Phase-Controlled MagnetronDevelopment for SPORTS: SpacePowerRadio Transmission System

N. ShinoharaH.MatsumotoK.Hashimoto

AbstractSince the first proposal of a Space Solar Power

Station (SPS) in 1968, many different types of SPS havebeen proposed. Someofthem are designed with microwavepower transmission (MPT) technology based on amicrowavepower transmitter with microwavetubes. Themicrowavetubes, such as klystrons or magnetrons, havehigh efficiency (> 70% ) and high power output (over1 kW). Weshowanewconcept for a microwavetransmitterwith phase-controlled magnetrons (PCM), for satisfyingboth requirements of high efficiencyandbeamcontrol. Wealso propose a newphased-array system with the phase-controlled magnetrons for the SPS.

Weuse the injection-locking technique and phase-locked-loop (PLL) feedback by controlling an anode currentfor the phase-controlled magnetron. Wecanstabilize andcontrol the frequency and phase ofthe microwaveemissionof the phase-controlled magnetron. However,wehaveapower loss after the phase-controlled magnetron for SPSuse, because the output power fromoneantenna is designedto be less than 1 W in a recent SPS design, and wehave toinsert a power divider and phase shifters after the phase-controlled magnetron. In order to decrease the power lossafter the phase-controlled magnetron, wehave recentlyproposed somenewconcepts, and have developed a phase-controlled magnetron array called SPORTS (Space POwerRadio Transmission System) in FY2000 and FY2001 atKyoto University.

1. Introduction

Microwavepowertransmission (MPT) technology isone ofthe most important technologies in the Space SolarPowerSystem (SPS). The dc-RF-dc conversion efficiency,the accuracy ofthe microwavebeamcontrol, and the cost of

the microwavepower transmission system are especiallyimportant. Webelieve that a magnetron is well-suited forthe microwavepowertransmission system ofthe SPS [1],especially a small experimental SPS, because its dc-RFconversion efficiency is high (>70% ) and it is veryinexpensive ( < $5 ). Weight is also important for a spacesystem, because it strongly affects cost. The weight-powerratio (g/W) ofthe magnetronsystem, which includes the dc-dc converter and heat radiation, is smaller than that for asemiconductor amplifier system, because the output powerofthe magnetron is high ( > 1 kW). Of cause, the magnetrondoes have someunsuitable characteristics for microwavepowertransmission including a communication system. Ithas been difficult to control the frequency and the phase ofthe microwaveoutput ofthe magnetron,because it is just amicrowaveoscillator.

Oneresult of our study is to show that the outputmicrowavespectrumofa magnetrondepends onthe stabilityofthe dc powersource andthe filament current [2]. Ifwe usea stabilized dc powersourcefor the magnetronand turn offthe filament current after stable oscillation occurs, thespectrum, including low frequencies and high frequencies,is quiet and pure, and it is adequate for use in a microwavepowertransmission system (Figure 1 ). The main frequency,the high frequency, and the low frequencyweremeasuredby a spectrum analyzer with a spiral antenna, a hornantenna, and a current probe, respectively. The harmonicsdid not show great improvementbetween turning on thefilament current and turning it off. This is because theharmonics ofthe magnetronare caused by distortion ofthefundamental, and the distortion is independent ofthe effectof the filament current. However,almost all spuriousemissions are suppressed except some spiky noise. Thecause ofthe other spurious signals is complex; however,weconsiderthat it is enough to suppress interference. However,there remains the problem ofthe frequency shift caused byheating of the magnetron; still, the spurious emissions aresuppressed.

Naoki Sh inohara, Hiroshi Matsumoto,andKozoHashimotoare with the Research Institute of SustainableHumanosphere, Kyoto University, Uji, Kyoto 611-0011,Japan; Tel: +81-774-38-3818; Fax: +81- 774-31-8463; E-

mail: shino@rish. kyoto-u. ac.jp

This invited paper is part ofthe Special Section on SpaceSolar Power Systems. An oral version was originallypresented at the 2003 Japan-US Joint Workshop on SpaceSolar Power System (JUSPS'03), July 3-4, 2003, KyotoUniversity, Uji, Kyoto, Japan.

