ELECTRONIC AND MECHANICAL IMPROVEMENT OF THE RECEIVING TERMINAL OF A FREE-SPACE MICROWAVE POWER TRANSMISSION SYSTEM NASA CR-135194 PT-4964 by William c. Brown RAYTHEON COMPANY Prepared for NATIONAL AERONAUTICS AND SPACE ADMINISTRATION NASA Lewis Research Center Contract NAS 3-19722
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NASA CR-135194 PT-4964 ELECTRONIC AND MECHANICAL IMPROVEMENT OF THE RECEIVING TERMINAL OF A FREE-SPACE MICROWAVE POWER TRANSMISSION SYSTEM by William C. Brown RAYTHEON COMPANY Prepared for NATIONAL AERONAUTICS AND SPACE ADMINISTRATION NASA Lewis Research Center Contract NAS 3-19722
1. Report No. NASA CR-135194 2. Government Accession No. 3. Recipient's Catalog No. 4. Title and Subtitle Electronic and Mechanical Improvement of the Receiving Terminal of a Free-Space Microwave Power Transmission System 5. Report Date August 1, 1977 6. Performing Organization Code 7. Author(s) William C. Brown 8. Performing Organization Report No. PT-4964 10. Work Unit No. 11. Contract or Grant No. NAS3- 19722 13. Type of Report and Period Covered Contractor Report 14. Sponsoring Agency Code 9. Performing Organization Name and Address Raytheon Company, Equipment Division and Microwave and Power Tube Division 430 Boston Post Road, Wayland, Mass. 01778 12. Sponsoring Agency Name and Address National Aeronautics and Space Administration Washington, D. C. 20546 15. Supplementary Notes Program Managers: Ira T. Myers and Stanley Domitz, Power Devices Section, NASA Lewis Research Center, Cleveland, Ohio 44135 16. Abstract Significant advancements were made in a number of areas: improved efficiency of basic receiving element at low power density levels ; improved resolution and confidence in efficiency measurements; mathematical modelling and computer simulation of the receiving element; and the design, construction, and testing of an environmentally protected two-plane construction suitable for low-cost, highly automated construction of large receiving arrays. 17. Key Words (Suggested by Author(s)) Rectenna Solar Power Satellite Microwave Power Microwave Transmission 18. Distribution Statement Unclassified - unlimited 19. Security Classif. (of this report) Unclassified 20. Security Classif. (of this page) Unclassified 21. No. of Pages 22. Price* 158 For sale by the National Technical Information Service, Springfield, Virginia 22151
TABLE OF CONTENTS 1 .0 INTRODUCTION 3 1. 1 Description of free-space power transmission by microwave beam and its early development 3 1.2 Major Microwave Collector-Converter Technology Developments 8 1.3 Progress in Rectenna Efficiency Using Progress in Rectenna Element Efficiency as an Index 19 1.4 The Energy Problem and the Solar Power Satellite Concept as Factors in Determining the Extent and Direction of Rectenna Development .22 1.5 Objectives of the Technology Development Reported Upon in this Report .24 2.0 IMPROVEMENTS IN TECHNIQUES FOR MEASURING THE EFFICIENCY AND LOSSES OF RECTENNA ELEMENTS; MATHEMATICAL MODELING AND COMPUTER SIMULATION OF THE RXCV RECTENNA ELEMENT 25 2. 1 Introduction 25 2. 2 Techniques for Measuring the Efficiency and Losses of Rectenna Elements 26 2. 2. 1 Measurement Equipment 26 2. 2. 2 Calibration of Microwave Power Input 26 2. 2.3 Measurement of Diode Losses 33 2. 2.4 A Check on Measurement Accuracies by Balancing Measurements of Input Microwave Power Against the Sum of the Measurements of DC Power Output, Diode Losses, and Circuit Losses 34 2.3 The Development of a Mathematical Model of the Rectenna Element Together with Computer Simulation Program and its use. 41 2.3.1 Introduction 41 2.3.2 Mathematical Model of the Rectenna Element 43 2.3.3 A Representative Set of Data Resulting from the Use of the Computer Simulation Program 43 2.4 Agreement of Computer Simulation Data with Experimental Measurements 47 2.4. 1 Comparison of Simulated Efficiency and Losses with those Measured Experimentally 47 3.0 RECTENNA ELEMENT CIRCUIT MODIFICATION 49 3.1 Introduction 49 3.2 Circuit Modifications to permit more efficient operation at reduced microwave power input levels 49 3.2 . 1 Introduction and summary of results 49 3.2 .2 The Design and Construction of Circuits for more efficient operation at lower power levels 50
3.2.3 Other Approaches to Efficient Operation at Lower Power Levels 58 3.3 Initial Effort in Integration of Rectenna Element into a Two Plane Structure 58 3.4 The Reduction of Second and Third Harmonic Radiation with the use of Stub Lines 59 3.5 Reduction in radiated harmonic power by metallic shielding 60 3.6 Improvement in the efficiency and in the consistency of efficiency measurements by refinements in the construction of the RXCV rectenna element 60 4.0 SCHOTTKY BARRIER DIODE DEVELOPMENT 64 4. 1 The Diode Design and Construction Matrix 68 4. 2 Life Test on Rectenna Elements and Diodes 69 5. 0 INTEGRATION OF IMPROVED DIODES AND CIRCUITS INTO A DESIGN COMPATIBLE WITH THE LONGER RANGE OBJECTIVE OF A LOW-COST RECTENNA SUITABLE FOR SSPS DEPLOYMENT 75 5. 1 Introduction 75 5.2 Outline of a Production Process for the SSPS Rectenna 77 5.3 Interrelationships Between the Losses in DC Bussing, the Cross Section of the DC Busses, the DC Power Collected by Each Element, the Density of the Rectenna Elements, and the Required DC Output Voltage Level 82 5.4 The Design and Construction of the 5-element Foreplane 84 5.4 . 1 Considerations in the Design of the Outer Metallic Shield 85 5.4 .2 Considerations in the Design of the Core Assembly 88 5. 5 Tests of the Separate Rectenna Elements in the Foreplane Structure with the use of the Expanded Waveguide Fixture 89 5.6 Smith Chart Presentation of Reflection Data 89 5. 7 Test of the 5-element Foreplane as an Integrated Part of a Larger Array 92 6 .0 SUMMARY OF RESULTS 102 7 .0 CONCLUDING REMARKS AND RECOMMENDATIONS HO Appendix A Appendix B
