Toward space solar power: Wireless energy transmission experiments past, present

and future
Frank E. Little, James O. McSpadden, Kai Chang, and Nobuyuki Kaya

Citation: AIP Conference Proceedings 420, 1225 (1998); doi: 10.1063/1.54957

View online: https://doi.org/10.1063/1.54957
View Table of Contents: http://aip.scitation.org/toc/apc/420/1
Published by the American Institute of Physics
 TOWARD SPACE SOLAR POWER: WIRELESS ENERGY TRANSMISSION EXPERIMENTS PAST,
PRESENT AND FUTURE

Frank E. Little James O. McSpadden, and Kai Chang Nobuyuki Kaya

Center for Space Power Department of Electrical Engineering Department of Computer
Texas A&M University Texas A&M University and Systems Engineering
(409) 845-8768 (409) 845-5409 and (409) 845-5285 Kobe University
(078) 881-1212

Abstract

Solar power is a reality. Today, increasing numbers of photovoltaic and other solar-powered installations are in service
around the world and in space. These uses range from the primary electric power source for satellites, remote site scientific
experiments and villages in developing countries to augmenting the commercial electric grid and providing partial power
for individual businesses and homeowners in developed countries. In space, electricity generated by photovoltaic
conversion of solar energy is the mainstay of power for low Earth and geostationary satellite constellations. Still, for all
its acceptance as a benign and environmentally friendly energy source, terrestrial solar power has yet to be seriously
considered a viable technology for providing base electrical generating capacity. The obvious reason is sunshine on Earth
is too unreliable. In addition to the diurnal and seasonal cycles, inclement weather reduces the average daily period and
intensity of insolation. However, the Sun shines constantly in space. The challenge is to harvest and transmit the energy
from space to Earth. The concept of space solar power based on microwave wireless energy transmission was first put forth
more than 25 years ago by Dr. Peter Glaser. We review historical experiments in wireless energy transmission which have
brought the technology from a laboratory curiosity to its present status. Results from recent experiments and their
implications for wireless energy transmission as an enabling technology for space solar power are reviewed. Current
developments are discussed along with proposed terrestrial and space experiments.

INTRODUCTION

Wireless power transmission has been proposed as a means to provide virtually continuous power without the use of
transmission wires. The uses for wireless transmission can be divided into terrestrial (point-to-point power transfer on
Earth and Earth generated power broadcast to the upper atmosphere or space) and space (space generated power broadcast to
ground and space-to-space power transfer). Space-to-ground transmission is economically the most important the use of
wireless power transmission.
The idea for delivering solar power from space to help meet the growing energy needs of developed and developing nations
was conceived by Dr. Peter Glaser (1968) in 1968. Dr. Glaser's concept was orbiting satellites converting solar energy and
transmitting the energy to Earth via a radio frequency energy beam. Satellites placed in geostationary equatorial orbit
35,800 kilometers above the Earth's surface would be continuously illuminated for most of the year. As a result of the orbit
location, the amount of sunlight shining on the satellite during the year is five times more than is available to any
terrestrial location. At geostationary orbit, satellites have the same rotational period as the Earth and are therefore fixed
over one location at all times, enabling the satellite to deliver almost uninterrupted power to a ground receiving site.
Power beaming for orbit raising consists of using ground stations to supply augmentation power for a satellite propulsion
system capable of raising a satellite from low Earth orbit to geostationary orbit (Brown and Eves 1992), some intermediate
Earth orbit (Brown and Schupp 1993) or lunar orbit (Bozek et al 1993). By providing increased energy to the transfer
vehicle, performance of the orbit transfer vehicle could be substantially enhanced, with high potential economic value
(Woodcock and Eder 1993).

