near field antenna measurement

Near-field antenna measurements

Lectures at Brno University of Technology, April 26-28, 2010

Juha Ala-Laurinaho

Aalto University School of Science and Technology

MilliLab/SMARAD, Department of Radio Science and Engineering

Espoo, Finland

Dept. of Radio Science and Engineering, MilliLab/SMARADLectures at Brno University of Technology, April 26-28, 20102

Outline• Introduction • History• Field regions• Scanning geometries• Measurement system• Error sources• Gain measurement• Probe compensation• Recent high-frequency measurements• Near-field to far-field transformation


Introduction• Near field of the antenna under test (AUT) is measured on a near-

by surface and far-field characteristics are calculated numerically• Due to the unideal probe the coupling between the AUT and probe

is measured => effect of the probe is compensated• Allows antenna diagnostics

• Surface deformations of AUT main dish, panel alignment• Fault elements in antenna arrays• Phased array element tuning

• Basically, two scans with different probes for determination of polarisation characteristics


History• First experimental work around 1950

• ”Automatic antenna wave front plotter”• No probe correction• At first, no far-field transformation

• First probe corrected theories 1961-1975• Theory put into practice 1965-1975• Technology transfer 1975-1985• 1985-

• Higher frequencies• Larger scanners• Spherical near-field measurement theory further developed• Computer processing capacity increased


Field regions of the antenna• Reactive near-field

• Evanescent modes• Evanescent energy couples to the probe capacitively or

inductively• 1-3 wavelengths from the antenna

• Radiating near-field (Fresnel) • Near-field measurements in this zone

• Far-field (Fraunhofer) from the 2D2/λ

( , )ikreEr

θ φ ∼


Scanning geometries• Planar

• Rectangular, xy-scanner• Plane polar, rotating linear-scanner • Bi-polar and spiral, AUT rotator and rotating arm

• Cylindrical• Linear scanner and AUT rotator

• Spherical• AUT rotator for azimuth and elevation rotation

• Arrays can be used• Linear arrays, arc arrays

• Near-field range can be considered as synthetic aperture version of compact range• Phased array is used to create planar wave front


Planar scanning (rectangular)

• Coupling equation, probe receiving and AUT transmitting

• Transmission characteristics of the AUT (distance z0)

• Calculation of the radiation of the AUT

1( , , ) ( , ) ( , )2

yx ik yik xi zPROBE x y AUT x y x yb x y z k k k k e e e dk dkγ

π

∞ ∞

−∞ −∞

= ⋅∫ ∫ R T

0 1

0( , ) ( , ) ( , , )2

yx

i zik yik x

AUT x y PROBE x yek k k k b x y z e e dxdy

γ

π−

∞ ∞−−−

−∞ −∞

= ⋅ ∫ ∫T R

( , , ) cos ( sin cos , sin sin )ikr

AUTiker k kr

φ θ θ θ φ θ φ−= ⋅E T


Cylindrical scanning

• Coupling equation, probe receiving and AUT transmitting

• Transmission characteristics of the AUT

• Radiation of the AUT

{ }, ,1( , , ) ( , ) ( )

2im i z

m PROBE m AUTmb z e e dφ γρ φ γ ρ γ γ

π

∞∞

=−∞−∞

= ⋅∑ ∫ R T

12

, , 0 00

1( ) ( , ) ( , , )2

im i zm AUT m PROBE b z e e d dz

πφ γγ γ ρ ρ φ φ

π−

∞− −

−∞

= ⋅ ∫ ∫T R

,2( , , ) sin ( ) ( cos )

ikrm im

m AUTm

ker i k er

φφ θ θ θ∞

=−∞

−⎡ ⎤= ⋅ − ⎣ ⎦∑E T


Spherical scanning• Coupling equation

• Transmission characteristics of the AUT

• Radiation of the AUT

, , , ,1

( , , ) ( ) ( ) ( ) ( )n

E E M M imnm AUT n PROBE nm nm AUT n PROBE nm

n m nb r r r e φφ θ θ θ

∞

= =−

⎡ ⎤= + ⋅⎣ ⎦∑ ∑ T R M T R N

1 2, ,

01, , 0 0

( ( ))( , , ) sin

( ( ))