TheRadio Science Bulletin No 3 10 (September, 2004) 29

Figure 1. The output spectrumofacommonlyused magnetron(bold:

with destabilizedpower supply andfilament off; normal: with half-wave

voltage-doubler powersupply andfilament on)

2.4 2.45

Freq. (GHz)2.5 2.55

-anl_""0 0.1 02 0.3 0.4 0.6 0.6 0.7 0.8 0.9

Fraq. (QHz) B 9 10 ll 12 13 14Freq. (GHz)

Therefore, in order to stabilize the magnetronfrequency and to control its phase, we have developed aphase-controlled magnetron (PCM) with injection lockingand phase-locked-loop (PLL) feedback obtained bycontrolling the anode current of the magnetron (Figure 2)[1]. We use a commonlyused, inexpensive magnetron for

the phase-controlled magnetron.The phase ofthe magnetroncan be tuned in several different ways. One is to change themagnetic field by using an external coil [3]. Another is tochange the anode current flow. Wehave chosen the anodecurrent control, because the range of frequency control andphase control is wider with anode current control than is

Injection Locking

I CmfrdtatfrCumrtcfMagutam]

PLL feed-back

Figure 2. A diagram ofthephase-controlled magnetron

30 Radio Science Bulletin No 3 10 (September, 2004)

600

5 580

| 560o

H »f**MM»MfMtt«P"nMWt|MHM«l»MM« MfrtMinMnu-i

S\

«J4£ 560

a.å S 540t

O 520

5000

20

Time [Min]

40 60

10

0

-10- 20

0)oc

<D

« »-«CO

-30 I

-50

-60 a.

-70

-80

Figure 3. The timedependence ofthe phaseandpower ofa phase-controlled magnetron.

possible with control ofthe magnetic field. Figure 3 indicatesthe time dependence of the phase and the power output ofthe phase-controlled magnetron. Afteran initial time delay,the output power and the phase of the phase-controlledmagnetron's output are stabilized. We have carried out aphased-array experiment with the phase-controlledmagnetrons, and we successfully controlled the beamdirection with the phase-controlled magnetrons [4].

Based on the phase-contro Ued-magnetrontechnique,wehave developed some microwavepower transmissionsystems for equipment at Kyoto University. One is a2.45 GHz system called SPORTS-2.45 (Space POwerRadioTransmission System for 2.45 GHz), and the other is a5.8 GHzsystem called SPORTS-5.8 (Space POwerRadioTransmission System for 5.8 GHz). The phase-controlledmagnetronhas characteristics ofhigh efficiency, high power,high voltage, and small weight-power ratio. However, forsomeSPS applications, the high-output-power advantageof the phase-controlled magnetron is weakened, becausethe output power fromoneantenna is designed to be lessthan one watt in a recent SPS design [5]. Therefore, wehaveto put apower divider and phase shifters before the antenna,in order to control the beam direction electrically and tosuppress grating lobes. Commonlyused power dividers andphase shifters have a large power loss. However, the SPSmusthave alow-loss microwavepowertransmission systembecause of cooling problems in dissipating generated heat,and cost considerations. The SPORTS system has thereforebeen developed with meansfor decreasing the power lossafter the phase-controlled magnetron, in order to realize abeam-control system.

There is no law regarding the use ofthe frequencyfor microwavepowertransmission. Only the I SM (Industrial ,Scientific, and Medical) band is nowopen for microwavepower transmission. Wewould like to use microwavesforpower transmission fromspace to the ground because of(system and propagation) efficiency and cost. 2.45 GHzand 5.8 GHz belong to the ISM band, and these are

microwaves. Therefore, wehave chosen the frequencies of2.45 GHz and 5.8 GHz. If other microwave frequenciesbecome available for microwave power transmission, wewill use the new frequencies.

