IEC PAS 62431:2005

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IEC
AVAILABLE
PAS 62431
SPECIFICATION
First edition
Pre-Standard
2005-07
Measurement methods for reflectivity
of electromagnetic wave absorbers
in millimetre wave frequency
Reference number
IEC/PAS 62431:2005(E)
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PUBLICLY
IEC
AVAILABLE
PAS 62431
SPECIFICATION
First edition
Pre-Standard
2005-07
Measurement methods for reflectivity
of electromagnetic wave absorbers
in millimetre wave frequency
 IEC 2005  Copyright - all rights reserved
No part of this publication may be reproduced or utilized in any form or by any means, electronic or
mechanical, including photocopying and microfilm, without permission in writing from the publisher.
International Electrotechnical Commission, 3, rue de Varembé, PO Box 131, CH-1211 Geneva 20, Switzerland
Telephone: +41 22 919 02 11 Telefax: +41 22 919 03 00 E-mail: inmail@iec.ch Web: www.iec.ch
PRICE CODE
Commission Electrotechnique Internationale
X
International Electrotechnical Commission
Международная Электротехническая Комиссия
For price, see current catalogue

– 2 – PAS 62431 © IEC:2005(E)
CONTENTS
1 Scope .5

2 Normative references .5

3 Terms, definitions and acronyms.5

3.1 Terms and definitions .5

3.2 Acronyms .8

4 Specimen .8

4.1 Specimen specification .8
4.2 Reference metal plate.8
4.3 Reference specimen for calibration .9
5 Specimen holder.9
6 Measurement equipment.9
6.1 Network analyser.9
6.2 Antenna .10
6.3 Amplifier.10
7 Measurement condition.10
7.1 Temperature and environment .10
7.2 Calibration temperature of measurement equipment .10
7.3 Warming-up of measurement equipments.10
7.4 Electromagnetic environment .10
7.5 Calibration of measurement equipment .11
7.6 Cable calibration .11
8 Calibration of measurement system and measurement conditions.11
8.1 Calibration of measurement system .11
8.2 Measurement conditions .11
9 Horn-antenna method .11
9.1 Measurement system.11
9.2 Measurement conditions .14
9.3 Measurement procedures .14
10 Dielectric lens antenna method – Focused beam type - .15
10.1 Outline .15
10.2 Measurement system.16

10.3 Measurement procedures .18
11 Dielectric lens method – Parallel beam method .19
11.1 Principle.19
11.2 Measurement system.20
11.3 Specimen .22
11.4 Measurement procedures .23
12 Test report .24
12.1 Content .24
12.2 Specimen .24
12.3 Measurement data.24
12.4 List of test equipment .24
12.5 Unit .24
12.6 Measurement method .24
12.7 Measurement condition.24

PAS 62431 © IEC:2005(E) – 3 –
Annex A (informative) Reflection and scattering from metal plate – Horn antenna

method.25

A.1 Reflection characteristics.25

A.2 Scattering characteristics θ ≒ 0°.26

Annex B (informative) Reflectivity of reference specimens using the horn-antenna

method.27

Annex C (Informative) Specifications of commercially available antennas.28

C.1 Horn antennas.28

C.2 Antennas consisting of dielectric lens.29

Annex D (informative) Calibration using network analyzer.30
D.1 Type of calibration .30
D.2 Calibration procedures.30
Annex E (informative) Dynamic range and measurement errors .32
Annex F (informative)  Enlargement of dynamic range – Calibration by isolation.33
Annex G (informative) Example of method of calculation of directional gain of horn
antenna.34
Annex H (informative) Relative permittivity of Styrofoam and foamed polyethylene
based on foam ratio.35
Annex I (informative) Calculation of Fraunhofer region – Horn antenna method .36
Annex J (informative) Electromagnetic field distribution near the focus - .37
Annex K (informative)  Time-domain and gating function of network analyser .38
K.1 Time domain.38
K.2 Time resolution and aliasing .38
K.3 Time resolution.38
K.4 Maximum measurable range（aliasing-free range） .39
K.5 Time gating and optimal gate limit.39
Annex L (informative) Specimen holder and antenna positioner .42
Annex M (informative) Example of design of dielectric lens .43
M.1 Design of dielectric lens.43
M.2 Design of antenna .43

