Beam control of Microwave Power Beam for a Solar Power Station ...

The 2 nd Joint International Conference on “Sustainable Energy and Environment (SEE 2006)”
A-023 (O) 21-23 November 2006, Bangkok, Thailand
Beam control of Microwave Power Beam for a Solar Power Station/Satellite
Kozo Hashimoto 1,∗ , Naoki Shinohara 1 and Hiroshi Matsumoto 2
1 Research Institute for Sustainable Humanosphere, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan
2 Kyoto University, Kyoto, Japan
Abstract: Solar power satellite (SPS) is paid attention to as a clean, inexhaustible large-scale base-load power supply, where carbon
dioxide is not exhausted. In order to send power a receiving site correctly, the ground receiving site sends a pilot signal and the SPS
sends power by a microwave to the arrival direction of the signal. Two technologies related to beam control are reviewed. A system
which uses a spread spectrum pilot signal is introduced. An arrival direction is estimated from the pilot signal after despreading the
SS modulation. This does not respond to fake or wrong signals and is more reliable under power transmission. A single frequency can
be used for both monochromatic power transmission and a carrier of the pilot signal and the antennas are shared for both power
transmission and pilot signal reception. Antenna radiation patterns are optimized for high transmission efficiency and low sidelobes
under a uniformly excited array through a genetic algorithm. Radiation patterns directed to multiple directions can also be created.
Keywords: Solar Power Satellite, Microwave Power Transmission, Spread Spectrum, Beam Forming, Retrodirective System
1. INTRODUCTION
Solar power satellite (SPS) is paid attention to as a clean, inexhaustible large-scale base-load power supply where carbon dioxide
is not exhausted. The expectation is great as renewable energy since the SPS is not affected by weather and can supply electric
power as a base load for 24 hours a day. The SPS is a gigantic satellite designed as an electric power station orbiting in the
Geostationary Earth Orbit (GEO) as shown in Fig. 1. The SPS sends a high power microwave to a ground power receiving site, wher
e the microwave power beam is rectified by a rectenna (rectifying antenna) system. The rectenna system converts the microwave to d
irect current (DC) and is connected to an existing electric power grid [1].
Fig. 1 Solar Power Satellite (Artist’s Concept)
The SPS system has an advantage of producing electricity with much higher efficiency than a photovoltaic system on the ground.
Since SPS is placed in GEO, where there is no atmospheric absorption, the solar input power density is about 30% higher than that
on the ground, and solar power is available 24 hours a day except for near the equinoxes without being affected by weather
conditions. Solar flux is approximately eight times higher in space than the long-term surface average on the ground if the insolation
is, for example, 4 kWh/m 2 /day [2]. With 50% MPT efficiency, this gives a net average of four. For the terrestrial system, however, the
efficiency, additional space due to the loss of the storage system, and their costs should be considered in order to supply electricity 24
hours a day. This ratio would become more than ten times depending on the minimum insolation and the efficiency of the storage
system [1].
Global warming and energy problems are key issues in the 21st century. SPS is one of the most import and clean power sources
which can solve these issues. The CO 2 emissions per kWh are compared for SPS, various fossil fuels, and nuclear power in Table 1
[3]. The CO 2 from the operations of the fossil fuel power generation systems mainly comes from burning of fuel, whereas the CO 2
emission from nuclear power plants mainly comes from the use of energy to produce nuclear fuel. Almost zero CO 2 emission is
expected from SPS operation. The SPS System would release less CO 2 / kWh than nuclear power generation.
Table 1 Comparison of relative CO2 emissions from different electricity generation systems (units: g CO 2 /kWh)
Generating system Operations Construction Total
SPS 0 20 20
Coal 1222 3 1225
Oil 844 2 846
Liquefied Natural Gas (LNG) 629 2 631
Nuclear power 19 3 22
Corresponding auhror: kozo@rish.kyoto-u.ac.jp
1

The 2 nd Joint International Conference on “Sustainable Energy and Environment (SEE 2006)”
A-023 (O) 21-23 November 2006, Bangkok, Thailand
A huge, clean power source is to be developed for sustainable economic activities with a sufficient suppression of CO 2 emission.
