Mathematical Absorber Reflection Suppression (MARS) for ...

Mathematical Absorber Reflection Suppression(MARS)for Anechoic Chamber Evaluation &ImprovementAMTA 200820-Nov-2008Greg Hindman & Allen C. NewellNearfield Systems Inc.19730 Magellan DriveTorrance, CA 905021

Outline• Review Chamber Accuracy Evaluation Methods• Mathematical Absorber Reflection Suppression (MARS)• MARS Usage with 0.5 to 18 GHz Broadband Log Periodicantenna for Chamber Reflection Evaluation• MARS Usage with 2.6 – 3.95 GHz Open Ended Waveguide(OEWG) Probe for Chamber Reflection Evaluation• MARS Results with 0.49 to 0.75 GHz SGH in Open Test Labwith no Absorber!• Summary2

NSI‟s New SNF Chamber• NSI-700S-75 SNF Scanner• 24‟x24‟x20‟ Chamber with36” pyramidal absorber• Tested with NSI-RF-DLP-03(0.5-18 GHz) Dual-Pol LogPeriodic Antennas as AUTand probe• MARS Analysis Used3

Range Accuracy Evaluation Methods• Against independent measurements of antenna in other testsystems• Best approach• Time consuming, costly• Simple near-field reflection test• Compare theta cut with AUT at 0 deg vs 180 deg• Easy to do, quick look at reflection levels• Chamber reflection test with AUT offset in Z direction• Full 2D NF tests with AUT offset by ¼ lambda• Time consuming, results affected by AUTremoval/realignment• NSI 180 phi vs. 360 phi vs redundant comparison test• Full 2D NF test with AUT on two measurement spheres• Easy to do, automatic processing, no AUT remounting needed4

Amplitude (dB)Plot Subtraction Explained• Converts measured patterns from powerinto voltage• Performs subtraction in pattern angles ofinterest• Residual error plot is the error level thatwould cause the observed patternchangesFar-field amplitude of WR137SGH020.nsi - Far-field amplitude of WR137SGH019.nsi0Plot 1 Plot 2 Plot 1 - Plot 2-10-20Pattern difference-30-40-50Measured signal1 st configMeasured signal2 nd config-60-70-75 -50 -25 0 25 50 75Azimuth (deg)5

Amplitude (dB)Simple Reflection Test in Chamber• A simple reflection test involvescomparing 0° and 180° Phi cuts• For a correctly alignedmeasurement system, thedifferences in the patterns aredue to the range reflections• Subtraction of the two patternsgives a preliminary estimate ofexpected error signal level• Chamber test result at 10 GHzon the DLP• Shows relatively good result ofabout -40 dB error level0-5-10-15-20-25-30-35-40-45-50-55-60Scattering Estimated for File TECOM_509H_TECOM 495H_103.NSIComparing 0 and 180 Phi Cuts at Freq = 10 GHzPhi=0 Phi=180 SCAT. SIG-150 -100 -50 0 50 100 150Theta scan with AUT Phi = 0°Theta scan with AUT Phi = 180°6

Simple Chamber Reflection Test• Theta cut with AUTphi = 0 deg from – to +• Theta cut with AUTphi = 180 deg from + to-7

Chamber Reflection with AUT OffsetProbeProbe• AUT offset using fixed length standoffs• Requires AUT removal, realignmentAUT positionchange in ZAUT (SGH)8

Typical AUT Offset Test ResultWR SizeFrequencyBand(GHz)ChamberReflection(dB)WR-650 1.12 – 1.7 -32 dBWR-430 1.7 – 2.6 -40 dBWR-284 2.6 – 3.95 -42 dBWR-187 3.95 – 5.85 -40 dBWR-137 5.85 – 8.2 -46 dBWR-90 8.2 – 12.4 -40 dBWR-62 12.4 - 18 -44 dBWR-42 18 – 26.5 -42 dBWR-28 26.5 - 40 -38 dBUsing AUT offset test method9

Self-Comparison Measurements• Self-comparison technique often used to evaluate the effectsof error sources on the measurement results• Useful to establish a „truth model‟, or our best estimate ofthe „correct‟ measurement result• Use this for comparison to results derived with compromises inaccuracy in favor of reduced test time. For this, we use threesteps:1. take the data with minimal or controlled position errors (stop motion orslow scanning)2. average out range reflection and alignment errors with a redundantscanning technique3. apply NSI‟s Mathematical Absorber Reflection Suppression (MARS)technique• These three steps will help to derive the „true‟ antenna radiationpattern for use in subsequent self-comparison measurements toderive the effect of error sources.10

360 phi vs. 180 phi SNF ScanningAUT points to + θ and – θ sides of chamberTwo different scanninggeometries – both give acomplete AUT data setAUT points to only + θ sides of chamber11

Redundant dataComparison Method Description+ =Best Estimate of“Truth Model”Best Estimate of“Truth Model” - =Chamber ReflectionsBest Estimate of- + = “Truth Model”Chamber Reflectionswith MARS12

