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Chapter Chapter 17 17 : Antenna Measurement : Antenna Measurement Introduction Antenna Ranges Radiation Patterns Gain Measurements 1 Directivity Measurements Impedance Measurements Polarization Measurements Scale Model Measurements

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Page 1: Chapter Chapter 17 17 : Antenna Measurement: Antenna ...pongsak.ee.engr.tu.ac.th/le428/chap17.pdf · Chapter Chapter 17 17 : Antenna Measurement: Antenna Measurement ... select proper

Chapter Chapter 17 17 : Antenna Measurement: Antenna Measurement

• Introduction

• Antenna Ranges

• Radiation Patterns

• Gain Measurements

1

• Gain Measurements

• Directivity Measurements

• Impedance Measurements

• Polarization Measurements

• Scale Model Measurements

Page 2: Chapter Chapter 17 17 : Antenna Measurement: Antenna ...pongsak.ee.engr.tu.ac.th/le428/chap17.pdf · Chapter Chapter 17 17 : Antenna Measurement: Antenna Measurement ... select proper

Introduction

• Practical antennas : complex structural

configuration & excitation method

• Need experimental results to validate

theoretical data.theoretical data.

• Ideal condition : test antenna in receiving

mode and illumination of test antenna by

plane wave, i.e., uniform amplitude and phase.

• Actual system : “large enough” separation ->

far-field region.

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Page 3: Chapter Chapter 17 17 : Antenna Measurement: Antenna ...pongsak.ee.engr.tu.ac.th/le428/chap17.pdf · Chapter Chapter 17 17 : Antenna Measurement: Antenna Measurement ... select proper

Phase Error

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Measurement Drawbacks

• Pattern Measurements : far-field distance too long ->

difficult to suppress “unwanted” reflections from ground

and surrounding objects.

• Impractical to move antenna to measuring site.

• Take time to measure characteristics, e.g., antenna array.• Take time to measure characteristics, e.g., antenna array.

• For outside measuring systems, uncontrolled

environment and no all-weather capability

• For enclosed systems, cannot accommodate large

antenna systems, e.g., ship, aircraft, antenna arrays, etc.

• Measurement techniques are expensive.

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Page 5: Chapter Chapter 17 17 : Antenna Measurement: Antenna ...pongsak.ee.engr.tu.ac.th/le428/chap17.pdf · Chapter Chapter 17 17 : Antenna Measurement: Antenna Measurement ... select proper

Antenna Ranges• By antenna facilities : Outdoor & Indoor ranges.

• Receiving mode & far-field required -> Ideal

incident field : uniform plane wave -> large

space required.

• Antenna Ranges:• Antenna Ranges:

– Reflection : suppress reflection effects

– Free-space : suppress contributions from surrounding

environments; Elevated ranges, Slant ranges,

anechoic chambers, compact ranges, near-field

ranges.

– Near-field ranges use Near-field/Far-field method to

convert measured near-field data to far-field.5

Page 6: Chapter Chapter 17 17 : Antenna Measurement: Antenna ...pongsak.ee.engr.tu.ac.th/le428/chap17.pdf · Chapter Chapter 17 17 : Antenna Measurement: Antenna Measurement ... select proper

Reflection Range

• “constructive interference” is desirable -> “quiet

zone”.

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Page 7: Chapter Chapter 17 17 : Antenna Measurement: Antenna ...pongsak.ee.engr.tu.ac.th/le428/chap17.pdf · Chapter Chapter 17 17 : Antenna Measurement: Antenna Measurement ... select proper

Elevated Ranges• Reduce contributions from surrounding environments by 1.

select proper directivity & side lobe of source antenna. 2.

clear between line-of-sight. 3. redirect or absorb reflected

energy that cannot removed. 4. utilize some special signal-

processing techniques.

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Page 8: Chapter Chapter 17 17 : Antenna Measurement: Antenna ...pongsak.ee.engr.tu.ac.th/le428/chap17.pdf · Chapter Chapter 17 17 : Antenna Measurement: Antenna Measurement ... select proper

Slant Ranges• Pattern maximum of free-space radiation toward center of the

test antenna

• First null toward the ground to suppress reflection.

• More compact than

elevated ranges.

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Page 9: Chapter Chapter 17 17 : Antenna Measurement: Antenna ...pongsak.ee.engr.tu.ac.th/le428/chap17.pdf · Chapter Chapter 17 17 : Antenna Measurement: Antenna Measurement ... select proper

Anechoic chambers• Controlled environment, all-weather capability, security &

minimize EM interference -> Indoor anechoic chambers.

• After high-quality absorbing

materials are available.

• Two basic types to minimize

specular reflections.specular reflections.

• Rectangular : maximize quiet

zone and simulate free-space.

Require absorbers.

