IEC 61000-4-20:2010

IEC 61000-4-20
Edition 2.0 2010-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
BASIC EMC PUBLICATION
PUBLICATION FONDAMENTALE EN CEM
Electromagnetic compatibility (EMC) –
Part 4-20: Testing and measurement techniques – Emission and immunity
testing in transverse electromagnetic (TEM) waveguides

Compatibilité électromagnétique (CEM) –
Partie 4-20: Techniques d’essai et de mesure – Essais d’émission et d’immunité
dans les guides d’onde TEM
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IEC 61000-4-20
Edition 2.0 2010-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
BASIC EMC PUBLICATION
PUBLICATION FONDAMENTALE EN CEM
Electromagnetic compatibility (EMC) –
Part 4-20: Testing and measurement techniques – Emission and immunity
testing in transverse electromagnetic (TEM) waveguides

Compatibilité électromagnétique (CEM) –
Partie 4-20: Techniques d’essai et de mesure – Essais d’émission et d’immunité
dans les guides d’onde TEM
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
XC
CODE PRIX
ICS 33.100.10; 33.100.20 ISBN 978-2-88912-149-6
– 2 – 61000-4-20 © IEC:2010
CONTENTS
FOREWORD.4
INTRODUCTION.6
1 Scope and object.7
2 Normative references .7
3 Terms, definitions and abbreviations .8
3.1 Terms and definitions .8
3.2 Abbreviations .11
4 General .11
5 TEM waveguide requirements.12
5.1 General .12
5.2 General requirements for the use of TEM waveguides .12
5.2.1 TEM mode verification .12
5.2.2 Test volume and maximum EUT size .12
5.2.3 Validation of usable test volume .13
5.3 Special requirements and recommendations for certain types of TEM
waveguides .15
5.3.1 Set-up of open TEM waveguides .15
5.3.2 Alternative TEM mode verification for a two-port TEM waveguide .16
6 Overview of EUT types .16
6.1 General .16
6.2 Small EUT .16
6.3 Large EUT.16
7 Laboratory test conditions .17
7.1 General .17
7.2 Climatic conditions .17
7.3 Electromagnetic conditions.17
8 Evaluation and reporting of test results.17
Annex A (normative) Emission testing in TEM waveguides.19
Annex B (normative) Immunity testing in TEM waveguides.40
Annex C (normative) HEMP transient testing in TEM waveguides .46
Annex D (informative) TEM waveguide characterization.53
Annex E (informative) Calibration method for E-field probes in TEM waveguides .61
Bibliography.71

Figure A.1 – Routing the exit cable to the corner at the ortho-angle and the lower edge
of the test volume .30
Figure A.2 – Basic ortho-axis positioner or manipulator .31
Figure A.3 – Three orthogonal axis-rotation positions for emission measurements.32
Figure A.4 – Twelve-face (surface) and axis orientations for a typical EUT .33
Figure A.5 – Open-area test site (OATS) geometry .34
Figure A.6 – Two-port TEM cell (symmetric septum) .35
Figure A.7 – One-port TEM cell (asymmetric septum) .36
Figure A.8 – Stripline (two plates) .38
Figure A.9 – Stripline (four plates, balanced feeding).39

61000-4-20 © IEC:2010 – 3 –
Figure B.1 – Example of test set-up for single-polarization TEM waveguides .44
Figure B.2 – Uniform area calibration points in TEM waveguide .45
Figure C.1 – Frequency domain spectral magnitude between 100 kHz and 300 MHz .52
Figure D.1 – Simple waveguide (no TEM mode).59
Figure D.2 – Example waveguides for TEM-mode propagation.59
Figure D.3 – Polarization vector.59
Figure D.4 – Transmission line model for TEM propagation .59
Figure D.5 – One- and two-port TEM waveguides .60
Figure E.1 – An example of the measurement points for the validation.62
Figure E.2 – Setup for validation of perturbation .63
Figure E.3 – Setup for measuring net power to a transmitting device .66
Figure E.4 – Example of setup for calibration of E-field probe .67
Figure E.5 – Setup for calibration of E-field probe by another method.69
Figure E.6 – Equivalent circuit of antenna and measurement apparatus.70