MARS Processing• MARS uses the standard NIST Spherical Near-field to Far-FieldTransformation Algorithm• NIST algorithm uses a mode filtering technique• Mode cutoff is based on the fact that modes above a certainindex number are exponentially attenuated and not detected bythe probe.• Mode cutoff is determined by the physical dimensions of the AUT• NSI‟s MARS technique applies a mathematical position shift to theAUT measured data during processing to optimize the way thespherical transform filtering is done.13

Mode Amplitude in dBMARS Mode AnalysisSpherical Modes for M=1• Spherical near-field to far-fieldtransformation involves sortingmeasured energy into sphericalmodes• Higher order modes are caused byreflections and can be filtered outif identified to be outside thephysical boundary of the antenna• Plot at right shows• Normal SNF modes• Modes after phase center trans.• Mode cutoff chosen based onphysical aperture size limit0-5-10-15-20-25-30-35-40-45-50-55-60-65-70Meas Trans maxmodes = 610 20 30 40 50 60Mode N Number14

Key MARS Requirements• Over sampling the data in Theta and Phi helps the process• Offsetting antenna phase center helps the process• Need to estimate or determine the phase center of the AUT• Not required for MARS• Knowledge of the AUT pattern• Knowledge of the test range configuration• Not frequency limited15

DLP Results With MARS ON/OFFReflection level at 1, 10, 17 GHz16

dBError Level & Improvement for DLPError Level Improvment with MARSError level Error level with MARS MARS Improvement30.020.010.00.0-10.0-20.0-30.0-40.0-50.0-60.0MARS ImprovementError Level0 5 10 15 20Frequency (GHz)17

WR-284 OEWG ResultReflection level at 2.5, 3.2, 4 GHz18

dBError Level Improvement for WR-284 OEWG20.0Error Level ImprovementError level Error level with MARS MARS Improvement0.0MARS Improvement-20.0-40.0-60.0Error Level2.500 2.750 3.000 3.250 3.500 3.750 4.000Freq (GHz)19

Open Lab Test Results – WR-1500 SGH• Open assemblylab at NSI• No absorber!• Good resultsachieved withMARSprocessing20

Amplitude (dB)Amplitude (dB)Open Lab Test Results – WR-1500 SGHFar-field amplitude of NSI-RF-SG1500_on_NSI-700S-95 SNF in lab_009.NSIFar-field amplitude of NSI-RF-SG1500_on_NSI-700S-95 SNF in lab_009.NSI00.49 GHz 0.56 GHz 0.63 GHz 0.70 GHz 0.75 GHz00.49 GHz 0.56 GHz 0.63 GHz 0.70 GHz 0.75 GHz-5-5-10-10-15-15-20-20-25-25-30-30-35-35-40-40-45-45-50-50-55-55-60-80 -60 -40 -20 0 20 40 60 80Azimuth (deg)-60-80 -60 -40 -20 0 20 40 60 80Azimuth (deg)21

Directivity(dBi)Directivity(dBi)Open Lab Test Results – WR-1500 SGHDirectivity vs. Frequency of C:\i-drive\NSI97\data\NSI-RF-SG1500\on 700S-95 NSF in open lab at NSI\NSI-RF-SG1500_on_NSI-700S-95 SNF in lab_009.NSIWorst-case difference 1.33 dB; average 0.76 dBDirectivity vs. Frequency of C:\i-drive\NSI97\data\NSI-RF-SG1500\on 700S-95 NSF in open lab at NSI\NSI-RF-SG1500_on_NSI-700S-95 SNF in lab_009.NSIMARS correction ON - Worst-case difference 0.66 dB; average 0.30 dB18.0Measured DirectivityCalculated NRL Directivity18.0Measured DirectivityCalculated NRL Directivity17.517.517.017.016.516.016.515.516.015.015.514.514.013.513.012.515.01.3 dB p-p 0.7 dB p-p14.514.013.512.00.50 0.55 0.60 0.65 0.70 0.75Frequency (GHz)13.00.50 0.55 0.60 0.65 0.70 0.75Frequency (GHz)22

Open Lab Test Results – WR-1500 SGHReflection level at 0.49, 0.63, 0.75 GHz23

dBOpen Lab Test Results – WR-1500 SGHError Level vs FreqError level Error level with MARS MARS Improvement20.010.00.0-10.0-20.0-30.0-40.0Error Level0.450 0.500 0.550 0.600 0.650 0.700 0.750Freq (GHz)MARS Improvement24

MARS Improvement SummaryAntenna AverageErrorLevelAverageError Levelw/MARSMARSimprovementFrequencyAntenna / Chamberconfiguration1 -30 dB -47 dB 17 dB 0.5 - 18 GHz Dual LP antenna in newNSI chamber with36” pyramidalabsorber2 -41 dB -52 dB 11 dB 2.6 – 3.95 GHz WR-284 OEWG in newNSI chamber with36” pyramidalabsorber3 -11 dB -23 dB 12 dB 0.49 – 0.75 GHz WR-1500 SGH in openlab environment withno absorber25

Summary• New Chamber at NSI characterized for reflection level from0.5 to 18 GHz• Used relatively simple comparison technique and MARSreflection suppression technique to establish chamberreflection level with and without MARS• -30 dB to -40 dB error level for NSI chamber• -40 dB to -50 dB error level improvement with MARS• Showed MARS ability to extend frequency coverage ofchamber26