2. SPORTS-2.45

SPORTS-2.45 is composed of four subsystems(Figure 4) : a phase-controlled magnetron phased array as amicrowave transmitter subsystem, a microwave receiver(rectenna array), solar panels for dc supply, and a near-fieldscanner for measurement.SPORTS-2.45 wasdesigned anddeveloped at Kyoto University as a test bed for a spacemicrowavepowertransmission experiment in FY2000.

The solar panels provide 8.4 kW of dc power to themicrowavetransmitter subsystem, composed of a phasedarray with twelve phase-controlled magnetrons. The designspecifies 200 V dc, as would be suitable in space use. Forthe phase-controlled magnetron of the SPORTS-2.45, weused acooker-type magnetron, 2M234, madeby MatsushitaCo. Typical characteristics ofthe 2M234 are as follows: thefrequency (matched load) is 2.455 GHz; the peak anodevoltage is 3.85 kV; the average output power (matchedload) is 550 W. Wealways turn off the filament currentduring power transmission after stable oscillation occurs.Weachieved a frequency stability of better than 10 8,relative to the frequency stability of an input referencesignal. The transmitted microwave power and frequencywere 4 kW and 2.45 GHz, respectively. One referencesignal was divided and injected into each phase-controlledmagnetron through a phase shifter. This was the first phase-controlled magnetron array in the world.

The SPORTS-2.45 system has two different types ofantenna arrays. One is a twelve-horn antenna array, withlow power loss but with a limited narrow-beamscanningcapability. The gain of each horn antenna is 17.73 dB. Theother antenna is a 96-dipole antenna array with reflector and

TheRadio Science Bulletin No 310 (September, 2004) 31

Discrete phase control with

2.45GHz2kW

12 PCMs(2.45GHz, 4kW)

12 element horn antenna

Rectenna Array

2m <£

Figure 4. The SPORTS-2. 45 system.

sub-phase shifters. The sub-phase shifter can control thephase ofa microwave signal with only the options of -60° ,0°, +60°. It is a 96-antenna array with twelve eight-waypower dividers and 96 sub-phase shifters, with which themicrowave beam can be scanned into a much wider rangeof directions. Wehave shown that we can keep high beam-collection efficiency when we control a beam to a two-times-larger direction with the sub-phase shifters in the SPSsystem [1].

For target detection, we use a retro-directive systemwith a CW pilot signal of400 MHz in the SPORTS-2.45.Weuse three receiving antennas for the pilot signal, and wecalculate the direction ofa target with a computer. Weputthe receiving antennas in triangular locations, and theelementspacing ofthe receiving antennas is 1.3 m ( 1.7A )à"Wecan easily change the frequency ofthe pilot signal by

hardware and software changes because we calculate thedirection of the target with a computer. Wesucceeded indetecting the target under conditions of power transmissionwith the phase-controlled magnetrons.

The rectenna array is composed of four types ofrectennas, which are placed accordingly for optimummicrowave input power, yielding the highest RF-dcconversion efficiency [6] in the presence of a spatially-varying gradient powerdensity ofthe microwavebeam.Allrectennas in an array cannot be ofhighest RF-dc conversionefficiency because the RF-dc conversion efficiencyof therectenna has an input-power dependence. Weuse a Yagi-Uda antenna and a power divider for the rectenna. ThemaximumRF-dc conversion efficiency of each rectennawas 77.7%, 75.7%, 73.0%, and 68.6% with 1 W, 2.1 W,

4. 1 W, and 6.7 Wof input microwave power, respectively

Figure 5. The SPORTS-5.8 system

PLL far Phase Cortrnl+Injection Ruck

32 TheRadio Science Bulletin No 3 10 (September, 2004)

Hlf f.O di*to à"».'