Table 1 – Values K for expanded uncertainty with normal distribution .15
Table B.1 – Uniform area calibration points.42
Table B.2 – Test levels .42
Table C.1 – Radiated immunity test levels defined in the present standard .52
Table E.1 – Calibration frequencies .63
Table E.2 – Calibration field strength level.64

– 4 – 61000-4-20 © IEC:2010
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ELECTROMAGNETIC COMPATIBILITY (EMC) –

Part 4-20: Testing and measurement techniques –
Emission and immunity testing in
transverse electromagnetic (TEM) waveguides

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
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and non-
governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely
with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
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.
International Standard IEC 61000-4-20 has been prepared by 77B: High-frequency
phenomena, of IEC technical committee 77: Electromagnetic compatibility, in cooperation with
CISPR (International Special Committee on Radio Interference) subcommittee A: Radio
interference measurements and statistical methods.
This second edition cancels and replaces the first edition published in 2003 and its
amendment 1 (2006), and constitutes a technical revision.
It forms Part 4-20 of IEC 61000. It has the status of a basic EMC publication in accordance
with IEC Guide 107.
The main changes with respect to the first edition of this standard and its amendment are the
following:
• consistency of terms (e.g. test, measurement, etc.) has been improved;

61000-4-20 © IEC:2010 – 5 –
• clauses covering test considerations, evaluations and the test report have been added;
• references to large TEM waveguides have been eliminated;
• a new informative annex has been added to deal with calibration of E-field probes.
The text of this standard is based on the following documents:
FDIS Report on voting
77B/637/FDIS 77B/641/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts of the IEC 61000 series, published under the general title Electromagnetic
compatibility (EMC), can be found on the IEC website.
The committee has decided that the contents of the base publication and its amendments will
remain unchanged until the stability result date indicated on the IEC web site under
"http://webstore.iec.ch" in the data related to the specific publication. At this date,
the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
– 6 – 61000-4-20 © IEC:2010
INTRODUCTION
IEC 61000 is published in separate parts according to the following structure:
Part 1: General
General considerations (introduction, fundamental principles)
Definitions, terminology
Part 2: Environment
Description of the environment
Classification of the environment
Compatibility levels
Part 3: Limits
Emission limits
Immunity limits (in so far as they do not fall under the responsibility of the product
committees)
Part 4: Testing and measurement techniques
Measurement techniques
Testing techniques
Part 5: Installation and mitigation guidelines
Installation guidelines
Mitigation methods and devices
Part 6: Generic Standards
Part 9: Miscellaneous
Each part is further subdivided into several parts, published either as International Standards,
Technical Specifications or Technical Reports, some of which have already been published as
sections. Others are and will be published with the part number followed by a dash and a
second number identifying the subdivision (example: IEC 61000-6-1).
This part of IEC 61000 is an International Standard which gives emission, immunity and
HEMP transient testing requirements.

61000-4-20 © IEC:2010 – 7 –
ELECTROMAGNETIC COMPATIBILITY (EMC) –

Part 4-20: Testing and measurement techniques –
Emission and immunity testing in
transverse electromagnetic (TEM) waveguides

