RCS Ranges: Basics of Absorber Layout Design and Operation and Calibration

Transcription

1 RCS Ranges: Basics of Absorber Layout Design and Operation and Calibration Dr. Vince Rodriguez, Ph.D. Senior Principal Antenna Design Engineer and Antenna Product Manager ETS-Lindgren List of Topics Introduction and Background Antenna vs. RCS measurements RF absorber Absorber treatment for Antenna and RCS ranges Typical treatment for APM Typical treatment for RCS Jack of all trades Operation: pay now, or pay later. Conclusion. 1

2 Introduction and Background: Antenna vs. RCS Radiation Pattern Book definition a 3D map that displays the strength of the radiated fields or power density as a function of direction 2

3 Radiation Pattern The radiation is then a representation of how much Electromagnetic energy is concentrated in each direction around the antenna 5 Antenna Pattern Measurement 3

4 Radar Cross Section Book definition IEEE Standard Definitions of Terms. RCS. For a given scattering object, upon which a plane wave is incident, that portion of the scattering cross section corresponding to a specified polarization component of the scattered wave. Scattering Cross Section. For a scattering object and an incident plane wave of a given frequency, polarization, and direction, an area that, when multiplied by the power flux density of the incident wave, would yield sufficient power that could produce, by isotropic radiation, the same radiation intensity as that in a given direction from the scattering object. RCS Incident Power Density POWER Isotropic Radiator Radiated Power Density Scattered Power Density 4

5 RCS, ;, 4,, = =4π = =4π RCS 5

6 Plane wave incidence RCS Far Field distance, ;, 4,, = =4π = =4π RF Absorber materials 6

7 The Absorber Family 1 Microwave Pyramidal absorber. EMC and EHP series Electric Losses Preferred technology for High frequencies It can be used for low frequencies if size (length) is increased The Absorber Family 2 Ferrite Tile. Magnetic Losses Preferred technology for Low frequencies (up to 1GHz), it has low profile (typically 7mm). It cannot be used for high frequencies Ferrite doped plastics shaped as pyramids will give about 15dB normal incidence 7

8 The Absorber Family 3 Hybrid Absorber. Electric and Magnetic Losses Preferred technology for EMC Applications. foam has to have special formula for good matching with ferrite tile at the bottom. At High frequencies its performance is not as good as MW pyramid of equal size. Flat top causes undesired reflections at MW range. Pyramidal Absorber Theory (Example) At the tip of the absorber The wave impedance is that of air Along the length of the pyramid the wave impedance falls between those two values. At the base of the pyramid The wave impedance becomes 8

9 Pyramidal Absorber Theory (Example) Curvilinear absorbers have a special shape to ensure better penetration and absorption of the wave On Absorbers Oblique incidence Although absorbers are cut in pyramidal shape for better normal incidence they are commonly used in oblique incidence The largest reflection is going to be in the bi-static direction 9

10 On Absorber 1. For a given pyramid element size chosen there is no expected backscattering component. The scattered field is a sum of all the possible grating lobe waves which propagate in different directions, Only those where the following condition is satisfied contribute to the scattering at a distance [*] 2. For m=0 and n=0 we have specular reflection only. For higher order modes to propagate we see that the period of the structure has to be larger than the wavelength 3. [*] W. Sun, C. Balanis Analysis and Design of Periodic Absorbers by Finite-Difference Frequency-Domain Method report No. TRC-EM-WS-9301 Telecommunications Research Center, Arizona State University, Tempe, AZ On Absorbers Wedge and pyramid Electric Losses A variant of pyramidal absorber wedge does not show backscattering. Preferred technology for QZ treatment and for RCS chambers. 10

11 Flammability and Fire Retardancy Absorbers convert EM energy into Thermal Energy If too much EM energy is applied a lot of thermal energy is generated Material will burn Flammability and Fire Retardancy Some materials can take more EM energy (power). Non flammable substrates are used and/or forced air cooling is used If enough power is used even high-power absorber will ignite. Flammability test standards are used to rate the absorber materials. NRL(Naval Research Laboratory) tests are the true anechoic chamber RF material standards. Other Standards such as UL-94 or DIN standards deal with flammability of foams or of construction materials 11