Only solar technologies can provide such a huge, clean power source in the near future. The terrestrial photovoltaics, wind,
geothermal, and other natural resources depend on the environmental conditions and are neither stable nor sufficient.
This system transmits electric power generated by solar cells in space through microwave. The microwave power is sent to a
power-receiving site on the ground or in space. The retrodircetive array is generally used for transmitting the power correctly to the
receiving site. Two technologies related to beam control are reviewed in the present paper.
A microwave power transmission system is introduced, which can send the power to multiple directions [4]. Power receiving sites
send pilot signals and high power microwave beams are sent to respective arrival directions of the signals. The spread spectrum (SS)
modulation is used for the pilot signals to differentiate them. This system is a kind of software retrodirective array. Pilot signals are
sent from power receiving sites. Based on the information of the arrived signals, the array patterns can be synthesized for a single or
multiple beams, sidelobe suppression, etc. The power is sent to the directions of the pilot signals. The conventional hardware
retrodirective system is fast but complex, and the adjustment of the system is difficult. Although this system is not so fast as the
hardware system, this has flexibility. Because the direct sequence spread spectrum is used for the signals, this does not respond to
fake or wrong signals and is expected to be more reliable under noises and the power transmission. This is a useful technique even
for a single receiving site.
Antenna radiation patterns are optimized for high transmission efficiency and low sidelobes under a uniformly excited array
through a pareto evolutionally algorithm, a kind of genetic algorithm [5]. This is expected to contribute cost reduction as well. This
method can also send the beams to multiple directions or multiple receiving sites.
2. RETRODIRECTIVE SYSTEM
In proposed SPS plans, the electric power of 1GW is supplied from a geosynchronous orbit at an altitude of 36,000 km to a power
reception target with a diameter of a few km by 5.8GHz microwave. The electric power density is highest at the center of the target.
The reception area is decided in consideration of the peak density and the safety to the living body. Because the product of the
sending and receiving areas is constant to obtain a given transmission efficiency, the area of the power transmission side is naturally
decided. It is necessary to direct the beam to the target in extremely high accuracy in order to avoid interference to existing
communication networks and to transmit electricity accurately, efficiently, and safely.
The high accuracy more than that generally used in communications is requested, for example, assuming accuracy 200m or less on
the ground to concentrate the microwave electric power of 90% or more on the receipt site of 0.0003° or less in beam angle. Since
mechanically obtaining this accuracy is impossible, a pilot signal is sent from the target on the ground, and the retrodirective system
which transmits a microwave power beam to the direction of arrival is used.
Based on the recent JAXA (Japan Aerospace Exploration Agency) model [6], a size of a transmitting antenna is 2kmφ at 5.8 GHz
and 1.3 GW output in a geostationary orbit (GEO). The diameter of a rectenna on the ground becomes 2.5 km. The angle to the edge
of the rectenna site from the SPS is 0.002º. One tenth of the value would be necessary at least. The phase difference, φ, between the
antenna elements with a spacing ofd when the direction of arrival (DOA) is θ measured from the broad side, is
2π
d
φ = sinθ
. (1)
λ
2
For an array of M elements with uniform amplitude, if the phase errors of the antenna elements are random, and their variance is Φ ,
the variance of beam pointing deviation is given by [7]
2 2 3
= 12Φ
/ M
δθ . (2)
This value is very small since M is more than 10 4 in one dimension. Systematic errors like errors in direction of arrival could cause
pointing errors. Doppler effects are negligible in GEO [8].
Fig. 2 Retrodirective system
A retrodirective system (Fig. 2) send a microwave power beam correctly to the direction of arrival of a pilot signal which is sent
2

The 2 nd Joint International Conference on “Sustainable Energy and Environment (SEE 2006)”
A-023 (O) 21-23 November 2006, Bangkok, Thailand
from a power receiving site. This is generally realized by hardware in its applications to communications [6]. The power distribution
in the SPS transmitter array is Gaussian-tapered. This focuses the power in the main beam to increase the transmission efficiency and
decrease the sidelobe levels. It is expensive to make the same number of retrodirective systems as transmitting antenna elements.
