MICROWAVE POWER TRANSMISSION SYSTEM STUDIES NASA CR-134886 ER75-4368 VOLUME IV SECTIONS 9 THROUGH 14 WITH APPENDICES RAYTHEON COMPANY EQUIPMENT DIVISION ADVANCED DEVELOPMENT LABORATORY SUDBURY, MASS. 01776 prepared for NATIONAL AERONAUTICS AND SPACE ADMINISTRATION NASA Lewis Research Center Contract NAS 3-17835
1. Report No. NASA CR- 134886 2. Government Accession No. 3. Recipient's Catalog No. 4. Title and Subtitle MICROWAVE POWER TRANSMISSION SYSTEM STUDIES Volume IV - Sections 9 through 14 with Appendices 5. Report Date December 1975 6. Performing Organization Code 7. Author(s) O. E. Maynard, W.C. Brown, A. Edwards, J. T. Haley, G. Meltz, J. M. Howell - Raytheon Co.; A. Nathan - Grumman Aerospace Corp. 8. Performing Organization Report No. ER75-4368 10. Work Unit No. 11. Contract or Grant No. NAS 3-17835 13. Type of Report and Period Covered o Contractor Report 14. Sponsoring Agency Code 9. Performing Organization Name and Address Raytheon Company Equipment Division 528 Boston Post Road Sudbury, Massachusetts 01776 12. Sponsoring Agency Name and Address National Aeronautics and Space Administration Washington, D. C. 20546 15. Supplementary Notes Mr. Richard M. Schuh NASA Lewis Research Center Cleveland, Ohio 44135 16. Abstract A study of microwave power generation, transmission, reception and control was conducted as a part of the NASA Office of Applications1 joint Lewis Research Center/Jet Propulsion Laboratory five-year program to demonstrate the feasibility of power transmission from geosynchronous orbit. This volume (4 of 4) is comprised of Sections 9 through 14 which present receiving antenna, frequency interference and allocation, risk assessment, system analysis and evaluation, critical technology and ground test program, critical technology and orbital test program. Section 9 reviews and discusses the microwave rectifier technology, approaches to the receiving antenna, topology of rectenna circuits, assembly and construction, with ROM cost estimates. It includes analyses and coot estimates for the equipment required to transmit the ground power to an external user. Section IC m -o.sents the analyses and discussion associated with radio frequency inter fei and allocation. Noise and harmonic considerations are presented for both the amplitron and klystron. Interference limits are identified and evaluated. Section 11 presents the risk assessment discussion wherein technology risks are rated and ranked with regard to their importance in impacting the microwave power transmission system. Section 12 presents the system analyses and evaluation of parametric studies of system relationships pertaining to geometry, materials, specific cost, specific weight, efficiency, converter packing, frequency selection, power distribution, power density, power output magnitude, power source, transportation and assembly. Capital costs per kW and energy costs as a function of rate of return, power source and transportation costs as well as build cycle time are presented. Appendices include estimated annual operations and maintenance cost for 5 and 10 GW systems, systems analysis examples and format for the readers use. Section 13 presents the critical technology and ground test program including objectives, configurations, definition of test Phases I through III and critical technology development with ROM costs and schedule. Section 14 presents the orbital test program with associated critical technology and ground based program based on full implementation of the defined objectives. ROM costs and schedule estimates are included. An appendix is included which provides further detail of the ground and orbital test programs such that the reader may readily modify configurations as studies and technology developments mature leading to modification of the driving objectives. 17. Key Words (Suggested by Author(s)) Microwave power transmission; power from space; satellite power transmission; phased array power transmission; rectifying antenna (rectenna). 18. Distribution Statement Unclassified - Unlimited 19. Security Classif. (of this report) Unclassified 20. Security Classif. (of this page) Unclassified 21. No. of Pages 234 22. Price* $3. 00
TABLE OF CONTENTS VOLUME I - EXECUTIVE SUMMARY Se cti on Pag q 1. Introduction 2 2. DC to RF Conversion 4 3. Transmitting Antenna and Phase Front Control 9 4. Mechanical Systems 16 5. Flight Operations 22 6. Receiving Antenna 25 7. Systems Analysis and Evaluation 28 8. Critical Technology 34 9. Critical Technology and Test Program 37 10. Recommendations for Additional Studies 42 VOLUME II - Sections 1 through 7 with Appendices A through G Page. 1. INTRODUCTION, CONCLUSIONS AND RECOMMENDATIONS 1.1 Introduction 1-1 1.2 Conclusions and Recommendations 1-3 1.2.1 General 1-3 1.2.2 Subsystems and Technology 1-5 1.2.2.1 Environmental Effects - Propagation 1-5 1.2.2.2 DC-RF Conversion 1-5 1.2.2.3 Power Interface and Distribution (Orbital) 1-7 12.2.4 Transmitting Antenna 1-7 1.2.2.5 Phase Front Control 1-8 1.2.2.6 Mechanical Systems and Flight Operations 1-9 1.2.2.7 Receiving Antenna 1-10 1.2.2.8 Radio Frequency Interference and Allocation 1-11 1.2.2.9 Risk Assessment 1-12 1. 2. 3 System Analysis and Evaluation 1-15