12 Power Measurements on Absorber Regular foam substrate Smaller foot print from 8 inch by 8 inch per pyramid to 3inch by 3inch (more surface more heat transfer) Cooling flow A test cell was set to measure the absorber Field levels at 3.1GHz 12

13 Field levels at 10.35GHz Temperature vs. time and air flow 13

14 Some IR images of absorber Image on conductive walls IR images of absorber 3.1GHz 5.522kW/sq m. or 1443v/m 14

15 IR images of absorber 10.35GHz 1.664kW/sq. m 792v/m 2.498kW/sq. m 970v/m 3.331kW/sq. m 1121v/m ABSORBER TREATMENT FOR ANTENNA AND RCS RANGES APM and RCS 15

16 Rectangular Chambers for Antenna Pattern Measurement ABSORBER TREATMENT FOR ANTENNA PATTERN MEASUREMENTS Chamber for APM 16

17 APM Chamber: undesired reflections QZ level Defines quality of the chamber Level of the highest reflected signal entering the Quiet Zone versus the direct path QZ level=20 log 10 17

18 Antenna Chamber: The Absorber Treatment Back wall (receive end wall) Normal Reflectivity better than QZ level Side wall Oblique incidence Reflectivity with off main beam gain better than QZ level Antenna Chamber: The Absorber Treatment Side wall absorber is only needed on those areas where a specular reflection exists between the source and the QZ Everywhere else shorter absorber can be used 18

19 Antenna Chamber: The Absorber Treatment Transmit end wall absorber can have a reflectivity that when added to the front to back ratio of the source antenna it meets the required QZ level Enter Chebyshev Analogous to Chebyshev impedance matching transformers in MW circuits. It is possible to have the reflections from the absorber field with different phases so that phase cancellation can be achieved at some frequencies. 19

20 Enter Chebyshev Choosing the proper step and polynomial weights is possible to improve the performance of the absorber field by a given number of db. The improvement shown on the right is based on a plane wave incident but even with a spherical wavefront an improvement can be accomplished. 48 absorber can give us 27dB, with an additional 15dB from Chebyshev and 3dB from the source antenna pattern will place us at 40dB for the side wall treatment. Enter Chebyshev 20

21 Chebyshev Measurement Sample Chebyshev Measurement Sample 21

22 An example of a typical treatment showing the staggering of absorber in the specular region for better performance. 36ft EHP-36PCL EHP-18PCL 24ft 4ft QZ EHP-18PCL 12ft EHP-12PCL EHP-12PCL EHP-18PCL EHP-36PCL Walkway absorber has a lower absorption so it is placed on the sides or behind the Quiet zone 36ft EHP-12PCL EHP-36PCL EHP-18PCL EHP-12PCL 24ft 4ft QZ EHP-18PCL 16ft EHP-18PCL EHP-36PCL 22

23 RCS RANGES ABSORBER TREATMENT FOR RADAR CROSS SECTION MEASUREMENT REANGES RCS is a far field parameter To put it on simple terms, the near field has spherical waves and standing wave behaviors The far field the sphere is large that it resembles a plane and the wave is traveling 23

24 RCS and far field Need to be far enough from the source to get an incident plane wave on the target The plane wave must cover the entire target Need to have plane waves Need to move very far away from the source to get plane wave behavior Or use a structure that will create a plane wave in a relatively short distance 24

25 Compact Range The compact range reflector can generate plane waves in a relatively short distance and these waves can illuminate a large object RCS RANGE Reflector and feed system 25

