ElectroScience Laboratory 1 Development of a Hemispherical Near Field Range with a Realistic Ground...

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ElectroScience Laboratory

Development of a Hemispherical Near Field Range with a Realistic Ground –

Part 3

E. Walton1, T-H Lee1, G. Paynter1, C. Buxton2 and J. Snow3

1The Ohio State University2FBI Academy3Naval Surface Weapons Ctr., Crane Div.

2006 AMTA Meeting; Austin TX.© ERIC WALTON, 10/2006


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NF RANGE WITH REALISTIC GROUND

THIS IS PART THREEOF A CONTINUING SAGA

OF PEOPLE VS. MOTHER NATURE


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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.



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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)

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).


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GROUND REFLECTIONS IN NF MEASUREMENTSGROUND REFLECTIONS IN NF MEASUREMENTS

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

Radius = 4 m; Freq. = 0.7 GHz


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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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Radius = 3 m; Freq. = 0.7 GHz

NF RANGE WITH REALISTIC GROUND YR-1

Early results, note various mechanisms.YR-1


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STATUS –YEAR 1STATUS –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)


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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.


YR-2


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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


DAMP SAND IS VERY LOSSY: BOREHOLE DATA

YR-2


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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 = 2.75loss 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


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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


13CRANEBENCH(C++ GUI by Dr. Frank Paynter)

Crane DDAS Workbench User Interface

YR-2


14INTERESTING 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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INTERESTING EXAMPLE

E-theta E-phi

RESULT OF TRANSFORMATIONYR-2


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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.


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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 area EE

SAME AREADIFFERENT # POINTS

YR-3


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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


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1 m DIAMETER DISK5 CM THICK

2.338 GHzMONOPOLE

ANTENNA UNDER TEST

We obtained “real” data from:Hemispherical range with

5.8 m Radius Arch

Absorber floor

EDGE DIFFRACTIONIS 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


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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.)

C:\Documents and Settings\walton.OSUESL\Desktop\AGC_AMER\AGC NFFF\agc testing_3.oaa

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

-180

-90

0

90

180

Pha

se A

ngle

(deg

)

E-THt

V Monopole at 2.338 GHz <Cranebench>C: \Docum ent s and S et t ings \walton.O S UE S L\ Desktop\A G C_A M E R\A G C NF F F \agc t es t ing_2. oaa

0 45 90 135 180Ph i (D e g ) a t Th e ta = 5 0 .0 0

-40

-30

-20

-10

0

10

Ma

gn

itu

de

(d

B)

E -T H gE -P H gE -T H uE -P H uE -T H tE -P H t

Maximum = 7.231800

Vert. Monopole at 2.338 GHz <Cranebench data>

GROUND REFLECTIONS

co-pol

x-pol

Note that monopole was likely not at the center.

NF RANGE WITH REALISTIC GROUND - YR 3


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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


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MEASURED MONOPOLE DATA

UNCOMPENSATED THETA CUTS COMPENSATED THETA CUT

EVERY 45 DEG. AZIMUTH

0 DEG. AZIMUTH

YR-3


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0 10 20 30 40 50 60 70 80 90 100-55

-50

-45

-40

-35

-30

-25

THETA ANGLE

ampl

itude

(dB

)

COMPARISON BETWEEN NF-FF DERIVED AND RAW DATA

FROM MEASURED PROBE DATA

NF-FF DERIVED

RAW DATA

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 resultbased on CraneBench transform of that dataat R=5.8 m as compensated.

YR-3


25COMPARISON

theoretical gain of 2.33 GHz monopole on 1-m dia diskcompensated Cranebench Far Field result using monopole conical cut data (1 deg increment)

NEC

YR-3


26Comparison between raw measured scanner data (1 deg.) and

spherical mode expansion algorithm result

Red = spherical mode expansion of measured data

YR-3


27COMPARE OSU PLANE WAVE SYNTHESIS WITH SPHERICAL MODE EXPANSION

(INCLUDE NEC-BSC THEORY)

Blue = Spherical mode expansionRed = OSU – plane wave expansionGreen = NEC-BSC model

NEC

YR-3


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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 expansionGreen = Spherical mode expansion

WE DON’T KNOW WHICH ONE IS “BEST”


YR-3


29DOES 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


30TRANSFORMATION 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 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


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Dielectric Probe Antenna Progress - YR 3

THE 2-LAYER ROD IS PREPARED 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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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 ABOUTMECHANICAL OFFSET COMPENSATIONS.


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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. 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


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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


40WE NOW HAVE ALL COORDINATES IN TURNTABLE (ie: AUT)

CENTERED COORDINATE SYSTEM

x’roomy’room

z’roompPP ˆ

ttt xRx ˆ

ttt yRy ˆ

ttt zRz ˆ

AUT

Turntable axis is a cross product: ttt yxz ˆˆˆ

This can all be done in Cartesian coordinates.

θ can be computed using a dot product: pzt ˆˆ)cos(

tRT ̂

YR-3


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xroomyroom

zroompPP ˆ

ttt xRx ˆ

ttt yRy ˆ

ttt zRz ˆ

AUT

γ

Knowing θ, we use an intermediate angle, γ (as shown)and some spherical trigonometry.

pt ˆˆ)cos( )cos()sin()cos(

tRT ̂

YR-3


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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


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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


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MODELING


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skewed


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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


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HEY !IS THAT

A HEMI ?

Dr. Eric K. WaltonDr. 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

ElectroScience Laboratory 1 Development of a Hemispherical Near Field Range with a Realistic Ground...

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