1.2.4 Technology Development and Test Programs 1-16 1.2.4.1 Technology Development and Ground Test Program 1-16 1.2.4.2 Technology Development and Orbital Test Program 1-16 1.2.5 Additional Studies 1-17 1.3 Report Approach and Organization 1-17 2. ORGANIZATION AND APPROACH 2.1 Organization 2-1 2.2 Approach 2-3 3. ENVIRONMENTAL EFFECTS - PROPAGATION 3.1 Introduction 3- 1 3.2 Atmospheric Attenuation and Scattering 3-2 3.2.1 Molecular Absorption 3-2 3.2.2 Scattering and Absorption by Hydrometeors 3-4 3.3 Ionosphere Propagation 3-12 3.3. 1 Ambient Refraction 3-12 3.3. 2 Scintillations Due to Ambient Fluctuations and Self-Focusing Instabilities 3-13 3.4 Ionospheric Modification By High Power Irradiation 3-19 3.5 Faraday Rotation Effects 3-21 3.5.1 Introduction 3-21 3.5.2 Diurnal and Seasonal Changes 3-21 3.5.3 Midlatitude Geomagnetic Storms 3-22 3.6 Conclusions and Recommendations 3-24 4. DC-RF CONVERSION 4.1 Amplitron 4- 1 4.1.1 RF Circuit 4-2 4.1.2 Pyrolytic Graphite Radiator 4-4 4.1.3 Magnetic Circuit 4-4 4.1.4 Controlling the Output of Amplitrons 4-5 4.1.5 Weight 4-5
4.1. 6 Cost 4- 7 4.1,7 Noise and Harmonics 4-7 4.1..8 Parameters Versus Frequency 4-9 4.1.9 Parameters Versus Power Level 4-15 4.2 Klystron 4-16 4.2.1 Periodic Permanent Magnetic Focusing 4-17 4.2.2 Circuit Efficiency 4-20 4.2.3 Klystron Efficiency With Solenoidal Focusing 4-25 4.2.4 Heat Dissipation and Beam Collection 4-27 4.2.5 Variations of Supply Voltages 4-34 4.2.6 Noise, Gain and Harmonic Characteristics 4-37 4.2.7 Tube Designs 4-40 4.2.8 Tube Lifetime 4-43 4.2.9 Weight and Cost 4-43 4.2.10 Conclusions 4-46 4.3 System Considerations 4-48 4.3.1 Amplitron Gain and Efficiency 4-48 4.3.2 Cascaded Vs Parallel Configurations 4-50 4.3.3 Cascaded Amplitron Gain 4-56 4.3.4 Amplifier Noise 4-56 4.3.5 Klystron Power Level 4-61 4.3.6 Converter Filter Requirements 4-64 4.4 Conclusions and Recommendations 4-69 5. POWER SOURCE INTERFACE AND DISTRIBUTION 5.1 Power Source Characteristics 5-1 5.2 Power Source-Converter Interface 5-3 5.3 Power Distribution Flow Paths 5-6 5.4 Magnetic Interaction 5-12 5.5 DC to RF Converter Protection 5-15 5.6 Power Distribution System 5-19 5.7 Power Distribution Cost and Weight 5-23 5.8 Power Budget 5-24 5.9 Conclusions and Recommendations 5-24
6. TRANSMITTING ANTENNA 6.1 Aperture Illumination and Size 6-1 6.2 Array Types 6-10 6.3 Subarray Types 6-16 6.4 Subarray Dimensions 6-16 6.5 Subarray Layout 6-20 6.6 Tolerance and Attenuation 6-26 6.6.1 Frequency Tolerance 6-26 6.6.2 Waveguide Dimensional Tolerances 6-27 6.6.3 Waveguide Attenuation 6-28 6.7 Mechanical Design and Analysis 6-29 6.7. 1 Thermal Analysis and Configuration 6-29 6.7.2 Materials 6-38 6.7.3 Transportation, Assembly and Packaging 6-43 6.8 Attitude Control and Alignment 6-47 6.9 Conclusions and Recommendations 6-49 7. PHASE FRONT CONTROL 7.1 Adaptive Phase Front Control 7-4 7.2 Command Phase Front Control 7-10 7.2.1 Phase Estimation 7-10 7.2.2 Bit Wiggle 7-14 7.3 Conclusions and Recommendations 7-14 APPENDIX A - RADIO WAVE DIFFRACTION BY RANDOM IONOSPHERIC IRREGULARITIES A. 1 Introduction A-l A. 2 Model for Electron Density Irregularities A-2 A. 3 Phase Fluctuations and Their Spatial Correlation at the Diffracting Screen A-3 A. 4 Phase and Amplitude Fluctuations and Their Spatial and Temporal Correlation Functions on an Observational Plane A-4
APPENDIX B - SELF-FOCUSING PLASMA INSTABILITIES B-l APPENDIX C - OHMIC HEATING OF THE D-REGION C-l APPENDIX D - CAVITY CIRCUIT CALCULATIONS D.1 Input Impedance D-1 D.2 Input Power and Gain at Saturation D-2 D.3 Intermediate- and Output-Gap Voltages D-3 D.4 Cavity Tunings D-3 D.5 Output Cavity and Circuit Efficiency D-4 APPENDIX E - OUTPUT POWER OF THE SOLENOID-FOCUSED KLYSTRON E-1 APPENDIX F - KLYSTRON THERMAL CONTROL SYSTEM F.1 Heat Conduction F-l F.2 Temperature, Area and Weight of Radiators F-2 F.2.1 Collector F-3 F.2.2 Collector Reflector and Heat Shield F-4 F.2.3 Body Radiator F-4 F.2.4 Body Reflector and Heat Shield F-5 F. 3 Weight of Heat Pipes F-5 APPENDIX G - CONFINED-FLOW FOCUSING OF A RELATIVISTIC BEAM G-1 VOLUME III - MECHANICAL SYSTEMS AND FLIGHT OPERATIONS (Section 8) Page 1. INTRODUCTION 1-1 2. SUMMARY 2.1 Task 1 - Preliminary Design 2-1 2.1.1 Control Analysis 2-1 2.1.2 Thermal/Structural Analysis 2-1 2.1.3 Design Options and Groundrules for Task 2 Concept Definition 2-5
2.2 Task 2 - Concept Definition 2-9 2.2.1 Mission Analysis 2-9 2.2.2 Antenna Structural Definition 2-9 2.2.3 Configuration Analysis 2- 14 2.2.4 Assembly 2-21 2.2.5 Cost 2-31 2.3 Recommendations 2-33 L TECHNICAL DISCUSSION 31. Mission Analysis 3. 1-1 3.1.1 SSPS Configuration and Flight Mode Descriptions 3. 1-1 3.1.2 Transportation System Performance 3. 1-1 3.1.3 Altitude Selection 3. 1-8 3.1.4 SEPS (Ion Engine) Sizing 3. 1—14 3.2 Antenna Structural Concept 3. 2-1 3.2.1 General Arrangement 3. 2- 1 3.2.2 Rotary Joint 3.2-1 3.2.3 Primary/Secondary Antenna Structure 3. 2- 15 3.2.4 Structure/Waveguide Interface 3. 2- 15 3.2.5 Antenna Weight and Mass Properties 3.2-18 3.3 Configuration Analysis 3. 3 - 1 3.3.1 Control Analysis 3. 3-1 3.3.2 Thermal Evaluation 3. 3-9 3.3.3 Structural Analysis 3. 3-41 3.4 Assembly and Packaging 3.4-1 3.4.1 Detail Parts 3.4-1 3.4.2 Structural Assembly 3.4-9 3. 5 Cost 3. 5. 1 3.5.1 Task 1 - Preliminary Design Results 3. 5-1 3.5.2 Task 2 - Concept Definition Results 3.5-5 3.5.3 MPTS Structural Costs 3. 5- 19
4. TECHNOLOGY ISSUES 4.1 Control System 4-1 4.1.1 Evaluation of Alternate Power Transfer and Drive Devices 4-1 4.1.2 Detailed Control System Analysis 4-2 4.2 Structural System 4-3 4.2. 1 Composite Structures and Assembly Techniques 4-3 4.2.2 Tension Brace Antenna Feasibility Assessment 4-4 4.2.3 Local Crippling Stress Evaluation 4-4 4.2.4 Design Environments 4-5 4.2.5 Optimum Antenna Structures 4-5 4.2.6 Finite Element Model Development 4-6 4.2.7 Composite Waveguide 4-6 4.3 Thermal System 4-7 4.3.1 Maximum Temperature 4-7 4,3.2 Transient Analysis 4-8 4.4 Assembly 4-9 4.4.1 Assembly Cost 4-9 4.4.2 Man’s Role in Assembly and Maintenance 4-10 5. REFERENCES 5-1 VOLUME IV - Sections 9 through 14 with Appendices H through K Page 9. RECEIVING ANTENNA 9.1 Microwave Rectifier Technology 9~1 9.2 Antenna Approaches 9“9 9.3 Topology of Rectenna Circuits 9“ 14 9.4 Assembly and Construction 9’21 9.5 ROM Cost Estimates 9~21 9.6 Power Interface Estimates 9“25 9.6.1 Inverter System 9~30 9.6.2 Power Distribution Costs 9~30 9.6.3 System Cost 9~31 9.7 Conclusions and Recommendations 9~31