LIST OF ILLUSTRATIONS 1-1 First experiment in the efficient transfer of power by means of microwaves at the Spencer Laboratory of Raytheon Company, in May 1963. In this experiment microwave power generated from a magnetron was transferred 18 feet and then converted with DC power with an overall efficiency of 16%. A conventional pyramidal horn was used to collect the energy at the reviewing end and a close-spaced thermionic diode was used to convert the microwaves into DC power. The DC power output was 100 watts. 5 1-2 Microwave powered helicopter in flight 60 ft above a transmitting antenna. The receiving array for collecting the microwave power and converting it to DC power was made up of several thousand point contact silicon diodes. DC power level was approximately 200 watt. 6 1-3 The first rectenna. Conceived at Raytheon Company, it was built and tested by R. George of Purdue University. Composed of 28 half-wave dipoles spaced one-half wavelength apart, each dipole terminated in a bridge-type rectifier made from four IN82G point-contact semiconductor diodes. A reflecting surface consisting of a sheet of aluminum was placed one-quarter wavelength behind the array. 10 1-4 The special rectenha made for the first microwavepowered helicopter. The array is 2 feet square and contains 4480 IN82G point-contact rectifier diodes. Maximum DC power output was 200 watts. 12 1-5 Greatly improved rectenna made from improved diodes (HP2900) which are commercially available. The one foot square structure weighs 20 grams and can deliver 20 watts of output power. 12 1-6 Test set-up of microwave power transmission system at Marshall Space Flight Center in 1970. The magnetron which converts de power at 2450 MHz is mounted on the waveguide input to the pyramidal horn transmitting antenna. The rectenna in the background intercepts most of the transmitted power and converts it to de power. Ratio of de power out of rectenna to the rf power into the horn was 40. 8%. Overall dc-to-dc efficiency was 26. 5% 13
LIST OF ILLUSTRATIONS (Conf d) 1-7 Close-up view of the first rectenna developed by Raytheon under MSFC contract. Microwave collector, rectification, and DC bussing of rectified power are all carried out in one plane. Rectenna elements are conneced in series 14 1-8 Experimental set-up comprised of dual-mode horn and improved rectenna. The efficiency ratio of the de power from the rectenna to the microwave power at the input to the dual-mode horn was measured and found to be 60.2% 16 1-9 Sketch of the Marshall Space Flight Center rectenna which was constructed in spring of 1974. Cutaway section of rectenna element shows the two section input low pass filter, the diode, and a combination tuning element and by-pass capacitor. 1? 1-10 Photograph of the MSFC rectenna constructed in 1974 under test. Horn at left of picture illuminates the rectenna (white panel) with a Gaussian distribution of power. Rectified DC power is collected from rectenna in circular ring path and dissipated in resistive loads on the test panel at the right. 17 1-11 Simplified Electrical Schematic for the rectenna element used in the RXCV receiving array at the Venus site of the JPL Goldstone facility. 18 1-12 Photograph of rectenna element designed for JPL RXCV demonstration at Goldstone. This element represents the departure point for the technology development being reported upon. 18 1-13 Photograph of the microwave power transmission system at the Raytheon Co. in which a certified overall DC to DC efficiency of 54% was obtained in March 1975. Magnetron at left of picture converts de power into microwave power which is fed into the throat of the dual-mode horn. The horn illuminates the rectenna panel with a gaussian distribution of power. Rectified de power is collected from the rectenna in circular ring paths and is dissipated in resistive loads on the test panel at the right. 20 1-14 Distribution of system and subsystem efficiencies (measured and estimated) in the experiment to obtain a certified measurement of DC to DC efficiency in March 1975 at the Raytheon Company. 20
1-15 Photo of the 264 Square Foot Rectenna at the Venus Site of the Goldstone Facility of the Jet Propulsion Laboratory. Power was transferred by microwave beam over a distance of one mile and converted into over 3 0 kW of cw power which was dissipated in lamp and resistive load. Of the microwave power impinging upon the rectenna, over 82% was converted into de power. The rectenna consisted of 17 subarrays, each of which was instrumented separately for efficiency and power output measurements. Each rectenna housed 270 rectenna elements, each consisting of a half-way dipole, an input filter section, and a Schottky-barrier diode rectifier and rectification circuit. The de outputs of the rectenna elements were combined in a series-parallel arrangement that produced up to 200 volts across the output load. Each subarray was protected by means of a selfresetting crowbar in the event of excessive incident power or load malfunction. Each diode was self-fused to clear it from short-circuiting the array in the event of a diode failure. 21 1-16 Progress in rectenna element efficiency as a function of time. 23 2-1 The Rectenna Element Test Arrangement Utilizing the Expanded Waveguide Test Fixture. 2-2 The Rectenna Element Test Arrangement Utilizing the Test Fixture with RF Ground Plane. Also shown is added directional coupler and HP analyzer to measure harmonic power level. 28 2-3 Close-up View of the "Split" Rectenna Element Mounted on the Ground-Plane Test Fixture 29 2-4 Diagram of the Arrangement for Measuring Diode Losses 29 2-5 View of the Back Side of the Ground-Plane Test Fixture Showing the Mounting of the Two Thermistors Which are Used in a Bridge to Measure the Diode Losses. The Thermistors Measure the Temperature Drop Across the Heat Flow Path from the Diode Heat Sink to the large Heat Sink of the Heavy Brass Plate. 3 0 2-6 Schematic Arrangement of Test Equipment for Calibration of Incident Microwave Power at the Input to the Rectenna Element Test Fixture 3 2