EARLY HISTORIC EXPERIMENT

The first wireless power transmission was recorded in scientific experiments by Heinrich Hertz in which high-frequency
power was generated, transmitted and received with parabolic reflectors, and detected at the receiver (Brown 1984). Wireless
power transmission experiments were continued by Nikola Tesla just before and during the early part of this century. Tesla
envisioned wireless power transmission as an alternative to the terrestrial transmission line and distribution grid. Tesla
hoped to use a central beaming station to set up a pattern of standing waves with optimally placed receivers. His idea, to
transmit energy without wires, was far ahead of the technology. In 1899, Tesla first attempted to transmit power from a 200
foot tower at his laboratory in Colorado Springs, Colorado. The resulting level of power broadcast and collected are not

CP420, Space Technology and Applications International Forum-1998

edited by Mohamed S. E1-Genk
DOE CONF-980103 9 1998 The AmericanInstitute of Physics 1-56396-747-2/98/$10.00
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recorded (Brown 1984). His final project, a demonstration transmission system on Long Island was discontinued in 1917
for lack of funding (Cheney 1981).

High power transmission of microwaves became possible with the invention of the magnetron and its subsequent
improvement by Japanese researchers in the 20s, and the invention of the klystron in the 30s. However, wireless power
transmission remained a distant possibility although in 1926 H. Yagi and S. Uda presented "Feasibility of Electric Power
Transmission by Radio Waves" at the Third Pan-Pacific Academic Conference in Tokyo (Matsumoto 1995).

During the Second World War, the secret Japanese "Z-project" to shoot down aircraft with a ground-based high power
microwave beam resulted in the development of a 100 kW magnetron (Matsumoto 1995).

A demonstration of point-to-point wireless power transmission was finally achieved by the Westinghouse Electric
Company in their laboratory in the thirties (Brown 1984), but it was not until the work of William Brown at the Raytheon
Company in the sixties, that point-to-point, focused power beaming with microwaves became practical.

THE GOLDEN AGE

Beginning in the 1950s Goubau and others demonstrated that microwave power could be transmitted via a beam waveguide
of lenses or reflecting mirrors at near 100% efficiency (Goubau and Schwering 1961 and Degenford et al 1964). This set the
stage for directional power beaming in a set of experiments using high power microwave tubes developed by Raytheon
capable of transmitting 100s of kilowatts of RF power (Showron et al, 1964). In a demonstration of a complete power
beaming system, 400 watts of power was generated at the transmitter and 100 watts received (Brown 1984, 1997). The
development of the microwave semiconductor diode (George and Sabbagh, 1963) and its incorporation into a planar array
rectenna (Brown et al 1969) led to the famous Raytheon long duration tethered helicopter (Brown 1965) and subsequent
beam riding helicopter (Brown 1969) experiments conducted by William Brown.

By 1975, Brown (1977) had developed the rectenna to nearly 100% absorbtion of incident microwave radiation, leading to
a measured dc-to-dc power conversion efficiency of greater than 54 percent (Dickinson and Brown 1975). The same year saw
a large-scale demonstration of power beaming with Raytheon and Jet Propulsion Laboratory personnel at NASA's
Goldstone Facility in which over 30 kilowatts of DC power was retrieved from a rectenna bank (Dickinson 1975). The
transmitted power was very high and only a small portion of the beam "hit" the rectenna which was located a mile from the
transmitter. This was the last of the "Golden Age" power beaming experiments.

First T r a n s m i s s i o n Svstem D e m o n s t r a t i o n

The transmission and conversion system components in this experiment were primitive by current standards. Four Hundred
Watts of 2.45 GHz microwave power was transmitted into an ellipsoidal reflector through a feed horn at the near focus and
detected at a receiving horn at the far focus of the ellipse. The ratio of power in the receiving horn to that in the feed was
52%. The microwave to DC conversion was 50%, yielding 104 W output DC power. Overall DC to DC efficiency was
estimated to be 13%, based on assuming 50% for the DC to microwave efficiency.

Helicovter Flight Demonstrations

The long duration helicopter flight in 1964 was the first demonstration of a microwave rectenna. Microwave power at 2.45
GHz from a magnetron source was fed to a dish antenna and received by a 0.37 m2 rectenna on the bottom of the helicopter.
The output power from the rectenna was 270 W with a power to mass ratio of 300 W/kg. The duration of the flight was 10
hours, with the helicopter sustaining an altitude of 14.7 m.