10. FREQUENCY INTERFERENCE AND ALLOCATION 10-1 10.1 Noise Considerations 10-3 10.1.1 Amplitron 10-3 10.1.2 Klystron 10-4 10.1.3 Interference Limits and Evaluation 10-6 10.2 Harmonic Considerations 10-6 10.3 Conclusions and Recommendations 10-12 11. RISK ASSESSMENT 11.1 Technology Risk Rating and Ranking 11-1 11.2 Technology Assessment Conclusions and Recommendations 11-16 12. SYSTEM ANALYSIS AND EVALUATION 12-1 12.1 System Geometry 12-1 12.2 Parametric Studies 12-3 12.2.1 System Relationships 12-3 12.2.2 Efficiency, Weight and Cost 12-8 12.2.3 Converter Packing 12-12 12.2.4 Capital Cost Vs Power and Frequency Results 12-13 12.2.5 Ground Power Density and Power Level Selection 12-19 12.2.6 Frequency Selection 12-22 12.2.7 Characteristics of 5 GW and 10 GW Systems 12-22 12.2.8 Energy Cost 12-36 12.3 Final System Estimates 12-41 12.3.1 Cost and Weight 12-41 12.3.2 Efficiency Budget 12-43 12.3.3 Capital Cost and Sizing Analyses 12-45 12.4 Conclusions and Recommendations 12-45 13. CRITICAL TECHNOLOGY AND GROUND TEST PROGRAM 13.1 General Objectives 13-1 13.2 Detailed Ground Test Objectives 13-2 13.3 Implementation - Ground Test 13-3 13.3.1 Summary 13-3
13.3.2 Phase I 13-5 13.3.3 Phase II 13-5 13.3.4 Phase III 13-9 13.3.5 Alternate Phase I Converter Implementation 13-11 13.4 Critical Technology Development 13-14 13.4.1 Amplitron 13-14 13.4.2 Klystron 13-14 13.4.3 Phase Control 13-14 13.5 Schedule and Cost 13-15 13.6 Conclusions and Recommendations 13-17 14. CRITICAL TECHNOLOGY AND ORBITAL TEST PROGRAM 14.1 Orbital Test Objectives 14-1 14.2 Implementation 14-3 14.2.1 Geosatellite (Mission 1) 14-4 14.2.2 Shuttle Sorties (Missions 2 through 11) 14-4 14.2.3 Orbital Test Facility 14-23 14.3 Cost and Schedule 14-25 14.4 Conclusions and Recommendations 14-30 APPENDIX H - ESTIMATED ANNUAL OPERATIONS AND MAINTENANCE COST (5 GW System) H-l APPENDIX I - ANNUAL OPERATIONS AND MAINTENANCE COST (10 GW System) I-1 APPENDIX J - SYSTEM ANALYSIS EXAMPLES J. 1 Introductory Analysis of Initial Operational System With Minimum Size Transmitting Antenna J-1 J.2 Analysis of the Final Operational System and Their Goals J-10 J.3 Analysis of the Initial Operational System Based On the Final System Configuration J-21 J.4 Weight and Cost Analysis for the Initial and Final Operational Systems J-25 J.5 Energy Cost J-27
APPENDIX K - DETAILS OF GROUND AND ORBITAL TEST PROGRAM K.1 Introduction K-l K.2 Objectives Implementation Equipment and Characteristics K-l K.3 Implementation of Objectives Hl, H2, DI and D2 Using Low Earth Orbit Sortie Missions K-3 K.4 Defining an MPTS Orbital Test Facility Program K-13 K.4.1 Assumptions K-13 K.4.2 Sizing the Phased Array Antennas K- 14
LIST OF ILLUSTRATIONS Figure Page 9-1 Microwave Rectifier Device Technology 9-3 9-2 Chronology of Collection and Rectification of Microwave Power 9-5 9-3 Major Rectenna Development Programs 9-6 9-4 Simplified Electrical Schematic for the Rectenna Element 9-7 9-5 Rectenna Element Efficiency Vs Frequency 9-8 9-6 Comparison of Antenna Approaches in Meeting Requirements for Reception and Rectification in Space-to-Earth Power Transmission 9-U 9-7 DC Power from Center and Edge Rectenna Elements as Function of Rectenna Dia and Total DC Power Received 9-12 9-8 Rectenna Element Efficiency as Function of Microwave Power Input 9-13 9-9 Microwave Losses in an Optimally Designed Diode as a Function of Input Power Level for a Microwave Impedance Level of 120 Ohms 9-15 9-10 Losses at Low Values of Microwave Input Power 9-16 9-11 Schematic Arrangement of Rectenna 9-17 9-12 Full-Wave Configuration 9-19 9-13 Bridge-Rectifier Configuration 9-19 9-14 Full-Wave and Bridge-Rectifier Configurations in Relationship to Wave Filter Terminals 9-19 9-15 Pseudo Full-Wave Two-Conductor Rectifier 9-20 9-16 Rectenna Construction 9-22 9-17 Rectenna Elements 9-23 9-18 Approach to Environmental Protection of Rectenna Elements 9-24 9-19 Industry Accumulated Production Experience (Billions of Units) 9’26 9-20 High Speed Rectenna Production 9-27 9-21 Basic Rectenna Distribution Layout 9-28 9-22 Estimated dc-ac Interface Losses 9-29 9-23 Inverter Unit Cost Derivation 9-30 9-24 Power Distribution ROM System Cost 9-30 9-25 Total Power Interface ROM Cost 9-31 10.1 RF Spectrum Utilization 10-2
10-2 Estimated Noise Power Density at Earth 10-5 10-3 MPTS Ground Power Densities for Harmonics 10-11 11-1 Technology and Hardware Development Risk Rating Definition 11-1 11-2 Satellite Power System Technology Risk Assessment 11-2 12-1 MPTS Geometry 12-2 12-2 MPTS Functional Diagram 12-4 12-3 Beam Efficiencies (n^) for Truncated Gaussian Tapers 12-7 12-4 MPTS Efficiency (n) Vs Frequency for Parametric Studies 12-10 12-5 Parametric Study Specific Costs and Weights 12-11 12-6 SPS Capital Cost Vs Frequency - 300 $/kg 12-14 12-7 SPS Captial Cost Vs Frequency - 100 $/kg 12-14 12-8 Transmitting Antenna Diameter for Lowest Cost SPS 12-15 12-9 Receiving Antenna Minor Axis for Lowest Cost SPS 12-16 12-10 Transmitting Antenna Diameter for Lowest Cost SPS (300 $/kg, 500 $/kW) 12-17 12-11 Receiving Antenna Minor for Lowest Cost SPS (300 $/kg, 500 $/kW) 12-18 12-12 SPS Capital Cost for Klystron Configurations 12-20 12-13 Peak Ground Power Density Vs Frequency 12-21 12-14 Receiving Antenna Size Vs Beam Efficiency and Taper 12-23 12-15 Transmitting Antenna Size Vs Beam Efficiency and Taper 12-24 12-16 Peak Ground Power Density Vs Beam Efficiency and Taper 12-25 12-17 Cost Matrix - 5 GW - Case LMM 12-27 12-18 Cost Matrix - 5 GW - Case MMM 12-28 12-19 Cost Matrix - 5 GW - Case LLH 12-29 12-20 Cost Matrix - 5 GW - Case HHL 12-30 12-21 Cost Matrix - 10 GW - Case LMM 12-31 12-22 Cost Matrix - 10 GW - Case MMM 12-32 12-23 Cost Matrix - 10 GW - Case LLH 12-33 12-24 Cost Matrix - 10 GW - Case HHL 12-34 12-25 Amplitron-Aluminum MPTS Comparison 12-35 12-26 Comparison of 5 GW Systems 12-35