2-1 Typical Calibration Curve 3 5 2-8 Zero Drift on Thermistor Bridge 35 2-9 Transient Response to Transistor Bridge to step function of DC Power Input 3 6 2-10 The DC power output, losses in the microwave diode, and losses in the input filter circuit are shown as a percentage of the microwave power absorbed by the rectenna element as a function of incident microwave power level. The sum of all of these is then compared with the absorbed microwave power. 40 2-11 Simplified Math-Model Schematic Diagram for Interpreting Computer-simulation results presented in Figures 2-12 and 2-13. 44 2-12 Time Behavior of Input Current to Rectenna Element, Diode Current, Microwave Filter Output Current, and Input Current to Rectifier Tank Circuit, as Computed 45 2-13 Time Behavior of Input Voltage to Rectenna Element, Diode Voltage, Diode Junction Voltage, and Voltage Across Output Capacitance Filter, as Computed. 45 2-14 Comparison of Computer Simulation Computations of Efficiency, Diode Losses, and Circuit Losses with those obtained experimentally. 48 3-1 A summary of the efficiencies achieved with various new rectenna and diode configurations as a function of power level, compared with performance of a standard RXCV element. 51 3-2 Rectenna Element Test Vehicle 54 3-3 RXCV Rectenna Element modified to provide higher characteristic impedance of second section of the input filter which serves as a X/4 matching section for higher impedance operation of the rectifier circuit. 56 3-4 Expanded waveguide section modified to permit testing of rectenna element with axis normal to regular position. New orientation corresponds to that in the two-plane rectenna construction format. 56 4-1 Comparison Between Voltage Current Characteristics for GaAs-Pf. 67
4-2 Diode Matrix and Manufacturing Sequence 70 4-3 Diode Life Test Results Using Test Arrangement Shown in Figure 1-13. 74 5-1 Proposed design of Rectenna motivated by environmental protection and cost considerations. 76 5-2 Physical construction of two-plane rectenna. With the exception of covers (white teflon sleeves in photograge) this is the same five element foreplane that was electrically tested in Figure 5-12. Reflecting plane made from hardware cloth is representative of what could be used in SSPS rectenna. 76 5-3 Basic core structure design illustrating the joining of individual rectenna elements to each other to form a linear, easily-fabricated structure performing the functions of DC power bussing and microwave collection and rectification. 78 5-4 Proposed method of continuous fabrication of the core assembly of rectenna elements. 79 5-5 A mechanical mockup of the proposed design of Figure 5-1 showing how the metal envelope can be assembled to the core rectenna in a continuous-flow type of manufacturing assembly. The metal envelope is an early design and has been superceded 79 5-6 Artists1 concept of a moving rectenna factory. Materials brought in at one end of factory are basic ingredients to high speed automated manufacture and assembly of rectenna panels which flow continuously from other end of factory. Panels are placed on footings also placed in the ground by the moving factory. 81 5-7 Suggested assembly method in which staked-in ceramic pins provide the dual function of assembling the rectenna element and behaving as electrical capacitance in the low pass filter. 83 5-8 Schematic electrical drawing showing how the sections of parallel diodes are connected in series to build up to the desired voltage level at the output. 83 5-9 Fabricated Metal Shield Halves Which Will Support and Shield, both electrically and environmentally, the Rectenna-Element Core. 86
5-10 Completed rectenna fore-plane assembly consisting of metallic shield and the core assembly of five rectenna elements. 86 5-11 Input admittance to the foreplane rectenna element measured at the junction of the half-wave dipole to the low- pass filter section. Input admittance is a function of the input microwave power level and the DC load resistance. Data were obtained using the expanded waveguide test fixture and one of the elements in the foreplane construction. The rectenna element tested was connected in the intended manner to the adjacent rectenna elements. The susceptance component can be varied by resetting the position of the movable short near the diode rectifier. By adjusting this setting and the DC load, it is possible to obtain a zero power reflection for a wide range of incident power. The 5-watt input level was placed at the center of the Smith Chart because this is the intended power level of operation when the foreplane structure is put into the 199- element rectenna for test. 91 5-12 The test set-up for checking the foreplane type of rectenna array. The five element foreplane structure is placed at the center of the array as shown. The DC output is dissipated in a resistive load. The collected power from the foreplane can then be compared with the power that would have been collected from the five elements that it replaced by a procedure discussed in the text. 93 5-13 Schematic Showing that all Rectenna Elements are Parts of Sets with Six or Twelve Elements Per Set, which lie on Circles Concentric with the Rectenna Center. In the 199-Element Array there are 7 additional sets to the 14 of this figure. 95 5-14 The average DC power output per element in a set of elements is plotted as a function of the radial distance of the set. The resulting points of data may be easily fitted to a Gaussian curve which is the approximate power density distribution in the beam. 96 5-15 Experimentally observed power output of the 5-element fore-plane structure is compared with predicted power based upon power picked up by remaining elements in sets whose total number of elements included elements in the foreplane structure. Data set Not 1 and No. 2 were taken at different times. Inconsistency of data at the "relative incident microwave power" value of 1.0 is caused by difficulty in resetting the incident microwave power level from one set of measurements to another. 96