– 4 – PAS 62431 © IEC:2005(E)
INTERNATIONAL ELECTROTECHNICAL COMMISSION

____________
MEASUREMENT METHODS FOR REFLECTIVITY

OF ELECTROMAGNETIC WAVE ABSORBERS

IN MILLIMETRE WAVE FREQUENCY
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote

international co-operation on all questions concerning standardization in the electrical and electronic fields. To
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agreement between the two organizations.
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
A PAS is a technical specification not fulfilling the requirements for a standard but made
available to the public.
IEC-PAS 62431 has been processed by subcommittee 46F: RF and microwave passive
components, of IEC technical committee 46: Cables, wires, waveguides, r.f. connectors, r.f.
and microwave passive components and accessories.
The text of this PAS is based on the This PAS was approved for
following document: publication by the P-members of the
committee concerned as indicated in
the following document
Draft PAS Report on voting
46F/26/NP 46F/29/RVN
Following publication of this PAS, which is a pre-standard publication, the technical committee
or subcommittee concerned will transform it into an International Standard.
This PAS shall remain valid for an initial maximum period of three years starting from
2005-07. The validity may be extended for a single three-year period, following which it shall
be revised to become another type of normative document or shall be withdrawn.

PAS 62431 © IEC:2005(E) – 5 –
MEASUREMENT METHODS FOR REFLECTIVITY

OF ELECTROMAGNETIC WAVE ABSORBERS

IN MILLIMETRE WAVE FREQUENCY
1 Scope
This PAS specifies the measurement methods for the reflectivity of electromagnetic wave

absorbers (EMA) for the normal incident, oblique incident and each polarized wave in the

frequency range from 30 GHz to 300 GHz. In addition, these methods are also equally
effective for the reflectivity measurement of other materials.
This PAS is applicable not only to those EMA which are widely used as the counter-measures
against communication faults, radio interference etc., but also to those used in an anechoic
chamber in some cases. EMAs may be any kind of material and may have any arbitrary shape,
configuration, or layered structure as indicated below.
Material: Conductive material, dielectric material, magnetic material
Shape: Planar, pyramidal-type, wedge-type, etc.
Layer structure: Single layer, multi layers, and graded-index material
This PAS may give the measurement method of reflectivity applicable to various EMAs or
materials. However, it may not be applicable to all EMAs.
This PAS may be supplemented with additional methods if necessary so that a future demand
may be fulfilled.
The PAS specifies the measurement methods for the reflectivity of EMA in the millimetre-
wave range:
– measurement frequency range: 30 GHz to 300 GHz
– reflectivity: 0 to –50 dB
– incident angle: 0° to 80°.
2 Normative references
The following referenced documents are indispensable for the application of this document.
For dated references, only the edition cited applies. For undated references, the latest edition
of the referenced document (including any amendments) applies.

ISO/IEC 17025, General requirements for the competence of testing and calibration
laboratories
IEEE 1128, IEEE Recommended Practice for EMA evaluation in the range from 30 MHz to
5 GHz
3 Terms, definitions and acronyms
3.1 Terms and definitions
For the purposes of this document, the terms and definitions of IEEE 1128, as well as the
following apply.
– 6 – PAS 62431 © IEC:2005(E)
3.1.1
ambient level
value of radiation power or noise which exists when no measurement is being carried out at

the experiment site
3.1.2
dynamic range
difference in decibels between the receiving level from a reference metal plate and the

receiving level measured when the metal plate is removed

3.1.3
directional gain
ratio of the radiated power density in a particular direction to the average power density that
would be radiated in all directions
3.1.4
dielectric lens
electromagnetic wave lens that is composed of dielectric material, usually mounted in front of
a pyramidal or conical horn
3.1.5
electromagnetic wave absorber
material ingredient which absorbs the electromagnetic wave energy and dissipates it thermally
3.1.6
focused beam
focused electromagnetic wave converged by the dielectric lens mounted in front of the horn
antenna. The focused beam diameter is a few times the wavelength or more at the beam
waist, which depends on the focal distance of the lens
3.1.7
Fraunhofer region
The region where the angular radiation pattern of an aperture antenna is nearly independent
of the distance.
3.1.8
Fresnel region
region where the angular radiation pattern of an aperture antenna depends on the distance
except for the extremely near region from the aperture
3.1.9
free-space method
measurement method that employs a single or a pair of horn antennas where the specimen
and the antennas are put in free space
3.1.10
horn antenna
aperture antenna where impedance matching is taken gradually from the waveguide aperture
to free space
3.1.11
normal incidence
incident electromagnetic wave striking normally to the specimen surface. The reflectivity in
normal incidence is usually measured in the configuration where the incident angle of a
transmit antenna and that of a receive antenna are within 0～5° with respect to the normal
direction of the specimen surface