12-27 SPS Capital Cost for Various Power Source Characteristics 12-40 12-28 SPS Energy Cost for Various Power Source Characteristics 12-40 12-29 SPS Energy Cost for Various Rates of Return 12-40 12-30 SPS Energy Cost for Various Construction Cycles 12-40 12-31 MPTS Cost Matrix 12-42 12-32 MPTS Efficiency Budget 12-44 12-33 Summary of Initial and Final Operational System Characteristics 12-46 13-1 MPTS Ground Test Functional Block Diagram 13-4 13-2 Instrumentation System Block Diagram 13-6 13-3 Ground Test Program Array Characteristics 13-7 13-4 Phase II Subarray - 2 x 2M - 40 kW 13-8 13-5 Received Power Density 13-9 13-6 Candidate Location for Phase III Demonstration 13-10 13-7 Phase III Rectenna 13-11 13-8 Phase III Received Power 13-12 13-9 MPTS Ground Test Siting Profile Phase III - Goldstone 13-13 13-10 Technology Development and Ground Test System Schedule 13-16 14-1 Microwave Orbital Program 14-5 14-2 Geosatellite Concept 14-6 14-3 Five Kilowatt Geosatellite Payload 14-6 14-4 Geosatellite Weight Estimate and Predicted Interim Upper Stage Performance and IUS Performance Estimate 14-7 14-5 Mission Schedule 14-8 14-6a Mission 2 - Structural Fabrication Technology 14-10 14-6b Mission 2 - Test Matrix 14-10 14-7a Mission 3 - Joint and Fastener Technology 14-12 14-7b Mission 3 - Test Matrix 14-12 14-8a Mission 4 - Waveguide Fabrication Technology Sortie 14-14 14-8b Mission 4 - Test Matrix 14-14 14-9a Mission 5 - Electronics Integration 14-15 14-9b Mission 5 - Test Matrix 14-15
14-10a Mission 6 - Subassembly Build-up 14-17 14-10b Mission 5A - Test Matrix 14-17 14-Ila Mission 7 - Rotary Joint Assembly 14-18 14-llb Mission 7 - Test Matrix 14-18 14-12a Mission 8 - Antenna to Rotary Joint Interface 14-20 14-12b Mission 8 - Test Matrix 14-20 14-13a Mission 9 - Central Mast Assembly and Integration Test 14-21 14-13b Mission 9 - Test Matrix (Conducting Mast Assembly) 14-21 14-14 Mission 11 - Test Matrix 14-24 14-15 OTF Antenna 14-24 14-16 OTF Power Densities 14-25 14-17 MPTS Orbital Test Program ROM Costs 14-27 14-18 Critical Technology Schedule 14-29 14-19 MPTS Orbital Test Program ROM Cost Summary 14-29 J-1 Total Cost Summary Format J-2 J-2 Total Cost Summary - Initial Operational Systems With Minimum A^ J-4 J-3 Total Cost Summary - Operational System (Goal) J-11 J-4 Total Cost Summary - Initial Operational System Using At = 64. 7 x 104 m2 J-22 J-5 Summary of Initial and Final Operational System Characteristics J-29 J-6 Capital Cost to Energy Cost Conversion Versus Rate of Return J-30 J-7 SPS Capital Cost/Transportation Cost for Various Power Source Characteristics J-31 J-8 SPS Energy Cost/Transportation Cost for Various Power Source Characteristics J-32 J-9 SPS Energy Cost/Transportation Cost for Various Rates of Return J-33 J-10 SPS Energy Cost/Transportation Cost for Various Construction Cycles J-34 K-1 Summary of Ground Test Objectives/Implementation K-2 K-2 Ionospheric Effects K-4
K-3 Utilization of Arecibo to Accomplish Ionosphere Test Requirements K-4 K-4 Ionosphere Test Requirements for F Layer K-4 K-5 Power Subarray Assembly Options for Meaningful Orbital Tests K-5 K-6 Recommended Microwave Payload Assemblies Build-Up K-5 K-7 Critical Technology Required for Defined Microwave Power Ground and Orbital Test Program K-6 K-8 Configurations to be Investigated on Orbit (Subarray and Below) K-ll K-9 Development Configuration (Subarray and Below Incorporating Control and Support Equipment) K-12 K-10 Large Array and Subarray Sizes for Cost, Inertia and Performance Estimation Purposes K-16 K-ll Array Flight Test Hardware K-22 K-12 Summary of Altitude Range and Associated Power Densities K-23
LIST OF NON-STANDARD TERMS AFCRL Air Force Cambridge Research Laboratory ATC Air Traffic Control ATS Applications Technology Satellite CFA Crossed Field Amplifier CPU Central Processor Unit GaAs Gallium Arsenide HLLV Heavy Lift Launch Vehicle Met Meteorological MPTS Microwave Power Transmission System MW Microwave N. F. noise factor PPM periodic permanent magnet ROM Rough Order of Magnitude SCR Silicon Controlled Rectifier SEPS Solar Electric Propulsion Stage Sm-Co(SMCO) Samarium Cobalt SPS Satellite Power System SSPS Satellite Solar Power Station TDRS Tracking and Data Relay Satellite TEC Total Electron Content
SECTION 9 RECEIVING ANTENNA The collection and rectification of microwave power from space is conceptually a two step process and in the early stages of development of microwave power transmission it was treated in this fashion. However, it was determined early that the individual requirements placed upon the collection of microwave power and upon the rectification of microwave power in a two step process could not be met by any available technology or any forseeable technology development. The rectenna concept, however, which in effect combined collection and rectification into a one-step process was found to meet all of the requirements for the collection and rectification of the power in a free-space microwave beam. Furthermore, it was found that the concept could be experimentally established immediately from available components and technology. It is also of interest to note that the portion of a microwave power transmission system represented by the collection and rectification of microwave power has grown in the last decade from the weakest and most insecure portion of the of the system to the strongest and most secure. This has come about not only because of the soundness of the rectenna concept but also because this is the portion of the system whose development has received the mo$t attention. The most recent portion of this development process has considered not only the efficiency and reliability aspects, but also those aspects dealing with low-cost fabrication. This has resulted in a large amount of ’Winnowing" of design approaches to arrive at a rather high level of design specificity. Since much of this winnowing has occurred in the period of the last three years, it is not highly documented although the general direction and the motivational influences are recorded in Reference 1. Therefore, it is desirable initially to review some of the factors which have led to the present detailed design. 