5-16 The power output of the foreplane structure and other sets of rectenna elements, which contain elements coupled to elements in the fore-plane structure, as a function of the foreplane DC load resistance. 98 5-17 Agreement between experimentally measured DC power from the foreplane structure and predicted value as a function of foreplane DC load resistance. 99 5-18 VSWR ratio and min position as a function of the DC load resistance of the foreplane construction and of the distance of the probe from the dipole plane. 99 5-19 Rectenna Edge to Horn Mouth Spacing 67 Inches, Frequency 2382 MHz, Center Rectenna Element Matched at 9.4 Watts in Expanded Waveguide Fixture. 101 7-1 Sketch of Recommended System. Ill
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SUMMARY This technology development was concerned with improvements in the reception and rectification of microwave power at the receiving terminal of a free-space power transmission system anymore specifically, with its application to the rectenna receiving array in the solar power satellite concept. In this system, large areas of the array operate at relatively low power density levels. A 20 percent improvement in the efficiency of elements operating at these levels was obtained by a combination of circuit and diode redesign. GaAs-W Schottky barrier diodes were designed and constructed to provide a lower voltage drop across the diodes to improve efficiency. A major accomplishment was the adaptation of previous rectenna technology to the more economically desirable two-plane construction format in which the foreplane provides the functions of collection, rectification, filtering, and DC power bussing and collection. A metal shield in the foreplane was designed to provide environmental protection and to act as a structural element in the rectenna array. Improvements in measurement and analytical techniques that were achieved were: mathematical modeling and computer simulation of the basic receiving element that checked out well with experimental data; quantitative measurement techniques for experimentally measuring diode and circuit losses; and an accounting method for balancing the microwave power input against the sum of DC power output and diode and circuit losses to achieve better confidence in efficiency measurements.
1 .0 INTRODUCTION The RF to DC Collector/ Converter technology development with which this report is concerned is in support of the larger technological development of free space power transmission by means of a microwave beam. The efficient free- space transportation of energy by electromagnetic beam brings a new three- dimensional aspect to the transfer of electrical power and permits the coupling of terrestrial power transmission systems to power sources and sinks located in the earth’s atmosphere and space. This report relates to the application of free space power transmission in which the sun’s energy is captured in equatorial geosynchronous orbit, converted to electrical power, and then sent to Earth by means of a microwave beam. At the earth’s surface the microwave power is efficiently collected and converted back into DC electrical power. Improvements in the performance of this receiving system and its reduction to a practical design are the specific subject matter of this report. To place the work to be reported upon in proper perspective, it is desirable to define and review the technology of free-space power transmission with particular emphasis upon prior RF to DC collector/converter technology. 1. 1 Description of free-space power transmission by microwave beam and its early development. Free-space power transmission by microwave beam is defined as the efficient point-to-point transfer of energy through free space by a highly collimated microwave beam. As a technology H) it includes the interchanging of de and microwave power at the transmitting and receiving ends of the system. Free- space is defined to exclude the use of any physically injected material such as waveguides or reflectors between the transmitting and receiving points of the system but not to exclude the presence of gaseous, liquid, or congealed material that exists naturally in the earth’s atmosphere. Free-space power transmission as a technology is differentiated from the use of microwaves in free space for point to point communication purposes by its very high efficiency and by the magnitude of the power which is handled at the receiving point - being in many cases over 90 percent of the microwave power launched at the transmitting point. The efficient collection and conversion of this incoming microwave power to conventional electrical power comprises a unique technology which bears little relationship to the traditional methods of receiving and processing microwave energy in communication and radar applications. The concept of power transfer by radio waves was first pioneered by Tesla (3) at the turn of the century. An acknowledged genius in low-frequency electric power generation and distribution, Tesla became interested in the general concept of resonance and sought to apply this to the transmission of electrical power from one point to another without wires. He built a large "Tesla coil" with which he hoped to produce oscillations of electrical energy around the surface of the earth and set up standing waves into which he could immerse his receiving antennas at the optimum point.
