Phased Array Antenna Measurement in Near Field Range, Jorge Salazar

Practical considerations for a Near-Field
antenna measurements

MICROWAVE REMOTE SENSING LABORATORY SEMINAR
UNIVERSITY OF MASSACHUSETTS, AMHERST
Presented by:
Jorge Luis Salazar-Cerreño
R
jlscerreno@gmail.com

casa Engineering Research Center for
Collaborative Adaptive Sensing
of the Atmosphere
March 28th, 2011

Collaborative Adaptive Sensing of the Atmosphere

Outline
• Introduction of antenna pattern measurement I
• Practical considerations for Near-Field antenna measurement
– Alignment of AUT and probe
– Span size and sampling spacing
– Probe correction
– Scattering multireflection in the room
– Leakage
– Error budget
• CASA Phase-tilt antenna patterns
• Recommendations for antenna test plan


Outdoor antenna measurements
• Outdoor Antenna Test Range
(OATR) is used for moderate FF
distances.

• One critical factor in outdoor
range systems is to find a place
free of clutter and free of RF
interferences.

• Another critical factor is
environmental changes (rain,
humidity, temperature, etc) that
can delay the test.

• The scanners required are
inexpensive in comparison with
dB Systems Inc.
the indoor range systems. The 2005 So. Turf Sod Rd. Hurricane, Utah 84737 U.S.A.
equipment cost of the scanners
can be lower than $40K.


Example of FF antenna measurement 0

-5

-10

-15

pattern (dB)
-20

-25

-30
Simulated
FF-Measured
-35

-40
-40 -35 -30 -25
theta(deg)

X-Band Frequency
Scan Array Antenna
64x64 Microstrip Patch
Antenna Elements

Eric Knap and Jorge Salazar


Indoor FF measurements
Advantages
– Provide a controlled environment and
an all-weather capability. Can eliminate
delays due to weather (rain, humidity,
temperature changes, etc)
– The measuring system is time and cost
effective. It requires only a small area.
– Patterns are as accurate as those
measured in a FF range
– Can provide a full characterization of
the antenna patterns.
– Compatibility with security
requirements
Disadvantages:
– FF distances FF=2D2/ Lambda
– Cost is driven by the absorbers and
scanner .

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Limitations of indoor FF measurements

2
2L
d FF 



Measured Elevation Patterns in Indoor Far field range

Example of the limitations FF INDOOR
1rst SLL higher in
of indoor FF measurements 2-3 dB than
expected

Measured Elevation Patterns in Planar Near Field range

PLANA NF

• X- Band CASA array antenna
• AUT size: 0.5mx0.25m
• Frequency: 9.36 GHz
• Far-field distance :16m
• Length of range:7m


FF distance limitations in FF range systems
(MIRSL antennas)

S-band C-band X-Band Ku- Band Ku- Band Ka- Band W- Band
CASA
Units FM-CW IWRAP IWRAP AMFR AMFR AMFR
X-POL
Frequency GHz 2.94 5.11 9.36 13.25 13.25 33 95
Lambda m 0.1020 0.0587 0.0321 0.0226 0.0226 0.0091 0.0032
3dB BW deg 3.00 6.58 2.08 6.40 0.75 0.70 0.70
Aperture size m 2.21 0.58 1.00 0.23 1.80 0.91 0.35
FF Distance m 95.7 11.5 62.4 4.7 286.2 182.2 77.6

FM-CW: Frequency Modulated Boundary Layer Profiler
2 L2
2 L2 f
 
IWRAP: Imaging Wind and Rain Airborne Profiler
CASA/X-POL: Ip1 ,Phase-Tilt and X-POL Radar Systems d FF
AMFR: Advance Multi-Frequency Radar (AMFR)  c


Compact antenna test range (CATR)
• Compact Antenna Test Range (CATR)
uses a reflector, or set of reflectors,
designed to collimate the radiated
pattern and create a plane wave at
considerably shorter distances.
• CATR provides a good protection
against weather , RF interference
and security.
• CATR is costly (3X-4X FF indoor).
– Requires a heavy 3 axes positioner
– Requires larger space than FF Indoor X-band surface surveillance radar test in the compact
– Requires expensive rolled-edge reflector antenna test range at the company's facility in Hengelo,
to reduce diffraction and quiet-zone the Netherlands.
(ex. $250K for only the reflector)


Near-Field (NF) range system
• NF provides the same advantages of any indoor
range system in terms of controlled environment,
real estate, interferences and accuracy.
• NF provides additional features, such as back
field projection, that are helpful to detect
anomalies in the surface of the AUT, especially in
active phased-array antennas.
• The Near-field range consists of a interferometer
connected to a field-probing antenna carrier by a
precise robotic system. The antenna probe is
moved through a planar, cylindrical or spherical
surface near the antenna under test (AUT)
• The near-field system operates by measuring the
phase front of the AUT and mathematically
transforms the phase front into the equivalent
far-field angular spectrum. For a planar near field,
the phase front and angular spectrum is related
to the Fourier Fast Transform (FFT)

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What kind of scan surface NF use?

