1. ElectroScience Laboratory 1 Development of a Hemispherical Near Field Range with a Realistic Ground – Part 3 E. Walton 1, T-H Lee 1, G. Paynter 1, C. Buxton 2 and J. Snow 3 1 The Ohio State University 2 FBI Academy 3 Naval Surface Weapons Ctr., Crane Div. 2006 AMTA Meeting; Austin TX. © ERIC WALTON, 10/2006

2. ElectroScience Laboratory 2 NF RANGE WITH REALISTIC GROUND THIS IS PART THREE OF A CONTINUING SAGA OF PEOPLE VS. MOTHER NATURE

3. ElectroScience Laboratory 3 SUMMARY / INTRODUCTION BUILD A NEAR FIELD ANTENNA MEASUREMENT RANGE The Ohio State University ElectroScience Laboratory The Naval Surf. Weap. Center; Crane Div. The FBI Academy OPTIMIZED FOR GROUND VEHICLES Hemispherical scanning system Over a realistic roadway/ground surface. THE CHAMBER 12.2 m high by 17.7 m wide by 21.3 m long Absorber covered walls and ceiling Concrete floor over damp sand pit VHF to S-band. NF RANGE WITH REALISTIC GROUND

4. ElectroScience Laboratory 4   NF RANGE WITH REALISTIC GROUND Normal spherical mode expansion techniques * will not work in such an environment. So … A plane wave synthesis algorithm will be used along with an “outside the sphere” ground reflection term. H-FRAME (no turntable) Probe corrected near-field scanning on a spherical surface was first solved in 1970 by Jensen in a doctoral dissertation at Technical University of Denmark. Much of the history of near field scanning and transformation development is given in a 1988 special issue of the IEEE AP-S Transactions (V. 36, No. 6, June 1988).

5. ElectroScience Laboratory 5 GROUND REFLECTIONS IN NF MEASUREMENTS Transformation Software The classical method of transforming from the near field to the far field consists of taking advantage of the efficiency of the Fourier transform. The data are transformed into a spectrum of plane waves in the geometrical system to be used. plane wave spectral components; cylindrical wave components spherical waves But we have a problem because we can only scan the upper hemisphere and the ground surface is penetrable. YR-1 Radius = 4 m; Freq. = 0.7 GHz

6. ElectroScience Laboratory 6 PLANE WAVE SYNTHESIS AUT Synthesized Wavefront Radiating elements Surface of ground Individual spatial displacements Synthesized below- ground elements (green) Sketch of plane wave synthesis geometry.

7. ElectroScience Laboratory 7 Radius = 3 m; Freq. = 0.7 GHz NF RANGE WITH REALISTIC GROUND YR-1 Early results, note various mechanisms. YR-1

8. ElectroScience Laboratory 8 STATUS –YEAR 1 AMTA 04 We developed a NF to FF algorithm that separately computes the direct signal, the ground reflected signal and the sum signal. We consider external ground reflections to obtain accurate FF patterns from NF probe data. We must make assumptions about the ground reflection coefficient in order to compute the FF patterns (of course this is in the case where there is significant ground reflection outside the domain of the probe hemisphere)

9. ElectroScience Laboratory 9 2005 WORK Complete the NF to FF algorithm development for the omnidirectional probe data in order to explore the behavior of the algorithm Include probe correction in the algorithm development work. Include full polarization development work in the algorithm development work. Begin the deliverable software development with an initial transition of the algorithm to the C++ programming. NF RANGE WITH REALISTIC GROUND YR-2 YR-2

10. ElectroScience Laboratory 10 ARM INTERACTION 3-element x-directed dipole array located 28’ above the ground plane at 150 MHz. Phi=0 (x-z) plane cut. Studies involved various probe types and arm shapes. Spurious signals can be reduced to better than 25 dB below the direct signals even at the lowest frequencies. Performance is better at the higher frequencies. 3 ele. array NF RANGE WITH REALISTIC GROUND YR-2 DAMP SAND IS VERY LOSSY:  BOREHOLE DATA YR-2

