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7/29/2019 Antenna Measurements
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ELE-6100 Suurtaajuustekniikan perusmittaukset
ELE-6106 Basic RF Measurements
Tiiti Kellomäki 2011This material is licensed under a creative commons licence.
Use for non-commercial and educational purposes is allowed.
1
Antenna
Measurements
Contents
1. What do we want to measure?
• Radiation pattern
• , ,
2. How do we measure it?
• Spherical coordinates
3. Error sources
4. Measurement ranges
Radiation Patterns –
All of Same Data!dB or not dB?
normalised or not?
1d/2d/3d?
How to read a Radiation Pattern
• Picture is actually ”gain pattern”
• Copolarisation: transmitting and receiving antennasboth in same polarisation (e.g. both vertical)
3 dB beamwidthabout 80 degrees
Largest side lobe 20 dBsmaller than main lobe: side
lobe level SLL = –20 dB
Main lobe directionis 0 degrees
Front-to-back ratio(F/B) = –20 dB
How to read a Radiation Pattern
dBi scale
(see next slide)Gain6 dBi
-
Green = co-polarisation,Blue = cross polarisation
maximum –18 dBi
Cross-polarisation
level (XPOL LVL):
–18 dBi – 6 dBi
= –24 dB
Cross-polarisation
should be zero in the
direction of the main
lobe. This comes
from the definition…
figure out!
Antenna Gain
• Antennas are passive, so they cannot add power.
• Imagine an ’isotropic antenna’ that radiates equally inall directions.
• Compare all fields to isotropic – dBi = decibel over isotropic (sometimes just ’dB’)
– dBd = dB over (half-wave) dipole, 0 dBd = 2 dBi
• Similarly, if reference antenna gain is known (in dBi),then unknown antenna gain can be compared – G X = G ref P X / P ref , called gain transfer method
4 dB
directive antenna
has 4 dBi gain
isotropic antenna
radiates same
power density in
all directions
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ELE-6100 Suurtaajuustekniikan perusmittaukset
ELE-6106 Basic RF Measurements
Tiiti Kellomäki 2011This material is licensed under a creative commons licence.
Use for non-commercial and educational purposes is allowed.
2
θ-directional
electric field
Polarisation Ellipse
• In general, polarisation is elliptical
• Axial ratio E max/E min
– often 1 (0 dB): circular polarisation
∞
φ-dir.
E-field
–
• Direction of rotation: polarisation
sense, left- or right-handed
• Copolarisation is ’the desired
polarisation’
• Cross-polarisation is orthogonal to
copolarisationE max/E min = 2
Axial ratio 6 dB
Axial Ratio Measurement
•
lin. pol. TX antenna
rotates rapidly
around the ’line of
sight’ (40 rpm)
AUT rotates slowly on
turntable, horizontally
(0,5 rpm)
rotation angle of AUT
r e c i e v e d p o w e r
using a linearly polarisedtransmit antenna
• Pit: polarisation mismatch
• Peak: polarisation match(lin. polarisation is parallelto ellipse major axis)
• Goal usually < 3 dB
• AUT = antenna under test
Spherical Coordinate System
= 0°0° ≤ ≤ 180°
0° ≤ ≤ 360° or
–180° ≤ ≤ 180°
z
= (90°, 0°)
= (90°, 90°)
x
y
Note: Satimo
StarLab coordinate
system is different.
Principal Planes
• Gain is often measured only in the principal planes(cuts of sphere). – E-plane: spanned by main lobe direction and E-field vector.
– H-plane: spanned by main lobe direction and H-field vector. – E- and H- lanes are ortho onal .
– different definitions for circularly polarised antennas.
• Note: you can make co- and cross-polarisation
measurements in both principal planes – result: four measurements
• Nice to know: – in principal planes: vertical polarisation is often simply θ-
polarisation or φ-polarisation–the other is then horizontal
– anywhere else in the radiation pattern:ver = cos φ – sin φ (or something like that)
Bold: vector
Error Sources
• Reflections
• Phase error: antennas too close
• Not exactly a plane wave illuminating AUT
-measurements should be done in far-field)
• Angle errors in mounting and directing
• Dynamic range of power meter, stability of generator…
• External noise and interference (nearby basestations!)
