Wireless Power Transfer at Higher Frequency

for SPS and for Commercial WPT

Naoki Shinohara

Research Institute for Sustainable Humanosphere, Kyoto University, GOkasho, Uji, Kyoto, Japan

shino@rish.kyoto-u.ac.jp

Keywords: Wireless Power Transfer, Microwave Power Transmission, Solar Power Satellite, Millimeter Wave

Abstract: One of the promising future power stations is a solar power satellite (SPS) station in geostationary orbit

(36,000 km above the surface of Earth) that uses wireless microwave power-transfer technology. In this

system, the power generated would be transmitted to the ground by a microwave beam. The SPS would be a

very large satellite with a large transmitting phased-array antenna that would work at 2.45 or 5.8 GHz. The

size of the transmitting antennas is theoretically determined by Maxwell’s equations. However, we must

reduce the size of the antennas to reduce the cost and to produce a small prototype satellite as a first step to

the SPS. The only way to reduce the size of the antennas is to use a higher frequency. We developed

rectennas that are optimized for 24 and 60 GHz transmission. In addition, we developed a monolithic

microwave integrated circuit (MMIC) rectenna for 24 GHz transmission and with dimensions of 1 mm × 3

mm. The maximum radio-frequency to direct-current (RF-DC) conversion efficiency is 47.9% for a 210

mW microwave input power with a 120  load. We also designed a rectenna for 60 GHz transmission

whose maximum RF-DC conversion efficiency is 46.2% for a 80 mW input power at 60 GHz with a 100 

load. Finally, based on rectenna technology, we propose other satellite experiments.

1 INTRODUCTION

A solar power satellite (SPS) station is a very

suitable application for a wireless power transfer

(WPT) via radio waves, especially via microwaves

(microwave power transfer or MPT). SPSs are one

of the promising future power stations for a

sustainable power source that uses solar cells

(Mankins, 2014). Current SPS design envisions a 2

km diameter antenna that would transmit at 5.8 GHz

in space. The beam efficiency between such a

transmitting antenna and a 2 km receiving rectenna

36,000 km away is approximately 90% (Shinohara,

2014). The size of antennas is determined by

Maxwell’s equations and cannot be reduced

(Shinohara, 2014). In working toward the SPS, we

must carry out small-scale satellite experiments. The

low Earth orbit (LEO) of 400 km would be used for

such an experiment. If the small satellite system is

the same as the SPS at 5.8 GHz, the antenna and the

rectenna must each be approximately 200 m long.

However, a 200 m antenna is too large for a small

satellite; the size limit is more on the order of 10 m.

Therefore, instead of simulating a SPS, we use a

small satellite to study the other objectives of MPT.

The same problem arises with commercial

applications of MPT. Theoretically, the size of

antenna required is over 10 m in diameter for MPT

over 1 km at 5.8 GHz with 90% beam efficiency.

This size is too large for a commercial MPT system

to compete with wired power transmission. This is

the reason that no commercial MPT system exists in

the world. Therefore, we propose WPT at a higher

frequency, for example, 24 or 60 GHz. However,

there are two problems with WPT at a higher

frequency: (1) an increase in absorption by air and

(2) a decrease in circuit efficiency and power. The

latter problem can be solved by technical means.

2 DEVELOPMENT OF 24 AND 60

GHZ RECTENNA

At Kyoto University, we propose a wireless system

that simultaneously transmits information and power

in the millimeter wave range. Higher frequency

Shinohara N.

Wireless Power Transfer at Higher Frequency for SPS and for Commercial WPT.

DOI: 10.5220/0005420700210024

In Proceedings of the Third International Conference on Telecommunications and Remote Sensing (ICTRS 2014), pages 21-24

ISBN: 978-989-758-033-8

results in higher communication speed and lower

antenna sizes. The first application considered is a

fixed wireless access system proposed by NTT

Corp., Japan (Seki, 2011).

First, we choose 24 GHz, which is in the industrial,

science, and medical (ISM) band, and develop a

rectifying circuit in the MPT receiver. We normally

use a Schottky diode with 

/4 distributed line and a

capacitance that is called a “single-shunt rectifier”

and with theoretical radio-frequency to direct-

current (RF-DC) conversion efficiency is 100%. For

example, the maximum efficiency of the proposed

single-shunt rectifier is over 90% at 2.45 GHz and is

80% at 5.8 GHz. For the millimeter wave system,

we consider that the capacitance is a weak point that

prevents optimizing the efficiency. Therefore, we

propose a new single-shunt rectifier with a class-F

load, which is composed of open stub resonators for

even and odd harmonics, instead of the capacitance.

The rectifying principle exploited by a conventional

single-shunt rectifier and the class-F load rectifier is

the same and both have a theoretical RF-DC

conversion efficiency (with only one diode) of 100%.

The rectifying circuit is composed of a microstrip

line and two diodes in parallel. Dimensions of the

developed 24 GHz MMIC rectenna are 1 mm × 3

mm on GaAs [Fig. 1(a)], with a maximum RF-DC

conversion efficiency of 47.9% for a 210 mW

microwave input signal at 24 GHz with a 120  load

[Fig. 1(b)] (Hatano, 2013).

(a)

(b)

Figure 1 : (a) Developed 24 GHz MMIC rectenna

and (b) RF-DC conversion efficiency (Hatano, 2013).