#_«-!*à" B.BIank MHR S.V37 6H2'3.Ml ilUn

SPAN2.000 GHz'

CFHTFB S.735 OtrttW S Mb VOHK Mtr SUP SO :

SPAM?.M» (MrAPT20 A

Figure 6a. The output spectrum ofthe 5.8 GHzmagnetron of SPORTS-5. 8 (the span is 2 GHz)

Marker.5.800000000 GHz

sfl« "-31.346 dBrn

^^^VAvA/^^,^^^ '

nun Sb«*d 1.525 s (601 Dts)

Figure 6b. The output spectrum ofthe semiconduc-tor amplifier ofSPORTS-5.8 (the span is 4 GHz).

[7]. There are 2,692 rectenna elements, and the diameter ofthe rectenna array is approximately 2 m.

3. SPORTS-5.8

In FY2001, we also developed SPORTS-5.8, with5.8 GHz magnetrons for the phase-controlled magnetron(Figure 5). SPORTS-5.8 is composed ofseveral transmittingsubsystems and rectenna arrays. The main microwavetransmitting system is composed of nine phase-controlledmagnetrons, based on a newly developed magnetron with afrequency of 5.8 GHz. Typical performance values of the5.8 GHz magnetron, developed by Matsushita Co., are asfollows: the frequency (matched load) is 5.800 GHz; thepeak anode voltage is 4.0 kV; the average output power(matched load) is 300 W. Wealways turn off the filamentcurrent during power transmission after stable oscillationoccurs. Four-stage eight-way power dividers were developedto distribute the microwave power from onephase-controlledmagnetron to 32 microstrip-antenna elements. Thetransmitted microwave power from the 288 antennas wasover 1.26 kW. In the main transmitter system, weadopted

a random array, in order to decrease the level of gratinglobes and to decrease power loss after the phase-controlledmagnetron. The power loss after the phase-controlledmagnetron was below -1.5 dB.

For target detection, we use a retro-directive systemwith a pilot signal of 4.8 GHz that is modulated with aspread spectrum [8]. We use four receiving antennas for apilot signal, and we calculate the direction ofa target witha computer. This does not respond to fake or wrong signals,because direct-sequence spread-spectrum is used for thesignals. It is expected to be more reliable in the presence ofnoise and power transmission. The pilot signal can bemodulated, e.g., forsending information on powerreceptionand authentication. This is a useful technique, even for asingle receiving site. It has been confirmed that the newdirection-finding system for multiple spread-spectrum pilotsignals works well. Figure 7 shows the result of directionfinding with a modulated spread-spectrum pilot signal inthe SPORTS-5.8 system [9]. The distance between Tx andRxwasapproximately 4 m.Weused two receiving antennasfor the pilot signal, which were put in the horizontal planewith a spacing of 0.5A. The error in the direction finding

2U)1>蝣obocllLLco9> Oe-ioL.LUffis>c<

蝣

^ ^ ^ fc l>蝣 蝣蝣S i^ M i蝣 蝣 H !l̂ H H

E r r o r : - 0 .7 ~ 0 .3 ( d e g r e e )

1 0 2 0

A n g le o f C o m in i P ilo t S ig n a l f d e g )

Figure 7. The result ofdirectionfinding with a

modulated spread-spectrumpilot signal [9].

Radio Science Bulletin No 3 10 (September, 2004) 33

CO

-10a

I -209

.g

| "30£

-40

w m ¥ti M M M L M

a m d ir e c t io n : 1 1 .2 (d e g r e e )

= > e r ro r 1 .2 ( d e g re e )I 蝣 I

- 2 0 - 1 0 0 1 0

A n g g l eo f m i c r o w a v eb e a m( d e g r e e )

2 0

F i g u r e8 . T h er e s u l to f b e a m f o r m i n gw i t h t h e r e t r o - d i r e c t i v es y s t e m[ 1 0 ] .

w a sb e l o w0 . 7 ° .T h i s i s s u f f i c i e n tf o r t h e f i n a l S P S s y s t e mb e c a u s ew eu s ea l a r g en u m b e ro f t h e d i r e c t i o n - f i n d i n gs y s t e m si n t h e S P S , a n d t h e d i r e c t i o n - f i n d i n g e r r o r i se x p e c t e dt o b e m u c hs m a l l e ri n t o t a l .