11. ElectroScience Laboratory 11 EXAMPLE RESULTS Consider a single horizontal dipole.  oriented in the x-direction  1.2 feet (0.366 meters) above a lossy dielectric half space. relative permittivity = 2.75 loss tangent = tan (δ) = 0.042. frequency of operation = 500 MHz (wavelength = 60 cm) probe hemisphere radius is 12 feet (3.66 meters). The raw probe data was synthesized using a geometrical theory of diffraction computer code written by Dr. Ron Marhafka at the ESL (called NEC-BSC). The near field to far field transformation was written in MATLAB. PLANE WAVE SYNTHESIS YR-2

12. ElectroScience Laboratory 12 (a) (b) (c) Result of transformation to the far field; E-theta and E-phi vs. Theta (a) Phi = 0 deg.; Phi = 45 deg., Phi = 90 deg.) φ=0º φ=90º φ=45º YR-2

13. ElectroScience Laboratory 13 CRANEBENCH (C++ GUI by Dr. Frank Paynter) Crane DDAS Workbench User Interface YR-2

14. ElectroScience Laboratory 14 INTERESTING EXAMPLE (probe data) E-theta E-phi E-r H-dipole; 1.2 ft. above realistic gnd; 7 ft. offset in x direction 12 ft. radius scanner; 500 MHz; (note non-zero r-component) YR-2

15. ElectroScience Laboratory 15 INTERESTING EXAMPLE NOTE THE RECOVERED SYMMETRY RESULT OF TRANSFORMATION YR-2

16. ElectroScience Laboratory 16 INTERESTING EXAMPLE E-thetaE-phi RESULT OF TRANSFORMATION YR-2

17. ElectroScience Laboratory 17 NF RANGE WITH REALISTIC GROUND –YR 3 IT IS COMMON TO BUILD A SCANNER THAT SCANS IN EQUAL INCREMENTS OF THETA AND PHI IF THIS IS DONE, THE DATA IS NOT PRESENTED IN EQUAL INCREMENTS OF ANGLE SPACE (NOT IN EQUAL STERRADIAN “PIXELS”) THE DATA MUST THUS BE COMPENSATED BEFORE BEING PASSED TO CRANEBENCH PS: NOTE THAT WE DON’T RECOMMEND THIS PRACTICE BECAUSE MUCH MORE DATA THAN NECESSARY WILL BE COLLECTED, AND THUS MUCH MORE SCANNING TIME (I.E.: $$$) WILL BE NEEDED.

18. ElectroScience Laboratory 18 Data point and associated Sterradian area Two measurement points representing half the associated area each Conservation of energy requires that the power per unit area (Sterradian) must be the same in both cases. So we let SAME AREA DIFFERENT # POINTS YR-3

19. ElectroScience Laboratory 19 Lets look at probe data where sterradian area compensation has been done UNCOMPENSATED THETA CUTS COMPENSATED THETA CUT EVERY 45 DEG. AZIMUTH ONLY 0 DEG. AZIMUTH NF RANGE WITH REALISTIC GROUND - YR-3

20. ElectroScience Laboratory 20 1 m DIAMETER DISK 5 CM THICK 2.338 GHz MONOPOLE ANTENNA UNDER TEST We obtained “real” data from: Hemispherical range with 5.8 m Radius Arch Absorber floor EDGE DIFFRACTION IS VERY STRONG 1.The scan angle is every 1 degree on the elevation plane. Absorbers sat on the table during the measurement. Axis of rotation is centered with plumb bob. 2. Physical radius from the outer edge of the probe antenna to the center of rotation is 168.44 inches (4.278 m). STUDY OF FULL HEMISPHERICAL SCAN OF A VERTICAL MONOPOLE ON A GROUND PLANE DISK

21. ElectroScience Laboratory 21 Use CraneBench to compute a conical Phi cut of the data at Theta = 50 deg. (remember that we do not expect to get exact results until the raw data is compensated because CraneBench expects equal sterriadian pixels and because CraneBench is not gain calibrated.) GROUND REFLECTIONS co-pol x-pol Note that monopole was likely not at the center. NF RANGE WITH REALISTIC GROUND - YR 3

