Dr. Takafuji considers energy harvesting
In order to use radio waves as an energy source, radio waves with a certain level of electric field strength must be present in space. Considering Japan, it is a broadcasting station that constantly emits a wide range of strong radio waves. Other radio waves that may be available are cell phones and WiFi radio waves, but these do not always emit strong radio waves from the base station. Because the radio waves are emitted at the required intensity when necessary, the variation is large. Therefore, in the sense of a stable energy supply, it is a radio wave transmitted from a broadcasting station. In Japan, the frequency band for terrestrial digital broadcasting is in the range of 470 MHz to 710 MHz. For example, from Tokyo Skytree, NHK and five key stations transmit radio waves with 10 KW of power (the effective radiation power is 69 KW). In addition, radio waves (around 80 MHz: NHK J-WAVE 7 KW ERP 57 KW) are also available from Tokyo Skytree. From December 2015, radio waves from 3 FM supplementary relay stations (output 7KW) were also added.
Fig. 1 Distance from antenna and received power ¥ diode threshold target
In order to use radio waves as an energy source, the problem is how to efficiently capture radio waves and convert the captured radio waves into a power source.
However, since the circuit of the wave generator is almost completed, I think that the next development is an antenna.
We already have noise not only from radio energy, but also from fluorescent lights, TVs and other appliances.
We have also succeeded in generating electricity from the noise (low frequency) from the motor.
The remote control has already been developed. The drone also has extended battery life with extended battery life.
The following is a list of research topics, so please check.
Finally, energy harvesting = wave power generation equipment, there is no end to the development of this equipment.
However, many development products will be born in the development stage.
In terms of familiar things (devices that do not require batteries),
1. All remote control devices 2. Mobile phones 3. Transmitters 4. Permanent travel in electric vehicles
Camera, electric toothbrush, electric shave, pacemaker, personal computer, flashlight, etc.
There will be no limit.
 Antenna design  Circuit design  Device design
 Antenna design
The energy source is mainly radio waves of digital terrestrial broadcasting, and is the radio waves of AM, FM and mobile phones. In order to capture the energy of as many channels as possible, the tuned antenna is required to broaden the reception frequency. In addition, high impedance is required to improve the compatibility with the rectifier circuit described later. Furthermore, in consideration of the form of use, the antenna shape is targeted for miniaturization, weight reduction, non-directionalization, and so on.
The following is a summary of these requirements: 1 Broadbanding
2 High impedance
3 Compact and lightweight
Photo 4 shows an IC card type antenna. It is miniaturized to the IC card size, and its directional characteristics are almost omnidirectional.
Photo 4 IC card type antenna (long side about 8 cm long)
The reception bandwidth is 24.5 MHz, and the impedance is 2 kΩ.
Fig. 3 VSWR characteristics (IC card type)
If this characteristic can be secured with this size, I think that practical use is near, but try to twist further.
 Circuit design
The Rectenna rectification method was the Cockcroft-Walton method, but I devised a different circuit. Here, I will refrain from the display. (Convenience of patent)
Cockcroft circuits are convenient rectifier circuits for obtaining high voltage power.
It is said that if you further increase the number of stages, you can get ultra-high voltage, so let me know for reference.
Fig. 4 Three-stage Cockcroft-Walton rectifier circuit
 Device design
We are studying Schottky barrier diodes (SBDs), PN junction diodes (PNDs), and gated diodes (GCDs) as rectifying diodes. GCD has the advantages of easy integration, fast frequency response, low junction capacitance, etc. If it can be optimized it will be a good diode candidate for rectenna.
Fig. 5 Structure of GCD (gate control diode)
※ SOI (Silicon on Insulator) is a technology to improve the high speed and low power consumption of CMOS LSI.
Fig. 6 IV measurement results of various diodes (Threshold characteristics) Fig. 7 Diagram of 500 MHz band weak power rectenna
Fig. 8 Example of sensor network For the rectenna,
1 Examination of other frequency bands (radio waves from mobile phones, AM, FM broadcast waves, fluorescent lamp noise, motor noise, etc.)
Higher efficiency at 2 poles low input (using backward tunnel diode etc.)
Also, as a whole system including sensors etc.,
1 Estimated power consumption of equipment equipped with rectenna
2 Study the power management circuit corresponding to the specific usage system.
In the future, I would like to propose a highly efficient rectification method for extremely low input RF power generation, and I think it would be wonderful if this research and development could contribute to the development of the Japanese semiconductor industry. I have witnessed the expansion of the field of using radio waves other than communication and broadcasting, and I feel that it is coming soon to the point where that realization is coming.
I look forward to future developments. Please stay tuned.
Freedom Co., Ltd. (Plan to establish a new company tentatively named Saito Research Institute, Inc.)