9. 1 MICROWAVE RECTIFIER TECHNOLOGY The efficient conversion of microwave power directly into dc power is a technology that is specifically related to the concept of power transmission by microwaves in either a free-space or confined waveguide mode. As contrasted to dc to microwave conversion which has received broad support from many areas, there has been little support of the reverse process.
Early investigators of the use of microwaves for power transmission in the 1957-1960 time frame resorted to the conversion of microwave energy at the receiving point into heat, which was used either directly or to run a heat engine. However, this approach leads to many mechanical complications and in any event can provide an overall efficiency of at most 30%. These considerations led immediately to the desire for an efficient electronic device that would convert the microwave power directly into dc power. The earliest known rectifier development projects in which the end use was intended for power purposes rather than information were those supported by two contracts from the laboratories of the U.S. Air Force at Wright Field. One of these contracts was awarded to Purdue University to examine broadly the development of devices to rectify microwave power. The other was awarded to the Raytheon Company for the study of a rectifier device that was the rectifier analog of the magnetron. The findings of these two investigations were important background in determining the course of subsequent investigations and in attempts to develop and operate complete systems making use of microwave power transmission. Rectifiers may be classified in several different ways. One division of classification is into solid-state and electron-tube devices. Another division would be into microwave-tube analog devices and diode rectifiers. Still another classification would be into low-impedance devices and high-impedance. Microwave-tube analog devices are characterized by low-current and high-voltage output, whereas diode rectifiers of both the solid-state and electron-tube types tend to be low impedance devices. There was considerable interest from private and industrial organizations in addition to the limited interest of the Department of Defense in the technology of microwave power rectification in the 1958 to 1962 time period. This interest is well documented in Okress, "Microwave Power Engineering", Volume I . Figure 9-1 summarizes a number of these concepts and their state of development. One of these concepts, the close spaced thermionic diode rectifier reached a state of development in which it could be used as a rectifier in the first known demonstration of microwave power transmission. However, it had serious reliability and life problems.
Figure 9-1* Microwave Rectifier Device Technology **
Although many rectifier divices which were the analogs of various microwave generators were proposed, only the development of the rectifier analog of the magnetron was supported. This device proved to be impractical for reasons of a very basic physical nature. The point-contact semiconductor diode was earlier demonstrated to be an efficient converter of microwaves into dc power, but its power handling capability was so low as to cause it to be initially dismissed from serious consideration. Later, with the introduction of the "rectenna" concept, its true potential as a microwave rectifier was recognized. The limited but broad interest in microwave power rectification devices of all kinds that was initiated in the 1958 to 1962 time frame did not continue beyond that period. Residual interest was focused upon the Schottky-barrier diode because of its high demonstrated efficiency and its relationship to the rectenna concept. As a result there is today no broadly based microwave power rectification technology, and any approaches to the collection and rectification of microwave power must rely upon the semiconductor diode, whose power handling capability is limited. The chronology of the collection and rectification of microwave power is given in Figure 9-2 and major development programs are outlined in Figure 9-3. The introduction of the Gallium Arsenide Schottky-barrier diode proved very significant in terms of high efficiency and power handling capacity. The combination of this device with a harmonic filter to attenuate radiation of harmonics and to store energy for the rectification process led to the configuration shown in Figure 9-4. This was used in construction of a 4 foot diameter rectenna for 2 Marshall Space Flight Center, and in the recently completed 25m rectenna built for the Jet Propulsion Laboratory that demonstrated 82% efficiency at an output power level of 32 kW. Verification of rectenna element efficiency during this same program established a reference point on the curve of Figure 9-5. The variation of efficiency with frequency is estimated from the equivalent circuit, and is of value for system studies to establish a desirable MPTS operating frequency.