PAS 62431 © IEC:2005(E) – 7 –
3.1.12
oblique incidence
incident electromagnetic wave striking to the specimen surface at an oblique angle. The

reflectivity in oblique incidence is usually measured with a transmit and a receive antenna set

up so that the incident and reflected angle of the EM wave may be equal

3.1.13
parallel beam
EM wave, which has a nearly flat phase front on the surface normal to the antenna axis, and

which is formed using a dielectric lens set up in the front of a horn antenna

3.1.14
monostatic measurement
measurement where the incident and reflected waves follow the same direction which lie at an
arbitrary angle with respect to normal to the specimen surface
3.1.15
bistatic measurement
measurement where the incident and reflection angle is equal
3.1.16
beam waist
portion at which the diameter of the focused beam becomes minimum when the
electromagnetic wave radiated from a transmit antenna is converged using a dielectric lens
3.1.17
focal point
centre of beam waist when the electromagnetic waves are converged using a dielectric lens
3.1.18
focal distance
distance between the centre of the dielectric lens and the focal point
3.1.19
reflectivity
reflectivity is expressed by
V
s
reflectivity = 20 log [dB],
V
m
where V is the reflected EM wave voltage received by the receive antenna when the
s
specimen is irradiated by the EM wave, and V is the voltage of the EM wave reflected from a
m
metal plate with equal size and with the same projection shape as normal to specimen surface

V V
s m
Key
1 Tx antenna
2 Rx antenna
3 EMA
4 Metal plate
Figure 1 – Definition of reflectivity

– 8 – PAS 62431 © IEC:2005(E)
3.1.20
reference metal plate
metal plate with the same shape and equal surface projected area as normal to the specimen

3.1.21
transverse electromagnetic wave

EM wave in which both the electric and magnetic fields are perpendicular to the plane of

incidence when an EM wave is incident normally to the specimen surface

3.1.22
transverse electric wave
EM wave in which the electric field is perpendicular to the plane of incidence when the EM
wave is incident to the specimen surface at an oblique angle
3.1.23
transverse magnetic wave
EM wave in which the magnetic field is perpendicular to the plane of incidence when the EM
wave is incident to the specimen surface at an oblique angle
3.1.24
time-domain function
VNA generally has a function to transform the measured frequency domain data to time
evolution data using Fourier transform because the VNA can measure both the amplitude and
phase of EM wave. Therefore, the reflected wave only from the specimen can be extracted by
applying a suitable time gating to the time evolution output signal and inverse Fourier
transform
3.2 Acronyms
Acronyms
EMA Electromagnetic wave absorber
NWA Network analyser
VNA Vector network analyser
TEM Transverse electromagnetic
TE Transverse electric
TM Transverse magnetic
4 Specimen
4.1 Specimen specification
It is recommended that the specimen have a flat surface and rigid structure having a
dimension equal to, or larger than 15 λ where λ is the wavelength of the EM wave at the
lowest frequency in the measurement frequency range. However, the detailed specifications
are given in each type of the two measurement methods described below.
4.2 Reference metal plate
4.2.1 Material and thickness
Aluminium, copper, stainless steel etc., which has a thickness of 1 mm to 2 mm, is preferred.
4.2.2 Surface roughness
The surface roughness of a reference metal plate should be less than λ/10, although less than
λ/20 is preferred, where λ is the wavelength that corresponds to the maximum frequency in

PAS 62431 © IEC:2005(E) – 9 –
the measurement frequencies range. For example, if the maximum frequency is 300 GHz,

then λ is 1 mm, and the preferable roughness becomes 0,05 mm.