CASCA antenna lab has a P-C and S Near-Field system
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CollaborativeAdaptive Sensing of the Atmosphere

Antenna range systems at UMASS
CASCA antennas Lab. has two range systems.
• NSI Taper FF range system
– Frequency Range : 1-40 GHz,
– Absorber size: 6”
(CF-6, 32dB@3GHz-50dB @50GHz)
– Length: 21’ (6.4m)
– 1 axes-positioner
– Base on PNA 8360 (1-40GHz)
• NSI S-C and Planar NF range
– Absorber size: 8” (30dB @1GHz-50dB @50GHz)
– Length: 14’x10’ (4.2mx3.0m)
– 1 axes-positioner for S and C
– Only in operates in CW mode

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Practical considerations
to perform a NF antenna measurement

• Alignment of the AUT and probe
• Span size and sampling
• Truncation effect
• Scattering and reflections

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Effects of misalignment between
probe and AUT
Beam Pointing Errors in P-NF (in deg)
• Alignment is more critical Band S-band X-Band Ka- Band
Aligment CASA , X-
in NF spherical and NF Accuracy
FM-CW
POL
AMFR

cylindrical in NF scanners Frequency GHz 2.9 9.4 33.0
Lambda m 0.10 0.03 0.01
• In NF planar scanners the 3dB BW deg 3.00 2.08 0.70

alignment depends on the Antenna size
Visual (1cm)
m
deg
2.21
0.130
1.00
1.146
0.91
6.274
AUT size and mechanical Tape measur. (5mm) deg 0.065 0.573 3.147
Laser (0.1mm) deg 0.001 0.011 0.063
stress

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Effects of misalignment between
probe and AUT (cont)
2 deg

X-band CASA Dual-Polarized
Phased Array Antenna
(By: Jorge Salazar and Rafael Medina)

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Effect of misalignment in Xpol

4 dB
PUT PICTURE OF
HORN AUT

17dB

Setup of standard Ku-band Horn antenna

Zo=4mm

ro=2deg


How to improve the alignment
• Use lasers and levels to
verify the axis
orthogonality
• Use the electrical
alignment technique
developed for S-NF by NSI
• Use the post-processing
techniques such as Motion
Tracking Interferometer
developed also by NSI

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Motion tracking interferometer
• Measures time variant
Az, El and Z motion
between the AUT and
probe, including effects
of:
– Thermal effects
– Cables
– Mechanical stress of
Note:
AUT.
Motion Tracking Interferometer is a new
• US patent #5,419,631 feature of NSI P-NF that cost $10K

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Important points about alignment
• Misalignment is more critical in Spherical than in
Cylindrical and Planar scanners.
• Misalignment in P-NF is critical for large array
antennas that operate at higher frequencies
• Misalignment is critical for low cross-polarization
measurements
• For S-and C-NF we can use the electrical alignment
and for P-NF we can use the post-processing
techniques such as Motion Tracking Interferometer
• Beside there are several techniques to align the
antenna and probe, those are expensive.


Span setup and Sample Spacing
• Span size (L) is calculated as function of D,
Z, and θ. t=0.5λ sample
• Z is the distance of the probe to the AUT, spacing in
that is recommended use a value between X and Y (hight
3λ to 5λ. frequency)

• D< 3λ is not recommended because the
reflections of the probe and AUT and also
because at than be possible capture
evanesces waves that can affect the
antenna performance (ripples).
• D< 5λ, is ok but the span is larger and the Z
measurement takes so long.
• Sample spacing recommended is around
0.5 λ. Oversampling lower that S< 0.48 λ
can introduce aliasing that can be
represented as peaks in the sidelobes.

NOTE:
• CASCA NF range system scan size is 5’x5’ (1.5m x 1.5m)


Truncation in P-NF
• Truncation eliminates information about the AUT
sidelobes beyond an angle determined by the
measurement geometry and filters out all
information about the evanescent modes.
• Truncation of planar near-field data is the major
source of uncertainty.
• Advantages: reduces time, reduces scattering

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Effect of the truncation
in the FF antenna patterns

With truncation at 50”

No Truncation (Span 60”)

MIRSL Ku-band Interferometer
(Courtesy Anthony Swochak)