1958 First interest in microwave power transmission. 1958 No rectifiers available - turbine proposed and studied. 1959 - 1962 Some government support of rectifier technology a. Semiconductors at Purdue b. Magnetron analogue at Raytheon 1962 Semiconductor and close-spaced thermionic diode rectifiers made available. 1963 First power transmission using pyramidal horn and closespaced thermionic diode rectifiers - 39% capture and rectification efficiency not practical for aerospace application. 1964 RADC microwave powered helicopter application demanded non-directive reception, light weight, high reliability. 1964 Rectenna concept developed to utilize many semiconductor rectifiers of small power handling capability to terminate many small apertures to provide non-directive reception and high reliability. 1968 - 197 5 Continued development of rectenna concept to format with high power handling capability, much higher capture and rectification efficiency, and potentially low production cost. 1975 Development of rectenna for transmission of kilowatts of rf power over 1.54 km with reception and conversion of incident rf power to dc at high rf to dc efficiency (JPL RXCV Program). 197 5 Initiation of contracted effort for improvement of rf to de collector/converter technology (LeRc-NAS3-19722). Figure 9-2. Chronology of Collection and Rectification of Microwave Power
Figure 9-3. Major Rectenna Development Programs
Figure 9-4. Simplified Electrical Schematic for the Rectenna Element
Figure 9-5. Rectenna Element Efficiency Vs Frequency
9.2 ANTENNA APPROACHES The requirements for reception and rectification of microwave power from a transmitter in synchronous orbit are listed below. a. Non-directive aperture b. High absorption efficiency c. High rectification efficiency d. Very large power handling capability e. Passive radiation of waste heat f. High reliability g. Long life h. Low radio frequency interference (RFI) i. Capable of being constructed in large aperture size j . Easy mechanical tolerance requirements k. Low cost These requirements must be matched up with the capabilities of various approaches to performing this function. Each candidate approach must consist of a means of collecting the microwave power and then converting it into dc power. While there are a number of existing technologies that can be used to collect the power, there is only one existing rectifier technology that is at all practical and that is the semiconductor Schottky barrier diode. The diode may be used singly or grouped in large numbers to provide the load for any collection approach, although it is obvious that auxiliary cooling will be necessary if large numbers are grouped together. The number of ways in which the power may be collected is limited. It may be collected by an array of contiguous horns with independent microwave load, an array of contiguous reflectors and feed horns with independent microwave load, a phased array of small-aperture elements with a common microwave load, or an array of small aperture elements with independent microwave load (rectenna). There is a basic objection to horn or reflector-horn collectors because of their inability to collect close to 100% of the power that impinges upon them. The near uniform power density of the microwave power impinging upon them will result in uniform illumination of the aperture and this will not match the natural aperture
power density distribution of the horn or reflector and horn aperture. A number of steps may be taken to improve this efficiency but they will increase the cost of the collector and in any event will not make it possible to approach closely to 100% capture efficiency. The phased array with common microwave load can improve upon this situation since the matching of its individual elements can be tailored to a uniform incident illumination. However, the common microwave load makes the phased array highly directive and would involve auxiliary cooling for the common microwave load. A comparison of the various approaches with the requirements for reception and rectification of space-to-earth power transmission is given in Figure 9-6. It will be noted that all of the approaches with the exception of the rectenna approach fail in four or more ways to meet the criteria for ground collection and rectification. The rectenna approach meets them all. There is one condition, however, in which the rectenna approach may be unfavorable. That condition is where the density of the illumination is so low that a single dipole cannot collect enough power to operate efficiently. Under these conditions it may be necessary to use one of the other collection schemes such as an array of dipoles which would feed enough power into a single diode to make it operate efficiently. Under these conditions the increased directivity may be acceptable. The variation of power from the center of the receiving antenna to the edge for various system power levels is given in Figure 9-7, and the variation in efficiency with input power of a rectenna element using presently designed diodes is given in Figure 9-8. It may be seen that for a 10 km, 5 GW case that only the elements at the very center provide high efficiency. Under these circumstances it is appropriate to undertake development of a rectenna element with higher efficiency at lower power levels. Several design parameters are involved in this development. These include an increase in the circuit impedance of the rectenna element to increase the dc voltage at a given power output, a reduced junction area in the diode to optimize efficiency at the lower power levels, and finally a change in the junction materials from GaAs-Pt to GaAs-W which will produce less
Figure 9-6. Comparison of Antenna Approaches in Meeting Requirements for Reception and Rectification in Space-to-Earth Power Transmission
Figure 9-7. dc Power from Center and Edge Rectenna Elements as Function of Rectenna Dia and Total dc Power Received
Figure 9-8. Rectenna Element Efficiency as Function of Microwave Power Input
power loss in the junction area. These are currently subjects of investigation at Raytheon under contract NAS3-19722 to Lewis Research Center. The results of a preliminary study of the impact of these variables upon the losses in the diode are summarized in Figures 9-9 and 9-10. 9. 3 TOPOLOGY OF RECTENNA CIRCUITS Both the efficiency and low radio frequency interference requirements make it necessary to incorporate a low-pass filter into the rectenna element. A further requirement is a design configuration which can eventually be directly incorporated into a printed circuit, stripline, or similar configuration. Such a configuration has long been the ultimate objective of rectenna development programs because of its light weight and low cost. A rectenna must be a two-level structure to achieve high efficiency. However, the second level is merely a reflecting surface which need not be physically coupled to the front surface. The front surface plane can then be used to: a. Absorb microwave power b. Rectify it c. Collect rectified power in dc collecting busses which carry the dc power to the edges of the rectenna section for collection into larger busses d. Prevent radiation of power at harmonic frequencies The use of the front plane for the first three of these functions was characteristic of several early experimental rectennas. However, these rectennas did not have filters which would prevent the reradiation of all power at harmonic frequencies. To prevent harmonic radiation it is necessary to insert a low-pass filter between the antenna element, which absorbs the power from space, and the rectifying element. This is shown schematically in Figure 9-H. In Figure 9-11, the large capacitance to the left of the dipole is placed a quarter wavelength from the dipole and therefore an infinite impedance is seen by the dipole terminals to its left. A low pass filter must be constructed with inductance and capacitance if the losses are to be minimized, and the resulting configuration is shown in Figure 9-4 for a single section. It also shows how a single diode could be incorporated
Figure 9-9 Microwave Losses in an Optimally Designed Diode as a Function of Input Power Level for a Microwave Impedance Level of 120 Ohms
Figure 9-10. Losses at Low Values of Microwave Power Input
Figure 9-11. Schematic Arrangement of Rectenna
as a rectifier, but there are other arrangements which could incorporate several diodes in a function other than pure parallel operation. For the present discussion, however, attention will be focused on the filter. It will be noted first that the low-pass filter, shown in Figures 9-4 and 9-11, allows the top and bottom of the network to be at different dc potentials. It therefore follows that the conductors which form the top and bottom of the filter can be used as dc busses to transport the rectified power to the edges of the array. A second aspect of the filter that must be taken into consideration is that a physical space is required for the construction of the filter. The space required is roughly proportional to the number of filter sections required, and there are likely to be at least two. A convenient place to put these filters is in the space between two of the half-wave dipole antennas as shown. A second consideration is the other possible rectifier configurations that could be employed. If a full-wave rectifier is employed as in Figure 9-12, an additional bus will be required, and if it is kept in the same plane as the other conductors without intersecting them, it must pass through the center line of the capacitances. This is probably not practical. If a full-wave bridge-type rectifier is used as in Figure 9-13, the problem becomes even more acute, since two additional terminals are created. If the terminals of successive rectifiers are connected in parallel, two additional busses will be required. The early rectennas built internally at MSFC and at Raytheon used bridge-type rectifiers and the power was collected by a single dc bus, connecting the elements in series. But these rectennas contained no filters between the rectifiers and the dipole antennas. If filters were inserted, the schematic would then have to look like that of Figure 9-14 and there is no single-plane topological solution since the filter is a two-terminal pair device. There is also the problem of a strong second harmonic content at terminals B-B1 and the suppression of its radiation from the series bus. It would therefore appear that if a full-wave rectifier were to be used an additional plane would be required for bussing the power. This does not necessarily rule out these configurations but there is no doubt that it places them at a disadvantage with the half-wave rectifier configuration shown in Figure 9-11.
