Microwave Power Transmission for Various Applications

It is known that electromagnetic energy also associated with the propagation of the electromagnetic waves. The energy of the electromagnetic waves can be described as (W/m²). The vector is called Poynting Vector. In the plane wave propagation, the vector results in a vector along the direction of propagation that is the direction of the energy flow. It means that all electromagnetic waves are energy themselves. We can use theoretically all electromagnetic waves for a wireless power transmission (WPT). The difference between the WPT and communication systems is only efficiency. The Maxwell’s Equations indicate that the electromagnetic field and its power diffuse to all directions. Although we transmit the energy in the communication system, the transmitted energy is diffused to all directions. Although the received power is enough for a transmission of information, the efficiency from the transmitter to receiver is quiet low. Therefore, we do not call it the WPT system.

In 1864, James C. Maxwell predicted the existence of radio waves by means of mathematical model. In 1884, John H. Poynting realized that the Poynting Vector would play an important role in quantifying the electromagnetic energy. In 1888, bolstered by Maxwell's theory, Heinrich Hertz first succeeded in showing experimental evidence of radio waves by his spark-gap radio transmitter. The prediction and Evidence of the radio wave in the end of 19^th century was start of the wireless power transmission.

At the same period of Marchese G. Marconi and Reginald Fessenden who are pioneers of communication via radio waves, Nicola Tesla suggested an idea of the wireless power transmission and carried out the first WPT experiment in 1899. He said “This energy will be collected all over the globe preferably in small amounts, ranging from a fraction of one to a few horse-power. One of its chief uses will be the illumination of isolated homes”. He actually built a gigantic coil which was connected to a high mast of 200-ft with a 3 ft-diameter ball at its top. He fed 300 kW power to the Tesla coil resonated at 150 kHz. The RF potential at the top sphere reached 100 MV. Unfortunately, he failed because the transmitted power was diffused to all directions with 150 kHz radio waves whose wave length was 21 km.

To concentrate the transmitted power and to increase transmission efficiency, we have to use higher frequency than that used by Tesla. In 1930s, much progress in generating high-power microwaves, 1-10 GHz radio waves, was achieved by invention of the magnetron and the klystron. After World War II, high power and high efficiency microwave tubes were advanced by development of radar technology. We can concentrate a power to receiver with microwaves. We call the wireless power transmission with microwaves as microwave power transmission (MPT).

Based on the development of the microwave tubes during the World War II, W. C. Brown started the first MPT research and development in 1960s. First of all, he developed a rectenna, rectifying antenna which he named, for receiving and rectifying microwaves. The efficiency of the first rectenna developed in 1963 was 50 % at output 4WDC and 40% at output 7WDC, respectively. With the rectenna, he succeeded in MPT experiments to wired helicopter in 1964 and to free-flied helicopter in 1968. In 1970s, he tried to increase DC-RF-transmission-RF-DC total efficiency with 2.45 GHz microwave. In 1970, overall DC-DC total efficiency was only 26.5 % at 39WDC in Marshall Space Flight Center. In 1975, DC-DC total efficiency was finally 54 % at 495WDC with magnetron in Raytheon Laboratory . In parallel, He and his team succeeded in the largest MPT demonstration in 1975 at the Venus Site of JPL Goldstone Facility . Distance between a transmitting parabolic antenna, whose diameter was 26m, and a rectenna array, whose size was 3.4 m x 7.2 m, was 1 mile. The transmitted microwave of 2.388GHz was 450 kW from klystron and the achieved rectified DC power was 30 kWDC with 82.5% rectifying efficiency. Based on the Brown’s work, P. E. Glaser proposed a Solar Power Satellite (SPS) in 1968.

In 1980s, Japanese scientists progressed the MPT technologies and research. In 1983 and 1993, Hiroshi Matsumoto’s team carried out the first MPT experiment in space. The rocket experiments were called MINIX (Microwave Ionosphere Nonlinear Interaction eXperiment) in 1983 and ISY-METS (International Space Year - Microwave Energy Transmission in Space) in 1993, respectively. They focused nonlinear interaction between intense microwave and ionospheric plasmas. In the MINIX experiment, they used cooker-type 800W-2.45GHz magnetron for microwave transmitter. New wave-wave-particle interaction phenomenons were observed in the MINIX. Plasma theory and computer experiments supported the observations.

In 21^st Century, digital device technologies were advanced to reduce the power. They can be used below mW-uW. Therefore, new MPT concept was applied for the low power digital device as ‘Ubiquitous power source’ and ‘Energy harvesting’. This is the same as Tesla’s dream.