Truncation effect
in the scan
AUT patterns
No Truncation

“Bad” Truncation

Radiation of the
corporate fed
Load
resistor

MIRSL Ku-band IWRAP antenna


Summary of Sampling and Truncation
• Use ~0.5λ sample spacing in X and Y (for the highest
frequency)
• Use between 3 λ to 5 λ as separation between the
probe and the AUT
• Truncation can affect the antenna patterns principally
in the far region of sidelobes. However in scanning
antenna the beam pattern can be affected significantly
• Perform first a full scan measurement and then define
the truncation area when the fields are below -40dB
using NN plots or fields projected to the surface.
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Scattering and reflections inside the
anechoic chamber
• Absorbers are not perfect, they
can provide an absorption up
to 40dB (1-40GHz)
• Sources of reflections:
– AUT and probe
– AUT and scanner/support
– AUT and walls (including floor
and ceiling)
• Taper anechoic chambers
minimize the reflection in walls

J. Appel-Hansen, “Reflectivity level of radio anechoic chambers,” IEEE
Trans. Antennas Propag., vol. AP-21, pp. 490–498, 1973.


RF Absorber and quiet zone room
GROUND OR SHIELDED ROOM 6" PYRAMIDAL RF ABSORBER WITHOUT GROUND
6" PYRAMIDAL RF ABSORBER
55
3GHz
50 6GHz
10GHz
45 18GHz

40 35dB

Reflectivity (dB)
Reflectivity (dB)
35 30dB

30

25

20

15

10
0 10 20 30 40 50 60 70 80
Theta (deg)


Horizontal Setup
Effect of reflections in
2010
Azimuth patterns NO ABSOBERS
IN FLOOR

Horizontal Setup

• Ku Band Interferometer Antenna 2011
• AUT size: 45”x1.25”
• Frequency: 13.195 GHz- 13.295 GHz

Patterns obtained using Slot Array Antenna designed by Ahtony Swochak.,
“Development, implementation, and Characterization of a Ku Band


Horizontal Setup
Effect of reflections when 2011
AUT is in vertical position

Vertical Setup

2011


Vertical setup
Effect of reflections when
AUT is in vertical position

Horizontal setup

• X- Band CASA array antenna
Size: 22”x12”
• Frequency: 9.36 GHz


Techniques to estimate the reflections in anechoic
chamber in NF NSI system
a. Self comparison technique:
The multipath effect can be identified
by observing the changes in the FF-
patterns changing the NF test
parameters such as:
• AUT-to-probe separation
• AUT-to-scanner separation
• AUT-to-wall separation
• AUT orientation in Az and El
• AUT lateral movement
• This test requires the probe
translation stage in Z (~$7.5K)
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EVALUATING NEAR-FIELD RANGE MUTI-PATH., Gregory F. Masters., Nearfield Systems.,
1330 E. 223rd St. #524 Carson, CA 90745


Techniques to estimate the reflection
in anechoic chamber in NSI NF system
2’x2’ metal
b. MARS: Mathematical Absorber obstruction
Reflection Suppression, is a post-
procesing technique developed by AUT
NSI useful to mitigate unwanted Planar slot
reflections. waveguide probe
antenna
MARS analisis of the measured
data and a special mode filtering
process to suppress the
undesirable scattered signals.
The technique is a general
technique that can be applied to
any S –NF and C-NF range systems.
NOTE:
Add this feature to CASCA NF range system requires additional cost of $15K

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IMPROVING AND EXTENDING THE MARS TECHNIQUE TO REDUCE SCATTERING ERRORS, Greg Hindman & Allen C. Newell
Nearfield Systems Inc. 19730 Magellan Drive Torrance, CA 90502


Summary of Reflections in the room
• Absorbers are not perfect, the reflectivity values
depend on the size, frequency and installation.
• Reflectivity of the absorbers are critical for lower
frequencies .
• It is important to keep in mind that the reflections of
waves on the ground, ceiling and uncover scanner
support can affect the main beam, sidelobes and
cros-pol patterns of the AUT considerably.
• Two common techniques can be used to minimize the
scattering and reflections inside the room.
– In P-NF use the Z-spacing
– In S-NF, C-NF use MARS


Probes and probe correction
• Probe types: There are
several probe types that you
can use: OEWP, Dipoles, MS
Patches, Horns, Conical
antennas, etc. Dual -log periodic
antennas 3GHz-
• Probe correction: This is the The QR-1 is a broadband, dual
polarized quad-ridged horn. It
17GHz

process of removing the effect operates from 750 MHz to 6 GHz.

of the pattern probe in the
AUT pattern (Co and Xpol
components).
• Probe data are calibrated and
certified by entities such as
National Institute of Standards MS Patch antenna
& Technology (NIST)
OEWP antenna probes


