ElectroScience Lab Compact and Spherical Range Design, Application and Evaluation Walter D. Burnside and Inder J. Gupta The Ohio State University ElectroScience Laboratory 1320 Kinnear Road Columbus, Ohio (614) and (614) Presented on September 21-22, 2005 for Raytheon (Tucson, AZ).

ElectroScience Lab Course Outline Basic Range Design Guidelines (Burnside) Compact Range Reflector Design (Gupta) Absorber Design and Layout (Burnside) Critical Range Evaluation (Gupta) Second Half Day First Full Day R-Card Fences for Outdoor Ranges (Gupta) Summary of Range Design Issues (Burnside)

ElectroScience Lab Walter D. Burnside You Want to Build a New or Refurbish an Existing Chamber: How is That Done Correctly? The Ohio State University ElectroScience Laboratory 1320 Kinnear Road, Columbus, Ohio (TEL) (614) (FAX) (614)

ElectroScience Lab What are the Important Issues? Commercial absorber vendors are not capable of creating state-of-the-art ranges that provide the superior performance required by modern systems. The problem is associated with 50 year old absorber and chamber designs versus modern proven concepts. The finest chamber designs are presently performed at universities. Their significant achievements have not been successfully transferred to industry, even though numerous papers and presentations have been published that describe their important findings. The major contributor to this problem has been that the RF industry, which has relied on traditional absorber companies to provide the specified performance. Thus, they have not been forced to change or update their practices, which require investments that they do not want to make.

ElectroScience Lab The RF Industry has Created RFQ’s that Require Improved Performance - How is that Performance Achieved or is it? Along with the lack of modern chamber design concepts, commercial vendors use antiquated evaluation methods. Thus, they respond to RFQ’s with traditional evaluation tests that do not have the dynamic range to expose their failure to meet the specifications. The RF industry accepts these inferior evaluation approaches because they appear to be following accepted practices. “That is the way we have always done it”. Industry must require proper evaluation methods that can provide the dynamic range necessary to verify that the desired performance has been achieved. This requires the most advanced testing methods possible.

ElectroScience Lab How Does One Overcome This Very Unfortunate Situation? 1) One must first determine what it is that they want measured and what are the requirements for that measurement, insuring that the full radiating or scattering system performance is being truly and accurately evaluated in this new or modified chamber. 2) The chamber design should begin by examining what type of chamber and dimensions are needed by simply using absorber but including changing the normal doping concentrations and using modern curved shapes and applying Chebyshev layouts. 3) During the absorber design phase, the chamber dimensions must be examined to determine if it has become excessive because the absorber only works for normal or near normal incidence. 4) If the chamber dimensions were increased significantly to avoid grazing or near grazing incidence, R-Card fences should be designed to reduce the chamber size and enhance performance. 5) Once the chamber has been built or modified, it must be probed and the resulting data processed using modern transform techniques to ascertain whether or not the desired performance has been achieved. If not, the source of the errors must be identified and corrected or at least better understood so that it does not happen again. This design approach was used to modify and evaluate SIMFAX. Thus, it will be used in the following slides to illustrate these principles.

ElectroScience Lab Anechoic ChamberCurrent CapabilityUpgrade Requirement Chamber Length 48 ft. shield, 40 ft. anechoic Chamber Width x Height 32 feet x 32 feet Spherical Array Dish37.67 ft. radius, 19 ft. chord Array Antenna Spacing 2 ft. (3 deg.) nominal, 1 ft. (1.5 deg.) high resolution Quiet Zone 6 ft. flight table 6’x6’ base, 8’ into chamber Quiet zone Reflectivity Performance (dB) 4 GHz 8 GHz >10 GHz 2 GHz 4 GHz 9-18 GHz 35 GHz Specular Zone Absorber 36 inch pyramidal TBD Walls, ceiling, floor absorber (non- specular) 8 inch wedge Array (TX) wall absorber 5 inch pyramidal Flight Table (RX) wall 8 inch pyramidal SIMFAX Chamber Parameters

ElectroScience Lab Avery Davis at Raytheon did a Very Careful System Performance Study and Found these Chamber Stray Signal Level Requirements Frequency Stray Signal Level Versus Desired Illumination 2 GHz 4 GHz 9-18 GHz 35 GHz -45 dB -50 dB -55 dB -50 dB The absorber design must achieve these stray signal levels within this existing chamber. This must be true for all the SIMFAX antennas, including the critical one’s near the sidewalls.

ElectroScience Lab SIMFAX Sidewall Illumination is Very Close to Grazing Incidence for First Few Antennas. Absorber on Sidewall Quiet Zone Sidewall Scattering Desired Illumination Normal pyramidal absorber can not be used for this illumination angle of incidence, which will be shown in the next few slides.