1 Scope and object
This part of IEC 61000 relates to emission and immunity test methods for electrical and
electronic equipment using various types of transverse electromagnetic (TEM) waveguides.
These types include open structures (for example, striplines and electromagnetic pulse
simulators) and closed structures (for example, TEM cells). These structures can be further
classified as one-, two-, or multi-port TEM waveguides. The frequency range depends on the
specific testing requirements and the specific TEM waveguide type.
The object of this standard is to describe
• TEM waveguide characteristics, including typical frequency ranges and EUT-size
limitations;
• TEM waveguide validation methods for EMC tests;
• the EUT (i.e. EUT cabinet and cabling) definition;
• test set-ups, procedures, and requirements for radiated emission testing in TEM
waveguides and
• test set-ups, procedures, and requirements for radiated immunity testing in TEM
waveguides.
NOTE Test methods are defined in this standard for measuring the effects of electromagnetic radiation on
equipment and the electromagnetic emissions from equipment concerned. The simulation and measurement of
electromagnetic radiation is not adequately exact for quantitative determination of effects for all end-use
installations. The test methods defined are structured for a primary objective of establishing adequate repeatability
of results at various test facilities for qualitative analysis of effects.
This standard does not intend to specify the tests to be applied to any particular apparatus or
system(s). The main intention of this standard is to provide a general basic reference for all
interested product committees of the IEC. For radiated emissions testing, product committees
should select emission limits and test methods in consultation with CISPR standards. For
radiated immunity testing, product committees remain responsible for the appropriate choice
of immunity tests and immunity test limits to be applied to equipment within their scope. This
standard describes test methods that are separate from those of IEC 61000-4-3.
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.
IEC 60050(161), International Electrotechnical Vocabulary – Chapter 161: Electromagnetic
compatibility
IEC 61000-2-11:1999, Electromagnetic compatibility (EMC) – Part 2-11: Environment –
Classification of HEMP environments
___________
These other distinct test methods may be used when so specified by product committees, in consultation with
CISPR and TC 77.
– 8 – 61000-4-20 © IEC:2010
IEC 61000-4-23, Electromagnetic compatibility (EMC) – Part 4-23: Testing and measurement
techniques – Test methods for protective devices for HEMP and other radiated disturbances
IEC/TR 61000-4-32, Electromagnetic compatibility (EMC) – Part 4-32: Testing and measure-
ment techniques – High-altitude electromagnetic pulse (HEMP) simulator compendium
IEC/TR 61000-5-3, Electromagnetic compatibility (EMC) – Part 5-3: Installation and mitigation
guidelines – HEMP protection concepts
CISPR 16-1-1, Specification for radio disturbance and immunity measuring apparatus and
methods – Part 1-1: Radio disturbance and immunity measuring apparatus – Measuring
apparatus
CISPR 16-1-4, Specification for radio disturbance and immunity measuring apparatus and
methods – Part 1-4: Radio disturbance and immunity measuring apparatus – Antennas and
test sites for radiated disturbance measurements
CISPR 16-2-3:2006, Specification for radio disturbance and immunity measuring apparatus
and methods – Part 2-3: Methods of measurement of disturbances and immunity – Radiated
disturbance measurements
CISPR 22, Information technology equipment – Radio disturbance characteristics – Limits and
methods of measurement
3 Terms, definitions and abbreviations
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60050(161), as well
as the following, apply.
3.1.1
transverse electromagnetic mode
TEM mode
waveguide mode in which the components of the electric and magnetic fields in the
propagation direction are much less than the primary field components across any transverse
cross-section
3.1.2
TEM waveguide
open or closed transmission line system, in which a wave is propagated in the transverse
electromagnetic mode to produce a specific field for testing purposes
3.1.3
TEM cell
closed TEM waveguide, often a rectangular coaxial transmission line, in which a wave is
propagated in the transverse electromagnetic mode to produce a specific field for testing
purposes and with an outer conductor completely enclosing an inner conductor
3.1.4
two-port TEM waveguide
TEM waveguide with input/output ports at both ends
3.1.5
one-port TEM waveguide
TEM waveguide with a single input/output port