4.2.3 Flatness
It is recommended that the flatness be less than 0,5 mm for a reference metal plate with size

1 m × 1 m.
4.2.4 Size and shape
Reference metal plate should have the same size and same projection shape normal to the
specimen surface. However, it is desirable to use the size specified by each method
described below. Care should be taken in selecting the size of the reference metal plate
because the reflection and scattering characteristics may depend on its size due to the
Fresnel refraction. The dependence of the reflection and scattering characteristics on the size
in the case of the horn antenna method is illustrated in Annex A.
4.3 Reference specimen for calibration
A reference specimen for calibration should be silica-glass plate or sapphire single-crystal
(001) plate with uniform thickness and smooth surface roughness. Relative permittivity should
be known in advance. When the dielectric material is selected, it is necessary to measure the
reflectivity of the specimen without putting anything on the backward surface of the reference
specimen. The reference specimen should be fixed by foamed plastics, which have a relative
permittivity, of near to 1, and EM waves do not reflect as in free space. It is recommended
that the accuracy of the measurement system be measured by comparing the measured
reflectivity with the theoretical one. The reflectivity of a silica-glass plate or sapphire plate
measured in the millimetre wave range is given in Annex B.
5 Specimen holder
A specimen holder may be different from any type of measurement method mentioned below.
The specimen holder should possess functions of adjusting azimuth and elevation.
6 Measurement equipment
Correct usage of the measurement equipment is very important in order to obtain the exact
results. The measurement of the reflectivity of EMA shall be performed using either a VNA or
scalar network analyser. When there are discrepancies in the measured results, it is
necessary to make calibration of the measurement system using a reference specimen. The
necessary equipment should be selected according to the type of measurement methods used,

as shown in below.
6.1 Network analyser
6.1.1 Vector network analyser
The VNA is recommended because it can measure both the amplitude and phase and time
domain function.
6.1.2 Scalar network analyser
The scalar network analyser can only measure the amplitude, and does not have time-domain
function, which is mainly used for relatively low accuracy measurement.

– 10 – PAS 62431 © IEC:2005(E)

6.2 Antenna
6.2.1 Horn antenna
Both a commercial as well as an in-built horn antenna can be used for the reflectivity
measurement of EMA except in special cases. However, The commercial horn antenna is

recommended in order to obtain the required measurement accuracy, which has an accurate

gain, VSWR, and size. The commercial coaxial-waveguide transducer is also recommended

where the VSWR or sizes are verified in each frequency band. The specifications of some

commercial horn antennas are shown in Annex C.

6.2.2 Lens antenna
Not only a dielectric lens antenna but also a metal-plate lens antenna or Luneberg lens
antenna can be used for the reflectivity measurement of EMA in this PAS. Either a
commercially available or an in-built product can also be applicable. However, the use of a
commercial antenna, in which the antenna gain, VSWR, and sizes are specified, will be
recommended in order to realize the required measurement accuracy. The specifications of
commercial horn antennas and dielectric lenses are illustrated in Annex C.
6.3 Amplifier
An amplifier is generally used in order to get sufficient dynamic range of the measurement
system. The warming-up of the amplifier is required, and the temperature should be kept as
constant as possible because the total gain of the amplifier will vary due to the temperature
drift as described in Clause 7.
7 Measurement condition
7.1 Temperature and environment
The measurement should be carried out in the atmosphere from 860 hPa to 1 060 hPa, and in
the room from 5 °C to 35 °C, and relative humidity from 45 % to 85 %. If the operation
temperature and humidity range of the measurement equipment are narrower than the above
range, the specifications of the measurement equipment should be followed. It is desirable to
control the measurement temperature within ±3 °C in order to suppress the influence of the
temperature drift of measurement equipment to a minimum. The measurement temperature of
the specimen should be selected to be 20 °C, 23 °C or 25 °C. In the case of high humidity,
relative humidity should be maintained at either 50 % or 65 % in measurement.
7.2 Calibration temperature of measurement equipment
If the temperature at which measurement equipment is calibrated is within ±3 °C around the

measurement temperature, measurement errors can be minimized. However, if the
measurement temperature exceeds the range of ±3 °C, then it is recommended to carry out
the calibration again.
7.3 Warming-up of measurement equipments
The warming-up time must be kept, typically 15-45 min, written in the specifications of the
measurement equipment or systems. Moreover, the warming-up time should be taken to be
longest in all of the measurement equipments.
7.4 Electromagnetic environment
When the EM wave power density in the measurement environment exceeds the public
regulation, and when the EM environment is judged to be not so good, the measurement
should be carried out in an anechoic room. When the directional gain of an antenna is large,
however, an anechoic chamber may not necessarily be required.