W eh a v e a n o t h e r ,a l t e r n a t i v e m i c r o w a v ep o w e rt r a n s m i s s i o ns y s t e mi n t h e S P O R T - 5 . 8 .I t i s c a l l e d a " b e a m -f o r m i n gs u b s y s t e m , "w i t h o n es e m i c o n d u c t o ra m p l i f i e r ,p o w e rd i v i d e r s , 1 4 4 f o u r - b i t p h a s e s h i f t e r s ,a n d 1 4 4 ( 1 2 *1 2 ) m i c r o s t r i p a n t e n n a s .T h ep e r f o r m a n c ec h a r a c t e r i s t i c so f t h e s e m i c o n d u c t o ra m p l i f i e ra r ea s f o l l o w s: t h e f r e q u e n c yi s 5 . 7 7 G H z ;t h e f r e q u e n c ys t a b i l i t y i s b e t t e r t h a n 1 . 4 p p m ;t h e s a t u r a t i o no u t p u tm i c r o w a v ep o w e ri s 2 6 . 9 W ; a n dt h es a t u r a t i o ng a i ni s 8 .5 d B . T h et o t a l m i c r o w a v ep o w e re m i t t e df r o mt h e a n t e n n aa r r a yt h r o u g ht h e p o w e rd i v i d e r s a n dp h a s e s h i f t e r si s o v e r 7 W . F i g u r e8 s h o w st h e r e s u l t o fb e a mf o r m i n gw i t h t h e r e t r o - d i r e c t i v es y s t e mw i t ha d i s t a n c eo f 4 m .T h e e l e m e n ts p a c i n go f t h e t r a n s m i t t i n ga n t e n n aw a s0 . 7 Aà "T h ee r r o ro f d i r e c t i o nf i n d i n g a n d b e a mf o r m i n gw a sb e l o w1 . 2 ° .W ec a nu s et h e s a m er e t r o - d i r e c t i v es y s t e mb o t h f o rt h e b e a m - f o r m i n gs u b s y s t e ma n df o rt h e t r a n s m i t t i n gs u b s y s t e m .W ec a nc a r r yo u te x p e r i m e n t sc o n c e r n i n gb e a mf o r m i n gw i t h t h e s u b s y s t e m .

W es h o ws p e c t r a ld a t a f o r t h e 5 . 8 G H zm a g n e t r o na n dt h es e m i c o n d u c t o ra m p l i f i e ro f t h e S P O R T S - 5 .8 s y s t e mi n F i g u r e 6 . U n f o r t u n a t e l y , t h e p a r a m e t e r s o f t h e

m e a s u r e m e n t sw e r ed i f f e r e n tb e t w e e nF i g u r e1 a n dF i g u r e6 ,a n dw ec a nn o te x a c t l yc o m p a r et h e s ed a t a .H o w e v e r ,t h e r ei s n o i n t e r f e r e n c ef r o mt h e s et r a n s m i t t i n gs y s t e m sw i t h t h er e t r o - d i r e c t i v es y s t e m ,a n dw ee x p e r i m e n t a l l ys u c c e e d e di nd e t e c t i n gt h e t a r g e t w i t h t h e r e t r o - d i r e c t i v es y s t e mw i t h a4 . 8 G H zp i l o t s i g n a l .