The required response time in SPS is not so fast as communications. A software retrodirective system would be useful because of the
above reasons. This system measures the direction of arrival of a pilot signal and sends the beam back to its direction. Flexible beam
forming becomes possible. Even power beams can be sent to multiple directions.
Intensity (m W /cm 2)
10 1
10 0
10 -1
10 -2
10 -3
-5 -4 -3 -2 -1 0 1 2 3 4 5
10 2 Rectenna Range (km )
Intensity (m W /cm 2)
10 1
10 0
10 -1
10 -2
10 -3
-5 -4 -3 -2 -1 0 1 2 3 4 5
10 2 Rectenna Range (km )
Fig. 3 Received intensity (power density) profiles at a rectenna site
a (Left) Array with 10-dB Gaussian power distribution, b (Right) Array with uniform excitation
For reduced sidelobes and efficient energy transmission, 10-dB Gaussian tapering is generally proposed for its output power
distribution of the transmitting array antenna for SPS [1]. The beam intensity pattern at the rectenna site has a non-uniform
distribution with a higher intensity in the center of the rectenna and a lower intensity at its periphery as shown in Fig. 3a. The peak
microwave power density at the rectenna site is 27 mW/cm 2 for 1GW output. A typical rectenna site is 4 km in diameter for a
transmitting antenna diameter of 1km operating at 5.8 GHz. Under these conditions, 93% of the transmitted power is collected. The
safety requirement for the microwave power density for humans is set to 1mW/cm 2 in most countries, which is satisfied at the
periphery (±2km). In the uniform excitation as shown in Fig. 3b, the main beam is narrower, side lobe levels are higher, and the
collected energy is lower.
3. SPREAD SPECTRUM PILOT SIGNAL AND ITS APPLICATION
A beam control system for microwave power transmission with Spread Spectrum (SS) pilot signals is developed for SPS. The SS
modulation is used to differentiate pilot signals sent from power receiving sites. An arrival direction is estimated from the phase
difference between two channels after despreading the SS modulation. This system is a kind of software retrodirective array. This
does not respond to fake or wrong signals and is more reliable under power transmission. The proposed system is useful even for a
single receiving site [4].
A new system is introduced in the SS system where a single frequency can be used for both monochromatic power transmission
and a carrier of the pilot signal and the antennas are shared for both power transmission and pilot signal reception [4]. Band
elimination filters (BEFs) and software synchronization detection are used in this system. Fundamental characteristics of the phasedifference
detection are evaluated. The phase detecting circuit works well and the direction of arrival has been successfully detected
under transmitting power from the same antennas. We can use this system to transmit energy in SPS, when we have BEFs that have
larger elimination and amplifiers that have a larger dynamic range and lower noise.
System Configuration
Pilot signal transmitter
Power Transmitting Signal
(2.45GHz)
2.45GHz Antenna
Spectrum Spread Pilot Signal
(⇒2.45GHz)
Phase Shifter
Down Converter
Phase Shifter
Down Converter
Phase Shifter
Down Converter
Phase Controlled
Magnetron
or
FET AMP
Despread circuit
(including MIXER) Spread code
A/D Converter
Control signal
Computer
MIXER
A/D Converter
MIXER
A/D Converter
Fig. 4 Block diagram of spread spectrum pilot system [4]
3

The 2 nd Joint International Conference on “Sustainable Energy and Environment (SEE 2006)”
A-023 (O) 21-23 November 2006, Bangkok, Thailand
The SPS needs to send a microwave power beam to a receiving site correctly. A pilot signal is sent from the receiving site to the
SPS and then the power is beamed back to its original arrival direction. This function is the same as a retrodirective antenna system.
This type of beam control is one of the most important techniques for microwave power transmission system used on SSPS. The
objective is to develop a new microwave power transmission system using a SS pilot signal with a single frequency for the power
transmission and the pilot signal, and to evaluate this system through experiments. The M-series PN code with the length of 1023 is
used for the SS modulation. This system is based on software retrodirective response. The antennas for power transmission and pilot
signal share a common frequency band.