Figure 9-12. Full-Wave Configuration Figure 9-13. Bridge-Rectifier Configuration Figure 9-14* Full-Wave and Bridge-Rectifier Configurations in Relationship to Wave Filter Terminals
Before ending the discussion of rectifier configurations, attention is called to a pseudo full-wave rectifier using only two conductors. Figure 9-15 shows a two-terminal pair structure that is a low-pass filter element made up of the capacitance of the diodes themselves with an intervening inductance whose value is such that the filter operates at or near the upper cutoff frequency. This filter section then behaves as a full-wave rectifier in the sense that current flows into the dc busses on both halves of the rf cycle. * Such an element could have a considerable amount of energy storage, i. e. , a significant Q. If the device were fed from one side only, the symmetry of the rectification process would be affected, being less affected with the higher Q values. The symmetry could be restored, regardless of the Q value, by feeding the network from adjacent half-wave dipoles assumed to be excited in the same phase. In most of the experimental work to date, only a single dipole has been involved with the rectifier. This permits designing and testing a single element of the rectenna according to the procedure that has been used successfully. This procedure makes use of a section of expanded waveguide into which the complete element is matched. Accurate measurements of efficiency can be made, and the cross-section of the expanded waveguide has been correlated wi th the area taken up by the element in the finished rectenna, Figure 9-15. Pseudo Full-Wave Two-Conductor Rectifier
9. 4 ASSEMBLY AND CONSTRUCTION The construction approach suggested for the rectenna is illustrated in Figure 9-16 where wire mesh is supported by a simple framework to be normal to the incoming power beam phase front. The angle is not critical due to the wide beam pattern of the dipole antenna elements. The open mesh reduces wind loads and the amount of material needed, and the relatively simple support arrangement keeps the foundation and site preparation costs as low as possible. A detail of the suggested mounting for the rectenna elements is given in Figure 9-17. dc power is collected by the elements in parallel and then summed in series as was indicated in Figure 9-H. The voltage level for summation involves a tradeoff of I R losses at low voltage and high current, versus the insulation penalties at higher voltages and lower currents. A level of 1 kV was somewhat arbitrarily selected as that level for power inversion up to 66 kV for distribution to a power grid. (An integrated rectenna industrial complex would perhaps eliminate the associated extra cost and efficiency loss. ) Environmental protection for the extremely large area of rectenna poses a unique problem in that many effective techniques are too costly to consider. The conditions to be considered are rain, wind, snow and ice, temperature extremes, hail, blown sand, salt spray, and ultraviolet solar radiation. The approaches considered were: radome over the whole assembly, exposed assembly with conformal coating, and exposed assembly with a dielectric tubing shield as shown in Figure 9-18 (top and bottom halves would be heat sealed). The radome would be too expensive; the conformal coating may pose difficulties with power loss; and the tubing may be too expensive and have cooling problems. However, the latter two concepts are proposed for further study. The main threat to damage with these methods would be the impact of large hailstones. This should be a consideration in site selection. 9. 5 ROM COST ESTIMATES Costs were generated on the basis of cost per square meter except for power distribution. It is assumed that diodes are developed to handle the full range of power densities involved and/or that power from several dipoles can be collected for a single diode at the same cost or less than for the single diode-dipole combination.
Figure 9-16. Rectenna Construction
4.^ i X..—« ........ .... .. ...... . ... ...... .... Figure 9-17* Rectenna Elements
Figure 9-18. Approach to Environmental Protection of Rectenna Elements
Nominal costs are: ? Real Estate 0.25 $/m 2 Site Preparation 0.40 $/m 2 Support Structure 6. 00 $/m 2 RF-DC Subarrays 4.00 $/m Power Distribution and Control 45. 00 $/kW (See Section 9. 6) {2. 50 $/m for 5 GW) The RF-dc subarray cost is madc up of: , Schottky Barrier Diode 2. 84 $/m Rectenna Circuit and Diode Assembly 3. 16 $/m We see that the support structure is the highest cost item. The diodes are the single most costly component and must be produced at about 1 cent each in quantities of billions to meet the target. The learning curve behavior for diodes shown in Figure 9-19 lends support to this estimate. The rectenna element assembly must also be produced in a high speed, low cost process. The scheme illustrated as an example in Figure 9-20 starts out with two spools of rectangular aluminum wire and one spool of dielectric ribbon material. Three forming machines produce the three pieces which flow together in a continuous process. 9. 6 POWER INTERFACE ESTIMATES Figure 9-21 is a simplified plane view of the general configuration analyzed. The total rectenna area has been subdivided into 5 main feeders, with each feeder handling 1000 MW, for a total rectenna output power of 5000 MW, Each main feeder receives power from 1000 - 1 kW inverters. These inverters serve the multiple functions of dc to ac inversion, phase synchronization and switchgear. The analysis assumes that a three phase ring inverter (see Figure 9-22) is suitable for the intended application. Input power to the inverter will be 1000 volts de and the output voltage of the inverter will be 66 kV rms, three phase 60 Hz. Further conversion of the voltage can be performed at the interface with the transmission system if required.