PAS 62431 © IEC:2005(E) – 11 –

7.5 Calibration of measurement equipment

The equipment shall be calibrated according to the standard established by the manufacturers,

or according to ISO/IEC 17025, or other corresponding standard. The items to be calibrated

include frequency, voltage, and attenuation, which depend on the measurement accuracy or

uncertainty of the measurement equipment.

7.6 Cable calibration
Degradation in the transmission characteristics of cables shall be checked when the cables

are connected direct without the intervention of an EMA or free space.

8 Calibration of measurement system and measurement conditions
8.1 Calibration of measurement system
Calibration of the measurement system shall be carried out according to the recommended
methods by NWA. Typical calibration methods are shown in Annex D.
8.2 Measurement conditions
8.2.1 Dynamic range
Both the receive levels with and without the reference metal plate shall be measured firstly
when the measurement system is set up. The dynamic range is defined as the difference of
these measured values in decibels. Annex E illustrates the relation between the dynamic
range and the measurement error. If the dynamic range of the measurement system is 40 dB
and the reflectivity of the specimen is –20dB, respectively, an error bar lies from –0,92 dB to
+0,83 dB with respect to –20 dB.
8.2.2 Setting up of the network analyser for keeping adequate dynamic range
The dynamic range of the measurement system can be increased by modifying the IF band or
by utilizing the averaging function, etc. of NWA when the dynamic range does not exceed a
necessary value. The dynamic range increases by use of the isolation calibration of VNA, as
shown in Annex F.
9 Horn-antenna method
9.1 Measurement system
9.1.1 Configuration of the measurement system

Figure 2 shows a block diagram of the measurement system. The arrangement of the
transmitting and receiving antennas, and the block diagram of the measurement system in the
horn-antenna method are illustrated below for normal and oblique incidence measurement. In
the measuring transmission coefficient S , a pair of antennas is used whereas only one
antenna is used for measuring the reflection coefficient S in normal incidence.

In the case of oblique incidence, the transmitting antenna should be arranged in such a way
that the central axis makes the same angle to the normal direction of the specimen surface
with that of the receive antenna. Here, if S is measured using two horn antennas in normal
incidence, then the vertical alignment of the transmit and receive antennas should be fixed
within 5°. The measurement equipment including a NWA were given in Clause 6.

– 12 – PAS 62431 © IEC:2005(E)

測定試料測定試料
ネッネットトワーワーククアアナラナライイザザ
受信ア受信アンンテテナナ
試料架台試料架台
コンコンピピュューータタ 2
送送信信アンアンテナテナ
プリプリンタンタ
a) Normal incidence (S )
Key
受受信信アンアンテナテナ
1 Tx antenna
2 Rx antenna
3 NWA
4 Computer
5 Printer
θθ
6 Specimen holder
7 Specimen
θθ
送送信信アンアンテナテナ
b) Oblique incidence (S )
Figure 2 – Configuration of the measurement system

9.1.2 Horn antenna
Both a commercial as well as an in-built horn antenna can be used for the reflectivity

measurement of EMA except in special cases. Before the measurement of reflectivity it is
necessary to calculate the directional gain of the horn antenna to determine the distance from
the antenna to the specimen. Annex G calculates the directional gain of the horn antenna.
Further, the directional gain, VSWR and sizes must be checked from the catalogue when a
commercial horn antenna is used.
9.1.3 Specimen holder
9.1.3.1 Material and shape
a) Material
The reflection from the specimen holder can be minimized by making use of foamed
polystyrene a with high a foaming ratio as a specimen holder because foamed plastics have
very low relative permittivity (near 1). Annex H shows the relative permittivity of foamed
polystyrene as a function of foaming ratio.

