Antenna Measurements techniques

7/29/2019 Antenna Measurements techniques

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Antenna

Measurements

Antenna

Measurements


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Antenna Measurements

Introduction

Antenna range

Radiation patterns

Amplitude

Phase

Gain measurements

Antenna polarization

Scale model measurements


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Introduction

The basic parameters to describe an antenna:Far field patterns (amplitude and phase)

Gain and directivity

Polarization

Efficiency

Input impedance

Current distribution

Most antennas are measured in their receiving mode.

Most of the methodology for measuring the characteristics of antennas was

developed before and during World War II.

The ideal condition for measuring far-field radiation characteristics is the

illumination of the test antenna by plane wave: uniform amplitude and phase

r > 2D2/ Maximum phase error from an ideal plane wave : 22.50


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Antenna Ranges

The facilities for antenna measurements are called antenna ranges(AR).

They can be categorized as outdoor ranges and indoor ranges.

According to the principle of measurement, they can be also categorized as:

reflection ranges, free-space ranges, and compact ranges.

Reflection Ranges

R0

hs

RD

Source antenna

Source antenna image

Receive antenna

hr

Rs

Reflection ranges are usually of

the outdoor type..

They are used to measure the patterns

of moderately broad antennas operating

in the UHF (500-1000 MHz) to the

16GHz frequency region.


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Free-Space Ranges

The free-space ranges are designed to provide reflection-free propagation of the EM waves.

Outdoor free-space ranges: elevated ranges and slant ranges.

Indoor ranges: anechoic chambers, near-field ranges.

Free-Space Ranges

A. Elevated ranges

Elevated ranges are used to test physically large antennas.

Both antennas (the transmitting and the receiving) are

mounted on high towers or buildings.

The terrain beneath is smooth.

The transmitting antenna has very low side lobe level.

The line-of-sight is always clear.

B. Slant ranges

The slant ranges are more compact than the elevated ranges.

The test antenna is mounted at a fixed height on a non-conducting

tower while the source antenna is mounted near the ground. The

first null is pointed toward ground to suppress the reflection.


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Compact Ranges

C. Anechoic chambers

Tapered chamber

We can move the source near the apex at the

lower end of the frequency and further away as

the frequency increases to produce nearly

constructive interference with the direct rays

at the test location.

Anechoic chamber are mostly utilized in the microwave region.

provide convenience and controlled EM environment, an all-weather capability and security.

Inner surface are covered with special RF/microwave absorbers.two types: rectangular chambers and tapered chambers.

Compact Ranges

The Compact Antenna Test Range (CATR) is

a collimating device which generates nearly

planar wave in a short distance (about 10 to

20 m).

Parabolic

curve

feed

Quiet

zone

Single paraboloid compact range

Test zone


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Compact Ranges

An offset feed is used for the reflector to prevent aperture blockage and to reduce the

diffraction from the primary feed structure.

The quiet zone is typically 50-60% the size of the reflector.Acceptable deviations for most CATRs are: less than 100 phase error, less than 1dB

ripple and taper amplitude deviations.

Low-gain feed antennas are used to decrease the amplitude taper.

Amplitude and phase ripples in the quiet zone are primarily caused by diffraction

from the edges of the reflector. There are two popular ways to reduce it: serrated-edge

reflectors and rolled-edge reflectors.

serrated-edge reflector

rolled-edge

reflectors

Four configurations: the single paraboloid;

the dual parabolic cylinder, the dual shaped-

reflector, the single parabolic cylinder.


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Compact Ranges

Dual parabolic cylinder compact range

feed

Quiet

zone

First reflector

Second reflector

Dual shaped-reflector compact range

Quiet

zone

feed

Sub-reflector

Main

reflector

The single paraboloidal reflector CATR is easy to implement and costs less. Feed spillover

into the quiet zone is low.

Because of the boresight of the feed antenna is directed at almost 900 to the plane wave

propagation direction, quiet zone contamination is relatively high for the dual paraboliccylinder configuration. However the cross polarization is relatively low for this design.

The shape of the sub-reflector maps the high-gain feed pattern into a nearly optimum

illumination of the main reflector. The dual shaped-reflector design results in a very

high illumination efficiency. Thus the spillover is reduced and the sensitivity is increased.


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Near Field/Far-Field Methods

The field amplitude, phase and polarization are measured in the near field of the

test antenna, which is in radiating mode. The near-field data is then transformed to

far-field patterns using analytical methods.

The NF/FF method begins with measurement of the magnitude and phase of the

tangential electric field radiated by the test antenna at regular intervals over a well-

defined surface: a plane, a cylinder, or a sphere in the near field. By the principle of

modal expansion, The sampled E field data is used to calculate the angular spectrum

of plane, cylindrical or spherical waves, which matches closely the radiated field

angular distribution.

The far-field radiation pattern of any aperture (surface) is the Fourier transform of

the aperture field distribution.


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Far-field pattern measurementsThe far-field patterns are measured on the surface of a sphere of constant radius.