61000-4-20 © IEC:2010 – 9 –
NOTE Such TEM waveguides typically feature a broadband transmission-line termination at the non-port end.
3.1.6
stripline
terminated transmission line consisting of two or more parallel plates between which a wave
is propagated in the transverse electromagnetic mode to produce a specific field for testing
purposes
NOTE Striplines usually have open sides for EUT access and monitoring.
3.1.7
inner conductor or septum
inner conductor of a coaxial transmission-line system, often flat in the case of a rectangular
cross-section, and which may be positioned symmetrically or asymmetrically with respect to
the outer conductor
3.1.8
outer conductor or chassis
outer conductor of a coaxial transmission line system, often having a rectangular cross-
section
3.1.9
characteristic impedance
for any constant phase wave-front, the magnitude of the ratio of the voltage between the inner
conductor and the outer conductor to the current on either conductor and which is
independent of the voltage/current magnitudes and depends only on the cross-sectional
geometry of the transmission line
NOTE TEM waveguides are typically designed to have a characteristic impedance of 50 Ω. TEM waveguides with
a characteristic impedance of 100 Ω are often used for transient testing.
3.1.10
anechoic material
material that exhibits the property of absorbing, or otherwise reducing, the level of
electromagnetic energy reflected from that material
3.1.11
broadband transmission-line termination
broadband line termination
termination which combines a low-frequency discrete-component load, to match the
characteristic impedance of the TEM waveguides (typically 50 Ω), and a volume of high-
frequency anechoic material
3.1.12
correlation algorithm
mathematical routine for converting TEM waveguide voltage measurements to open-area test
sites (OATS), semi-anechoic chamber (SAC), or free space field strength levels
3.1.13
EUT type
grouping of products with sufficient similarity in electromagnetic characteristics to allow
testing with the same test installation and the same test protocol
3.1.14
exit cable
cable that connects the EUT to equipment external to the TEM waveguide or cable exiting the
usable test volume
NOTE Test volume is specified in 5.2.2.

– 10 – 61000-4-20 © IEC:2010
3.1.15
interconnecting cable
cable that connects subcomponents of the EUT within the test volume but does not exit the
test volume
3.1.16
test set-up support
non-reflecting, non-conducting, low-permittivity support and positioning reference that allows
for precise rotations of the EUT as required by a correlation algorithm or test protocol
NOTE A typical material is foamed polystyrene. Wooden supports are not recommended (see [4] ).
3.1.17
ortho-angle
angle that the diagonal of a cube makes to each side face at the trihedral corners of the cube;
assuming that the cube is aligned with the TEM waveguide Cartesian coordinate system, the
azimuth and elevation angles of the projection of the cube diagonal are 45°, and the angles to
the face edges are 54,7°
NOTE 1 Figure A.2a shows a diagram of the ortho-angle.
NOTE 2 When associated with the EUT, this angle is usually referred to as the ortho-axis.
3.1.18
primary (field) component
electric field component aligned with the intended test polarization
NOTE In conventional two-port TEM cells, the septum is parallel to the horizontal floor, and the primary mode
electric field vector is vertical at the transverse centre of the TEM cell.
3.1.19
secondary (field) component
in a cartesian coordinate system, either of the two electric field components orthogonal to the
primary field component and orthogonal to each other
3.1.20
resultant field (amplitude)
root-sum-squared values in V/m of the primary and the two secondary field components
3.1.21
manipulator
any type of manual or automatic non-metallic test set-up support similar to a turntable, and
capable of supporting an affixed EUT throughout numerous positions as required by a
correlation algorithm or test protocol
NOTE An example of a manipulator design is shown in Figure A.2.
3.1.22
hyper-rotated TEM waveguide
TEM waveguide that has been reoriented such that its ortho-axis is normal to the surface of
the Earth
NOTE Additional details are given in [6].
3.1.23
gravity-dependent / -independent
the gravitation force of the earth has a fixed direction. The EUT can be rotated around all
three axes. Due to different rotation positions, the EUT is affected by the gravitation force in
different directions. The EUT is gravity-independent if it is working properly in all positions,
___________
2 Figures in square brackets refer to the bibliography.

61000-4-20 © IEC:2010 – 11 –
which means working properly regardless of the direction of the gravity vector relative to the
EUT. The EUT is gravity-dependent if it does not work properly in one or more test positions
3.2 Abbreviations
BALUN balanced-to-unbalanced transformer
DFT discrete Fourier transform
EUT equipment under test
FFT fast Fourier transform
GTEM gigahertz transverse electromagnetic
HEMP high-altitude electromagnetic pulse
OATS open-area test site
PoE points of entry
RF radio frequency
SAC semi-anechoic chamber
SPD surge protective device
TDR time-domain reflectometer
TE transverse electric (mode), (H-mode)
TEM transverse electromagnetic
TM transverse magnetic (mode), (E-mode)
VSWR voltage-standing-wave-ratio