PAS 62431 © IEC:2005(E) – 13 –

b) Shape
The shape in normal projection to the specimen surface and area of a specimen holder which

mounts the specimen should be equal to those of the specimen in order to suppress the

reflection of the EM wave from the specimen holder. The uncovered portion of the specimen

holder should be covered by a pyramidal-type wave absorber, and the shape of the uncovered

portion should have a wedge form, as Illustrated in Figure 3.

Key
1 Pyramidal EMA
2 Specimen holder
3 Mounting point of specimen
4 Wedge form
Figure 3 – Mounting method of specimen
9.1.3.2 Azimuth and elevation angle adjustment function
Figure 4 shows the elevation angle adjustment as well as the lifting and descending
mechanism. An azimuth table under the specimen holder, which has the mechanism adjusting
elevation and azimuth angle, should be installed in order to enable the accurate installation of
the specimen with respect to the transmit antenna. The accuracy of elevation and directional
angle should be about 0,1°.
Key
1 Specimen holder
2 Azimuth angle adjustment function
3 Elevation angle adjustment function

Figure 4 – Mechanism for adjusting azimuth and elevation
9.1.4 Mounting of the specimen
Either double-sided adhesion tape, simple paste or thin cellophane tape is used to fix the
reference metal plate and specimen to the specimen holder.

– 14 – PAS 62431 © IEC:2005(E)

9.1.5 Antenna stand
Attention should be paid to cover up a tripod portion by a pyramidal-type wave absorber

although the quality of the material of the tripod stand which mounts a transmitting antenna

does not need to be made from rosin or wood.

9.2 Measurement conditions
9.2.1 Measurement environment
The measurement should not necessarily be performed in an anechoic chamber, which

depends on the directional gain of the antenna. However, there shall be no obstacle in the

direction of the main beam of the horn antenna. If the obstacle cannot be removed, a screen
of pyramidal-type wave absorbers shall be installed on the path of the EM wave. When the
oblique incidence characteristic is measured, the floor and roof, etc. should also be covered
with pyramidal-type EMA because the reflected EM wave from them has the same path length
as that from the specimen in many cases.
9.2.2 Measuring distance
When the EM waves are radiated from the rectangular aperture of a horn antenna, the
distance, R which separates the Fresnel region from the Fraunhofer region, the boundary
between the two may be arbitrarily taken to be at Eq. (1), where D is an effective maximum
dimension of the antenna aperture, and λ is the wavelength. The directional gain, G of the
d
horn antenna is represented by Equation (2). From Equations(1) and (2), the range of R
representing the Fraunhofer region can be expressed by Equation (3) using G .
d
R≥ 2D /λ (1)
2 2
G = 4πD /λ (2)
d
R≥ G λ / 2π (3)
d
It is desirable to keep the distance between the specimen and the antennas greater than the
right-hand side of Equation (3), which depends on the measurement frequency. Annex I
shows the relation of the directional gain of the antenna and measuring distance.
9.2.3 Size of specimen
The size of the specimen should be larger than 10λ × 10λ for the reflectivity measurement
using the horn-antenna method, whereλ is the maximum wave length in the measurement
frequency range. If the size of the specimen is smaller than 10λ × 10λ, a quite accurate

adjustment of azimuth and elevation angles should be done.
9.3 Measurement procedures
Measurement is carried out according to the following steps after installation of measurement
equipment, based upon the conditions described in Clause 7.
9.3.1 Adjustment of measurement system
a) Set up the transmitting antenna and specimen holder according to each measurement
condition, i.e. normal or oblique incidence, distance between specimen and antennas, etc.
b) Set up the transmit antenna in such a way that its height will be at the centre of the
specimen, and adjust the horn antenna so that the aperture may be perpendicular to the
horizontal plane using a spirit level.
c) Set up the reference metal plate on the specimen holder, and adjust the elevation angle
so that the reference metal plate is perpendicular to the horizontal plane using a spirit
level.
PAS 62431 © IEC:2005(E) – 15 –

d) Set up the position and normal direction of the reference metal plate so that the receiving

level of the scattered EM wave may become maximum by rotating the metal plate through

±10° of the directional angle using an azimuth turntable.

e) Check the dynamic range of the measurement system. Measure the receiving level of the

reference metal plate at the measurement frequency range. Remove the reference metal

plate and measure the receive level. Calculate the dynamic range and the difference of

the two levels in decibels. Carry out the isolation calibration according to 8.2.2 when the

desired dynamic range is not obtained.