Following the tethered helicopter demonstration, a free-flying, 10 GHz beam riding helicopter was developed. In this
design, the microwave beam provides control references for maintaining position and attitude (with the exception of
elevation). Pitch and roll were determined from the beam wave front, yaw from the linear polarization of the beam and
distance from the axis by beam density. The control circuitry was optimized such that in still air, the helicopter maintained
position within one to two degrees of pitch and roll and a fraction of an inch in position. In the demonstration, power for
the helicopter was supplied by a power cable and not from the microwave beam.

Efficiency Demonstration

The highest verified efficiency for all end-to-end wireless power transmission system (DC output power compared to DC
input power) was achieved by William Brown at Raytheon and certified by the NASA Jet Propulsion Laboratory. A gaussian
beam with negligible side lobes was transmitted over a short distance to a planar rectenna array. Overall efficiency was

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54.18 _+0.94%, with efficiency of DC power to microwave power conversion being 68.9%, aperture to aperture
transmission efficiency being 95% and beam collection and rectification to DC efficiency being 82.4%.

Although no subsequent measurements of overall DC to DC efficiency have been made, collection/rectification efficiency
has been improved to near 90% at 2.45 GHz and if the 90% efficiency which has been achieved for DC to microwave
conversion at 915 MHz could be transferred to 2.45 GHz, nearly 77% efficiency would be possible.

Goldstone

The long distance high power wireless power transmission demonstration at the Venus site of the Jet Propulsion
Laboratory's Goldstone facility was done largely to show that wireless power transmission was ready to graduate from the
laboratory to field applications. Microwave power at 2.388 GHz was transmitted from a 25 m antenna to a 3.6 m by 7.2 m
rectenna located at a distance of 1.54 km (0.96 mile). The demonstration consisted of scanning the microwave beam across
the rectenna, illuminating a set of lights associated with individual panels of the rectenna. Over 30 kW was converted to DC
output power from the rectenna, with the efficiency of the rectenna (collection and rectification) being measured at 82%.

THE RECENT P A S T

Beginning in the late 1980s, interest again developed in power beaming, both from Earth, for point-to-point terrestrial
use and to power sustained flight, as well as from space with renewed interest in the concept of solar energy from space.
Although 2.45 GHz remained the main frequency for wireless power transmission studies and experimentation, work was
begun at other frequencies. A rectenna array was developed at 5.8 GHz with greater than 80% RF to DC conversion
efficiency (Bharj et al 1992). ARCO Power Technologies, Inc. developed a 35 GHz rectenna in 1988 for which they reported
50% conversion efficiency. This was subsequently improved to 72% (Koert and Cha 1992) and has been proposed for a
mobile high altitude microwave powered aircraft (Koert and Cha 1993).

Several demonstrations of microwave powered vehicles have been reported; including the Canadian Stationary High
Altitude Relay Platform (SHARP) microwave powered airplane, flown at low altitude for 20 minutes using 2.45 GHz
microwave frequency (Schlesak, Alden and Ohno 1988); a remote microwave powered rover (using 5.86 GHz) which included
a tracking antenna at the Sarnoff Research Center (Bharj et al 1993); the Japanese Microwave Lifted Airplane eXperiment
(MILAX) which demonstrated an active phased array transmitting antenna (Kaya et al 1993); a Semi-Autonomous BEam
Rider (SABER) model helicopter (Houston, Hawkins and Brown 1995); and the Japanese Energy Transmission toward High
altitude long endurance airship ExpeRiment (ETHER) airship (Kaya et al 1995, Onda et al 1995)

Japanese researchers have developed and flown sounding rocket experiments to investigate the effects of power beaming
(at 2.41 GHz) on the ionosphere. MINIX (Microwave Ionosphere Nonlinear Interaction eXperiment) was flown in 1983
(Kaya et al 1986). ISY-METS (International Space Year-Microwave Energy Transmission in Space), an international
follow-on experiment to confirm and extend MINIX, included contributions from Texas A&M University and the
International Space University and was flown in 1993 (Kaya et al 1993).

