electroscience laboratory 1 e. k. walton*(1), t-h lee(1), g. paynter(1), j. snow(2), and c....

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ElectroScience Laboratory

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 11 - 15 JUNE 2007

Development of a Hemispherical Near Field Antenna Measurement Range

for use on a Realistic Ground


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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


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•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


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HISTORY:HISTORY:

• 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 MEASUREMENTSGROUND REFLECTIONS IN NF MEASUREMENTS


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Radius = 4 m; Freq. = 0.7 GHz

NF RANGE WITH REALISTIC GROUND – YR 1


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Transformation SoftwareTransformation 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


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RAY PATHS

PROPAGATION TO FAR FIELD

YR-1

RAY PATHS TO USE IN SYNTHESIS

……


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Radius = 3 m; Freq. = 0.7 GHz

NF RANGE WITH REALISTIC GROUND YR-1

Early results, note various mechanisms.


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ARM INTERACTION3-Element X-Directed Dipole Array

Element Spacing: 0.29 Excitations: (0.5,104.4o),(1.04,0.0o),(0.5,-104.4o)

Phi = 0 Degree Cut, Er ComponentFrequency = 150 MHz, Dipole Length=1.1m, Center Element Height=28'

Theta (Degrees)

-90 -75 -60 -45 -30 -15 0 15 30 45 60 75 90

Nea

r F

ield

Pat

tern

(dB

)

-70

-65

-60

-55

-50

-45

-40

-35

-30

-25

-20

-15

-10

Er, Dipole OnlyEr, Scattered Field

3-Element X-Directed Dipole ArrayElement Spacing: 0.29

Excitations: (0.5,104.4o),(1.04,0.0o),(0.5,-104.4o)Phi = 0 Degree Cut, EComponent

Frequency = 150 MHz, Dipole Length=1.1m, Center Element Height=28'

Theta (Degrees)

-90 -75 -60 -45 -30 -15 0 15 30 45 60 75 90

Nea

r F

ield

Pa

ttern

(dB

)

-70

-65

-60

-55

-50

-45

-40

-35

-30

-25

-20

-15

-10

E, Dipole Only

E, Scattered Field

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


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DAMP SAND IS VERY LOSSY:

Dielectric constants of sands with various moisture content.

AS SHOWN BY BOREHOLE DATA

YR-2


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PLANE WAVE SYNTHESIS

AUT

SynthesizedWavefront

Radiatingelements

Surface of ground

Individual spatial displacements

Synthesized below-ground elements(green)

Sketch of plane wave synthesis geometry.


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FF

Ptp

tf

fpff

delf

Ft

Pt

FfPf

X

Z

Y

Geometry for the near field to far field transformation

PLANE WAVE SYNTHESIS


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(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


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CRANEBENCH (by Frank Paynter)

YR-2

GUI-BASED C++ PACKAGE“CRANE BENCH”


15INTERESTING 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


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INTERESTING EXAMPLEC:\Documents and Settings\Sharon\Desktop\05-CRANE\CRANEBENCH_3-7-05\1hdipole-zd-x7_0-y0_0-z1_2_ra12ft_0500mhz_sphere_1.oaa

0 90 180 270 360Phi (Deg) at Theta = 80.00

10

20

30

40

50

60

Mag

nitu

de (d

B)E-THgE-PHgE-THuE-PHuE-THtE-PHt

Maximum = 57.264999C:\Documents and Settings\Sharon\Desktop\05-CRANE\CRANEBENCH_3-7-05\1hdipole-zd-x7_0-y0_0-z1_2_ra12ft_0500mhz_sphere_2.oaa

0 90 180 270 360Phi (Deg) at Theta = 50.00

10

20

30

40

50

60M

agni

tude

(dB

)

E-THgE-PHgE-THuE-PHuE-THtE-PHt

Maximum = 56.317501

FAR ZONE CONICAL CUTS

NOTE THE RECOVERED SYMMETRY

RESULT OF TRANSFORMATION

YR-2


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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.

area EE

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


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SPECIFIC EXAMPLE - YR-3

1 m DIAMETER DISK5 CM THICK

2.338 GHzMONOPOLE

ANTENNA UNDER TEST

We obtained data from:Hemispherical range with

5.8 m Radius Arch

Absorber floor

EDGE DIFFRACTIONIS STRONG


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STERRADIAN COMPENSATIONEXAMPLE

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


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COMPARE OSU PLANE WAVE SYNTHESIS WITH COMMERCIAL SPHERICAL MODE EXPANSION

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 expansionGreen = Commercial Spherical Mode Expansion

WE DON’T KNOW WHICH ONE IS “BEST”YR-3

STERRADIAN COMPENSATION EXAMPLE

DATA FROM A NF CHAMBER WITH ANABSORBER-COVERED FLOOR


21DOES 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


22TRANSFORMATION 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?)


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WE WILL 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


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Dielectric Probe Antenna Progress - YR 3

THE 2-LAYER ROD IS READY FOR ANTENNA PATTERN MEASUREMENT.

NEW PROBE

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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


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LOG PERIODIC MEASUREMENT SETUP

ESL BLDG

Instrumentation

antennaFiberglass 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


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COMMERCIAL LOG PERIODIC

INSTRUMENTATIONANTENNA THRUST BEARING

ANDGUY LINES

ROTOR

FIBERGLASSPOLE

WINCH

YR-3

LOG PERIODIC MEASUREMENT SETUP


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NOW LETS TALK ABOUTMECHANICAL OFFSET COMPENSATIONS.

YR-3


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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


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SUMMARY OF PROBLEM

xroom yroom

zroom

otpPP ˆ

ottt txRx ˆ

ottt tyRy ˆ

ottt tzRz ˆ

AUT

ottRT ˆ

Turntabletrajectory

Probe trajectory

Mechanical errors exaggerated for clarity

Turntableoffset

Turntable axis offset by ot

YR-3


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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


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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 2ND 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


33EXTRACTION OF T-TABLE PARAMETERS

FROM LASER TRACKING DATA

LASER TRACKING DATA (ROOM-REFERENCED)


34EXTRACTION OF T-TABLE PARAMETERS

FROM LASER TRACKING DATA


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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


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PROGRESSPROGRESS2004 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++

2006 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.


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Dr. Eric K. WaltonDr. Eric K. Walton, The Ohio State Univ.

[email protected]

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 ?