W eh a v et w ot y p e so f r e c t e n n aa r r a yi n t h e S P O R T S -5 . 8 .W ee m u l a t ea s m a l le x p e r i m e n t a ls a t e l l i t e w i t h r e c t e n n a s ,a n db o t h r e c t e n n aa r r a y sc a nb e s t o r e di n a s m a l l s h a p ea n de x p a n d e d .T h e s a m er e c t e n n ae l e m e n ti s u s e d f o r b o t ha r r a y s ,a n do n l y t h e s h a p eo f t h e a r r a y si s d i f f e r e n t . O n ei sap l a n er e c t e n n aa r r a y ,a n dt h e o t h e ri s a q u a s i - g l o b e - s h a p e dr e c t e n n aa r r a y .T h e m a x i m u mR F - d ce f f i c i e n c y o f t h er e c t e n n ae l e m e n ti s o v e r7 1 . 8 % w i t h 1 0 0 m Wo f 5 . 8 G H zm i c r o w a v ei n p u t p o w e ra n dw i t h a 2 0 0 - o h ml o a d .

4 . S P O R T S - 5 . 8 o p t i o n s

T h e S P O R T S - 2 . 4 5a n d S P O R T S - 5 . 8s y s t e m sw e r ed e v e l o p e d w i t h t h e c o n c e p to f " h i g h e f f i c i e n c y b e a mc o n t r o l . "I n o r d e rt o d e c r e a s et h e l o s s a f t e r a p h a s e - c o n t r o l l e dm a g n e t r o na r r a yw i t h h i g h e f f i c i e n c y , w eu s ea s u b - p h a s e -s h i f t e rs y s t e mf o r S P O R T S - 2 . 4 5 ,a n d a r a n d o ma r r a yf o rS P O R T S - 5 . 8 .B e s i d e st h e s ee x p e r i m e n t s ,w eh a v ed e v e l o p e dt w od i f f e r e n t t y p e s o f m i c r o w a v et r a n s m i t t e r sw i t h a

C o n t r o l l e rF o r P C M

8 5 %

M a g n e t r o n1 7 = 7 4 %

P h a s eC o n t r o l l e dM a g n e t r o n

6 3 %

4 X 8 P o w e rD i v i d e r7 7 = 9 2 %

< - 0 . 3 5 d B )

M ec h a n i c a lP h a s eS h i f t e r

? 7 = 8 7 %( 0 . 6 d B )

T o t a lE f f i c i e n c y1 0 /

F i g u r e9 . A d i a g r a ms h o w i n gt h e e f f i c i e n c yo ft h e S P O R T S - 5 .8 o p t i o n

s y s t e m .

3 4 T h eR a d i o S c i e n c e B u l l e t i n N o 3 1 0 ( S e p t e m b e r ,2 0 0 4 )

frequency of 5.8 GHz. One uses three parabolic antennas,which form a one-dimensional phased array of parabolicantennas. Wecandecrease the loss after the phase-controlledmagnetron because wedo not need phase shifters after thephase-contro lled magnetron in the parabolic phased array.

The other is a revised phase-controlled magnetronwithout the inj ection-locking technique and with a low-lossphase-shifter system. This is a circulator-less phase-controlled magnetron. The size ofthe circulator-less phase-controlled magnetron module is 400 mmx 400 mmx150 mm.Its weight is below 13 kg, which includes themagnetron, dc/dc converter, waveguide, PLL control unit,and structure. The weight ofthe magnetron is only 0.6 kg,and the output microwave power is approximately 300 W.The weight/power ratio is below 43 g/W. The frequencystability ofthe SPORTS-5.8 option is approximately 10*;however, wecan improve the frequency stability with animprovement in the performance ofthe PLL. For a low-lossphase shifter, weadopted a mechanical (moving dielectricsubstance) system, and its loss is -0.6 dB with analogphase shift. An efficiency diagram is shown in Figure 9. Wehave achieved 50% total efficiency of the microwavetransmitter with beamcontrol. The mechanical phase shifter,however, still has many problems: for instance, movingspeed and lifetime.

Wetried to decrease the size and weight of theSPORTS-5.8 option; however, wedid not try to decreasethe size and weight of both SPORTS systems. Generallyspeaking, the 5.8 GHz system is smaller than the 2.45 GHzsystem in microwave power transmission.

