IEC 62431:2008

IEC 62431 ®
Edition 1.0 2008-07
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Reflectivity of electromagnetic wave absorbers in millimetre wave frequency –
Measurement methods
Réflectivité des absorbeurs d’ondes électromagnétiques dans la plage des
fréquences des ondes millimétriques – Méthodes de mesure
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IEC 62431 ®
Edition 1.0 2008-07
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Reflectivity of electromagnetic wave absorbers in millimetre wave frequency –
Measurement methods
Réflectivité des absorbeurs d’ondes électromagnétiques dans la plage des
fréquences des ondes millimétriques – Méthodes de mesure

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX XA
ICS 17.120; 19.080; 29.120.10 ISBN 978-2-88912-802-0

– 2 – 62431  IEC:2008
CONTENTS
FOREWORD . 5
1 Scope . 7
2 Normative references . 7
3 Terms, definitions and acronyms . 7
3.1 Terms and definitions . 7
3.2 Acronyms and symbols . 10
4 Specimen . 12
4.1 Specimen specification . 12
4.2 Reference metal plate . 12
4.2.1 Material and thickness . 12
4.2.2 Surface roughness . 12
4.2.3 Flatness . 12
4.2.4 Size and shape . 12
4.3 Reference specimen for calibration . 12
5 Specimen holder . 13
6 Measurement equipment . 13
6.1 Type of network analyzer . 13
6.2 Antenna . 13
6.2.1 Horn antenna . 13
6.2.2 Lens antenna . 13
6.3 Amplifier . 13
6.4 Cable . 14
7 Measurement condition . 14
7.1 Temperature and environment . 14
7.2 Warming up of measurement equipment . 14
7.3 Electromagnetic environment . 14
8 Calibration of measurement system and measurement conditions . 14
8.1 Calibration of measurement system . 14
8.2 Measurement conditions . 14
8.2.1 Dynamic range . 14
8.2.2 Setting up of the network analyzer for keeping adequate dynamic
range . 14
9 Horn antenna method . 15
9.1 Measurement system . 15
9.1.1 Configuration of the measurement system . 15
9.1.2 Horn antenna . 16
9.1.3 Specimen holder . 16
9.1.4 Mounting of the specimen . 18
9.1.5 Antenna stand . 18
9.2 Measurement conditions . 18
9.2.1 Measurement environment . 18
9.2.2 Measuring distance . 18
9.2.3 Size of specimen . 18
9.3 Measurement procedures . 19
10 Dielectric lens antenna method – focused beam method . 20
10.1 Outline . 20

62431  IEC:2008 – 3 –
10.2 Measurement system . 20
10.2.1 Transmitting and receiving antennas . 20
10.2.2 Focused beam horn antenna . 21
10.2.3 Specimen size . 22
10.2.4 Reference metal plate size . 22
10.2.5 Specimen holder . 22
10.2.6 Method of fixing the specimen and the reference metal plate . 23
10.3 Measurement procedures . 23
11 Dielectric lens antenna method – parallel beam method . 25
11.1 Principle . 25
11.1.1 Outline . 25
11.1.2 Parallel EM wave beam formed using a EM wave lens . 25
11.2 Measurement system . 26
11.2.1 Composition of measurement system . 26
11.2.2 Dielectric lens antenna . 29
11.3 Specimen . 29
11.3.1 General . 29
11.3.2 Reference metal plate . 29
11.3.3 Size of specimen . 30
11.4 Measurement procedures . 30
11.4.1 Normal incidence . 30
11.4.2 Oblique Incidence . 30
12 Test report. 31
Annex A (informative) Reflection and scattering from metal plate – Horn antenna
method . 33
Annex B (informative) Reflectivity of reference specimens using horn antenna method . 38
Annex C (informative) Specifications of commercially available antennas . 39
Annex D (normative) Calibration using VNA . 42
Annex E (informative) Dynamic range and measurement errors . 50
Annex F (informative) Enlargement of dynamic range – Calibration by isolation . 52
Annex G (informative) Relative permittivity of styrofoam and foamed polyethylene
based on foam ratio . 53
Annex H (informative) Calculation of Fraunhofer region – Horn antenna method . 54