A point-to-point microwave power transmission experiment was developed in Japan in collaboration with Kansai Power
Company to demonstrate the feasibility of terrestrial power transmission (Matsumoto 1995).

SHARP

The Canadian government Communications Research Center conceived the idea of providing satellite-like specialized
electronic communications with electric powered airplanes equipped with transponders and flying in a 2 km circular pattern
at 21 km altitude. The flight system would consist of a light weight aircraft with 70 m 2 of rectenna surface. Power would be
supplied from an approximately 500 kW microwave transmitting source consisting of multiple transmitters in an 80 m
diameter antenna grid. The 5.8 GHz beam would focus to a half-power spot of 20 m with 1000 W/m 2 at flight altitude. The 50
kW captured by the rectenna would supply 40 kW power for the electric motor of the airplane and 10 kW for the payload.

A scale model prototype of the SHARP aircraft was flown in 1987 for 20 minutes to a height of about 300 m using power
beamed at 2.45 GHz to demonstrate the concept of beam powered flight.

MILAX

This experiment was carried out by researchers at Kobe University using a model airplane vehicle similar to the Canadian
SHARP experiment to demonstrate the feasibility of microwave power transmission and to test new components. Whereas
the SHARP flight relied on existing microwave tube and antenna technology, the MILAX sought to demonstrate an active

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phased array solid-state antenna including beam steering. The antenna was composed of 96 microstrip antenna subarrays.
The microwave reference oscillator signal (at 2.41 GHz) was distributed to each subarray through its phase shifter and
amplified for a total output greater than 1.2 kW. The antenna efficiency (DC input to RF output) was 42%. A two camera TV
system was used to determine the position of the airplane. Position information from the TV was fed through a computer
which controlled the phase shifters to point the beam at the airplane.

The experiment consisted of a free-flying model plane with a rectenna on the bottom surface flying above a moving car
with an antenna mounted on its roof. The model was flown under microwave power for about 40 seconds per trial (on a
straight path) at a height of 10 m. The steerable phased array antenna tracked the plane and maintained nearly constant
output to the plane rectenna despite variations in their relative position.

MINIX

The first rocket experiment of microwave power transmission was MINIX, carded out by a team from Kyoto University and
Kobe University in 1983. The experiment was devised to test the non-linear interaction of microwave radiation with the
ionosphere and to demonstrate microwave power transmission in space. Two 850 W magnetrons fed 2.45 GHz microwaves
into a truncated waveguide antenna on the mother section which beamed to receiving instruments on a daughter section.
Variations in the plasma wave spectrum due to the presence of the beam were measured.

|SY-METS

The Kobe-Kyoto team developed the MEFS experiment to extend the range of data and verify the results obtained with
MINIX on microwave-plasma interaction and to test a solid state phased array transmitting antenna in space conditions.
The antenna elements were similar to those used in MILAX, but were arranged on four deployable paddles in a cross
configuration. The total power radiated by the antenna was 800 W. As with MILAX, two CCD cameras connected to a
computer detected the position and range of the receiving rectenna and sent control signals to the phase shifters to direct
and focus the beam. Two rectennas, one Japanese and one from Texas A&M University making the experiment truly
international, were used to detect the microwave signal. The receiving rectennas were deployed from a daughter section
which separated from the mother to vary the microwave transmission distance. (As an aside, the Texas A&M rectenna panel
was designed, built, tested, qualified and delivered for integration in three months.)

Kansai Power E x v e r i m e n t

This pilot point-to-point terrestrial power transmission project used a 5 kW magnetron at 2.45 GHz to feed a high
performance parabolic transmitting antenna through a truncated waveguide giving a Gaussian distribution at the aperture.
The beam was transmitted 40 m to a rectenna. The efficiency for capture and conversion of the transmitted beam was
measured at 75%.

PRESENT R E S E A R C H

Frequencies other than 2.45 GHz, particularly 5.8 GHz and 35 GHz are being given greater attention as candidates for
wireless powel~ transmission in studies and experiments. The mass and size of components and systems for the higher
frequencies are attractive. However, the component efficiencies are less than for 2.45 GHz, and atmospheric attenuation ,
particularly with rain, is greater.