22. ElectroScience Laboratory 22 COMPARE NEC-BSC FF EXACT VS. P-WAVE SYN FF COMPUTATION BASED ON NEC-BSC NF SYNTHESIZED DATA THE NEC-BSC NF DATA WERE DONE ON A SPIRAL CUT BOTTOM LINE; IT WORKS VERY WELL YR-3

23. ElectroScience Laboratory 23 MEASURED MONOPOLE DATA UNCOMPENSATED THETA CUTS COMPENSATED THETA CUT EVERY 45 DEG. AZIMUTH 0 DEG. AZIMUTH YR-3

24. ElectroScience Laboratory 24 Monopole NF-FF study Comparison between NF-FF derived and raw scanner data. Remember: the raw data is near field measurement data and the “derived” result is far field result based on CraneBench transform of that data at R=5.8 m as compensated. YR-3

25. ElectroScience Laboratory 25 COMPARISON theoretical gain of 2.33 GHz monopole on 1-m dia disk compensated Cranebench Far Field result using monopole conical cut data (1 deg increment) NEC YR-3

26. ElectroScience Laboratory 26 Comparison between raw measured scanner data (1 deg.) and spherical mode expansion algorithm result Red = spherical mode expansion of measured data YR-3

27. ElectroScience Laboratory 27 COMPARE OSU PLANE WAVE SYNTHESIS WITH SPHERICAL MODE EXPANSION (INCLUDE NEC-BSC THEORY) Blue = Spherical mode expansion Red = OSU – plane wave expansion Green = NEC-BSC model NEC YR-3

28. ElectroScience Laboratory 28 COMPARE OSU PLANE WAVE SYNTHESIS WITH MI SPHERICAL MODE EXPANSION (INCLUDE NEC-BSC THEORY) OSU @ 4.71 = R, DELTA = 1 DEG. Sph. Mode @ 4.28 = R, DELTA = 1 DEG. F= 2.338 GHZ Blue = NEC-BSC model Red = OSU – plane wave expansion Green = Spherical mode expansion WE DON’T KNOW WHICH ONE IS “BEST” NF RANGE WITH REALISTIC GROUND YR-3

29. ElectroScience Laboratory 29 DOES THE 5.8 VS. THE 4.7 RADIUS MAKE MUCH DIFFERENCE? THE DIFFERENT R VALUES MAKE A DIFFERENCE, BUT IT IS UNCLEAR WHICH ONE IS “BETTER” OSU PLANE WAVE SYNTHESIS OF MEASURED DATA YR-3

30. ElectroScience Laboratory 30 TRANSFORMATION ALGORITHMS: CONCLUSIONS DISAGREEMENT BETWEEN SPH MODE EXPANSION AND THE OSU PLANE WAVE SYNTHESIS RESULTS ARE +/- 1.5 DB OR LESS MEASURED DATA HAS SMALL GROUND REFLECTIONS NOT MODELED IN NEC-BSC THE NEC-BSC THEORY IS NOT QUITE EXACT NEC-BSC MODELS AN INFINITELY THIN DISK (ACTUAL DISK WAS ~5 CM THICK) NEC-BSC DISK IS MADE OF SHORT SEGMENTS, IS NOT A CIRCLE THERE ARE NO GROUND REFLECTIONS IN THIS NEC-BSC FF RESULT IN AREAS OF DISAGREEMENT, WE DON’T KNOW WHICH ONE IS “BEST” THE 1 DEG. DELTA DATA IS VERY CLOSE TO ALIASING AT THE RADIUS USED (BUT THE “MINIMUM SPHERE” IS SMALLER THAN THE PROBED SPHERE) THE RIPPLE (PERHAPS TRUNCATION EFFECTS) IN THE SYNTHESIZED RESULTS CAN BE SUPPRESSED BY FILTERING. IT WOULD BE GOOD TO SEE SOME OTHER DATA IN ORDER TO EXPLORE THE DETAILS OF THE TEST RANGE BEHAVIOR AND COMPARE THE PERFORMANCE OF THE SPHERICAL MODE EXPANSION TECHNIQUE TO THE PLANE WAVE SYNTHESIS TECHNIQUE. (WHO CAN HELP! ANYONE WITH SOME NF SCANNER DATA?)