Figure 1 – Definition of reflectivity . 10
Figure 2 – Configuration of the measurement system normal incidence (S ) . 15
Figure 3 – Configuration of the measurement system oblique incidence (S ) . 16
Figure 4 – Mounting method of specimen . 17
Figure 5 – The mechanism of adjusting azimuth and elevation . 17
Figure 6 – Measurement system for normal incidence (side view) . 20
Figure 7 – Measurement system for oblique incidence (top view) . 21
Figure 8 – Structure of a dielectric lens antenna . 22
Figure 9 – Structure of specimen holder. 23
Figure 10 – EM wave propagation using a horn antenna and a dielectric lens . 26
Figure 11 – Block diagram of the measurement system . 27
Figure 12 – A measurement system for normal incidence . 28

– 4 – 62431  IEC:2008
Figure 13 – Measurement system for oblique incidence . 28
Figure 14 – Position of a shielding plate . 29
Figure 15 – Items to be mentioned in a test report . 32
Figure A.1 – Reflection from the reference metal plate versus measurement distance
between the antenna and the metal plate . 33
Figure A.2 – Reflectivity of reference metal plate versus size. 34
Figure A.3 – Reflectivity of reference metal plate at 40 GHz . 35
Figure A.4 – Reflectivity of reference metal plate with cross section of 200 mm ×
200 mm at 40 GHz . 35
Figure A.5 – Analysis of reflection from a metal plate . 37
Figure B.1 – Reflectivity of a 200 mm × 200 mm silica-glass plate in millimetre wave
frequency . 38
Figure C.1 – Representative specifications of a horn antenna . 39
Figure C.2 – Structure of cylindrical horn antenna with dielectric lens in Table C.2, A
used at 50 GHz - 75 GHz . 40
Figure C.3 – A structure of dielectric lens and horn antenna in Table C.2, D . 41
Figure D.1 – Measurement configuration for the case of normal incidence with a
directional coupler connected directly to the horn antenna . 42
Figure D.2 – Configuration for response calibration using a reference metal plate in the
case of normal incidence . 43
Figure D.3 – Configuration for response calibration using a reference metal plate in the
case of oblique incidence . 44
Figure D.4 – Configuration for response and isolation calibration in the case of normal
incidence . 45
Figure D.5 – Configuration for response and isolation calibration in the case of oblique
incidence . 45
Figure D.6 – Configuration for S 1-port full calibration in the case of normal
incidence . 46
Figure D.7 – Precision antenna positioner configuration. 47
Figure D.8 – TRL calibration procedure. 48
Figure D.9 – Measurement and TRL calibration of transmission line . 49
Figure E.1 – An example of receiving level of a reference metal plate and that without
a specimen . 50
Figure E.2 – Dynamic range and measurement error of reflectivity . 51
Figure F.1 – A method to remove spurious waves . 52
Figure H.1 – Fraunhofer region and antenna gain . 54

Table 1 – Acronyms . 11
Table 2 – Symbols . 11
Table C.1 – Antenna gain 24 dB (example A) . 39
Table C.2 – Some specifications of antennas with dielectric lenses . 40
Table G.1 – Relative permittivity and foam ratio of styrofoam . 53
Table G.2 – Relative permittivity and foam ratio of foamed polyethylene . 53

62431  IEC:2008 – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
REFLECTIVITY OF ELECTROMAGNETIC
WAVE ABSORBERS IN MILLIMETRE WAVE FREQUENCY –
MEASUREMENT METHODS
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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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.
International Standard IEC 62431 has been prepared by subcommittee SC46F: RF and
microwave passive components, of IEC technical committee 46: Cables, wires, waveguides,
R.F. connectors, R.F. and microwave passive components and accessories.
IEC 62431 replaces and cancels IEC/PAS 62431 with corrections of obvious errors as noted
in 46F/29A/RVN.
This bilingual version published in 2011-11, corresponds to the English version published in
2008-07.
The text of this standard is based on the following documents:
CDV Report on voting
46F/65/CDV 46F/72/RVC
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.