E Enm AUT nm PROBE imnmM M

nm AUT nm PROBE nm

T R r Nr e d d

T R r M

π πφφ θ θ φ θ

−−

−

⎧ ⎫ ⎧ ⎫ ⎧ ⎫⎪ ⎪ ⎪ ⎪= ⋅ ⋅ ×⎨ ⎬ ⎨ ⎬ ⎨ ⎬⎪ ⎪ ⎪ ⎪ ⎩ ⎭⎩ ⎭ ⎩ ⎭

∫ ∫ r b

, ,1

( , , ) ( 1) ( ) ( )ikr n

n E M imnm AUT nm nm AUT nm

n m n

ier er

φφ θ θ θ∞

= =−

− ⎡ ⎤= − + ⋅⎣ ⎦∑ ∑E T M T N


Measurement environment

• Compact measurement site• Indoor measurements• Control of temperature, humidity etc.

• Less facility space required than in CATRs • Multiple reflections between AUT and probe antenna and its

surroundings critical => high quality absorbers around probe• Reflections from measurement chamber walls not as critical


Error sources• Multiple reflections between the AUT and probe

• Scans with different separation, smaller probe • Linearity of the receiver

• E.g. 0.02dB/dB nonlinearity causes several tenths of dB error ingain and several dB error in -35 dB sidelobe level

• Calibration of receiver with precision attenuator and correctionof the near-field data with calibration curve

• Scanner inaccuracies• Especially at high frequencies • Large errors in sidelobe levels

• Finite scan area• Errors outside the ”solid angle”


Gain measurement• Three options for absolute gain measurement

1) Direct gain measurement- basic equations of near-field theory- probe antenna is the gain standard- insertion loss between AUT and probe has to be measured - calibrated attenuator is used when the generator and load portsare connected

2) Gain comparison technique- standard antenna is measured- multiple reflections and truncation errors may be significant if the standard antenna is much smaller

3) Three-antenna method- two probes and AUT- three near-field scans required- multiple reflections can be problem


Probe compensation • Coupling between the AUT and probe antenna is measured

• Probe compensation to correct the near-field data• In planar geometry probe orientation is fixed during scans

• Probe response can be deconvolved by dividing the complex AUT angular spectrum by the complex probe angular spectrum

• Cross-polar components can also be taken into account• Different correction equations for different measurement

geometries• Effect of probe compensation largest in planar geometry• Use of an orthomode transducer (OMT) instead of use of two probes


Probe Antenna• Selection of probe antenna affects the spatial filtering • Open-ended waveguide

– Almost omnidirectional– May be loaded with dielectric material

• Loop antenna– Measures magnetic field

• High-gain antennas, horns, dishes etc.– Provides spatial filtering for minimizing multipath errors– For high quality probe correction, the probe gain should be high

enough over the range of far-field angles required in the output data– For high-gain antenna measurement– Increases signal-to-noise ratio

• Synthetic probe antenna– E.g. movement in z-direction for suppressing axial mutual coupling

between AUT and probe– Increases measurement time


Measurement system• Near-field measurement system consists three

subsystems: 1) Computer for controlling and post-processing2) RF-system 3) Mechanical instrumentation

• Measurement environment affects the measurement accuracy– E.g. reflectivity level of the anechoic chamber


Computer for Controlling and Post-Processing

1. Control of scanner movement 2. Data acquisition, synchronisation to the scanner movement3. Post processing of the obtained data

– FFT-computation for the far-field pattern– Probe compensation– Back projection, holographic image for antenna diagnostics

• Capabilities of modern computers are sufficient for use in planar near-field measurements systems

• FFT computing time proportional to N*log2(N)• DFT computing time proportional to N*N • For 1024 samples transformation with DFT takes about 100 times

longer than using FFT


RF-Instrumentation• Vector network analyser for complex sampling of the field

– Amplitude and phase• Stability

– Long data collection time requires good stability– Long term drifts can be corrected with tie scans

• Sampling rate affects the measurement time• E.g. 1.5 m antenna at 300 GHz

– Scanning area 1.8 m x 1.8 m– Sampling interval 0.5 mm (λ/2)– 3600 x 3600 samples– For sampling rate of 10 samples/s, acquisition time is 360 hours =