4 General
This standard describes basic characteristics and limitations of TEM waveguides, namely test
volume, field uniformity, purity of the TEM mode, and frequency ranges. Various general
properties of TEM waveguides are described in Annex D.
Radiated emission measurements in a TEM waveguide are usually correlated with the open-
area test site (OATS) and semi-anechoic chamber (SAC) methods, which provide valid and
repeatable measurement results of disturbance field strength from equipment. In this case so-
called correlation algorithms are used to convert TEM waveguide measurement results to
OATS-equivalent data, as described in Annex A.
TEM waveguides can also be used as field generators for testing the immunity of equipment
to electromagnetic fields. Details are given in Annex B. Immunity testing in TEM waveguides
is cited in several other standards listed in the Bibliography. Field generation properties can
also be used for measuring field strength, see Annex E and other publications listed in the
Bibliography.
TEM waveguide tests are not restricted to radiated measurements on fully assembled
equipment. They may also be applied to the testing of components, integrated circuits, and
the shielding effectiveness of gasket materials and cables. For further information see
Bibliography.
– 12 – 61000-4-20 © IEC:2010
5 TEM waveguide requirements
5.1 General
TEM waveguides can be used for emission and immunity tests when certain requirements are
met. For the validation of a TEM waveguide the following methods shall be applied.
This clause focuses on general validation aspects such as the dominant TEM mode and field
homogeneity. Specific validation requirements for emission, immunity, and transient testing
are given in the Annex A, Annex B, and Annex C, respectively.
5.2 General requirements for the use of TEM waveguides
5.2.1 TEM mode verification
TEM waveguides may exhibit resonances above a certain cut-off frequency determined by the
cross-sectional dimensions and/or the waveguide length. For practical use, the field in a TEM
waveguide is considered to propagate in a TEM mode when the following requirements are
met. This verification of the TEM mode applies to waveguides used either for immunity or
emissions testing. The TEM mode behaviour shall be confirmed at regular intervals (see
5.2.3).
NOTE 1 Generally, a TEM waveguide manufacturer should verify and document the TEM mode behaviour over
the desired frequency range and include verification data with the system documentation.
Using an immunity-type uniform-area verification procedure (according to 5.2.3) the
magnitudes of the secondary (unintended) electric field components shall be at least 6 dB
less than the primary component of the electric field, over at least 75 % of the tested points in
a defined cross-section of the TEM waveguide (perpendicular to the propagation direction).
−0
For this 75 % of test points, a primary electric field component tolerance greater than dB
+6
−0
up to dB, or a secondary electric field component level up to –2 dB of the primary field
+10
component, is allowed for a maximum of 5 % of the test frequencies (at least one frequency),
provided that the actual tolerance and frequencies are stated in the test reports. The
frequency range is 30 MHz up to the highest frequency of intended use of the TEM
waveguide. The first frequency step shall not exceed 1 % of the fundamental frequency and
thereafter 1 % of the preceding frequency in 80 MHz to 1 000 MHz, 5 % below 80 MHz and
above 1 000 MHz. One constraint on the sweep speed is the response time of the field probe.
NOTE 2 The TEM field is the dominant mode and the cavities are low Q values, therefore resonances are not
expected to be narrow. For this reason the use of logarithmic frequencies is acceptable for TEM mode verification
testing.
NOTE 3 For transient tests the start frequency should be 100 kHz.
NOTE 4 The 6 dB criterion from 5.2.1 specifies the dominant TEM mode and not the field uniformity, and is
separate from and not to be confused with the field uniformity requirements of 5.2.3. Further information about field
uniformity is given in [17].
5.2.2 Test volume and maximum EUT size
The maximum size of an EUT is related to the size of the “usable test volume” in the TEM
waveguide. The usable test volume of the TEM waveguide depends on the size, geometry,
and the spatial distribution of the electromagnetic fields.
The usable test volume of a TEM waveguide (see Figures A.6 to A.9) depends on the “uniform
area” as defined in 5.2.3. The propagation direction of the waveguide TEM mode (typically z-
axis) is perpendicular to a uniform area (transverse plane, typically xy-plane). In the xy-plane
the entire cross-section of the usable test volume has to fulfil the requirements of the uniform
area defined in 5.2.3. The minimum value for the distance h between EUT and each
EUT
conductor or absorber of the waveguide (see Figures A.6 to A.9) is given by the distance
between the boundary of the uniform area (see 5.2.3) and the conductor. However, h
EUT
61000-4-20 © IEC:2010 – 13 –
should not be zero, in order to avoid the possible change of the EUT operational condition by
the close coupling between EUT and conductors of the waveguide (recommended: h
EUT
should be larger than 0,05 h). Along the z-axis (propagation direction) the usable test volume
is limited by z ≤ z ≤ z . The length of the test volume is L = z − z . The
min max max min
requirements of a uniform area shall be validated for cross-sections at each z with
z ≤ z ≤ z . It can be assumed that the TEM mode requirements are fulfilled for
min max
z ≤ z ≤ z under the following conditions:
min max
• if TEM mode requirements are fulfilled at the position z , and the geometry of the
max
waveguide is similar to one of the types shown in Figures A.6 to A.9 with a constant
aspect ratio of h to w (inherent shape) for 0 < z < z , or,
max
• if TEM mode requirements are fulfilled at the positions z and z , and the waveguide
min max
cross-section is constant or uniformly tapered for z < z < z and the derivatives dh/dz
min max
and dw/dz are a smooth function for z < z < z (no kinks or steps in the conductor
min max
geometries).
The maximum size of an EUT is related to the size of the usable test volume. The EUT shall
be verified not to be larger than 0,6 w times 0,6 L (see Figures A.6 to A.9).
NOTE 1 The ISO 11452 series recommends an EUT size of 0,33 w × 0,6 L, and MIL-STD 462F recommends
0,5 w × 0,5 L.
The maximum usable EUT height is recommended to be 0,33 h, with h equal to the distance
between the inner and outer conductors (conductor spacing) at the centre of the EUT in the
test volume (for example, between septum and floor in a TEM cell). For all TEM waveguides,
the EUT shall fit within the usable test volume for all rotation positions.
NOTE 2 Most standards restrict EUT size to 0,33 h. Most data sheets from TEM cell suppliers limit the EUT height
to a maximum of 0,5 h. Except for highly accurate calibration, such as for field probes and sensors, the EUT height
can exceed 0,33 h, but it should not exceed the manufacturer’s recommendations. The maximum usable EUT
height can be higher than 0,33 h if the manufacturer provides information about the measurement uncertainty for
larger EUTs. More information about loaded waveguide effects is given in [25].
5.2.3 Validation of usable test volume
5.2.3.1 General considerations
This subclause uses the concept of a ”uniform area“ which is a hypothetical area in which
variations of the field magnitude are acceptably small (see [15]). The TEM waveguide
dimensions determine the size of this uniform area (plane), unless the EUT can be fully
illuminated in a smaller surface. The maximum size of an EUT is related to the size of the
usable test volume (see 5.2.2).
NOTE 1 In general the exact form and the location of the uniform area are not specified, but are determined using
the procedures of this standard.
NOTE 2 If no other definition is given, the uniform area should be a vertical plane orthogonal to the propagation
direction of the field. It should be one plane face area in front of the EUT.
NOTE 3 The vertical plane assumes that the direction of TEM mode propagation is near horizontal (aligned to the
z-axis) and plane wave propagation is given. If the TEM mode propagation direction is in some other direction, the
uniform area plane may be re-orientated accordingly.
The use of a transmission line set-up avoids perturbation due to ground-reflected fields as in
a semi-anechoic chamber set-up; thus, uniform fields may be established in the vicinity of the
inner and outer conductors (in the normal direction only).
In principle, the uniform area may be located at any distance from the input port; the location
will depend on the specific waveguide geometry. The uniform area is valid only for that
distance from the input port at which it is calibrated. To allow EUT rotation, the uniform area