Recent work by Brown (1995) has resulted in the conversion of a common 2.45 GHz magnetron oscillator, with the
addition of external circuitry, into a high-gain, phased-locked amplifier with independent control of the operating
frequency and of the power output level. This control was achieved over a broad range of power output and operating
frequency. In addition, an amplitude control system using a buckboost coil is incorporated into the packaging of the
magnetron. This controls the microwave power output level within narrow limits despite a wide variation in the DC voltage
applied to the magnetron, functioning as a low loss power conditioner when the magnetron is paralleled with other
magnetrons on a common high power bus. It also functions as an electronic filter to help counter the effects of a current
ripple on the power supply. This magnetron has been coupled with a slotted waveguide antenna element to form the basis of
a phased array antenna.

Although conversion efficiency is not yet as high as for magnetrons, solid state radio frequency transmitting systems
show promise and are improving. The Japanese in particular continue development of solid state devices for the Solar Power
Satellite systems and experiments they are planning (Kaya et al 1997, Matsumoto et al 1997).

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 Improvements are being reported for RF to DC conversion devices such as higher frequency rectennas (McSpadden, Fan and
Chang 1997) and cyclotron wave converters (Savvin 1997)

The prize for most ambitious wireless power transmission demonstration proposed since Tesla's Long Island tower
experiment before World War I goes to the Japanese SPS 2000 project (ISAS 1993, Sasaki, Naruo and Nagatomo 1997).
This project would launch a demonstration space solar power satellite capable of transmitting 10 MW of power. The
satellite would orbit the Earth along an east to west low earth orbit and could deliver bursts of power to sites located in a
narrow band along the equator. The purpose is to demonstrate a functioning solar power satellite system including the
wireless transmission link and develop the ground infrastructure in several locations to provide the basis for a space solar
power market.

Limited studies have been undertaken to understand the effects of non-ionizing radiation on biota. Regulatory agencies
have established widely varying standards and limits for human exposure to microwave radiation dependent both on the
wavelength and intensity of the radiation. The most widely cited non-human studies were performed with birds (Arthur D.
Little, Inc. 1980) and bees (Gary and Westerdahl 1980). Japanese researchers have embarked on a long term project to
determine the effect of microwave radiation on the biota (Murakami et al 1997).

Planning for terrestrial power beaming projects has moved beyond pilot scale. One project, planned for R6union Island
will supply electricity to the remote village of Grand-Bassin (Celeste et al 1997). A second, planned for the Southwest
United States, would use an existing utility owned large-scale photovoltaic electric generation site to supply power to be
converted to microwaves, beamed to a rectenna, reconverted to electricity and supplied to the utility's grid (Nansen 1996).

Rgtrodirective Phased Arrav Antenna/Reftenna Demonstration

Recent work at Kobe University has led to the development of a demonstration 5.8 GHz retrodirective phased array
transmission system. The antenna uses solid state amplifiers directly connected to the transmitting antenna elements to
reduce cable losses and reduce weight and was derived from the antenna design used for MILAX/METS. A half frequency pilot
beam (2.9 GHz) is broadcast from the rectenna. Receiving antennae for the pilot beam are integral with the antenna. The
phase of the pilot beam is determined by comparison with the 5.8 GHz master oscillator. The conjugate phase is fed to
phase shifters to accurately steer the beam onto the rectenna

Advantages of this system include smaller size and less mass when compared to 2.45 GHz, simple and accurate pointing
control and improved efficiency of the power amplifiers. The system was demonstrated at SPS '97 in Montreal.