31. ElectroScience Laboratory 31 NF RANGE WITH REALISTIC GROUND WE WILL ACTUALLY USE 2 PROBE ANTENNAS: LOG PERIODIC (Commercial) Low freq. EDO Corp. AS-48315 (dual polarized) Has been fully characterized DIELECTRIC ROD ANTENNA (in-house design) 1 – 6 GHz Designed at the OSU/ESL by Chi-Chih Chen Build at the OSU/ESL by Chi-Chih Chen Will be characterized fully by end of Oct. 2006 YR-3

32. ElectroScience Laboratory 32 Dielectric Probe Antenna Progress - YR 3 THE 2-LAYER ROD IS PREPARED FOR ANTENNA PATTERN MEASUREMENT. NEW PROBE

33. ElectroScience Laboratory 33 05/21/06 Two-layer-rod, er=6(1”)+er4(2”). Gain measurement in compact range. Dielectric Probe Antenna Progress THE TWO-LAYER ROD WITH THE EXTENDED TIP TESTED IN THE ANTENNA MEASUREMENT CHAMBER. NEW PROBE YR-3

34. ElectroScience Laboratory 34 LOG PERIODIC MEASUREMENT SETUP ESL BLDG Instrumentation antenna Fiberglass pole Nylon Guys Nylon & Steel Deployment Cable 30 ft. 45 feet Counterweight Rotator & tilt base Thrust Bearing 1,200 lb. Brake Winch entire pole rotates driven by the bottom rotator stabilized at the mid-pole thrust bearing. YR-3

35. ElectroScience Laboratory 35 COMMERCIAL LOG PERIODIC INSTRUMENTATION ANTENNA THRUST BEARING AND GUY LINES ROTOR FIBERGLASS POLE WINCH YR-3 LOG PERIODIC MEASUREMENT SETUP

36. ElectroScience Laboratory 36 1.GIVEN CARTESIAN (x-y-z; room based) LASER TRACKING COORDINATES FOR ARM AND TURNTABLE (with respect to encoder readouts) 2.GIVEN PHASE CENTER SHIFT OF PROBE ANTENNA AS A FUNCTION OF FREQUENCY 3.MEASURE RECEIVED SIGNAL AMPLITUDE AND PHASE AS A FUNCTION OF ARM AND TURNTABLE ENCODER OUTPUTS 4.CONVERT ENCODER ANGLE DATA INTO TRUE PROBE ANTENNA COORDINATES WITH RESPECT TO ANTENNA UNDER TEST NOW LETS TALK ABOUT MECHANICAL OFFSET COMPENSATIONS.

37. ElectroScience Laboratory 37 SUMMARY OF PROBLEM ASSUME TURNTABLE DOES NOT “WOBBLE” ON ITS BEARING. (CENTER POINT OF ROTATION AND AXIS OF ROTATION ARE FIXED) WE MAKE NO SUCH ASSUMPTION FOR THE ARM. It may sag and bend. ASSUME TURNTABLE AND ARM LOCATIONS ARE REPEATABLE WITH RESPECT TO ENCODER READOUTS. BUT ASSUME TURNTABLE AND ARM CENTERS OF ROTATION ARE OFFSET FROM ROOM COORDINATE AXIS CENTER. ASSUME AXIS OF ROTATION OF ARM AND TURNTABLE ARE NOT ALIGNED WITH ROOM COORDINATE NOR WITH EACH OTHER. ASSUME AXES OF ROTATION OF ARM AND TURNTABLE DO NOT INTERSECT. ASSUME AXES OF ROTATION OF ARM AND TURNTABLE ARE NOT ORTHOGONAL TO EACH OTHER (NOT AT 90 DEG. ANGLE). ASSUME PHASE CENTER OF PROBE ANTENNA VARIES WITH FREQUENCY. YR-3