Fig. 4 shows a retrodirective transmitting system. Three channels of the system are shown. This system receives a spread spectrum
pilot signal, detects the direction of arrival, and transmits a power beam in the desired direction by software control. Both the power
transmitting and receiving frequencies are equal to 2.45GHz. This frequency is selected since most SPS models use the ISM
frequency band for energy transmission. Transmitting power is basically generated by a phase controlled magnetron or sent from
FET amplifiers. This is sent to each antenna through a phase shifter for beam steering and a circulator. A signal generator is used as
the transmitter since quite low phase noise is required for better phase difference measurements. The transmission direction is fixed
to the broadside in the present experiment. The received signal goes through the circulator, is down-converted to 10.7MHz,
despread, down-converted to 10 kHz, and A/D converted to measure the phase difference by Fourier analysis. Because the direct
sequence spread spectrum is used for the pilot signal, it does not respond to erroneous signals and is more reliable under noise. The
despread circuit supplies the PN code for despreading to other mixers which share the code. This works under multiple pilot signals
and is a useful technique even for a single receiving site.
Effect of BEF and Soft Sync.
Fig. 5 Effect of band elimination filter and software synchronization [7]
Fig. 5 shows the effect of band elimination filter (BEF) and software SS synchronization. The two lines starting from a root mean
square error (RMSE) < 0.1 show the RMSE without BEF for ft (transmitter frequency) -fp (pilot signal frequency) = 0 and 6MHz for
reference. The other two lines marked with ■ (orange) and ▲ (green) show the RMSE with BEF, and with BEF and the software
synchronization detection system, respectively. The lines stop after the limit where SS synchronization cannot be locked.
Synchronization is attained up to a ratio of 40dB if software synchronization is used. The bad noise figure in the present simple BEF
causes the increase of RMSE. It is confirmed that arrival directions can be measured correctly under the transmitting conditions.
4. BEAM FORMING OF UNIFORMLY EXCITED PHASED ARRAY
For reduced sidelobes and efficient energy transmission, 10-dB Gaussian tapering is generally used for its transmitting array
antenna for SPS as stated in section 2. This causes a problem on the thermal design since elements at the array center are excited 10
times stronger than those near the edge. A high-efficiency and low-cost technology is required for SPS because of its large scale. We
focus on the beam forming of a software retrodirective system by controlling only phases of the antenna elements with constant
excitation. This system is expected not only to be low cost but also to reduce the MSLL (Maximum Side Lobe Level) for the high
efficiency. Uniformly excited array is proposed for transmitting antenna as a solution of thermal issues [5]. Our purpose is an
optimization of radiation pattern with high power collection efficiency and low MSLL.
0
-10
experimentalresult(opt) at v=0[m ],d=0.031[m ]
experim ent
calculation
gain[dB]
-20
-30
-40
-50
-60
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
u[m ]
Fig. 6 a (Left) Beam forming to boresight direction with the minimum MSLL. Red and green lines show calculated and
experimental values, respectively. b (Right) Beam forming to multiple directions. Boresight is 90º in this case
4

The 2 nd Joint International Conference on “Sustainable Energy and Environment (SEE 2006)”
A-023 (O) 21-23 November 2006, Bangkok, Thailand
We have a "Beam forming subsystem" in the SPORT-5.8 (Space POwer Radio Transmission System for 5.8 GHz) [10]. It consists
of one semiconductor amplifier, power dividers, 144 4-bit phase shifters and 144 (12x12) microstrip antennas. The element spacing
of the transmitting antenna is 0.6. We optimized the radiation pattern of SPORTS 5.8 as a small-scale verification of beam formation
of SPS. We optimized the phase distribution for a power concentration in the main beam and reduction of MSLL in the near field
with the constant excitation. Antenna radiation patterns are optimized for high transmission efficiency and low sidelobes under a
uniformly excited array through a pareto evolutionally algorithm in order to solve the problem. We acquired a radiation pattern of
6.6dB lower MSLL than that in the case when all phases are same. We experimentally confirmed that the calculated pattern (blue) is
very close to the experimental one (red) in Fig. 6a. It can be said that the influence of the mutual coupling is little when the distance
between antenna elements is 0.6 λ in the array of rectangular patch antennas.