SPS 2 0 0 0

The design calls for a gravity stabilized satellite capable of delivering 10 MW of electricity from a spherical 1100 km east-
to-west equatorial orbit. The phased array antenna will be capable of steering +30* along the orbital path (E-W) and +16.7 ~
perpendicular to the orbital path (N-S). This will limit the possible rectenna sites to close to the equator. In addition to
being limited to an equatorial band, the receiving sites must be at least t200 km apart to maximize the length of time for
power transmission to each individual site. Because power can only be received intermittently at any ground site (about 4
minutes out of the 108 minute orbit for a beam scan angle of 30*) energy storage is an important component of any ground
site. Further limitations are placed on the power available to any site by the diurnal rotation of the Earth, since the satellite
is incapable of delivering energy while in eclipse over a site during the night. With an average daily coverage of less than
30 minutes per site, 4 to 4.5 MWh of energy could be available to a site from the SPS 2000 satellite.

The satellite is in the form of a long prism. The base of the satellite is always Earth facing and mounts the transmitting
array. The "roof' faces of the satellite are paneled with photovoltaic cells. The phased array transmitting array is based on a
dense array of low energy solid state antenna elements (see MILAX description, the design assumes an efficiency of 60%,
which has not yet been achieved, the MILAX/METS antenna solid state elements achieved 42% efficiency). To assure target
acquisition and tracking, a retrodirective beam at 245 MHz transmitted from each rectenna site is used. The satellite would
be launched in sections and assembled on orbit.

Initial designs studies have been completed and a scale model mock-up of the satellite has been made. Several potential
receiving sites, from Pacific Islands to South American Andes locations have been visited by the SPS 2000 team, with a
generally enthusiastic reception.

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Microwave Garden

A long-duration microwave exposure facility was built in 1994 in Japan. A horn antenna is mounted at a height of 2 m on
a pole in the middle of an 8 m by 8 m test field. The plot is surrounded by a 4 m high shield fence. The radiation field has
been mapped to determine local intensities. Several experiments have been conducted with plants on the effect of 2.45 GHz
radiation at beam intensities up to 23 mW/cm2 ( the maximum proposed beam intensity for the DOE/NASA Reference
Study). Seeds sewn in the plot are allowed to germinate and grow while under constant radiation.

To date, the studies have been carried out under natural conditions, leading to the conclusion that whereas the radiation test
equipment and facility had been validated, any effect from microwave radiation was overwhelmed by the local weather
conditions.

Grand-Bassin Proiect

During its implementation, the French led Grand-Bassin project will accomplish several goals. Most important of these is
to provide an actual demonstration of point-to-point power beaming. Grand-Bassin is a small isolated mountain village on
La REunion. Set in the scenic environment of a river valley surrounded by steep cliffs, access is limited to trail or
helicopter. Several tourist lodges have been established in Grand-Bassin to accommodate sightseers. Further development
of the tourist potential of Grand-Bassin is hampered by the lack of electricity in the village to supply refrigeration for food
and laundry for overnight guests. Several options were investigated for providing up to 10 kW of electricity to Grand-
Bassin. For primarily aesthetic reasons, a microwave wireless power transmission link from the existing terminus of a
electric power line was chosen.

The primary constraint imposed on the system was cost. In order to compete with photovoltaic conversion and keep
overall energy costs low, the end-to-end electrical conversion and transmission system efficiency had to be at least 20%.
Although the aesthetic desire was to use as small a transmitter and rectenna as possible, concern for the perceived safety of
the human inhabitants and other biota argues for low energy density (maximum of 5 mW/cm2) in the beam, with an
attendant loss of efficiency. An "H" dipole design is used for the rectenna. The transmitter will consist of injection locked
phase and amplitude controlled magnetrons (based on William Brown's work, see above) feeding a multi-focus parabolic
reflector. This design consists of several parabolic reflector sectors with a common focus, a microwave analogue to the
Fresnel lens. The distance of the wireless link is 700 m. The design system will utilize a rectenna aperture diameter of 17 m,
with a 6 m transmitter diameter to give 95% collection efficiency. Overall ac-to-ac conversion efficiency is calculated to be
57%.

A prototype system demonstration is planned for 2000. The system will consist of four multi-focus parabolic reflectors fed
by 1 kW magnetrons transmitting over a distance of 150 m to a 180 m 2 H dipole rectenna to deliver 2 kW output power.