26 Reflections on the Range We are now measuring the scattered or reflected signal We want the scattered signal from the target only! Backwall reflection comes from the same direction, so the absorber has to be extremely efficient. In some cases time gating can help in eliminating that reflection Target will scatter energy to all the walls. The energy scattered towards the back of the chamber must be redirected to the backwall The one reflected towards the illumination system must be eliminated, the absorber and the high directivity of the illumination system will help on that. Recall the backscattering At high frequencies the absorber treatment will have some backscatter component that will give an error on the measurement Wedge will eliminate this backscatter energy 26

27 RCS RANGE Reflector and feed system Compact Range for RCS pyramid wedge Little or no treatment behind the reflector pyramid pyramid wedge Absorber fences may be required to avoid direct illumination from the reflector feeds 27

28 Jack of all trades, master of none COMBINED APM AND RCS RANGES Combined Ranges RCS and APM are two different sets of measurements The Absorber treatment requirements are different These facilities are very large investments Are there advantages/disadvantages to combine both types of tests into one? 28

29 Differences in Ranges Reflector and feed system Receive end wall is the most critical Backscattering from side wall absorber must be eliminated Wedge is used on side walls and around the quiet zone A plane wave must illuminate the target Side walls are the most critical Backscattering from the sidewall absorber not a problem typically Pyramidal absorber is used on side walls We want far field condition but near field to far field transformations allow for shorter distances when measuring patterns Reducing the Backscattering Reducing the backscattering can be accomplish by getting more penetration into the absorber of the incoming waves The flat surfaces of the sides of the pyramid provide an abrupt change in wave impedance Placing the specular treatment at 45 degrees (diamond arrangement) provides a less abrupt change in wave impedance and reduces the backscattering A/3 A/3 A/3 29

30 Compact Range for RCS and APM Pyramid at 45deg wedge Little or no treatment behind the reflector pyramid Pyramid at 45deg wedge Absorber fences may be required to avoid direct illumination from the reflector feeds Consider the Following The 45 degree arrangement will not reduce backscattering to the levels that wedge can. Such range may not reach the levels provided by wedge treated range The positioners (pylons) used in RCS are different from the APM positioners Given the cost of a RCS compact range an additional dedicated facility for APM will be a small percentage of the overall cost. If electrically large antennas are being tested (long far field distances) consider a Near to Far Field system. 30

31 A Sample of a dual purpose range Specular Absorber at 45º Top (or side view) B A/3 A/3 A/3 Absorber mounted at 45 degrees to reduce the backscattering during RCS Operations A 31

32 Pay now or pay later DAY TO DAY OPERATION Consider Shielding: how quiet is your area? Are you close to airports or military bases? How clean is your AC power? Do you know what the next wireless frequency band is going to be? Doors: larger is better. Modifying any chamber will always be more expensive than installing that door from the beginning Hoists and cranes: over 70% of test time is preparation the faster you can mount your target to the pylon and the smaller the disruption to the floor absorber treatment the faster your test time 32

33 Mounting the Target This chamber has a very large reflector The feed and subreflector is hidden in an absorber lined room under floor level Notice the person The pilon comes down into another room under floor level for mounting the target Target Handling The target is mounted in a room under the chamber This is a smaller range than the previous one so after the pylon is in position the floor is closed and the absorber treatment is not disrupted A small disruption is needed for the pylon base 33

34 Setting the Test A hoist in this chamber allows the technicians to set up the target The rotator on top of the pylon is clearly seen This chamber has a 2m reflector with serrations A side wall door with bridge allows technicians to reach the target on the pylon The hoist is operated by a second technician Feed and radar system are inside the chamber to reduce cable losses Bringing targets into the Range Hoist Hoist rails Hoist rails move into position to access chamber Shielded door 34

35 Calibration Spheres Flat plates NIST squat Cylinders RCS is determined numerically. NIST and the USAF conducted a series of analysis to determine their broadband RCS. They are easier to manufacture and position NIST squat cylinders Cylinder D (inches) H (inches) D H The tolerance in the dimensions is ±0.002 inches 35