15 days


RF-Instrumentation

• Dynamic range or signal-to-noise ratio (SNR)– The insertion loss in the AUT – probe path is approximately the

difference between the gains of the AUT and probe (or difference in the aperture areas)

– Transformation gain or process gain increases the dynamic range in far-field pattern by sqrt(N)

– E.g. 1000 samples increases dynamic range by 30 dB • Probe movement causes cable bending

– Corrections may be required• Thermal variations cause changes in the electrical

lengths of cables– Thermal control


Mechanical Instrumentation

• Accuracy in z-direction is critical– Granite reference plates – Tracking of the probe with laser interferometer

• Active probe position compensation from tabulated data or • Numerical correction in the near-field to far-field

transformation, phase correction 2πΔ/λ


Mechanical Instrumentation

• Transversal accuracy is typically not as critical as the accuracy in z-direction– Laser interferometers can be used for accurate measurement of

the position – Affects the side lobe levels far from main beam

• Mechanical vibrations of the probe affect the measurement accuracy

• Alignment of the AUT with respect to the scanner affects the accuracy of the measurement of the pointing of the antenna


Alignment

• AUT and the scanner system are aligned with respect to each other– Accuracy depends on the application

• Alignment can be done with mirrors, lasers, mirror cubes• Autocollimation with mirror cube can give an accuracy of

about 0.0003 degrees• Alignment of the AUT may require a rotator stage• Errors in the near-field data also affect the measurement

of the boresight– E.g. errors in bending cables could cause almost linear phase errors


Measurement Error Sources • 18 term list error sources in planar near-field scanning

by National Bureau of Standards or National Institute of Standards and Technology, USA is presented in ref. [11]

• Error sources can be evaluated with– Computer simulations– Test on measurement system– Error equations

• Error components are combined to give an estimate of the total error– Error components are assumed uncorrelated with each other– Root of sum of squares (RSS) is used


Measurement Error Sources• The first four are related to the probe errors1. Probe relative pattern

• Measured pattern• Calculated pattern• Error equations

2. Probe polarization ratio• Error equations

3. Probe gain measurement• Error equations

4. Probe alignment error• Error equations


Measurement Error Sources

• Items 5-18 contribute to the measured spectrum5. Normalization constant

• Error equations

6. Impedance mismatch factor• Error equations

7. AUT alignment error• Error equations and tests on measurement system• Calculation of boresight


Measurement Error Sources8. Data-point spacing (aliasing)

• Error equations and tests on measurement system• a priori knowledge of the antenna

9. Measurement area truncation• Computer simulations and error equations• A priori knowledge of the antenna

10. Probe x- and y-position errors• Computer simulations and error equations• Laser interferometer measurements

11. Probe z-position errors• Computer simulations and error equations• Laser interferometer measurements


Measurement Error Sources12. Multiple reflections (probe/AUT)

• Tests on measurement system• Scans with different separation, difference small compared to

wavelength

13. Receiver amplitude nonlinearity• Computer simulations, error equations and tests on measurement

system

14. System phase errors due toa) Receiver phase errorsb) Flexing cables/rotary jointsc) Temperature effects– Error equations and tests on measurement system


Measurement Error Sources

15. Receiver dynamic range• Test on measurement system

16. Room scattering• Test on measurement system

17. Leakage and crosstalk• Test on measurement system

18. Random errors in amplitude and phase• Computer simulations, error equations, and test on

measurement system


Example of Leakage; constant leakage signal

UncompensatedUncompensated CompensatedCompensated


Example at 310 GHz

•• Previous nearPrevious near--field field scanner of Radio scanner of Radio LaboratoryLaboratory

•• Scanning area 1.5 m x Scanning area 1.5 m x 1.5 m1.5 m

•• Used for quietUsed for quiet--zone zone testing of hologramstesting of holograms

–– Planar wavePlanar wave--frontfront•• Used for testing of dual Used for testing of dual

reflector feed system reflector feed system –– Spherical waveSpherical wave--frontfront


Example at 310 GHzFrom: J. Häkli, Shaped reflector antenna design and antenna measurements at sub-mm wavelengths, Doctoral Thesis, Helsinki University of Technology, 2006, 217 p.