ElectroScience Lab Bistatic Reflection Coefficients for 8 in Straight Pyramid Absorber FREQUENCY=500MHz FREQUENCY=1000MHz FREQUENCY=2000MHz FREQUENCY=4000MHz Note that the reflection levels approach unity as the angle of incidence approaches grazing (180 degrees). At 2 GHz, 8” pyramidal absorber can not be used even for normal incidence. -45dB stray signal level

ElectroScience Lab Bistatic Reflection Coefficients for 12 in Straight Pyramid Absorber FREQUENCY=500MHz FREQUENCY=1000MHz FREQUENCY=2000MHz FREQUENCY=4000MHz Note that the reflection levels approach unity as the angle of incidence approaches grazing (180 degrees). At 2 GHz, 12” pyramidal absorber could be used directly for angles incidence up to 35 degrees (70 degrees in plot). -45dB stray signal level

ElectroScience Lab Bistatic Reflection Coefficients for 18 in Straight Pyramid Absorber FREQUENCY=500MHz FREQUENCY=1000MHz FREQUENCY=2000MHz FREQUENCY=4000MHz Note that the reflection levels approach unity as the angle of incidence approaches grazing (180 degrees). At 2 GHz, 18” pyramidal absorber could be used directly for angles incidence up to 53 degrees (106 degrees in plot). -45dB stray signal level

ElectroScience Lab Pyramidal Absorber Issues and Limitations The angle of incidence on absorber is relative to the plane containing the absorber tips. Thus, thicker absorber has to deal with wider illumination angles. For the first several antennas, normal pyramidal absorber can not used because of its poor wide angle performance. Since pyramidal absorber was designed for near normal incidence, one can change the doping to examine its impact for wide angle illumination. The Chebyshev layout approach and curved pyramids can also be considered to improve the wide angle performance. All of these potential options were considered for SIMFAX. They are not options that are normally considered by absorber and chamber designers.

ElectroScience Lab Pyramidal Absorber with 4-Step Chebyshev Along Cross Range and Uniform Along Down Range Cross Range Down Range Pyramid

ElectroScience Lab Enhanced Reflection Coefficient Performance Using Chebyshev Absorber Layout 4-Layer Chebyshev 7-Layer Chebyshev The Chebyshev layout can be applied using any absorber type. The improvement shown here is added to the performance of that material.

ElectroScience Lab Rantec 8” Pyramidal Absorber with 18CL Doping Without Chebyshev Layout With Chebyshev Layout The doping and Chebyshev layout have greatly improved the stray signal performance but not sufficient to used alone for the first four antennas.

ElectroScience Lab Rantec 12” Pyramidal Absorber with 18CL Doping Without Chebyshev Layout With Chebyshev Layout The doping and Chebyshev layout have greatly improved the stray signal performance but not sufficient to used alone for the first four antennas.

ElectroScience Lab Rantec 18” Pyramidal Absorber with 24CL Doping Without Chebyshev Layout With Chebyshev Layout The doping and Chebyshev layout have greatly improved the stray signal performance but not sufficient to used alone for the first four antennas.

ElectroScience Lab Absorber Alone will not Meet SIMFAX Stray Signal Performance so Tapered R-Card Fences must be Used The R-Card Fences block the near grazing signals and reflect that energy back toward the transmit wall. The fences do not diffract significantly. Note that there is one fence for each of the four antennas nearest the sidewall.

ElectroScience Lab 3D View of SIMFAX Chamber Sidewall Chamber Wall 18”+3.6” Chebyshev Pyramid 12”+3.6” Chebyshev Pyramid 8”+3.6” Chebyshev Pyramid Test Zone Tapered R-Card Fences

ElectroScience Lab Tapered R-Card Fence Resistive Film Layups Fence #1 Fence #2 Fence #3 Fence #4 The tapered R-Card fences have a triangular shape to reduce the small diffraction from the outer edge of the fence. These thin film layups are held in place and mounted on the wall using light weight foam sandwich structures.

ElectroScience Lab 18” Pyramid with 4-Step Chebyshev 12” Pyramid with 4-Step Chebyshev 8” Pyramid with 4-Step Chebyshev Final Chebyshev Pyramidal Absorber Layout Located Between Each of the Four R-Card Fences Note that the R-Card fences and absorber are fully integrated together to completely cover the sidewalls.

ElectroScience Lab Probe Measurements of SIMFAX Chamber Horizontal Scan at Various Fixed Frequencies This probe data was collected across a horizontal scan through the center of the test zone. The data was collected every inch from -40” to 40”. The position data at fixed frequency can then be transformed to the angle domain which indicates the source of chamber stray signals. The following slides show the angle domain data on the top plot and raw probe data on the bottom one. The angle domain results will show that the side wall scattering is minimal. Finally, the angle domain results will show the angular scan direction associated with each of the feeds mounted on the source wall.