– 6 – 62431  IEC:2008
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IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
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colour printer.
62431  IEC:2008 – 7 –
REFLECTIVITY OF ELECTROMAGNETIC
WAVE ABSORBERS IN MILLIMETRE WAVE FREQUENCY –
MEASUREMENT METHODS
1 Scope
This International Standard specifies the measurement methods for the reflectivity of
electromagnetic wave absorbers (EMA) for the normal incident, oblique incident and each
polarized wave in the millimetre-wave range. In addition, these methods are also equally
effective for the reflectivity measurement of other materials:
– measurement frequency range: 30 GHz to 300 GHz;
– reflectivity: 0 dB to –50 dB;
– incident angle: 0° to 80°.
NOTE This standard is applicable not only to those EMA which are widely used as 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 pointed out
below.
Material: Conductive material, dielectric material, magnetic material.
Shape: planar-, pyramidal-, wedge-type, or other specific shapes.
Layer structure: single layer, multi layers, or graded-index material.
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
3 Terms, definitions and acronyms
For the purposes of this document, the following terms and definitions apply.
3.1 Terms and definitions
3.1.1
ambient level
the value of radiation power or noise which exists when no measurement is being carried out
at the experiment site
3.1.2
associated equipment
an apparatus or product connected for convenience or operation of the equipment
3.1.3
beam diameter
the diameter where the electric field strength decreases by 3 dB from the centre of the
focused beam
– 8 – 62431  IEC:2008
3.1.4
beam waist
the portion at which the diameter of the focused beam becomes minimum when the
electromagnetic waves radiated from a transmit antenna are converged using a dielectric lens
3.1.5
beam waist diameter
beam diameter at the beam waist
3.1.6
bistatic measurement
measurement where the incident and reflection angle are equal
3.1.7
dielectric lens
electromagnetic wave lens that is composed of dielectric material
Usually, it is used by mounting in front of a pyramidal or conical horn.
3.1.8
directional gain
ratio of the radiated power density in a particular direction to the average power density that
would be radiated in all directions
3.1.9
dynamic range
difference in decibels between the receiving level from the reference metal plate and the
receiving level measured when the metal plate is removed
3.1.10
electromagnetic wave absorber
material ingredient which absorbs the electromagnetic wave energy and dissipates it
thermally
3.1.11
focal distance
distance between the centre of the dielectric lens and the focal point
3.1.12
focal point
centre of the beam waist when the electromagnetic waves are converged using a dielectric
lens
3.1.13
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.
3.1.14
fraunhofer region
region where the angular radiation pattern of an aperture antenna is nearly independent of the
distance
62431  IEC:2008 – 9 –
3.1.15
free-space method
measurement method that employs a single or pair of horn antennas where the specimen and
the antennas are put in free space
3.1.16
fresnel region
region where the angular radiation pattern of an aperture antenna depends on the distance
except for the region extremely near to the aperture
3.1.17
horn antenna
aperture antenna where impedance matching is taken gradually from the waveguide aperture
to free space
3.1.18
monostatic measurement
measurement where the incident and reflected waves follow the same direction and which lie
at an arbitrary angle with respect to normal to the specimen surface
3.1.19
normal incidence
the incidence for which an electromagnetic wave strikes to the specimen surface normally
The reflectivity in normal incidence is usually measured in the configuration where the
incident angle of a transmitting antenna and that of a receiving antenna are within 0° to 5°
with respect to the normal direction of the specimen surface.
3.1.20
oblique incidence
the incidence for which an electromagnetic wave strikes to the specimen surface at an oblique
angle
The reflectivity in oblique incidence is usually measured with a transmitting and receiving
antenna set up so that the incident and reflected angle of EM wave may be equal.
3.1.21
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 front of a horn antenna
3.1.22
reference metal plate
metal plate with the same shape and an equal surface projected area in normal to the
specimen
3.1.23
reflectivity
the ratio between reflected EM wave voltage received by the receiving antenna when a
specimen is irradiated by the EM wave, and the voltage of the EM wave reflected from a metal
plate with equal size and with the same projection shape in normal to the specimen surface
expressed in decibel by
V
S
reflectivity = 20log = 20log V − 20log V [dB] (1)