Phase Retrieval• Accurate phase measurement is difficult at high

frequencies • Phase can be retrieved from two amplitude scans in two

parallel planes• Geometry of the AUT aperture gives one additional

constraintSCANNING PLANES

ANTENNA UNDERTEST


Phase Retrieval• Fourier iteration method

– Near-field is transformed between two planes, forward and backward propagation of plane wave components

– Iteration of phase, amplitude is replaced with measured amplitude

– Developed by UCLA, USA

• Conjugate gradient method – Also amplitude iteration– Developed by University of Naples, Italy

• Separation of measurement planes have to be large enough– Challenging to achieve adequate accuracy at high frequencies


SWAS measurement• Submillimeter Wave Astronomy Satellite (SWAS) was

measured with planar near-field scanning • Off-axis Cassegrain antenna

• Aperture 56 cm x 68 cm• Scans at 490 and 550 GHz• Scanning range 80 cm x 80 cm• 256 x 256 points • 50 minutes scan time• Scanner z-accuracy 1.2 microns


EOS MLS at 640 GHz

• Earth Observing System Microwave Limb Sounder (EOS MLS)

• Antenna has a 1.6 meter aperture and operates at frequencies as high as 640 GHz

• Nearfield Systems, Inc. (NSI) manufactured a planar scanner of 2.8 m x 2.8 m for Jet Propulsion Laboratory (JPL)

• Scanner z-planarity was 4 microns rms


NSI Tiltable Near-field Scanner

• 0.9 m x 0.9 m• Used up-to 950 GHz (λ = 316 micrometers)• RMS planarity 20 micrometers in any plane from

horizontal to vertical plane• Active structure correction for z-axis uses

predetermined structural data


Near-field to far-field transform

• Coordinate system• Sampling theory• Probe compensation• Fast Fourier Transformation (FFT)• Back projection


Coordinate system• Boresight of the antenna to +z-direction• Propagation vector k,

• Direction of propagation can be defined with

λπ /2=k

.222yx

yx

kkK −−=γ

yx ,kk += uuK

.cos

,sinsin,cossin

,

,2222

θ

φθφθ

=

==

++=

++=

z

y

x

zyx

zyx

k

kk

kkkk

kkk zyx uuuk


Sampling theory• All significant energy from AUT has to be obtained in the

synthesised near-field aperture– planar geometry suitable for high-gain antennas

• Nyquist sampling theorem has to be applied – sampling interval dense enough if the highest spatial frequency

changes less than 180 degrees between two adjacent sample points

– half of the wavelength is the minimum • Aliasing will cause spurious signals in the region of interest

if the sampling is not dense enough• Typically used band limits

,max ,max,x yk kx yπ π

= =Δ Δ


Sampling interval

• Sparse sampling grid can be used if limited angular range is of interest • E.g. 10 degrees bandlimit in x-direction, θ = 10°, φ = 0°:

• In the totally aliasing free case the band limit is

• E.g.

λλ

πλπ

9.210sin2

10sin2max

≈°

=Δ

⇒Δ

=°=

x

xk

,1 sin( )

x y λθ

Δ Δ ≤+

10 , 0.851 sin10

x y λθ λ= °⇒ Δ Δ = ≈+ °


Sampling area vs. angular range

• Angular range depends on the size of sampling area L, antenna dimensions D and measurement distance d

⎟⎠⎞

⎜⎝⎛ −

=dDL

valid 2arctanθ

DL

θ

probemeasurement plane

d valid


Truncation error

• All the radiated power should be go through the synthetic aperture formed during near-field scanning and thus accepted by the probe

• Due to the truncation of the scanning area, integrated power is lower

• Especially, power radiated to other directions than to main beam is lower

• Directivity tends to increase• Errors in far side-lobe levels


Transformation of the near-field to far-field

• Electromagnetic field to angular spectrum• Planar rectangular raster scan as an example1) 2-D Fourier transform of the measured complex fields

- Spatial frequency spectrum or PWS (plane wave spectrum)2) Probe compensation

- Measured spatial frequency spectrum is divided by probe spatial frequency spectrum

3) AUT spatial frequency spectrum to AUT angular spectrumor )/arctan(),/arccos( xyz kkkk == φθ

)/arctan( angleazimuth ),/arcsin( angleElevation zxaye kkkk == θθ


Back projection

• Back projection of holography can be used for antenna diagnostics– Surface accuracy of the AUT main mirror– Fault elements in an antenna array