With the advantage of historical perspective, we realize that these experiments were decades ahead of the unfolding of a technology that could accomplish his objective. This new technology was based upon the early microwave experiments of Hertz (4), but had to await the development of efficient generators of microwave power. This capability began to emerge with the microwave generators developed for the radar of World War II and later for microwave communications. The event which directly precipitated interest in the use of microwaves for power transmission was the support of the development of super-power microwave tubes by the Department of Defense in the early I960's, This program resulted in high-efficiency tubes with such high power handling capability (several hundreds of kilowatts) that the use of microwaves for the efficient transfer of large amounts of power became a distinct feasibility. The first demonstration of the efficient transmission of meaningful amounts of power by microwaves took place at the Spencer Laboratory of Raytheon Co. in May 1963 In this first demonstration, shown in Figure 1-1, the means used for collecting the power at the receiving end of the system utilized conventional antenna technology in the form of a pyramidal horn. The means used for rectifying the microwave power to DC power was a close-spaced thermionic diode. Neither of these technologies was completely satisfactory. The receiving horn was highly directive and because of the difficulty of matching its antenna pattern to that of the incoming beam its collecting efficiency was only 87%. The rectifier efficiency was only 50%. Nevertheless, as a result of this demonstration the Rome Air Development Center of the Air Force became interested in the concept of a microwave powered platform for communication purposes. The Raytheon proposal of a microwave powered helicopter to accomplish this objective and the resulting contract became crucial in determining the evolutionary path of the collection and rectification of microwave power from a free-space microwave beamj'Tt was recognized that the pyramidal horn would not be satisfactory because of a combination of its high directivity with the natural roll and pitch of the vehicle. It was also recognized that the limitations of the close-spaced thermionic rectifier would place severe limitations on the practicality of the platform. The "rectenna" device was proposed to the Air Force as a solution to this problem. The rectenna device made it possible to simultaneously solve the directivity and antenna pattern matching problems of microwave power collection and at the same time make practical use of the semiconductor device whose power handling capability had prevented it from being seriously considered for a system in which significant amounts of power were being handled. The Raytheon Company actually demonstrated a microwave powered helicopter using a rectenna prior to active work on the Air Force contract. But the Air Force contract was the basis for an extension of the effort and several notable demonstrations, including a ten hour continuous flight of the vehicle' Figure 1-2 shows the helicopter in flight. It was necessary, of course, to use laterally constraining tethers to keep the helicopter on the microwave beam but this limitation was later removed by a study and experimental confirmation
Figure 1-1. First experiment in the efficient transfer of power by means of microwaves at the Spencer Laboratory of Raytheon Co. in May 1963. In this experiment microwave power generated from a magnetron was transferred 5. 48 meters and then converted with DC power with an overall efficiency of 16%. A conventional pyramidal horn was used to collect the energy at the receiving end and a close-spaced thermionic diode was used to convert the microwaves into DC power. The DC power output was 100 watts.
Figure 1-2. Microwave powered helicopter in flight 18. 28 meters above a transmitting antenna. The receiving array for collecting the microwave power and converting it to DC power was made up of several thousand point contact silicon diodes. DC power level was approximately 200 watts.
that the microwave beam could be used successfully as a position reference in a control system in an automated helicopter which would keep itself positioned over the center of the bearm This progression of efforts established the rectenna device as a probable solution to the collection and rectification problem in a broad class of microwave power transmission applications, but much work remained to be done to make it a practical device in the context of the SSPS type of application. The opportunity to further evolve the rectenna device was largely the result of the interest of the Marshall Space Flight Center in applying microwave power transmission to the transfer of energy and power between satellites, (8) and the contractual effort supported at Raytheon Company. ' In a more recent time frame very substantial advances in overall system performance have been made. These advances include a certified overall transmission efficiency of 54% starting with the DC power applied to the microwave generator and ending with the DC power out of the rectenna at the receiving point. ' ' A particularly impressive demonstration was made at the Goldstone facility of the Jet Propulsion Laboratory. In this demonstration power was transmitted over a distance of 1. 6 kilometers and a DC power output of over 30 kilowatts was obtained at the receiving point. Table 1-1 presents a summary of the early chronology of the collection and rectification of microwave power. It will be noted that there was inter-' est in microwave power transmission prior to any capability of efficiently converting microwave power directly into DC power. ' ' TABLE 1-1 CHRONOLOGY OF COLLECTION & RECTIFICATION OF MICROWAVE POWER 1958 First interest in microwave power transmission 1958 No rectifiers available - turbine proposed and studied 1959-1962 Some government support of rectifier technology (1) Semiconductors at Purdue University (2) Magnetron analogue at Raytheon 1962 Semiconductor and close-spaced thermionic diode rectifiers made available. 1963 First power transmission using pyramidal horn and close-spaced thermionic diode rectifiers - 3 9% capture and rectification efficiency not practical for aerospace application. 1964 RADC microwave powered helicopter application demanded nondirective 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- Continued development of rectenna concept to format with high power Present handling capability, much higher capture and rectification efficiency, and potentially low production cost.
1. 2 Major Microwave Collector-Converter Technology Developments As a result of the early experience with the severe demands placed upon the receiving portion of a free-space microwave power transmission system and the discovery of the ability of the rectenna concept to cope with all of these demands, the history of microwave collector/converter technology is almost exclusively that of the development of the rectenna. The following general requirements are placed upon the collector/ conve rte r: • large aperture • high power handling capability • non-directive • high efficiency • ability to operate efficiently over a substantial frequency range • light weight • easy mechanical tolerances • ability to passively radiate any heat resulting from inefficient • operation • high rel lability • very long life • minimal radio frequency interference • low cost. The rectenna has been found to successfully meet all of these requirements, with the possible exception of radio frequency interference. RFI, however in the form of harmonic power, is a special problem that confronts both the transmitter and the receiver. Since the harmonic level must be down to such low levels to meet non-interference requirements and meeting it by wave filters would result in such higher cost and reduced efficiency, the proper solution may be to have an allocation of frequencies for the harmonics that are generated in the system. The rectenna has gone through a number of development stages whose nature was largely determined by the motivational influences of the period and the state of development of diodes. These stages are outlined with the aid of Table 1-2. The microwave powered helicopter application was the dominant early influence and was responsible for the initial development of two separate embodiments of the rectenna concept. The very first rectenna. Figure 1-3,which established its general properties made use of a rectenna element characterized by a halfwave dipole antenna terminated in a full-wave bridge. This development was based upon an early study of the solid-state diode as an efficient rectifier of microwave power by George (^4) and its adaptation as a rectifier of free-space radiation by Brown, et al. ^6) The rectenna elements were separated from each other by approximately one-half wavelength. Unfortunately, such a construction using the then existing point-contact diodes could not handle nearly enough power density to be used for the demonstration of a microwave-powered helicopter. A new configuration characterized by a dense compaction of diodes
TABLE 1-2 MAJOR RECTENNA DEVELOPMENT PROGRAMS
Figure 1-3. The first rectenna. Conceived at Raytheon Company, it was built and tested by R. George of Purdue University. Composed of 28 half-wave dipoles spaced one-half wave-length apart, each dipole terminated in a bridge-type rectifier made from four IN82G point-contact semi-conductor diodes. A reflecting surface consisting of a sheet of aluminum was placed one-quarter wavelength behind the array.