38. ElectroScience Laboratory 38 APPROACH TO PROBLEM 1.USE LASER TRACKER TO PROVIDE ROOM-COORDINATES (xyz) OF POINT ON TURNTABLE WITH RESPECT TO ITS ENCODER READOUT 2.USE LASER TRACKER TO PROVIDE ROOM-COORDINATES (xyz) OF TWO (or more) POINTS ON PROBE SUPPORT (points along the probe antenna support extension; under load) 3.FIT FOURIER SERIES TO TRACKS OF TURNTABLE TARGET POINTS AND ARM TARGET POINTS. 4.TRUNCATE FOURIER SERIES EXPANSION TO A SMALL NUMBER (INCLUDING THE DC VALUES) (eliminate higher order terms when insignificant) 5.USE THE FOURIER COEFFICIENTS TO GIVE THE ROOM COORDINATES OF THE PROBE PHASE CENTER AND THE TURNTABLE VECTOR COORDINATES BASED ON THE ENCODER VALUES. AT THIS POINT, WE CAN COMPUTE THE TURNTABLE ROTATION AXIS AND CENTER OFFSET. YR-3

39. ElectroScience Laboratory 39 APPROACH TO PROBLEM FINALLY, FOR EACH POSITION REPORTED BY THE ENCODERS, WE CAN COMPUTE THE TRUE LOCATION OF THE PROBE ANTENNA WITH RESPECT TO THE ANTENNA UNDER TEST YR-3

40. ElectroScience Laboratory 40 WE NOW HAVE ALL COORDINATES IN TURNTABLE (ie: AUT) CENTERED COORDINATE SYSTEM x’ room y’ room z’ room AUT Turntable axis is a cross product: This can all be done in Cartesian coordinates. θ can be computed using a dot product: YR-3

41. ElectroScience Laboratory 41 x room y room z room AUT γ Knowing θ, we use an intermediate angle, γ (as shown) and some spherical trigonometry. YR-3

42. ElectroScience Laboratory 42 CONCLUSIONS SO FAR WE NOW HAVE AN ALGORITHM TO USE TO COMPUTE THE TURNTABLE AND ARM COEFFICIENTS THE COEFFICIENTS CAN BE USED TO FIND x,y,z (room based) POSITIONS THE COEFFICIENTS CAN ALSO BE USED TO FIND THE TURNTABLE AXIS OFFSET AND TILT. ESPECIALLY NOTE THAT WE CAN USE THE COEFFICIENTS TO COMPUTE THE ARM (PROBE ANTENNA) POSITION IN ROOM COORDINATES AS A FUNCTION OF ARM ENCODER OUTPUT. THIS MEANS THAT WE DO NOT NEED TO COMPUTE THE ARM AXIS OFFSET AND TILT. (We only use the xyz (room) coordinates.) FINALLY: WE HAVE AN ALGORITHM TO COMPUTE R, θ AND φ AS ANGLES AND DISTANCE FROM THE ANTENNA UNDER TEST TO THE RANGE PROBE ANTENNA. YR-3

43. ElectroScience Laboratory 43 Z-Axis of T-Table computed using cross product - YR 3 Note the turntable tilt causes the normal axis to “miss” the probe antenna WE ARE NOW MODELING THIS SITUATION

44. ElectroScience Laboratory 44 NF RANGE WITH REALISTIC GROUND - YR 3 MODELING

45. ElectroScience Laboratory 45 NF RANGE WITH REALISTIC GROUND skewed

46. ElectroScience Laboratory 46 NF RANGE WITH REALISTIC GROUND - YR 3 CONCLUSIONS: WE WILL SOON HAVE AN OPERATIONAL HEMISPHERICAL NF RANGE FOR ANTENNAS ON A REALISTIC GROUND WE WILL USE A DIRECT FAR FIELD COMPUTATION WE WILL COMPENSATE THE INPUT DATA FOR: Known reflection coefficient of the ground Scanning in non-uniform solid angle increments Offset in scanning axes; turntable offset and axis angle arm offset, axis angle and sag antenna phase center offset vs. frequency

47. ElectroScience Laboratory 47 HEY ! IS THAT A HEMI ? Dr. Eric K. Walton Dr. Eric K. Walton,The Ohio State Univ. Dr. Teh Hong Lee, The Ohio State Univ. G. Frank Paynter, The Ohio State Univ. Carey Buxton, FBI Academy Jeff Snow, NSWC/Crane