5. Conclusion

The SPS must have a low-loss microwavepowertransmission system with beam control because of thecooling limitation problem and cost considerations.Generally, the SPS has been designed with a microwavetransmitting system with a phased array of over 80%efficiency. However, present microwavetechnology doesnot permit such a high-efficiency phased array. Our groupfocused on the magnetron as the microwavetransmitter formicrowave power transmission because of its cost,efficiency, and weight. We have developed the phase-controlled magnetron and its phased arrays, called SPORTS.

The SPORTS systems have been developed withconcepts to decrease the power loss after the phase-controlledmagnetron, in order to realize a low-loss beam-controlsystem. The concepts are the "sub-phase shifter," "randomarray," "parabolic phased array," and "mechanical phaseshifter." The SPORTS systems also include a retro-directivesystem for target detection, a beam-forming subsystemwith a semiconductor amplifier, and receiving rectennaarrays. The SPORTS systems have achieved totally highefficiency with beam control; however, wearestill trying to

increase the efficiency and stability ofthe phase-controlledmagnetron, and trying to decrease power loss after thephase-controlled magnetron with the SPORTS systems.

The eventual goal is a practical SPS. Weworktowardsa microwave power transmission system with higherefficiency and lighter weight at Kyoto University.

6. Acknowledgements

The authors gratefully appreciate the participation ofE&C Engineering K.K., Mitsubishi Heavy Industries, Ltd. ,Mitsubishi Electric Co., and IHI aerospace Co., Ltd., in theco-development of the SPORTS systems. This work ispartly supported by the 2 1 st COE program "EstablishmentofCOE on Sustainable Energy System" at Kyoto University.

7. References

1. N. Shinohara, H. Matsumoto, andK. Hashimoto, "SolarPowerStation/Satellite (SPS) with Phase Controlled Magnetrons,"IEICE Trans. Electron., E86-C, 8, 2003, pp. 1550-1555.

2. T. Mitani, N. Shinohara, H. Matsumoto, and K. Hashimoto,"Experimental Study on Oscillation Characteristics ofMagnetron after Turning Off Filament Current," Electronicsand Communications in Japan, Part II : Electronics, E86, 5,2003, pp. 1-9.

3. W. C. Brown, "The SPS Transmitter Designed around theMagnetron Directional Amplifier," Space Power, 7, 1, 1 988,pp. 37-49.

4. N. Shinohara, J. Fujiwara, and H. Matsumoto, "Developmentof Active Phased Array with Phase-Controlled Magnetrons,"Proceedings ofISAP2000, 2, 2000, pp. 713-716.

5. M. Mori, H. Nagayama, T. Hanyu, and H. Matsumoto, "Sum-maryof Studies on Space Solar Power System ofNASDA" (inJapanese), Proceedings of4th SPSSymposium, 2001 , pp. 12 1-

125.

6. T. Miura, N. Shinohara, and H. Matsumoto, "ExperimentalStudy of Rectenna Connection for Microwave Power Trans-mission," Electronics and Communications in Japan, Part 2,84, 2, 2001, pp. 27-36.

7. T. Miura, K. Hirayama, N. Shinohara, and H. Matsumoto,"Study of a High Power Rectenna for Microwave PowerTransmission" (in Japanese), IEICE Trans. B, J83-B, 4, 2000,pp. 525-533.

8. K. Hashimoto, K. Tsutsumi, N. Shinohara, and H. Matsumoto,"BeamControl System with Spread Spectrum Pilot Signals forSolar Power Satellite," Proceedings of the URSI GeneralAssembly, 2002, p. 1284 (H Special P.7).

9. K.Morishita, O. Hasegawa, K. Iizuka, T. Kimura, H.Makimura,H.Matsumoto, K. Hashimoto, and N. Shinohara, "Technologyand Applications of Microwave Energy Transmission" (inJapanese), Mitsubishi Juko Giho, 40, 6, 2003, pp. 340-343.

TheRadio Science Bulletin No 3 10 (September, 2004) 35