SPS End-to-End Terrestrial D e m o n s t r a t i o n

A test project to demonstrate all the major elements of a solar power satellite on the ground is being developed. In this
demonstration concept, the DC output of a photovoltaic array is used to power a transmitting array at the hundreds of kW
power level. The receiving rectenna, located at a distance of 1 to 5 km from the transmitter, would convert the RF power to
DC for a utility grid. The objective is to verify practical wireless power transmission and to establish the reliability of
components operated over time. In addition to operation data, environmental studies could be performed to ensure the safety
of the beam. Finally, it would be possible to test the concept of beam splitting (targeting multiple receivers) from the
transmitting antenna.

FUTURE D I R E C T I O N S

"'Would you tell me, please, which way I ought to go from here?'
'That depends a good deal on where you want to get to,' said the cat." (Lewis Carroll)

Much recent effort has concentrated on reexamining the technology base and commercial potential for a modem space
solar power system (Mankins et al 1997). A number of options for space solar power, including the original reference
definition system, have been studied.

Several technology and system demonstration experiments have been proposed both in space and on the ground to
advance awareness of and confidence in wireless power transmission technology. In addition to the Grand-Bassin prototype

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demonstration planned for 2000 (Celeste et al 1997), a terrestrial end-to-end demonstration (sunlight to power grid) to test
system reliability continues under development (Nansen 1996).

Space based experiments include equipment and system concept demonstrations (retrodirective antenna) and further tests of
the interaction between a high power density microwave beam and the space plasma both on the International Space Station
(Matsumoto et al 1997) and the Japanese led International Space Power ExpeRiment (ISPER) on a free free flying satellite
(Kaya 1996). Finally, discussions are beginning on an international experiment using the International Space Station and
the Japanese SFU.

Space Station Microwave Power Transmission Experiment

A Japanese led experiment to test a retrodirective transmission system and microwave space plasma has been proposed for
the International Space Station (Matsumoto et at 1997). The experiment would be mounted as an external experiment on
the Japanese Experiment Module Exposed Facility. The experiment uses a small 2.2 m by 1.6 m transmitting antenna with a
total power of 680 W beamed to a 0.5 m by 0.5 m rectenna fixed at the end of a boom extendible to 2.5 m. The pointing and
focusing of the microwave beam is accomplished by electronically reconstructing the pilot beam and using the information
to control digital phase shifters for individual transmitting elements. The transmitting antenna can direct and focus the
microwave beam onto the rectenna, achieving a range of energy densities through changing the beam size at the focal
point. Since the space station orbit lies within the upper plasma bands, long duration microwave beam space plasma
interaction experiments can be conducted.

ISPER

A Japanese led space experiment is being planned to demonstrate a new concept for a solar power satellite. The heart of the
concept is a sandwich structure with the retrodirective solid state phased array transmitting antenna integrated with solar
cells directly connected to the power amplifiers, eliminating long conducting lines. Light inflatable concentrators would
track the sun and reflect to the solar cells.

The experiment consists of a transmitting antenna-photovoltaic array sandwich array deployed from the Japanese Space
Flyer Unit (SFU) in low earth orbit beaming to daughter satellites and ground stations. The primary objective of the ISPER
experiment is to verify the space operation of the antenna-photovoltaic array sandwich. Secondary objectives are to
examine the nonlinear interaction between the ionospheric plasma and the microwave beam demonstrate the solar collector
and demonstrate microwave wireless power transmission from low earth orbit. The ground station will receive enough
power to light a small lightbulb.

A preliminary rocket experiment is planned which will deploy a circular phased array antenna to transmit microwave power
to the ground. The objective is to determine the performance of the retrodirective control at long range.

CONCLUSIONS

Wireless power transmission is an enabling technology for space solar power and remains a fruitful area for research and
development. Although much work on microwave transmission continues in the laboratory, critical space experiments
including a commercial pilot solar power satellite (SPS 2000) and large scale, long duration terrestrial demonstrations are
under development.
Acknowledgments
The authors wish to acknowledge the many contributions of Mr. William C. Brown to the field of microwave wireless
power transmission. Without Bill's pioneering efforts, this field would still be in its infancy.

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