9.3.2 Measurement using scalar network analyser

a) Set up the reference metal plate on the specimen holder, and measure the receiving level,

R (dB).
metal
b) Replace the reference metal plate by specimen on the specimen holder, and measure the
receiving level, R [dB].
absorber
c) Calculate the reflectivity of the specimen by subtracting the receiving level R [dB] from
metal
receiving level, R [dB].
absorber
9.3.3 Measurement using vector network analyser
a) Set up the reference metal plate on the specimen holder. Measure the vector quantities of
&
the receiving level, Γ .
metal
b) Remove the reference metal plate from the specimen holder, and measure the receiving
&
level, Γ without the specimen.
residual
c) Mount the specimen on the specimen holder, and measure the vector quantities of the
&
receiving level, Γ .
absorber
d) Remove the specimen from the specimen holder, and measure the vector quantities of the
&
receiving level, Γ .
residual
& &
e) Subtract the vector quantities of the receiving levels, Γ from Γ , and subtract
residual metal
the undesired waves other than those reflected directly from EMA.
f) Transform the vector quantities into the time-domain data from the frequency domain data,
and apply time gating for the main response from EMA only.
g) After the time gating is applied, transform the responses into the frequency domain
receiving level, R (dB) of the reference metal plate.
metal
&
h) Subtract the vector quantities of receiving level, Γ , from the vector quantities of the
residual
&
receiving level, Γ of the reference metal plate, and subtract the undesired waves.
absorber
i) Transform the vector quantities obtained into the time domain from the frequency-domain
data, and apply time gating for the main response only.
j) After the time gating is applied, these responses are retransformed to the frequency
domain data, receiving level, R (dB) of the specimen.
absorber
k) Calculate the reflectivity of specimen by subtracting the receiving level, R (dB) of the
metal
reference metal plate from the receiving level, R (dB) of specimen.
absorber
10 Dielectric lens antenna method – Focused beam type -
10.1 Outline
A method which uses the focused beam type horn antenna has the following characteristics.
— Large measurement space may not necessarily be required because the focused EM wave
has a beam waist of several wavelength and has nearly flat phase-front on the focal plane.

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— Sufficient dynamic range can be easily obtained because the EM wave does not spread

over into the surroundings.
— The measurement cannot necessarily be carried out in an anechoic chamber in the case

where the large dynamic range is not required because the scattered EM waves in the

surroundings cannot easily become a receiving antenna.

10.2 Measurement system
10.2.1 Transmitting and receiving antennas

The block diagram of the measurement system is shown in Figure 5.

1. Network analyzer 1. Network analyser
2. Dielectric lens 2. Tx (transmit) antenna
3. Horn antenna 3. Rx (receive) antenna
4. Incident wave 4. Dielectric lens
5. Reflected wave 5. Incident wave
6. Specimen 6. Reflected wave
a) Normal incidence b) Oblique incidence

Figure 5 – Block diagram of measurement system

10.2.2 Focused beam horn antenna

10.2.2.1 Antenna structure
Figure 6 shows a structure of an antenna with a dielectric lens used in the focused beam
method which is composed of a coaxial-waveguide transducer, mode conversion feed, which
converts a linearly-polarized EM wave to a circularly polarized one, circular horns, and
convex-type dielectric lens. The EM waves radiated from the antenna gradually converge at
the focus, where the minimum beam waist of the EM wave becomes several wavelengths. The
focal length is determined by both curvature of a convex-type dielectric lens and relative
permittivity of the lens material. The amplitude of the EM wave at beam waist changes as
Gaussian as a function of the radial distance away from the central axis of the lens, which is
at its maximum at the centre of the focus. The phase at the focus does not depend so strongly
on the radial distance because both the path (electric length) that is transmitted through the
centre of the lens and the path (electric length) that is transmitted through the peripheral part
of the lens are nearly equal. Some specifications of a commercial dielectric lens antenna,
such as diameter, focal length, and lens material etc., are shown in Clause C.2.

PAS 62431 © IEC:2005(E) – 17 –
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