antenna measurements

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ELE-6106 Basic RF Measurements

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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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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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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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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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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