in a string-like construction in which the diodes themselves were part of the collection process as well as the rectification process was developed and used successfully in the early helicopter work. (Figures 1-2 and 1-4. ) In time coincidence with the demonstration of the helicopter, Hewlett Packard Associates had developed a new physical format for a silicon Schottky barrier diode with the potential for much greater reliability and power handling capability than the point contact diode as well as offering considerably greater efficiency. A number of these were forwarded by HPA for evaluation and their superiority as rectifiers was confirmed by R.H. George (18), They were not put into a rectenna element format. In the time period from 1965 until 1970 there was no direct support of rectenna development from either government or industry. During this time period, the Air Force did support the development of a helicopter which would automatically position itself over the center of a microwave beam, a capability necessary for the practical use of a microwave powered helicopter. However, a substantial amount of development work on the rectenna was carried out by W. C. Brown using private funds and time during thel967 to 1968 time period. This, work was primarily aimed at designing a very light weight rectenna structure-which utilized a rectenna element format consisting of a half-wave dipole antenna terminated in a full-bridge rectifier made up of HPA 2900 diodes ' ’ " (Figure 1-5.) This work was important in that it established the physical format of the rectenna development effort that was to be undertaken later at MSFC and that was also to be supported under MSFC contract at Raytheon Company. It was also used in a demonstration of microwave power transmission to the MSFC Director, Werner von Braun, and his staff. This demonstration may have been a decisive factor in a decision to undertake the support of this work at MSFC during a time period of NASA contraction. In Spetember 1970 a demonstration involving the measurement of the various efficiencies in complete microwave power transmission system (DC to DC) was made at Marshall Space Flight Center ( 8) (Figure 1-6). The rectenna used for this purpose. Figure 1-7, employed rectenna elements patterned after those just discussed but developed to the point where their individual capture and rectification efficiencies approximated 70%. The configuration is important in the context of rectenna development for satellite power stations in that the collection, rectification, and DC collection was performed in a single plane positioned approximately a quarter wave-length above the reflection plane. This is the intended approach whose development was a part of the activity under this contract. The MSFC demonstration of 1970 indicated a number of deficiencies in the system including a rectenna collection efficiency of only 74% versus the theoretical maximum of 100%. This low collection efficiency was associated with improper spacing of the rectenna elements from each other in the rectenna array. It was therefore decided to space the elements more closely to each other and, in addition, terminate the DC output of each rectenna element in a
Figure 1-4. The special rectenna made for the first microwave-powered helicopter. The array is 0. 6 meters square and contains 4480 IN82G pointcontact rectifier diodes. Maximum DC power output was 200 watts. Figure 1-5. Greatly improved rectenna made from improved diodes (HP2900) which are commercially available. The 0.3 meter square structure weighs 20 grams and can deliver 20 watts of output power.
Figure 1-6. Test set-up of microwave power transmission system at Marshall Space Flight Center in 1970. The magnetron which converts de power at 2450 MHz is mounted on the waveguide input to the pyramidal horn transmitting antenna. The rectenna in the background intercepts most of the transmitted power and converts it to de power.’ Ratio of de power out of rectenna to the rf power into the horn was 40. 8%. Overall dc-to-dc efficiency was 26. 5%.
Figure 1-7. Close-up view of first rectenna developed by Raytheon under MSFC contract. Microwave collector, rectification, and DC bussing of rectified power are all carried out in one plane. Rectenna elements are connected in series.