ElectroScience Lab Pre-Probe Measurements of SIMFAX Chamber Antenna #L7 Horizontal Scan at 2 GHz Horizontal PolarizationVertical Polarization

ElectroScience Lab Pre-Probe Measurements of SIMFAX Chamber Antenna #L7 Horizontal Scan at 2.8 GHz Horizontal PolarizationVertical Polarization

ElectroScience Lab Pre-Probe Measurements of SIMFAX Chamber Antenna #L7 Horizontal Scan at 3.6 GHz Horizontal PolarizationVertical Polarization

ElectroScience Lab Pre-Probe Measurements of SIMFAX Chamber Antenna #L7 Horizontal Scan at 4.4 GHz Horizontal PolarizationVertical Polarization

ElectroScience Lab Post-Probe Measurements of SIMFAX Chamber Antenna #L7 Horizontal Scan at 2 GHz Horizontal PolarizationVertical Polarization Minimal side wall scattering

ElectroScience Lab Post-Probe Measurements of SIMFAX Chamber Antenna #L7 Horizontal Scan at 2.8 GHz Horizontal PolarizationVertical Polarization Minimal side wall scattering

ElectroScience Lab Post-Probe Measurements of SIMFAX Chamber Antenna #L7 Horizontal Scan at 3.6 GHz Horizontal PolarizationVertical Polarization Minimal side wall scattering

ElectroScience Lab Post-Probe Measurements of SIMFAX Chamber Antenna #L7 Horizontal Scan at 4.4 GHz Horizontal PolarizationVertical Polarization Minimal side wall scattering

ElectroScience Lab Post-Probe Measurements of SIMFAX Chamber Higher Resolution 2-18GHz Probe Data In order to resolve scattering in the horizontal plane which includes the other feeds and R-Cards, only horizontal scan data was collected for this high resolution study. The data was collected every inch from -40” to +40” and from 2 to 18 GHz every 80 MHz. With an 80 MHz frequency step size, the unaliased down range time window is 12.5 nanoseconds. It will be shown that this resolution will not be sufficient to resolve the undesired stray signal time delays as will be seen in the following TDOA results.

ElectroScience Lab Post-Probe Measurements of SIMFAX Chamber Raw Probe Data for L7 Antenna Horizontal Polarization Vertical Polarization

ElectroScience Lab Post-Probe Measurements of SIMFAX Chamber Direction-of-Arrival for L7 Antenna Horizontal Polarization Vertical Polarization Small Error

ElectroScience Lab Post-Probe Measurements of SIMFAX Chamber Time and Direction-of-Arrival for L7 Antenna Horizontal Polarization Vertical Polarization

ElectroScience Lab Post-Probe Measurements of SIMFAX Chamber Higher Resolution 2-6 GHz Probe Data In order to resolve scattering in the horizontal plane which includes the other feeds and R-Cards, only horizontal scan data was collected for this high resolution study. The data was collected every inch from -40” to +40” and from 2 to 6 GHz every 20 Mhz. The error term is more significant in the 2-6 GHz frequency band. With an 20 MHz frequency step size, the unaliased down range is 50 nanoseconds. This resolution will be sufficient to resolve the undesired stray signal time delays as will be seen in the following TDOA results.

ElectroScience Lab Post-Probe Measurements of SIMFAX Chamber Raw Probe Data for L7 Antenna Horizontal Polarization Vertical Polarization

ElectroScience Lab Post-Probe Measurements of SIMFAX Chamber Direction-of-Arrival for L7 Antenna Horizontal Polarization Vertical Polarization Small Error

ElectroScience Lab Post-Probe Measurements of SIMFAX Chamber Time and Direction-of-Arrival for L7 Antenna Horizontal Polarization Vertical Polarization Small Error

ElectroScience Lab Small Error Terms Caused by Scattering of the R-Card Fences Followed by Scattering from the Transmit Wall

ElectroScience Lab Post-Probe Measurements of SIMFAX Chamber Raw Probe Data for R7 Antenna Horizontal Polarization Vertical Polarization

ElectroScience Lab Post-Probe Measurements of SIMFAX Chamber Direction-of-Arrival for R7 Antenna Horizontal Polarization Vertical Polarization VeryS mallE rror

ElectroScience Lab Post-Probe Measurements of SIMFAX Chamber Time and Direction-of-Arrival for R7 Antenna Horizontal Polarization Vertical Polarization VeryS mallE rror

ElectroScience Lab Final Post-Probing Conclusions The post-probe results look very good in terms of the data collection, even for the high resolution scans. Also, the post-probe results look very good in terms of performance for all antennas tested even for vertical polarization. As stated earlier, the L7 and R7 have a small amount of attenuation near the sidewall caused by the R-Cards which was expected. Nevertheless, the beam is clearly positioned toward the antenna. One should note that the processed beam results show that the sidewall scattering is extremely small. The small remaining scattering occurs as a result of R-Card scattering bouncing off the source wall as shown by the high resolution data. This small scattering term is noticeable from 2-6 GHz but becomes very small above 6 GHz. In summary, Raytheon should be very pleased to see that the major error source in SIMFAX has been removed that being the sidewall scattering. The remaining errors are very small and result from interaction between the R-Card fences and the transmit wall scattering.