36 Range Certification RCS Measurement Facilities Documentation Standard Published by the Radar Committee of the Signature Measurements Standards Group Based on the ISO guide 25 (equivalent ANSI Z540) The same body approved for public release: DOCUMENT VOLUME I AND VOLUME II RADAR CROSS SECTION (RCS) CERTIFICATION FOR STATIC AND DYNAMIC RCS MEASUREMENT FACILITIES Process 36

37 Range Book TABLE I-1 RCS RANGE BOOK TABLE OF CONTENTS SECTION TITLE PAGE 1 Introduction and Endorsement 2 References 3 Glossary 4 Organization and Management 5 Quality System, Audit, and Review 6 Personnel 7 Accommodation and Environment 8 Equipment and Reference Materials 9 Measurement Traceability and Calibration 10 Calibration Methods 11 Handling of Calibration Items 12 Records 13 Certificates and Reports 14 Subcontracting of Calibration 15 Outside Support Services and Suppliers 16 Complaints 17 Interrange Comparison Programs 18 Data Processing Procedures 19 Range-Specific Uncertainty Analysis 20 Ongoing Research, Planned Improvements Range Book 1 Introduction and Endorsement This section should contain a brief statement of compliance with the standards set by this document in accordance with ISO, a policy statement regarding the organization's commitment to continuous quality improvement. 2 References This section should list the documents cited in the range book. 3 Glossary This section should list and define the specialized terms and acronyms used in the range book. 4 Organization and Management This section should fully define the organization that is chartered to operate the range. 5 Quality System, Audit, and Review The purpose of this section is to ensure that a complete overview of the company quality system and self-auditing process is provided. This section should contain procedures for the control and maintenance of documentation, arrangements for ensuring that the range reviews all new work to ensure that it has the appropriate facilities and resources before commencing such work, management arrangements for permitting departures from documented policies and procedures or from standard specifications, procedures for protecting confidentiality and proprietary rights, procedures for audit and review, procedures for a management quality audit at least once a year, procedures for documentation of all audit processes, and procedures for tracking and monitoring the quality of the primary RCS calibration data. The RCS Certification Review Committee s Certification Report should be attached to 5 upon completion of the certification review process. 37

38 Range Book II 6 Personnel This section should contain the qualifications of all personnel assigned to the range. Specialized training, training certificates. Company or resume format may be used. If an organization, due to personnel policy or union concerns, treats their records as company sensitive the range book should so state, and indicate what organization has these records, and who has the right to review and update. If the reviewers are not allowed to see these records, the range technical manager may certify that such records exist. 7 Accommodation and Environment This section should document the range environment. Some environmental variables that could affect radar cross section measurements include: electromagnetic interference, temperature variations, mechanical vibrations, and seasonal variations in vegetation, sea state, moisture levels, winds, precipitation, etc. 8 Equipment and Reference Materials Configuration control is essential for maintaining a repeatable, quality RCS system. This section of the range book may be one of the most significant. Here all electromechanical equipment that makes up the RCS range should be identified. References to appropriate equipment manuals should be given in sufficient detail so that a qualified technician can trace the major subsystems of the range. Maintenance and calibration histories should be kept for critical components, and complete system configuration should be tracked. The procedures for identifying equipment, maintenance, and configuration control should be documented here. Personnel responsible for following these procedures should be clearly identified. Range Book III 9 Measurement Traceability and Calibration This section should complement Section 8 as it specifically relates to primary RCS calibration. The range will document its calibration program. Equipment and system calibration intervals should be clearly established. Traceability of system components to a national standard should be maintained. Calibration certificates and reports should be referenced. System configuration used in RCS calibrations should be tracked The system configuration used in interrange comparisons should be clearly documented together with the results of calibration comparisons. 10 Calibration Methods Both measurement and calibration procedures should be fully documented. Separate documentation should be available for different procedures (i.e. wideband, monostatic and bistatic). 11 Handling of Calibration Items This section should summarize how primary calibration items are protected during storage, handling, and use. 12 Records This section describes the range's procedures for creating and maintaining records of all aspects of range operations. Procedures for maintaining calibration records, measurement records, system configuration, etc. are especially important. Procedures to record interrange comparison results should be clearly defined. 38