separate resistor to obtain a much greater range of data on the behavior of the rectenna. This latter decision involved a change in the manner in which the DC power was collected and instrumented/^) The output of each rectenna element was brought back through the reflector plane. This arrangement shown in Figure 1-8 provided such an enhanced capability to study and understand the performance of the rectenna that it was retained in the further development of the rectenna. (See Figure 1-9 for the later adaptation to a more recent MSFC rectenna. ) The construction, however, is not economical and is not recommended for most applications. The changed collection geometry as shown in Figure 1-8 improved the collection efficiency to about 93%. Other changes improved the overall transmission, collection, and rectification considerably . Because the diode rectifier is such an important element in the collection and rectification process, a search for diodes which would improve the efficiency and power handling capability of the rectenna has been a continuing procedure. In 1971, Wes Mathei suggested that the Gallium Arsenide Schottky-barrier diode that had reached an advanced state of development for Impatt devices might be a very good power rectifier and provided a number of diodes for testing.(^» ') These devices were indeed much better. Their revolutionary behavior in terms of higher efficiency and much greater power handling capability rapidly became the basis for the planning of improved rectenna performance. The knowledge of the superior performance of this device was coincident with the advancement of the concept of the Satellite Solar Power Station by Dr. Glaser of the A. D. Little Co. (21) The earliest investigation of a rectenna design for this concept indicated that the economics of its construction would be crucial and that mechanical and electrical simplicity of the collection and rectification circuitry would be of paramount importance. This factor, combined with the fact that no harmonic filters had existed in previous rectenna element designs but would be necessary in any acceptable microwave power transmission system, motivated a completely new direction of rectenna element development. This new direction was the development of a rectenna element employing a single diode in a half-wave rectifier configuration with adequate wave filters to attenuate the radiation of harmonics and to store energy for the rectification process. The construction of such a rectenna element and its insertion into a DC bus collection system is shown in Figure 1-9. This rectenna element was used, in the last phase of the MSFC sponsored work at Raytheon to construct a rectenna 1. 21 meters in diameter which was illuminated by a gaussian beam horn (Figure 1-10). The combined collection and rectification efficiency of this rectenna was measured at 80%. A lower cost and slightly more efficient form of this rectenna element was developed for the RXCV work sponsored by NASA at JPL. This element is shown in Figures 1-11 and 1<-12, together with a greatly simplified equivalent electrical circuit of the device. The same electrical circuit applies to the MSFC rectenna element of Figures 1-9 and IrlO.
Figure 1-8. Experimental set-up comprised of dual-mode horn and improved rectenna. The efficiency ratio of the de power from the rectenna to the microwave power at the input to the dual-mode horn was measured and found to be 60. 2%.
Figure 1-9. Sketch of the Marshall Space Flight Center rectenna which was constructed in spring of 1974. Cutaway section of rectenna element shows the two section input low pass filter, the diode, and a combination tuning element and by-pass capacitor. Figure 1-10. Photograph of the MSFC rectenna constructed in 1974 under test. Horn at left of picture illuminates the rectenna (white panel) with a Gaussian distribution of power. Rectified DC power is collected from rectenna in circular ring path and dissipated in resistive loads on the test panel at the right.
Figure 1-11. Simplified Electrical Schematic for the rectenna element used in the RXCV receiving array at the Venus site of the JPL Goldstone facility. Figure 1-12. Photograph of rectenna element designed for JPL RXCV demonstration at Goldstone. This element represents the departure point for the technology development being reported upon.
The last measurements of overall system efficiency and overall rectenna efficiency were made in March of 1975 at Raytheon Company with the experimental setup shown in Figure 1- 13 The rectenna elements used in the rectenna array were those developed for the JPL RXCV demonstration at Goldstone but optimized for performance at 2.45 GHz. In order to establish a greater degree of credibility to the values of efficiency that might be obtained from the setup the Quality Assurance department of the Jet Propulsion Laboratory supervised the taking of the data. The overall DC to DC efficiency was measured at 54. 18%with a probable error of ± 0. 94%. The overall collection efficiency of the rectenna was more difficult to ascertain because of the inaccuracy in determining the fraction of the generated microwave power which is intercepted by the rectenna. The most probable efficiency, however, was 82%. A schematic of the test set up and the breakdown of efficiencies and inefficiencies is given in Figure 1'14. The last major rectenna effort ^2) reported upon is the relatively large scale reception-conversion subsystem (RXCV) for a microwave power transmission system located at the Venus site of the JPL Goldstone facility in the Mojave desert. This effort was not undertaken as a technology development as such but nevertheless gave useful output in terms of (1) confirmation of the reliability and efficiency of advanced diode design, (2) evaluation of rectenna subarray performance with incident uniform power density, (3) protective measures to be taken to guard against rectenna failure with accidental load removal or with unusual wave-forms of the envelope of the transmitted microwave power, (4) protection of the rectenna elements from the atmospheric environment. The rectenna shown in Figure 1-15 consisted of seventeen subarrays each 1.22 meters square and containing 270 rectenna elements. The rectenna element shown in Figures 1-11 and 1-12 that was designed for this application constitutes the point of departure for the technology development program being reported upon. The collection and conversion efficiency of this array was measured to be 82% at a total DC output of 3 0 kilowatts. 1.3 Progress in Rectenna Efficiency Using Progress in Rectenna Element Efficiency as an Index. The rectenna efficiency is given by the product of the microwave power collection efficiency and the rectification or conversion efficiency. The maximum theoretical collection efficiency is 100% and it has been measured at over 99% efficiency by means of VSWR measurements of a probe in front of the array. The validity of measuring collection efficiency by this means rests upon a small amount of power being reflected from the rectenna and upon a gaussian distribution of energy in the incoming wave and in the reflected wave. These conditions are closely approximated by the setup shown in Figure 1-13 where the gaussian illumination is laid down by means of a dual-mode horn. If it is assumed then that the collection efficiency can be made close to 100%, it follows then that the efficiency of the conversion of the collected power
Figure 1-13. Photograph of the microwave power transmission system at the Raytheon Co. in which a certified overall DC to DC efficiency of 54% was obtained in March 1975. Magnetron at left of picture converts de power into microwave power which is fed into the throat of the dual-mode horn. The horn illuminates the rectenna panel with a gaussian distribution of power. Rectified de power is collected from the rectenna in circular ring paths and is dissipated in resisDve loads on the test panel at the right. Figure 1-14. Distribution of system and subsystem efficiencies (measured and estimated) in the experiment to obtain a certified measurement of DC to DC efficiency in March 1975 at the Raytheon Company.