How the probe can
affect the AUT patterns
in Planar NF
AUT OEWP

REF. PROBE CORRECTION EFFECTS ON PLANAR, CYLINDRICAL AND SPHERICAL NEAR-
FIELD MEASUREMENTS GREG HINDMAN, DAVID S. FOOSHE

casa
How the probe can
Engineering Research Center for

affect the AUT
patterns in
Cylindrical NF
AUT OEWP

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casaHow the probe can
Engineering Research Center for

affect the AUT
patterns in
Spherical NF
AUT OEWP



How the probe can affect the AUT
cross-polar patterns

Θ=45 deg
Θ=45 deg

Θ=45 deg
Xpol: 23dB

Planar NF Cylindrical or Θ=0 deg
Xpol: -40dB
Spherical NF



Leakage in room
• The main source of leakage in near-field
measurements are produced by cables,
connectors and bad connections with the
AUT and the probe.

• Leakage from cables, connectors and the
RF source can be identified and reduced
by using well shielded cables, tightening
connectors and placing instruments in
shielded enclosures.

• One way to verify the leakage between
cables, connectors and the probe or
antenna is terminate first the AUT and
measure in S-C or P NF.


Error budget for -30dB SLL measurement

EVALUATING NEAR-FIELD RANGE MUTI-PATH., Gregory F. Masters., Nearfield Systems.,
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1330 E. 223rd St. #524 Carson, CA 90745


Antenna range systems at UMASS
CASCA antennas Lab. has two range systems.
• NSI Taper FF range system
(CF-6, 32dB@3GHz-50dB @50GHz)
– Length: 21’ (6.4m)
– 1 axes-positioner
• NSI S-C and Planar NF range
– (30dB @1GHz-50dB @50GHz)
– Length: 14’x10’ (4.2mx3.0m)
– 1 axes-positioner for S and C
– Only in operates in CW mode

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CASA Phase-tilt
antenna azimuth
Patterns
V-pol

H and Hx at NN-FF of PTAA (64x32), θ=0º, SC

CASA Phase-tilt
antenna azimuth
Patterns
V-pol


CASA Phase-tilt
antenna azimuth
Patterns
H-pol

H and Hx at NN-FF of PTAA (64x32), θ=0º, SC

CASA Phase-tilt
antenna azimuth
Patterns
H-pol


CASA Phase-tilt antenna elevation patterns
Measured patterns of 32x1 linear array in LRU (32X18)
V-port H-Port

Parameter Value@9.36GHz Parameter Value@9.36GHz
BW 3.81 deg BW 3.62 deg
SLL1(L/R) -29/-26.5 dB SLL1(L/R) -23.2/-22.5 dB
Emax(dB) -43.83 dB Emax(dB) -43.30 dB
Xpol_brodside -34 dB (rel) Xpol_brodside -38.7 dB (rel)
ICPR2 -34.4 ICPR2 -34.0

Measured results:
• RL better than -13 dB at Resonant frequencies for H and V
Patch layer Foam layer
• Impedance bandwidth: 200 MHz at RL of -10 GHz (improved in 80 MHz)
Fed layer • Beam pattern bandwidth: 100 MHz (improved in 40 MHz)
Reflector layer • Isolation port -27 dB


Recommendation for an antenna
measurement in NF system
• Never perform an antenna measurement without a reference of patterns of
the AUT (simulated, previous measurement).
• Understand first how the errors (fabrication, random errors) can affect the
antenna patterns.
• Before performing a measurement, make sure that scattering and
reflection in the room are low enough to avoid contamination of the AUT
measurements.
• Make sure that the size of the scanner is larger than the span required for
the AUT.
• Perform a stability test ( to evaluate the effect of mechanical stress and
temperature versus time).
• Perform a leakage test to be sure that cables are well connected with the
AUT and receiver.
• Keep in mind that the cross-polar measurements are very sensitive to
alignment and also reflections in the room.
• Perform an antenna test plan and discuss details with your advisor or person
who has experience doing this type of measurement.
• Call me at 413-123456 if you need some help. I will charge you a beer/hour.


Consideration for a good Antenna Plan Test
• Have preliminary results of AUT (simulations previous meas. Paterns)
• Estimation the time for each specific test measurement
• Have in mind how the position errors can affect your measurments.
• Take in consideration the alignment of AUT and probe
• Do a stability test (mechanical stress, temperature)
• Monitoring the SNR vs Frequency (for large bandwidth antenna)
• Take in consideration the scattering issues at lower frequencies (more
if you wan very low cross-pol values ( better that 25dB)
• Take in consideration the Probe data to be used to correct the probe
effect
• Coordinate systems (Az/Ele, Theta/Phi, X /Y)

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Thanks

In this Picture Jorge Salazar and Rafael Medina taking antenna
patterns of CASA dual-polarized phased -Array antenna (Feb. 2011)
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Phased Array Antenna Measurement in Near Field Range, Jorge Salazar

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