ElectroScience Laboratory 1 E. K. Walton*(1), T-H Lee(1), G. Paynter(1), J. Snow(2), and C. Buxton(3) (1) The Ohio State University, Columbus, OH (2) Naval Surf. Weapons Ctr., Crane Div. (3) FBI Academy, Quantico, VA APS MEETING JUNE 2007 Development of a Hemispherical Near Field Antenna Measurement Range for use on a Realistic Ground

ElectroScience Laboratory 2 SUMMARY / INTRODUCTION BUILD A NEAR FIELD ANTENNA MEASUREMENT RANGE 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

ElectroScience Laboratory 3   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) 12.2 m high by 17.7 m wide by 21.3 m long

ElectroScience Laboratory 4HISTORY: 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). GROUND REFLECTIONS IN NF MEASUREMENTS

ElectroScience Laboratory 5 Radius = 4 m; Freq. = 0.7 GHz NF RANGE WITH REALISTIC GROUND – YR 1

ElectroScience Laboratory 6 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

ElectroScience Laboratory 7 GROUND REFLECTIONS IN NF MEASUREMENTS RAY PATHS PROPAGATION TO FAR FIELD YR-1 RAY PATHS TO USE IN SYNTHESIS … …

ElectroScience Laboratory 8 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.

ElectroScience Laboratory 9 FF P tp tf fp ff delf Ft Pt Ff Pf X Z Y Geometry for the near field to far field transformation PLANE WAVE SYNTHESIS

ElectroScience Laboratory 10 Radius = 3 m; Freq. = 0.7 GHz NF RANGE WITH REALISTIC GROUND YR-1 Early results, note various mechanisms.

ElectroScience Laboratory 11 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

ElectroScience Laboratory 12 DAMP SAND IS VERY LOSSY: Dielectric constants of sands with various moisture content. AS SHOWN BY BOREHOLE DATA YR-2

ElectroScience Laboratory 13 (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 single horz. dipole (NEC-BSC) 500 MHz 1.2 feet (0.366 meters) above ground ground is a lossy dielectric half space radius is 12 feet (3.66 meters) MATLAB code

ElectroScience Laboratory 14 CRANEBENCH (by Frank Paynter) YR-2 GUI-BASED C++ PACKAGE “CRANE BENCH”

ElectroScience Laboratory 15 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

ElectroScience Laboratory 16 INTERESTING EXAMPLE NOTE THE RECOVERED SYMMETRY RESULT OF TRANSFORMATION YR-2

ElectroScience Laboratory 17 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. SAME AREA … DIFFERENT # POINTS YR-3 Multiply each voltage by square root of the associated point area in Sterradians. Thus, as points crowd together, their power per unit area remains the same. Modify the raw data file to make this change and then feed modified data file to CraneBench. IT IS COMMON TO BUILD A SCANNER THAT SCANS IN EFFICIENT INCREMENTS OF THETA AND PHI BUT NOT IN EQUAL INCREMENTS OF SOLID ANGLE SPACE (NOT IN EQUAL STERRADIAN “PIXELS”) MOST REAL DATA MUST THUS BE COMPENSATED BEFORE BEING PASSED TO CRANEBENCH

ElectroScience Laboratory 18 SPECIFIC EXAMPLE - YR-3 1 m DIAMETER DISK 5 CM THICK GHz MONOPOLE ANTENNA UNDER TEST We obtained data from: Hemispherical range with 5.8 m Radius Arch Absorber floor EDGE DIFFRACTION IS STRONG

ElectroScience Laboratory 19 STERRADIAN COMPENSATION EXAMPLE YR-3 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 exact

ElectroScience Laboratory 20 COMPARE OSU PLANE WAVE SYNTHESIS WITH COMMERCIAL SPHERICAL MODE EXPANSION 4.71 = R, DELTA = 1 DEG. Sph = R, DELTA = 1 DEG. F= GHZ Blue = NEC-BSC model Red = OSU – plane wave expansion Green = Commercial Spherical Mode Expansion WE DON’T KNOW WHICH ONE IS “BEST” YR-3 STERRADIAN COMPENSATION EXAMPLE DATA FROM A NF CHAMBER WITH AN ABSORBER-COVERED FLOOR

ElectroScience Laboratory 21 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

ElectroScience Laboratory 22 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?)

ElectroScience Laboratory 23 WE WILL USE 2 PROBE ANTENNAS: LOG PERIODIC (Commercial) Low freq. EDO Corp. AS (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 YR-3

ElectroScience Laboratory 24 Dielectric Probe Antenna Progress - YR 3 THE 2-LAYER ROD IS READY FOR ANTENNA PATTERN MEASUREMENT. NEW PROBE

ElectroScience Laboratory 25 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

ElectroScience Laboratory 26 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

ElectroScience Laboratory 27 COMMERCIAL LOG PERIODIC INSTRUMENTATION ANTENNA THRUST BEARING AND GUY LINES ROTOR FIBERGLASS POLE WINCH YR-3 LOG PERIODIC MEASUREMENT SETUP

ElectroScience Laboratory 28 NF RANGE WITH REALISTIC GROUND NOW LETS TALK ABOUT MECHANICAL OFFSET COMPENSATIONS. YR-3

ElectroScience Laboratory 29 SUMMARY OF PROBLEM - YR 3 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

ElectroScience Laboratory 30 SUMMARY OF PROBLEM x room y room z room AUT Turntable trajectory Probe trajectory Mechanical errors exaggerated for clarity Turntable offset Turntable axis offset by YR-3

ElectroScience Laboratory 31 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

ElectroScience Laboratory 32 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.THE DC VALUES GIVE THE OFFSET 5.THE 2 ND COEFFICIENT PERMITS DETERMINATION OF THE TILTS 6.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

ElectroScience Laboratory 33 EXTRACTION OF T-TABLE PARAMETERS FROM LASER TRACKING DATA LASER TRACKING DATA (ROOM-REFERENCED)

ElectroScience Laboratory 34 EXTRACTION OF T-TABLE PARAMETERS FROM LASER TRACKING DATA

ElectroScience Laboratory 35 NF RANGE WITH REALISTIC GROUND - YR 3 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 polarization rotation due to arm sag antenna phase center offset vs. frequency

ElectroScience Laboratory 36PROGRESS 2004 WORK Develop a NF to FF algorithm that separately computes the direct signal, the ground reflected signal and the sum signal. Use external ground reflections to obtain accurate FF patterns from NF probe data. Use measurements of 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) 2005 WORK Complete the NF to FF algorithm development for the probe data and explore the behavior of the algorithm Characterize the sand pit using borehole measurements of dielectric properties Compute the interaction effects of the metallic support arm Include full polarization development work in the algorithm development work. Code a user-friendly algorithm using C WORK Characterize the low band log periodic probe antenna (gain, phase, beam pattern, phase center vs. freq.) Design and characterize the high band probe antenna (dielectric rod design) Begin the derivation of the turntable and arm offset and sag compensation technique 2007 WORK Finish the turntable and arm offset compensation algorithms (offset and polarization) Incorporate the turntable and arm compensation algorithms in a C++ user friendly “package” Test the computer systems using new near field data.

ElectroScience Laboratory 37 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 HEY ! IS THAT A HEMI ?