An energy harvesting from ambient radio waves is very important technology for mW applications to reduce a battery, e.g. IoT sensors, vital sensors, etc. Compared with the other energy harvesters, like PowerMEMS, power generation by heat, etc., the energy harvesting from the ambient radio waves is very stable to generate the power everywhere and every time because the distributed power source of the radio waves is originally artificial for wireless broadcasting and communications. However, it contains the same weak point with all harvesters that available power is not enough to satisfy the user requirement of the energy. Only in the energy harvesting from the ambient radio waves, we can increase the received power when we put a special transmitter for wireless power only, which can increase the transmitting power. It is named a "Radiative Wireless Power Transfer (WPT)". We can apply the same harvester (rectifying antenna = rectenna) technology both for the energy harvesting and the WPT.

In recent 10 years, the WPT technology and business is expanded in the world. It is based on the discussion of new radio regulation in consideration of suppression of unexpected interference to existing radio waves like WiFi and of keeping safety of human. In 2022, ITU-R (International Telecommunication Union - Radiocommunication Sector) recommends frequencies suitable for the WPT. In the same 2022, Japanese government establish a new radio regulation for the WPT at 920MHz band, 2.4GHz band, and 5.7GHz band. In US, the WPT system is considered as one of ISM (Industrial, Scientific, and Medical) applications and a lot of start-up companies of the WPT born and produce the commercial WPT equipment. Current WPT applications are similar with the application of energy harvesters, battery-less sensors. In the current step of the WPT business, good rectenna technology is required. When the input radio wave power to the rectenna is enough to excite a diode/CMOS on the rectifying circuit, over several tens of milliwatts, the RF-DC conversion efficiency of the rectenna has already reached at > 90% at 920MHz band and at 2.4GHz and at 85% at > 75% at 24/28GHz band in Japan. When input radio wave power is not enough like the energy harvesting system, the RF-DC conversion efficiency is still low and we need a special technique to increase the efficiency. This is the important technology in the first step of the WPT.

In the second step, we need a beam forming technology in the transmitter to increase the receiving power by focusing and/or chasing beam on the moving receiver. A phased array antenna is usually applied for beam forming. We can charge a mobile phone and can fly a drone by focusing radio wave with high beam efficiency when we have a good phased array antenna technology. In Kyoto University, we have developed a low cost and high efficiency phased array antenna at 28GHz in 2025. We also fly the battery-less micro drone only by the beamed microwave at 5.8GHz at distance of approximately 70 cm. In future, we expect to build a Solar Power Satellite (SPS) with 1GW solar cells and 2km phased array antenna in geo-stationary orbit in 26,000km above in 2050. There are some big R&D projects for the SPS not only in Japan but also in US and China. We use the same and advanced technology both for the SPS, WPT, and energy harvesting from ambient radio waves.

In this talk, I will introduce the recent advance of the energy harvesting technology from ambient radio waves and of the WPT. I also introduce the current commercial and future WPT applications and discussion status of the radio regulation of the WPT in the world.