Figure 9-19. Industry Accumulated Production Experience (Billions of Units)
Figure 9-20. High Speed Rectenna Production
Figure 9-2 lo Basic Rectenna Distribution Layout
In the analysis that follows, it is assumed that dc bus losses from the rectenna to the inverters are a part of the rectenna system. In addition, the substations located at points 1 through 5 in Figure 9-21 will be defined largely by individual site and specific transmission systems. Accordingly, these costs are not included. The overall efficiency of an inverter for the proposed application is difficult to estimate at this time. Figure 9-22 tabulates the probable losses, using what can be considered the lowest achievable values for each identified loss. Achieving these in an actual system will require a significant development program. Figure 9-22. Estimated dc-ac Interface Losses ROM costs for the dc-ac interface equipment has been developed in three steps. First, the basis cost of 1 kW inverter is estimated and then a learning curve applied for the total system cost. Secondly, the power distribution system, consisting principally of the 5 MW feeders have been estimated, and thirdly, the results of steps 1 and 2 have been combined for the total system cost.
9. 6. 1 INVERTER SYSTEM The unit costs are derived as shown in Figure 9-23. 9. 6. 2 POWER DISTRIBUTION COSTS For preliminary ROM estimating purposes the five main feeder cables have been considered to consist of five 1, 000, 000 circular mil single conductor oil filled paper insulated cables per phase. Eac.i cable diameter is approximately 2. 192 inches and cable weight is 8, 630 pounds per 1000 feet. For the 5 main feeders a total cable length of 104 miles is required. Power distribution ROM system costs are summarized in Figure 9-24. Figure 9-24. Power Distribution ROM System Cost Figure 9-23 Inverter Unit Cost Derivation
9. 6. 3 SYSTEM COST Figure 9-25 summarizes the total system costs including installation labor. A rough estimate is also included of related site costs. These costs include handling and test equipment, footings and support structures, cable laying equipment, etc. Figure 9-25. Total Power Interface ROM Cost 9. 7 CONCLUSIONS AND RECOMMENDATIONS For the receiving antenna: a. An array of small independent elements able to collect and rectify incident microwave power is required for low cost and high efficiency. b. A linearly polarized dipole with GaAs Schottky barrier diode is recommended. c. Development of rectifying antenna elements including diodes for low power density is needed. d. Rectenna collection and conversion efficiency is 84 percent and a realistic development goal is 90 percent. e. Support structure is major cost item requiring further in-depth study as types of terrain, soils, mechanics and environments are established. f. Power interface to the user network needs development to reach 92 percent and greater efficiency. For a total output power of 5000 MW, the normalized cost is $45/kW
REFERENCES (SECTION 9) 1. W.C. Brown, "Free-Space Microwave Power Transmission Study, Phase 2", Raytheon Report No. PT-3539 covering period from April 1, 1971 - August, 1972. NASA Contract No. NAS-8-25374. 2. E. M. Sabbagh, Microwave Energy Conversion, Rept. WADD-61-48, Pt. III. Air Force Systems Command, Wright-Patterson Air Force Base, Ohio, May 1962. 3. R.H. George and E. M. Sabbagh, "An efficient means of converting microwave energy to dc using semiconductor diodes", IEEE Intern. Conv. Record, Electron Devices, Microwave Theory Tech. , vol. 11, Pt. 3, pp. 132-141, March 1963. 4. J. Thomas, High Power Microwave Converter, Tech. Rept. No. ASDTR61-476, Pt. II, February 1963, ASTIA, Document No. AD-402975. 5. Okress, F.C., Microwave Power Engineering, vol. 1 and 2, New York: Academic, 1968. 6. W.C. Brown, "Thermionic Diodc Rectifier", in Okress, Microwave Power Engineering, pp. 295-297. 7. W. C. Brown and R. H. George, "Rectification of microwave power", IEEE Spectrum, vol. 1, pp. 92-97, October 1964. 8. W. C. Brown, "A Survey of the Elements of Power Transmission by Microwave Beam", 1961 IRE International Convention Record, Vol. 9, Part 3, pp. 93-105. 9. J. F. Skowron, G. H. MacMaster and W.C. Brown, "The Super Power CW Amplitron", Microwave Journal, October, 1964. 10. W. C. Brown, "Experiments in the Transportation of Energy by Microwave Beam", 1964 IEEE International Convention Record, Vol. 12, Part 2, pp. 8-17. 11. W.C. Brown, "Experiments Involving a Microwave Beam to Power and Position a Helicopter", IEEE Trans, on Aerospace and Electronic Systems, Vol. AES-5, No. 5, pp. 692-702, Sept., 196912. W. C. Brown, "Experimental Airborne Microwave Supported Platform", Technical Report No. RADC-TR - 65-188, Dec., 1965. Contract AF30 (602) 3481.
13. W.C. Brown, Experimental System for Automatically Positioning a Microwave Supported Platform’1, Technical Report No. RADC-TR- 68-273, Oct., 1968. Contract AF 30 (602) 4310. 14. R.H. George, Solid State Microwave Power Rectifiers, Rept. RADC- TR-65-224, Rome Air Develop. Center, Griffiss Air Force Base, New York, August 1965. 15. W.C. Brown, "Progress in the Efficiency of Free-Space Microwave Power Transmission", Journal of Microwave Power, Vol. 7, No. 3, pp. 223-230. 16. W.C. Brown, "Free-Space Microwave Power Transmission Study", Raytheon Report No. PT-2931 covering period of Dec. , 1969 to Dec. , 1970. NASA contract no. NAS-8-25374. 17. W. C. Brown, "Progress in the Design of Rectennas", Journal of Microwave Power, Vol. 4, No. 3, pp. 168-175, 1969. 18. W.C. Brown, "The Receiving Antenna and Microwave Power Rectification", Journal of Microwave Power, Vol. 5, No. 4, pp. 279-292, 1970. 19. Robinson, W. J. , "Wireless power transmission in a space environment, " J. Microwave Power, vol. 5, Dec. 1970. 20. Glaser, P. E. , "Power from the sun: its future", Science, vol. 162, pp. 857-861, Nov. 22, 1968. 21. "Reception Conversion System for (RXCV) for Microwave Power Transmission System, Final Report", JPL Contract No. 953968, Raytheon Company, Sudbury, Mass., September 1975. 22. R. E. Bettand and V. D. Nene, "Analysis and Performance of a Three- Phase Ring Inverter", IEE Transactions on Industry and General Applications, Vol IGA-6, pp. 488-496, September/October 1970. 23. Dickinson, R. M. , "Evaluation of a Microwave High-Power Reception- Conversion Array for Wireless Power Transmission", JPL Technical Memorandum 33-741, September 1, 1975.
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