• The plane wave spectrum in the measurement plane is transferred to the aperture plane by back propagating each PWS-component

• Inverse FFT is used to calculate the aperture field from the PWS• The phase of the aperture field can be scaled with wavenumber to

obtain a surface error map• Defocus and feed placement errors can be found by using e.g. least

squares methods


References1. D. Slater, Near-field Antenna Measurements, Artech House, Norwood, MA, 1991, 310 p.2. A. G. Repjar, A. C. Newell, M. H. Francis, "Accurate determination of planar near-field correction parameters for linearly polarized probes", IEEE

Transactions on Antennas and Propagation, Vol. 36, No. 6, June, 1988, pp. 855-868.3. A.D. Yaghjian, An overview of near-field antenna measurements, IEEE Transactions on Antennas and Propagation, Vol. 34, No. 1, January, 1986,

pp. 30-45.4. L. I. Williams, Y. Rahmat-Samii, R. G. Yaccarino, The bi-polar planar near-field measurement technique, part I: implementation and measurement

comparisons, IEEE Transactions on Antennas and Propagation, Vol. 42, No. 2, February, 1994, pp. 184 – 195.5. R. G. Yaccarino, Y. Rahmat-Samii, Phaseless bi-polar planar near-field measurements and diagnostics of array antennas, IEEE Transactions on

Antennas and Propagation, Vol. 47, No. 3, March 1999, pp. 574 – 583. 6. J. P. McKay, Y. Rahmat-Samii, Compact range reflector analysis using the plane wave spectrum approach with an adjustable sampling rate, IEEE

Transactions on Antennas and Propagation, Vol. 39, No. 6, June 1991, pp. 746 – 753. 7. D. Slater, A 550 GHz near-field antenna measurement system for the NASA Submillimeter Wave Astronomy Satellite, Antenna Measurement

Techniques Association Conference, October 3-7, 1994.8. N. Erickson, V. Tolls, Near-field measurements of the submillimeter wave astronomy satellite antenna, Proceedings of the 20th ESTEC Antenna

Workshop on Millimetre Wave Antenna Technology and Antenna Measurements, Noordwijk, The Netherlands, 1997, pp. 313−319.9. P. R. Foster, D. Martin, C. Parini, A. Räisänen, J. Ala-Laurinaho, T. Hirvonen, A. Lehto, T. Sehm, J. Tuovinen, F. Jensen, K. Pontoppidan: Mmwave

antenna testing techniques - Phase 2, MAAS Report 304, Issue No 2, ESTEC Contract No 11641/95/NL/PB(SC), December 1996, 224 p.10. http://www.nearfield.com/11. A.C. Newell, Error analysis techniques for planar near-field measurements, IEEE Transactions on Antennas and Propagation, Vol. 36, No. 6, June

1988, pp. 754-768. 12. E. B. Joy, Near-field range qualification methodology, IEEE Transactions on Antennas and Propagation, Vol 36, No. 6, June 1988, pp. 836-844.13. Y. Rahmat-Samii, L. I. Williams, R. G. Yaccarino, The UCLA bi-polar planar-near-field antenna-measurement and diagnostics range, IEEE Antennas

and Propagation Magzine, Vol. 37, no. 6, December 1995. R. C. Johnson, H. A. Ecker, and R. A. Moore, ”Compact range techniques and measurements,” IEEE Trans. Antennas Propagat., vol. AP-17, no. 5, pp. 568–576, Sept. 1969.

14. R. G. Yaccarino, Y. Rahmat-Samii, A comparison of conventional and phaseless planar near-field antenna measurements: the effect of probe position errors, Proceedings of IEEE Int. Conference on Phased Array Systems and Technology, Dana Point, CA, USA, 21-25 May, 2000, pp. 525-528.

15. L. Le Coq, M. Vaaja, B. Fuchs, O. Lafond, J. Ala Laurinaho, J. Mallat, M. Himdi, A. V. Räisänen, "IETR and TKK- MilliLab measurements cooperation in ACE 2 context: characterization of a Half Maxwell Fish Eye lens at 110 and 150 GHz," Proceedings of EuCAP 2009, 3rd European Conference on Antennas and Propagation, Berlin, Germany, March 23-27, 2009, pp. 2442-2446.

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