39 Range Book IV 13 Certificates and Reports This section should define the standard reporting formats used by the ranges when reporting RCS measurements to customers. 14 Subcontracting of Calibration This section details aspects of the RCS calibration process that are subcontracted. All subcontracted work must satisfy the standards set by this document in accordance with ISO 15 Outside Support Services and Suppliers : This section should document any services or suppliers used to produce calibrated RCS data. This section complements Section Complaints This section should document the formal complaint procedure used to resolve disputes between the range and the customer. 17 Interrange Comparison Programs This section shall include the company policy regarding interrange comparisons. Records of participation in calibration interrange comparisons, the results obtained, and conclusions drawn from such studies are to be presented or referenced here. 18 Data Processing Procedures Important details of processing measurement and calibration data should be documented here. Range Book V 19 Range-Specific Uncertainty Analysis Policies and procedures for establishing range uncertainties need to be completely documented or referenced here. A sample uncertainty table together with system parameters or other adequate uncertainty method should be linked or available in this section. Any system parameters should be displayed here. Any scientifically-based uncertainty analysis is acceptable, providing all appropriate assumptions and/or exclusions regarding the parameters comprising the analysis are clearly documented. 20 Ongoing Research, Planned Improvements The purpose of this section is to briefly summarize the three-year look ahead regarding any range research and/or other activities designed to improve range data quality, efficiency, repeatability, or traceability. A list of desirable research areas to be conducted in the future to improve specific and known deficiencies on the range or in the RCS industry should also be provided, as well as a single top-level roadmap. Ongoing plans for future upgrades of equipment should be included. Such research information will allow customers to quickly identify and evaluate ongoing range improvements in the context of their current or planned use of the range 39

40 Conclusion Basic notion of APM and RCS Basic understanding of RF absorber Basic understanding of the treatment on a APM rectangular range Basic understanding of the treatment on a RCS compact range The limitations of a mixed range Some important points to consider when designing a range. Approach for Range Certification About the Presenter Vince Rodriguez. Attended The University of Mississippi Ole Miss, in Oxford MS, where he obtained his B.S.E.E. M.S. and Ph.D. in 1994, 96 and 99 respectively. On August 1999 Dr. Rodriguez was visiting assistant professor of Electrical Engineering at Texas A&M University-Kingsville. In June 2000 Dr. Rodriguez joined EMC Test Systems (now ETS-Lindgren) as an RF and Electromagnetics engineer, During that period he was involved in the design and was principal engineer in several large chamber projects including the taper chamber at NRL in Washington and the satelliteautomotive-emc chamber at the INPE in Brazil. In September 2004 Dr. Rodriguez took over the position of Senior Principal Antenna Design Engineer, placing him in charge of the development of new antennas for different applications and on improving the existing antenna line. In addition to that in 2006 Dr. Rodriguez took over the duties of Antenna Product Manager placing him in charge of development, marketing and maintenance of the antenna product line. Dr. Rodriguez is the author of more than fifty publications including journal and conference papers as well as book chapters. Dr. Rodriguez holds patents for hybrid absorber and for a new dual ridge horn antenna. Dr. Rodriguez is a Senior Member of the IEEE and several of its technical societies including the AP, MTT and the EMC societies. He is a Senior Member of the Antenna Measurements Techniques Association (AMTA). Dr. Rodriguez is an active member of the Applied Computational Electromagnetic Society (ACES). He is an Associate Editor of the ACES Journal and has served as a reviewer for the ACES Journal and for the Journal of Electromagnetic Waves and Applications. He has chaired sessions and workshops at ACES, AMTA, ATMS and IEEE symposia Dr. Rodriguez is a Full member of the Sigma Xi Scientific Research Society and of the Eta Kappa Nu Honor Society. 40