• Tapered : phase difference

between direct and reflected

waves is small.9

Page 10: Chapter Chapter 17 17 : Antenna Measurement: Antenna ...pongsak.ee.engr.tu.ac.th/le428/chap17.pdf · Chapter Chapter 17 17 : Antenna Measurement: Antenna Measurement ... select proper

Compact Ranges• To obtain “ideal” plane wave illumination.

• Compact Antenna Test Range (CATR) can generate nearly

planar wavefronts in a very short distance.

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Page 11: Chapter Chapter 17 17 : Antenna Measurement: Antenna ...pongsak.ee.engr.tu.ac.th/le428/chap17.pdf · Chapter Chapter 17 17 : Antenna Measurement: Antenna Measurement ... select proper

Radiation Patterns• Spherical coordinate system -> 3-dimensional pattern.

• Impractical to obtain 3-d pattern.

• Alternatively, a number of 2-d patterns (pattern cuts) are

measured and used to construct 3-d.

• To obtain pattern cuts:

– Fix φ and vary θ. ->– Fix φ and vary θ. ->

Elevation patterns

– Fixθ and vary φ. −>

Azimuthal patterns.

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Page 12: Chapter Chapter 17 17 : Antenna Measurement: Antenna ...pongsak.ee.engr.tu.ac.th/le428/chap17.pdf · Chapter Chapter 17 17 : Antenna Measurement: Antenna Measurement ... select proper

InstrumentationClassification of instrumentations:

1. Source antenna &

Transmitting system

2. Receiving system

3. Positioning system

4. Recording system

5. Data-processing

system

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Page 13: Chapter Chapter 17 17 : Antenna Measurement: Antenna ...pongsak.ee.engr.tu.ac.th/le428/chap17.pdf · Chapter Chapter 17 17 : Antenna Measurement: Antenna Measurement ... select proper

Instrumentation (2)

• Source antenna : typically log-periodic antenna,

parabolas with broadband feeds, horn antenna;

polarization must be controllable.

• Source : frequency control, frequency stability, spectral

purity, power level, and modulation.

• Receiving system : bolometer detector, amplifier, • Receiving system : bolometer detector, amplifier,

recorder; or a heterodyne system.

• Recording system : linear plot or polar plot; record

relative field or power patterns -> relative pattern.

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Page 14: Chapter Chapter 17 17 : Antenna Measurement: Antenna ...pongsak.ee.engr.tu.ac.th/le428/chap17.pdf · Chapter Chapter 17 17 : Antenna Measurement: Antenna Measurement ... select proper

Instrumentation (3)

• Positioning system : must be capable to rotate in various

planes to generate pattern cuts.

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Page 15: Chapter Chapter 17 17 : Antenna Measurement: Antenna ...pongsak.ee.engr.tu.ac.th/le428/chap17.pdf · Chapter Chapter 17 17 : Antenna Measurement: Antenna Measurement ... select proper

Instrumentation (4)• Typical systems for measuring antenna & RCS pattern.

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Page 16: Chapter Chapter 17 17 : Antenna Measurement: Antenna ...pongsak.ee.engr.tu.ac.th/le428/chap17.pdf · Chapter Chapter 17 17 : Antenna Measurement: Antenna Measurement ... select proper

Amplitude Pattern• Amplitude pattern = vector sum of two orthogonally polarized

field components. Can be measured using same system as

radiation pattern measurement.

• In situ measurement : preserve environmental performance

characteristics.

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Page 17: Chapter Chapter 17 17 : Antenna Measurement: Antenna ...pongsak.ee.engr.tu.ac.th/le428/chap17.pdf · Chapter Chapter 17 17 : Antenna Measurement: Antenna Measurement ... select proper

Phase Measurements

• In general, far-field component given by

• E, ψ : amplitude, phase

• Phase is “periodic”.

r

eeE

jkrj

u

= ),(),(ˆ φθψφθuE

• Need reference.

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Page 18: Chapter Chapter 17 17 : Antenna Measurement: Antenna ...pongsak.ee.engr.tu.ac.th/le428/chap17.pdf · Chapter Chapter 17 17 : Antenna Measurement: Antenna Measurement ... select proper

Gain Measurements

• Two basic methods: absolute-gain and gain-transfer.

• Absolute-gain used to calibrate antennas that can be used

as standards for gain measurements and requires no a

priori knowledge of the gains.

• Gain-transfer (or gain-comparison) used in conjunction

with standard gain antennas to determine absolute gain with standard gain antennas to determine absolute gain

of the antenna under test.

• Typical antennas used for gain standards:

– Resonant λ/2 dipole (gain around 2.1 dB) : broad pattern,

affected by surrounding environments.

– Pyramidal horn antenna (gain 12-25 dB) : very directive, less

affected by environments.

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Page 19: Chapter Chapter 17 17 : Antenna Measurement: Antenna ...pongsak.ee.engr.tu.ac.th/le428/chap17.pdf · Chapter Chapter 17 17 : Antenna Measurement: Antenna Measurement ... select proper

Absolute Gain : Absolute Gain : 22--antennaantenna

• Gain equation:

G0t,G0r : transmitting,

receiving gains.