– 14 – 61000-4-20 © IEC:2010
shall be spaced a distance at least greater than the largest case dimension away from the
end of the usable test volume z defined in 5.2.2.
max
The uniform area is validated in the empty enclosure, for the frequency range and frequency
steps specified in 5.2.1 using a non-modulated signal.
Depending on the size of the uniform area, it is validated at least with 5 measurement points
(4 at the corners and one at the centre). The spacing between two test points has to be
smaller than 50 cm. If the 50 cm limit is exceeded, an equally spaced grid has to be used for
the test points. This means that 9 points shall be used.
5.2.3.2 Field uniformity and TEM mode measurement procedure
The procedure for carrying out the validation is known as the “constant forward power”
method and is as follows:
a) position the isotropic 3-axis probe at one of the points in the grid;
b) apply a forward power to the TEM waveguide input port so that the electric field strength
of the primary field component is in the range of the given limit E through the
Limit
frequency range and frequency steps specified in 5.2.1, and record all the forward power,
primary and secondary components field strength readings;
c) with the same forward power, measure and record the primary and secondary field
strengths at the remaining grid points;
d) calculate the standard deviation according to Equation (1). All measurement results are
expressed in dB(V/m);
e) the primary field component magnitude of the remaining points shall lie within a range of
6 dB. The level of the secondary field components shall not exceed –6 dB of the primary
field component at each of these points;
f) of the remaining points, take the location with the lowest primary field component E as
ref
−0
the reference (this ensures the dB requirement is met);
+6
g) knowing the forward power and the field strength, the necessary forward power for the
required test-field strength can be calculated using Equation (1), and shall be recorded.
E
test
P = P with E in V/m and P in W (1)
test fwd
E
ref
EXAMPLE If at a given point, 81 W gives 9 V/m, then 9 W is needed for 3 V/m.
Alternatively, an equivalent procedure is to establish a constant primary component electric
field strength in the range of the given E and record the forward power delivered to the
Limi
t
input port. Next, steps a), d), e), f) and g) shall be applied. This method is known as the
“constant field strength” method.
The uniformity validation is applicable for all EUTs whose individual faces (including any
cabling) can be fully enclosed by the ”uniform area“. It is intended that the full uniform area
validation be carried out annually or when changes have been made to the enclosure
configuration (e.g. TEM cell or stripline within a shielded enclosure).
5.2.3.3 Field uniformity criteria
The uniformity of the field is determined as follows.
At test point i the measured field strength is given as E . The mean-value and the standard
i
deviation are calculated for N test points.