Fig. 6b shows optimized examples of beam forming to multiple directions for an 80-element linear array. The black and blue lines
indicate a normal optimization and a optimization when the first side lobe is suppressed. When the side lobe is suppressed as shown
in the blue lines, the width of the main lobe is wider, MSLL is lower, and the power is more efficiently collected in beams to both
directions.
5. RESULTS AND DISCUSSION
The direction of arrival (DOA) is measured using 8-element CMSA array with 0.6λ spacing. The distance between the pilot TX and
the system is 4.32m. The output power of the pilot is -20dBm from a dipole. The transmitting power is 10dBm. The power of the
transmitting signal is 9dBm and that of the pilot is -60dBm at the input of the down converter. When two adjacent antennas are used
the errors are about ±1.5 degrees. When the 7 times longer element spacings are used, they are about ±0.4 degree and become much
better. Although the antenna distance is so large and the ambiguity of angles could occur, this would be removed using both results
[4].
For power transmission to a single direction, two new facts have been revealed. One is that antenna array with the phase-only
synthesis is more effective if element directivities are high. The other is that the performance for suppression of MSLL (maximum
sidelobe level) is improved if the number of elements is increased. The optimized results are experimentally confirmed by the beam
forming system. This system can send microwave beams to multiple directions. For multi-beam forming, high efficiency can be
obtained if the beam width of the main lobe is extended to the original angle of the 1st sidelobe [5].
6. CONCLUSION
It is clarified that the DOA can be correctly measured even if power is transmitted at the same frequency as that of the pilot signal.
The problem is a bad noise figure due to the very simple circuit. It is expected that better Pt/Pp ratio could be attained by optimizing
the circuit. The optimization system by controlling only phases of the antenna elements works well for forming beams to a single and
multiple directions. These results show some efforts for the beam control to realize the SPS.
7. ACKNOWLEDGMENTS
The authors gratefully acknowledge Dr. T. Mitani and Messrs. K. Tsutsumi, S. Niijima, and M. Eguchi. This work is partly
supported by the 21 st COE program on Establishment of COE on sustainable energy system.
8. REFERENCES
[1] H. Matsumoto, H., and Hashimoto, K. (Eds.), Supporting Document for the URSI White Paper on Solar Power Satellite
Systems (Draft), URSI, 2006.
[2] Other Insolation examples: 5.0 (Cairo, Egypt), 4.8 (Austin, Texas, USA), 4.52 (Sydney, Australia), 3.71 (Rome, Italy), 3.25
(Kagoshima, Japan), 3.83 (Madrid, Spain) by Glaser, P.E., Davidson, F.P., and Csigi, K.I., Eds. (1997) Solar Power Satellites,
Wiley-Praxis, New York.
[3] The earlier work of the Research Group for Environmental Issues in Keio Economic Observatory, Japan.
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[5] Hashimoto, K., Niijima, S., Eguchi, M., and Matsumoto, H. (2005) Optimization of uniformly excited phased array for
microwave power transmission (in Japanese), IEICE Technical Report, SPS2005-09, pp. 23-30.
[6] M. Mori, M., Kagawa, H., Nagayama, H., and Saito, Y. (2004) Current Status of Study on Hydrogen Production with Space
Solar Power Systems (SSPS), 4th International Conference on Solar Power from Space - SPS’04, Granada, Spain.
[7] Mailloux, R.J. (1993) Phased Array Antenna Handbook, Artech House, Boston.
[8] Hashimoto, K. and H. Matsumoto (2006) Retrodirective system for solar power satellites, 57th International Astronautical
Congress, IAC-06-C3.1.09, Valencia, Spain.
[9] Miyamoto,R.Y., and Itoh, T.(2002) Retrodirective Arrays for Wireless Communications, IEEE Microwave Magazine, vol. 3,
no. 1, 71-79.
[10] Shinohara, N., Matsumoto, H., and Hashimoto, K., (2004) Phase Controlled Magnetron Development for SPORTS - Space
Power Radio Transmission System -, URSI Radio Science Bulletin, (310) pp. 29-35.
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