Phase Error • If measurement distance is too short, AUT will be
illuminated by a spherical wave instead of plane
wave
• Phase error: phase difference between end and
centre of AUT
λ
22D r =
• Measurement distance for
phase error 22.5°:
– measured gain smaller
– side lobes larger
– minima filled
– note: 22.5° is often too much!
point
source
r
D
λ /16, or
22,5°
antenna
under
test
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ELE-6100 Suurtaajuustekniikan perusmittaukset
ELE-6106 Basic RF Measurements
Tiiti Kellomäki 2011This material is licensed under a creative commons licence.
Use for non-commercial and educational purposes is allowed.
3
Field Regions
field
distribution
λ = wavelength
don’t go
herenear-field
measurements
far-field
measurements
distance from
antenna
Reflections
-
Spurious plane wave
-
Pattern measurement in ideal quiet-zone Pattern measurement in distorted quiet-zone
Ville’s PhD thesis is
AUT
Spurious side lobe
True pattern
Measured pattern
AUT
True pattern
Measured pattern
Pictures: Ville Viikari / VTT Wireless sensors, Alumnus of TKK Radio laboratory
antenna
measurement errors
afterwards. Utterly
interesting!
http://lib.tkk.fi/Diss/20
07/isbn978951228714
7/
RFID Measurements
• Usually read range measurement usingan RFID reader
• More sophisticated: – frequency sweeps
–
– in anechoic chamber
• Impedance measurement is difficult: IC chip instead
of connector (impedance could be 16–j150 ohms)• Photos: RFID readers by Tagsense (left), Deister
electronics, Thingmagic, and Alien Technology (top).
Slide: Matti Nikkari, ELE/Rauma
Free-Space Far-Field
Measurement Range
Anechoic chamber: VTT antenna measurement hall: 12 m x 12 m x 17 m,
100 MHz…200 GHz, far-field distance < 10 m. Photo: Arttu Rasku / ELE
AUT had to be mounted on the roof of a car. It acts as a ground plane. VTT antenna
research hall, photo: Arttu Rasku / ELE.
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ELE-6100 Suurtaajuustekniikan perusmittaukset
ELE-6106 Basic RF Measurements
Tiiti Kellomäki 2011This material is licensed under a creative commons licence.
Use for non-commercial and educational purposes is allowed.
4
EMC Chamber
EMC Chamber
EMC Chamber Compact Range
• Create a plane wave around AUT – use reflectors or even holograms!
• Useful for millimetre waves – calculate far-field distance r > 2D 2/λ , for 100 GHz,
antenna diameter 3 m
quiet zone
feed, e.g. horn
subreflector main
reflector
paraboloid
Compact range: Courtesy of SELEX Sensors and Airborne Systems, Edinburgh(photo shows AUT) Courtesy of SELEX Sensors and Airborne Systems, Edinburgh (AUT shown)
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ELE-6100 Suurtaajuustekniikan perusmittaukset
ELE-6106 Basic RF Measurements
Tiiti Kellomäki 2011This material is licensed under a creative commons licence.
Use for non-commercial and educational purposes is allowed.
5
Courtesy of SELEX Sensors and Airborne Systems, Edinburgh
(front: AUT, back: main and subreflectors, right: feed Courtesy of SELEX Sensors and Airborne Systems, Edinburgh
Hologram• TKK Radio laboratory
• 650 GHz – diameter 3.16 m
– 4000 slots
– slot width 250 µm
– wavelength 0.5 mm
• Creates plane wavefrom spherical wave
• Picture: Juha Ala-Laurinaho / TKKDepartment of RadioScience andEngineering
Conclusion
• We want to measure radiation pattern
– Gain, directivity, polarisation
– E-plane, H-plane
• Reflections are a major error source.
– Others: phase error (antennas too close)
misalignment, …• Measurements usually in far-field.
– Measurement distance > 2D2/λ .
– Can be overcome with clever techniques.
More Material
• Antenna textbooks such as Balanis,Stutzman+Thiele
• ELE-3506, ELE-6216, SMG-8506
• Antenna measurement theor b ORBIT/FR linkon course webpage) – contains all the definitions, read it!
• When you really need it: IEEE Standards (links oncourse webpage): – IEEE Standard test procedures for antennas (135 p)
– IEEE Standard definition of terms for antennas (40 p)
Future Activities
• Wednesday: at EMC chamber,high voltage hall, SF216
near-field measurements,starting here, then moving toStarLab room (SM202)
• Thursday next week:Deadlines