensions [27].  
The 303 GHz gyrotron at the Fukui University Far Infrared Region Development Research Center was used for wireless power supply experiments [28; 29]. The measurement setup is shown in Figure 21. The beam output power was fixed at 33.4 kW. The rectenna was installed at a point 3 m away. The rectenna was connected to the load resistance. The oscilloscope was connected via a conducting wire. Then the DC output power was evaluated. The beam profile at the 3 m points was evaluated using the IR camera. By measuring the beam profile, the power density at the rectenna insertion point was ascertained. The power input to the rectifier was estimated. The RF-DC conversion efficiency was evaluated from the change in the DC output power when the rectenna was brought close to the beam center. The measurement results are presented in Figure 22. Maximum RF-DC conversion efficiency of 2.17% was recorded for input power of 342 mW. The load resistance was 130 \(\Omega\) at Rectenna A. The maximum DC output power 17.1 mW and rectenna power density 3.43 kW/m 2 when the load resistance was 200 \(\Omega\) at RectennaB.  
Figure 21: Experimental system of wireless power transmission using a 303 GHz gyrotron: (**a**) transmitting side and (**b**) receiving side.  
Figure 20: 303 GHz MSL rectenna: (**a**) rectenna A (with notch filter) and (**b**) rectenna B (with low-pass filter).