Figure 1-15. Photo of the 24. 5 Square Meter Rectenna at the Venus Site of the Goldstone Facility of the Jet Propulsion Laboratory. Power was transferred by microwave beam over a distance of 1.6 km and converted into over 30 kW of cw power which was dissipated in lamp and resistive load. Of the microwave power impinging upon the rectenna, over 82% was converted into de power. The rectenna consisted of 17 subarrays, each of which was instrumented separately for efficiency and power output measurements. Each rectenna housed 270 rectenna elements, each consisting of a half-wave dipole, an input filter section, and a Schottky-barrier diode rectifier and rectification circuit. The de outputs of the rectenna elements were combined in a series-parallel arrangement that produced up to 200 volts across the output load. Each subarray was protected by means of a self-resetting crowbar in the event of excessive incident power or load malfunction. Each diode was self-fused to clear it from short-circuiting the array in the event of a diode failure.
into DC power is really the measure of the overall rectenna element efficiency where the rectenna element is defined as shown in Figure Irll. The progress that has been made in rectenna element efficiency as determined by test equipment to be described in Section 2. 0 of this report is shown in Figure 1-16. According to Figure 1-16 the efficiency has now exceeded 90%. The validity of this figure is the subject of discussion in Section 2. 0 of this report. The progress in efficiency is closely as sociated with the use of improved diodes but the choice of circuitry is also important. 1.4 The Energy Problem and the Solar Power Satellite Concept as Factors in Determining the Extent and Direction of Rectenna Development The early stages of rectenna development were carried out in response to the need for high altitude atmospheric platforms that could stay aloft indefinitely propelled by the power beamed to them by microwaves, and for the transmission of power from one vehicle to another in space where wire transmission would be impractical. There was no generally recognized energy problem at that time and certainly no general recognition that our budding space capability could be associated with fulfilling an energy need should one exist. Now, of course, the energy problem is well recognized, as it is also recognized that the use of electrical power is growing at a faster rate than our requirements for energy as a whole and that there is a strong indication that the electrical growth rate will be further increased as energy consumers turn to electrical power as a substitute for natural gas and oil. Unfortunately, the present methods of generating electrical power pollute the environment and consume natural resources at a prodigious rate. Under these circumstances it is only natural that we turn to the sun and investigate it as an answer to our electrical energy requirements. However, two serious problems confront us when we seek to use if for this purpose. The first problem is its diffuse nature which makes it difficult to capture in large amounts without enormously large and expensive physical structures. The second problem is its low duty cycle and only partial dependability. We can be certain of its unavailability at night, but never certain of its availability in the daytime with an intensity sufficient for electrical energy producing purposes. Out in space in geosynchronous orbit, however, the sun is available over 99% of the time and its infrequent and short term eclipses by the earth can be precisely predicted and planned for. That desirable condition would be of no practical importance if it were not possible to place large energy collecting arrays into synchronous orbit and in some manner get that energy back to earth where it is needed. Dr. Glaser 2?) was the first to point out that we could combine three technologies, all developed within the past 20 years, to accomplish this task. These three technologies are (1) the new capability to transport material into space, (2) the solar photovoltaic cell which directly converts solar flux into DC electrical power, and (3) free space power transmission by means of a microwave beam. As a result of this proposal and initial feasibility study performed by a team made up of personnel from Arthur D. Little, Inc., Raytheon Company,
Figure 1-16. Progress in rectenna element efficiency as a function of time.
Grumman Aerospace Corporation, and Textron Inc., the concept was accepted for study by NASA. (23, 24) Subsequent studies supported by NASA (25, 26, 27) have not only confirmed its technical feasibility but have established the possibility of it being economically competitive in the future with conventional and other advanced approaches to electrical energy production. One of these studies was devoted to the microwave power transmission system associated with the solar power satellite. (26) The study confirmed the previous finding that to be most economical the power rating of the systems would be large. The typical rectenna would receive over five gigawatts of microwave power, and have an area of 70 square kilometers. The maximum power per rectenna element would be 1. 5 watts while the minimum would be 0. 15 watts, although there is a good reason to believe that in the eventual system these power levels may be increased by a factor of two. The rectenna would have to be fully environmentally protected and have to meet cost goals by a low material cost per unit area and by a low-cost material handling operation which would convert basic materials into completed rectenna subarrays at high speed. 1. 5 Objectives of the Technology Development Reported Upon in this_______ Report The previous sections have been included to serve as a background for understanding the appropriateness of the objectives of the technology development to be reported upon, and for understanding the approaches to achieving those objectives. The broad objective of the effort covered in the subsequent sections of this report is to improve those features of the rectenna which are important to its function in a full scale solar power satellite system. One feature of particular importance is the efficiency associated with the rating of the individual element in the system. Surprisingly the problem is not one of power handling capability since the element has more than enough power handling capability. The problem lies rather in the reduced efficiency that is obtained at the lower power levels which are more representative of the manner in which the rectenna is used in the solar power satellite. Hence, one objective was to do those things to both circuit and diode which would improve efficiencies at lower power levels. Another objective was to develop better instrumentation and procedures to provide better resolution in measurements and to provide a higher confidence level in the efficiency measurements being made. Finally, but of great importance, was the first iteration of an electrical and mechanical design aimed at the high speed, low-cost fabrication of a fully environmentally protected rectenna. The achievement of this latter objective is crucial to the credibility of the economic aspect of the satellite power system.
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