61000-4-20 © IEC:2010 – 15 –
Mean: (2)
E = E
∑ i
N
()N
Standard deviation: σ = ()E − E (3)
E ∑ i
N −1
()N
In the statistical sense N = 5 reflects a very small quantity but nevertheless a normal
distribution for the measurements E can be assumed. For the probability that 75 % of the
i
measurement results will fall in the range
E − K ⋅σ ≤ E ≤ E + K ⋅σ (4)
i
E E
the factor K is chosen to be 1,15.
Table 1 – Values K for expanded uncertainty with normal distribution
Factor
1 1,15 1,3 1,5 2 3
K
Probability
68,3 75,0 80,6 86,6 95,5 99,7
%
Dealing with dB-values often the probability is requested whether the measurement E falls
i
into the band according to Equation (5).
E ≤ E ≤ E + E (5)
i
Limit Limit Margin
Comparing this band with Equation (4) gives Equation (6).
E ≤ E ≤ E + 2 ⋅ K ⋅σ (6)
Limit i Limit
E
Margin
σ ≤ (7)
E
2 ⋅ K
For 175 % ⇒ K = 1,5 and a margin of 6 dB the standard deviation shall be
6 dB
.
σ ≤ = 2,61 dB
E
2 ⋅1,15
The largest dimension of the sensor shall be smaller than 10 % of the distance between the
inner and outer conductor. In this case, any field perturbation can be neglected. More details
are given in [18].
5.3 Special requirements and recommendations for certain types of TEM waveguides
5.3.1 Set-up of open TEM waveguides
To minimize ambient effects, open TEM waveguides should be installed inside a shielded
room.
NOTE 1 The permitted ambient signal levels are defined in Annexes A, B, and
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