Pt,Pr : transmitting,

receiving powers.

+

=+t

rdBrdBt

P

PRGG 101000 log10

4log20)()(

λπ

receiving powers.

R : antenna separation

λ : wavelength

• For identical antennas:

19

+

==

t

r

dBrdBt

P

PR

GG

1010

00

log104

log202

1

)()(

λπ

Page 20: Chapter Chapter 17 17 : Antenna Measurement: Antenna ...pongsak.ee.engr.tu.ac.th/le428/chap17.pdf · Chapter Chapter 17 17 : Antenna Measurement: Antenna Measurement ... select proper

Absolute Gain : Absolute Gain : 33--antennaantenna

• 2-antenna method is not applicable for

“nonidentical” antennas.

• Need 3 antennas with 3 following gain equations:

+

=+ta

rbdBbdBa

P

PRGG 1010 log10

4log20)()(

λπ

Then solve for Ga,Gb,Gc .

20

ta

+

=+ta

rcdBcdBa

P

PRGG 1010 log10

4log20)()(

λπ

+

=+tb

rcdBcdBb

P

PRGG 1010 log10

4log20)()(

λπ

Page 21: Chapter Chapter 17 17 : Antenna Measurement: Antenna ...pongsak.ee.engr.tu.ac.th/le428/chap17.pdf · Chapter Chapter 17 17 : Antenna Measurement: Antenna Measurement ... select proper

Gain TransferGain Transfer

• Most commonly-used.

• Use gain standards to determine absolute gain.

• Use two set, first AUT as receiving antenna,

second AUT replaced by standard gain antenna.

+

=+ log10

4log20)()(

PRGG Tπ

•Homework #4 [pdf] Solution [pdf]

21

+

=+0

10100 log104

log20)()(P

PRGG T

dBdBT λπ

+

=+0

10100 log104

log20)()(P

PRGG S

dBdBS λπ

+=

S

TdBSdBT

P

PGG 10log10)()(

Test:

Standard:

Thus,

Page 22: Chapter Chapter 17 17 : Antenna Measurement: Antenna ...pongsak.ee.engr.tu.ac.th/le428/chap17.pdf · Chapter Chapter 17 17 : Antenna Measurement: Antenna Measurement ... select proper

Directivity MeasurementDirectivity Measurement• Simplest method:

1. Measure two principal E- and H-plane patterns.

2. Determine half-power beamwidths of each pattern.

3. Compute directivity using

dd

D21

2

0

)/180(4

ΘΘ=

ππ22

2

0

)/180(181.22D

Θ+Θ=

π(2-27) OR (2-30b)

• Alternative method:

• If there are both θθθθ and φφφφ components:

22

dd 21 ΘΘ 2

2

2

1

0

dd Θ+Θ(2-30b)

∑ ∑= =

==M

j

N

i

ijirad

rad

FMN

BPP

UD

1 1

0max

0 sin),(2

;4

θφθπππ

φθ

φθφθφθ

π

)()(

4;

},{

},{0

radrad PP

UDDDD

+=+=

Page 23: Chapter Chapter 17 17 : Antenna Measurement: Antenna ...pongsak.ee.engr.tu.ac.th/le428/chap17.pdf · Chapter Chapter 17 17 : Antenna Measurement: Antenna Measurement ... select proper

Polarization MeasurementPolarization Measurement

• Polarization-pattern method.

• Need additional measurement to determine sense of

rotation:

– Use two antennas:

CW & CCW.

– Use dual-polarized probe.– Use dual-polarized probe.

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Page 24: Chapter Chapter 17 17 : Antenna Measurement: Antenna ...pongsak.ee.engr.tu.ac.th/le428/chap17.pdf · Chapter Chapter 17 17 : Antenna Measurement: Antenna Measurement ... select proper

Impedance MeasurementImpedance Measurement

• Two types of impedances : Self and Mutual

• If antenna radiating into unbounded medium, i.e. no

coupling, Self-impedance = driving-point impedance.

• If there’s coupling between AUT and other sources or

obstacles, driving-point impedance is a function of both

self and mutual impedances.self and mutual impedances.

• In practice, driving-point impedance = input impedance

• Typically, use vector network analyzer (VNA), slotted

lines.

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Page 25: Chapter Chapter 17 17 : Antenna Measurement: Antenna ...pongsak.ee.engr.tu.ac.th/le428/chap17.pdf · Chapter Chapter 17 17 : Antenna Measurement: Antenna Measurement ... select proper

Scale Model MeasurementScale Model Measurement

• In many applications (e.g., aircraft, ship, etc.), antennas

and structures are too “large” in weight/size to move.

• Furthermore, moving changes environments.

• Use Geometrical scale modeling to

– physically accommodate, within small ranges, measurements

that can be related to large structures,that can be related to large structures,

– experimental control,

– minimize costs.

• Need “exact” replicas,

physically & electrically.

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