Any position on the sphere can be identified by two angular coorGLQDWHV ) of

the spherical coordinate system.

In general, the pattern of an antenna is 3-dimensional. Because it is impractical tomeasure a 3-D pattern, usually two 2-D pattern cuts are measured and they must be

orthogonal.

Receiver

Polarization

positioner

Source

control

Positioner

control

Test positioner

indicators

Pattern

recorder

Test

antenna

source

Simple block diagram of a pattern

measuring system.

Phase

measurement

circuit

Test

antenna

Reference

Test

Probe

Phase pattern measuring system


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Gain measurements

Usually, free-space ranges are used to measure the gain of antennas operating above 1 GHz.

Between 0.1-1GHz, ground-reflection ranges are utilized. Below 0.1 GHz, the gain is

measuredin situ.

Gain is the most important figure-of-merit parameter of an antenna.

Two methods: absolute-gain and gain-transfer ( or gain-comparison)

Absolute-gain measurements

The methods are based on Friis transmission formula.

There are two basic methods: two-antenna method and three-antenna method.

A. Two-antenna method

)](log10)4

(log20[2

1)( 1010

t

r

dB

P

PRG +=

Two identical test antenna, one is transmitting and the other is receiving.

Assuming that the antennas are well matched in terms of impedance and polarization.

R: antenna separation

Pr: received power

Pt: transmitted power


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Gain measurements

B. Three-antenna method

If the two antennas in the measuring system are not identical, three antennas must be

employed and three measurement must be made.

All three combinations:

)(log10)4

(log20)()(1

2101021

t

r

dBdB

P

PRGG +=+

)(log10)4

(log20)()(1

3

101031

t

r

dBdB

P

PRGG +=+

)(log10)4

(log20)()(2

3

101032

t

r

dBdBP

PRGG +=+

From these equations, the gains of all three antennas can be determined.

antenna 1 and antenna 2




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Gain measurements

Gain-Transfer (Gain-Comparison) measurements

This technique needs a gain standard whose gain is known.

Two sets of measurements are performed.

I. The test antenna is in receiving mode, and the received power PTis recorded;II. The test antenna is replaced by standard gain antenna and its received power P

s

is measured. The geometrical arrangement is kept intact and the transmitted power

is maintained same.

)(log10)()( 10s

T

dBsdBT P

PGG

+=

GT: gain of test antenna G

s: gain of standard antenna

If the test antenna is circularly or elliptically polarized, two orthogonal linearly polarized

gain standards must be used in order to obtain the partial gains corresponding to each

linearly polarized component. The total gain of the test antenna is:

)(log10)( 10 THTVdBT GGG +=

GTV

: the partial gain with respect to vertical polarization

GTH: the partial gain with respect to horizontal polarization


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Polarization measurementsA general polarization of an antenna is described by polarization ellipse (the axial ratio and

the tilt angle), as well as the sense of rotation (clockwise or counter-clockwise, right-hand or

left-hand).

The polarization measurement techniques are classified into three general categories.

I. Partial methods: provide partial polarization information.

II. Comparison methods: yield complete polarization information but require a polarization

standard.

III. Absolute methods: yield complete polarization information and require no polarization

standard.

Polarization-pattern method

This method can only determine the polarization ellipse in a given direction of radiation but

not the sense of rotation.

The antenna under test can be either in transmitting or in receiving mode. The probe antennamust be linearly polarized. Usually it is a dipole.

probe

Test antenna


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Polarization measurements

Linearly polarized

If the test antenna

is linearly polarized, the output voltage will be proportional to sin .

is circularly polarized, the polarization pattern is a circle.

is elliptically polarized, a typical dumb-bell contour is obtained.

Circularly polarized

Polarization

pattern

Polarization

ellipse

Elliptically polarized

If the polarization of an entire plane is needed to know, the test antenna (working in the

receiving mode) is rotated over the desired plane while the probe is used as a source. The

pattern obtained is called as the axial ratio pattern. From this pattern, the ratio of the outer

envelope to the inner one for a given angle represents the axial ratio of the polarization in

that direction.


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Scale model measurements

In some cases, such as with antennas on the ships,

aircraft, spacecraft, etc., the antenna and its supporting

structure are too heavy in weight or too big in size to be

moved or accommodated by the antenna ranges. We use

geometrical scale model to perform antenna measurements.

The principle ofgeometrical scale model

Permittivity

PermeabilityVelocity

Impedance

Antenna gain

l=l/n

t=t/n= /n

C=C/n

L=L/n

Ae=A

e/n2

f=nf

=n

Length

TimeWavelength

Capacitance

Inductance

Echo area

Frequency

Conductivity

Unchanged parametersScaled Parameters

The whole structure is scale modeled by a factor of n.

The ideal scale modeling requires exact replicas both

physically and electrically.The 1/15th scale model of V-22 aircraft

in Villanovas anechoic chamber

Antenna Measurements techniques

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