10 10 S 10 m
V
m
where V is the reflected EM wave voltage received by the receiving antenna when a
s
specimen is irradiated by the EM wave, and V is the voltage of the EM wave reflected from a
m
– 10 – 62431  IEC:2008
metal plate with equal size and with the same projection shape in normal to the specimen
surface. See Figure 1.
2 2
V V
s m
3 4
IEC  1094/08
Key
1 Tx antenna
2 Rx antenna
3 EMA
4 Metal plate
Figure 1 – Definition of reflectivity
3.1.24
time domain function
a function that is implemented in VNA to transform the measured frequency domain data to
time evolution data using inverse Fourier transform because the VNA can measure both the
amplitude and phase of EM wave

Using this function, the reflected wave only from the specimen can be extracted by applying a
suitable time gating to the time evolution output signal and Fourier transform.

3.1.25
transverse electric wave
TE 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
3.1.26
transverse electromagnetic wave
TEM wave
EM wave in which both the electric and magnetic fields are perpendicular to the direction of
incidence when an EM wave is incident normally to the specimen surface
3.1.27
transverse magnetic wave
TM 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
3.2 Acronyms and symbols
The acronyms and symbols are shown in Table 1 and Table 2.

62431  IEC:2008 – 11 –
Table 1 – Acronyms
Acronyms
CW Continuous wave
EMA Electromagnetic wave absorber
FFT Fast Fourier transformation
IF Intermediate frequency
NWA Network analyzer
PTFE Polytetrafruorethylene
SNA Scalar network analyzer
TE Transverse electric
TEM Transverse electromagnetic
TM Transverse magnetic
TRL Thru-reflect-line
VNA Vector network analyzer
VSWR Voltage standing wave ratio

Table 2 – Symbols
Symbol Meaning
A A half size of the aperture of an ideal absorber wall (a half side of metal plate with
squared shape (Annex A)
a Longer side size at aperture of pyramidal horn antenna
b Shorter side size at the aperture of a pyramidal horn antenna
c Length of pyramidal horn antenna
C(x) Fresnel integral
D Diameter of the aperture of a conical horn antenna
d Distance from mirror image of transmit antenna to the aperture of ideal absorber wall
d Distance from the aperture of an ideal absorber wall to receiving antenna
D Effective maximum dimension of the antenna aperture
m
E
Electric field strength at the position of an ideal absorber（Annex A）
E Receiving electric field strength in the case that an ideal absorber has a square-
r
shaped aperture with size 2a（Annex A）
E
Receiving electric field strength in the case that ideal absorber is not put（Annex A）
s0
G Directional gain of horn antenna
d
R Distance from the aperture of horn antenna to the specimen
R
Receiving level in the case that the test specimen is put on the specimen holder (dB）
a
R Receiving level in the case that the reference metal plate is put on the specimen
m
holder（dB）
S Reflection coefficient
S Transmission coefficient
S(x) Fresnel integral
V Receiving voltage in the case that the test specimen is put on the specimen holder
a
V Receiving voltage by direct wave from the transmitting and receiving antenna
d
V Receiving voltage by the reflected wave from the metal plate with the same cross
m
section and shape as the test specimen
V Receiving voltage by the reflected wave except for the test specimen
r
– 12 – 62431  IEC:2008
Table 2 (continued)
Symbol Meaning
V Receiving voltage by the reflected wave only from the test specimen
s
β Phase constant (=2π/λ)
Γ
Receiving level in the case that the electromagnetic absorber is put on the specimen
a
holder (vector quantity)
Γ
Receiving level in the case that the reference metal plate is put on the specimen
m
holder (vector quantity)
Γ Receiving level in the case that the test specimen is not put on the specimen holder
r
(vector quantity)
ε ’ Real part of relative permittivity
r
θ Incident angle of electromagnetic wave
λ
Wavelength of electromagnetic wave in free-space