Next, we designed a rectifying circuit that operates

at 60 GHz. The rectifying circuit is composed of a

microstrip line on a Teflon substrate and two diodes

connected in parallel. For the 60 GHz rectifying

circuit, we focused on three points to increase the

RF-DC conversion efficiency: (1) First is the length

of the microstrip line between each diode with a

through hall, which in turn is connected to ground

plane. Upon changing the length of this microstrip

line, the efficiency goes through a maximum and a

minimum. (2) Second is the number of

corresponding harmonics of the class-F load. We

estimated a relationship between the number of

corresponding harmonics of the class-F load and the

efficiency, and concluded that to increase the

efficiency, it is sufficient to use only one stub

resonator for a fundamental wave. (3) Finally, the

impedance of the class-F load. We increased the

impedance of the class-F load. The rectifying circuit

designed is shown in Fig. 2. An ADS simulation of

the maximum RF-DC conversion gives an efficiency

of 46.2% for 80 mW input power at 60 GHz with a

100  load.

Fig.ure 2 : Designed and simulated 60 GHz

rectifying circuit.

Third International Conference on Telecommunications and Remote Sensing

Figure 3 shows the frequency dependence of the

RF-DC conversion efficiency of rectifying circuits

for rectennas developed since the 1960s. The star

marks are our contributions, which are at 24 and 60

GHz. The RF-DC conversion efficiency in the

millimeter-wave frequency range is sufficient to use

millimeter waves for WPT.

Figure 3 : Frequency dependence of RF-DC

conversion efficiency of rectifying circuits for

rectennas developed since the 1960s.

3 PROPOSED SATELLITE MPT

EXPERIMENT FOR SPS

In 2009 in Japan, the Basic Plan for Space Policy

was published, which states “As a program that

corresponds to following major social needs and

goals for the next 10 years, a Space Solar Power

Program will be targeted for the promotion of the 5-

year development and utilization plan.” We thus

need both a technical advance and a “surprise” in the

next space experiment based on the Basic Plan for

Space Policy. In 2013, the Basic Plan for Space

Policy was revised, but it still promoted the SPS for

Japan. In addition, the SPS figures in the Japanese

“Basic Plan for Energy Policy” from April, 2014.

As a first step to the SPS, a WPT experiment in

space or from space to ground is very important.

Only three MPT rocket experiments have been done

in the world, and they were done in Japan. In 1983,

Professor Matsumoto of Kyoto University conducted

the first MPT rocket experiment, which was called

the Microwave Ionosphere Nonlinear Interaction

Experiment (MINIX). This experiment was in

collaboration with Kobe University and the Institute

of Space and Astronautical Science (ISAS)

(Matsumoto, 1986). In the MINIX experiment, they

used a 2.45 GHz cooker-type magnetron and

waveguide antenna as microwave transmitter. In

1993, Professor Matsumoto’s group carried out their

second rocket experiment, which was called the

International Space Year Microwave Energy

Transmission in Space (ISY-METS) experiment

(Kaya, 1993). This experiment used a phased array

at 2.411 GHz. The MINIX and ISY-METS were

space-to-space MPT experiments. The third and last

WPT rocket experiment was carried out in 2006, by

Professor Kaya of Kobe University, ISAS, and the

European Space Agency (Kaya, 2006). This is the

only a rocket MPT experiment whose microwave

was transmitted from the rocket back to the ground.

However, the microwave was diffused and did not

qualify as a power beam.

The difficulty of the MPT experiment from space

to ground is caused by the low frequency of

microwave radiation. A small satellite must orbit at

300 to 400 km. A distance of several hundred

kilometers is too far to create a microwave beam at

2.45 or 5.8 GHz (these frequencies are too low).

Therefore, we propose an MPT space experiment at

24 GHz that is based on the technologies described

in Section 2. In the early 1990s, a 24 GHz MPT

satellite experiment was proposed and studied in

Japan (Matsumoto, 1993). However, this was a

space-to-space MPT experiment. Herein, we propose

a space-to-ground MPT experiment.

Using the microwave frequency of 24 GHz has

the following advantages and disadvantages:

Advantages

(1) The antenna size of the MPT can be decreased

to a tenth of the size of Tx × Rx antenna.

(2) 24 GHz is in the ISM band so there are very

few users.

Disadvantages

(1) Efficiency and power are lower than at 2.45

GHz.

(2) Absorption in air is greater.

(3) Technical obstacles are greater.

Wireless Power Transfer at Higher Frequency for SPS and for Commercial WPT

Orbital Velocity

7.5km/s

Satellite

Antennas for Beam

Shape Detection

Ground

Microwave Beam

Pilot Signal

Antenna

Frequency : 24GHz

Diameter of Tx Antenna : 20m

Power : 10kW

400km above

Peak Power Density

: 126mW/m

Half Power Distance

@ 400km : 240m

ex. Beam Efficiency 75%

with 400m Rectenna

Fig. 4 Parameters of proposed 24 GHz satellite MPT

experiment.

However, we can transmit microwave power from

a 400 km orbit to the ground with higher beam

collection efficiency if appropriately calibrate the

values for experimental parameters. Estimated

values for the parameters are shown in Fig. 4. With

these parameters, we can receive sufficient

microwave power from the space. Therefore,

provided the requisite technical advance and

“surprise,” the 24 GHz space experiment can be

conducted.

4 CONCLUSIONS

In working toward the SPS, an MPT satellite

experiment is desired as soon as possible. However,

Maxwell’s equations and the required satellite orbit

render an effective MPT experiment difficult.

Therefore, to perform the MPT satellite experiment,

we consider using frequencies of 24 GHz or higher.

We have already developed 24 and 60 GHz

rectifying circuits for a rectenna with sufficient RF-

DC conversion efficiency.

There are still technical problems preventing an

MPT experiment with higher frequency. For

example, the experiment would require a high-power

transmitter and amplifier with high DC-RF

conversion efficiency. We hope the advance of

radio-wave technologies will support the MPT

satellite experiment and realize the SPS.

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Third International Conference on Telecommunications and Remote Sensing