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 10 λ , where λ is the wavelength of the EM wave at the
l l
lowest frequency in the measurement frequency range. However, detailed specifications are
given in each type of the three measurement methods described in Clauses 9, 10, and 11.
4.2 Reference metal plate
4.2.1 Material and thickness
Aluminium, copper, stainless steel or other metal, which has thickness of about 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
m
than λ /20 is preferred, where λ is the wavelength that corresponds to maximum frequency
m m
in 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
The 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 in
Clauses 9, 10, and 11. 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
The 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

62431  IEC:2008 – 13 –
be known in advance. When 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 a material such as foamed plastics,
which has relative permittivity near to 1, and in which EM waves do not reflect like they do in
free space. It is recommended to verify the accuracy of the measurement system by
comparing the measured reflectivity with the theoretical one. The reflectivity of a silica-glass
plate measured in the millimetre wave range is given in Annex B.
5 Specimen holder
A specimen holder might be different by the any type of measurement method mentioned in
Clauses 9, 10, and 11. It should be recommended that the specimen holder possess functions
for adjusting azimuth and elevation.
6 Measurement equipment
The equipment must be calibrated according to the procedure established by the
manufacturers, or calibration laboratories accredited by ISO/IEC 17025. The items to be
calibrated include frequency, voltage, and attenuation, which depend on the measurement
accuracy or uncertainty of the measurement apparatuses. 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 SNA. When there are
discrepancies in the measured results, it is necessary to make calibration of the measurement
system using a reference specimen. Various necessary apparatuses should be selected
according to the type of used measurement methods as shown below.
6.1 Type of network analyzer
The VNA is recommended because it can measure both the magnitude and phase of S and
S and it has a time domain function.
The SNA can measure only the magnitude of S and S .
11 21
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 also be used for the reflectivity measurement of EMA in this standard. Either a
commercially available or an in-built product can also be applicable. However, the use of a
commercial lens antenna, in which 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 a sufficient dynamic range for the measurement
system. The warming up of the amplifier is required and the temperature should be kept as

– 14 – 62431  IEC:2008
constant as possible because the total gain of the amplifier will vary due to the temperature
drift as it is described in Clause 7.
6.4 Cable
Degradation in transmission characteristics of cables must be checked in the measurement
frequency range when the cables are connected directly.
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 apparatuses are narrower than the
above range, the specifications of the measurement apparatuses 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 apparatuses to a minimum. The
measurement temperature or humidity of the specimen should be specified when the
reflectivity depends on temperature or humidity.
7.2 Warming up of measurement equipment
The warming-up time, typically from 15 min to 45 min, must be written in the specifications of
the measurement equipment or systems. Moreover, the warming up time should be taken to
be longest among all of the measurement equipment.
7.3 Electromagnetic environment
When the EM wave power density in the measurement environment exceeds that specified in
public regulations, and when the EM environment is judged to be negative, 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.
8 Calibration of measurement system and measurement conditions
8.1 Calibration of measurement system
Calibration of the measurement system must be carried out according to the recommended
methods by NWA. Typical calibration methods are shown in Annex D. If the temperature at
which the measurement system is calibrated is within ±3 °C of the measurement temperature,
measurement errors can be minimized. However, if the measurement temperature is outside
of the range of ±3 °C, then it is recommended to carry out the calibration again.
8.2 Measurement conditions
8.2.1 Dynamic range
Both the receiving levels with and without the reference metal plate must be measured initially
when the measurement system is set up. 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
reflectivity of specimen is –20 dB, an error bar lies from –0,92 dB to +0,83 dB.
8.2.2 Setting up of the network analyzer 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 of NWA when the dynamic range does not exceed a

62431  IEC:2008 – 15 –
necessary value. The dynamic range increases by use of the isolation calibration of the VNA
as shown in Annex F.
9 Horn antenna method
9.1 Measurement system
9.1.1 Configuration of the measurement system
Figure 2 and Figure 3 show a block diagram of the measurement system. The arrangement of
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.
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