IEC TR 61917:1998

TECHNICAL
IEC
REPORT
First edition
1998-06
Cables, cable assemblies and connectors –
Introduction to electromagnetic (EMC)
screening measurements
Câbles, cordons et connecteurs –
Introduction aux mesures de blindage électromagnétique

Reference number
IEC 61917:1998(E)
Numbering
As from 1 January 1997 all IEC publications are issued with a designation in the
60000 series.
Consolidated publications
Consolidated versions of some IEC publications including amendments are

available. For example, edition numbers 1.0, 1.1 and 1.2 refer, respectively, to the
base publication, the base publication incorporating amendment 1 and the base

publication incorporating amendments 1 and 2.

Validity of this publication
The technical content of IEC publications is kept under constant review by the IEC,
thus ensuring that the content reflects current technology.
Information relating to the date of the reconfirmation of the publication is available
in the IEC catalogue.
Information on the subjects under consideration and work in progress undertaken by
the technical committee which has prepared this publication, as well as the list of
publications issued, is to be found at the following IEC sources:
• IEC web site*
• Catalogue of IEC publications
Published yearly with regular updates
(On-line catalogue)*
• IEC Bulletin
Available both at the IEC web site* and as a printed periodical
Terminology, graphical and letter symbols
For general terminology, readers are referred to IEC 60050: International
Electrotechnical Vocabulary (IEV).
For graphical symbols, and letter symbols and signs approved by the IEC for
general use, readers are referred to publications IEC 60027: Letter symbols to be
used in electrical technology, IEC 60417: Graphical symbols for use on equipment.
Index, survey and compilation of the single sheets and IEC 60617: Graphical symbols
for diagrams.
* See web site address on title page.

TECHNICAL
IEC
REPORT – TYPE 3
First edition
1998-06
Cables, cable assemblies and connectors –
Introduction to electromagnetic (EMC)
screening measurements
Câbles, cordons et connecteurs –
Introduction aux mesures de blindage électromagnétique

 IEC 1998  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 Geneva, Switzerland
Telefax: +41 22 919 0300 e-mail: inmail@iec.ch IEC web site http: //www.iec.ch
Commission Electrotechnique Internationale
PRICE CODE
U
International Electrotechnical Commission
For price, see current catalogue

– 2 – 61917 © IEC:1998(E)
CONTENTS
Page
FOREWORD . 3

Clause
1 Scope and object . 5

2 Reference documents. 5

3 Electromagnetic phenomena. 5
4 The intrinsic screening parameters of short cables . 7
4.1 Surface transfer impedance, Z . 7
T
4.2 Capacitive coupling admittance, Y . 7
c
4.3 Injecting with arbitrary cross-sections . 9
4.4 Reciprocity and symmetry . 9
4.5 Arbitrary load conditions. 9
5 Long cables – coupled transmission lines. 9
6 Transfer impedance of a braided-wire outer conductor or screen . 16
7 Test possibilities . 21
7.1 Measuring the transfer impedance of coaxial cables . 21
7.2 Measuring the transfer impedance of cable assemblies . 22
7.3 Measuring the transfer impedance of connectors. 22
Annex A List of symbols . 25
Annex B Bibliography . 27
Annex C Additional reading. 29

61917  IEC:1998(E) – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION

___________
–
CABLES, CABLE ASSEMBLIES AND CONNECTORS

INTRODUCTION TO ELECTROMAGNETIC (EMC)

SCREENING MEASUREMENTS
FOREWORD
1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of the 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, the IEC publishes International Standards. 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. The 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 the 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 National Committees.
3) The documents produced have the form of recommendations for international use and are published in the form
of standards, technical reports or guides and they are accepted by the National Committees in that sense.
4) In order to promote international unification, IEC National Committees undertake to apply IEC International
Standards transparently to the maximum extent possible in their national and regional standards. Any
divergence between the IEC Standard and the corresponding national or regional standard shall be clearly
indicated in the latter.
5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with one of its standards.
6) Attention is drawn to the possibility that some of the elements of this International Standard may be the subject
of patent rights. The IEC shall not be held responsible for identifying any or all such patent rights.
The main task of IEC technical committees is to prepare International Standards. In
exceptional circumstances, a technical committee may propose the publication of a technical
report of one of the following types:
• type 1, when the required support cannot be obtained for the publication of an International
Standard, despite repeated efforts;
• type 2, when the subject is still under technical development or where, for any other reason,
there is the future but not immediate possibility of an agreement on an International
Standard;
• type 3, when a technical committee has collected data of a different kind from that which is
normally published as an International Standard, for example “state of the art”.
Technical reports of types 1 and 2 are subject to review within three years of publication to
decide whether they can be transformed into International Standards. Technical reports of type
3 do not necessarily have to be reviewed until the data they provide are considered to be no
longer valid or useful.
IEC 61917 which is a technical report type 3 has been prepared by subcommittee 46A: Coaxial
cables, of IEC technical committee 46: Cables, wires, waveguides, r.f. connectors, and
accessories for communication and signalling.

– 4 – 61917 © IEC:1998(E)
The text of this technical report is based on the following documents:

Committee draft Report on voting

46A/267/CDV 46A/284/RVC
Full information on the voting for the approval of this technical report can be found in the report

on voting indicated in the above table.

A bilingual version of this technical report may be issued at a later date.

61917  IEC:1998(E) – 5 –
CABLES, CABLE ASSEMBLIES AND CONNECTORS –

INTRODUCTION TO ELECTROMAGNETIC (EMC)

SCREENING MEASUREMENTS
1 Scope and object
Screening (or shielding) is one basic way of achieving electromagnetic compatibility (EMC).

However, a confusingly large number of methods and concepts is available to test for the
screening quality of cables and related components, and for defining their quality. This
technical report gives a brief introduction to basic concepts and terms trying to reveal the
common features of apparently different test methods. It should assist in correct interpretation
of test data, and in the better understanding of screening (or shielding) and related
specifications and standards.
2 Reference documents
IEC 60096-1:1986, Radio-frequency cables – Part 1: General requirements and measuring
methods
Amendment 2 (1993)
IEC 60096-2:1961, Radio-frequency cables – Part 2: Relevant cable specifications
Amendment 1 (1990)
IEC 60096-4-1:1990, Radio-frequency cables – Part 4: Specification for superscreened cables
– Section 1: General requirements and test methods
IEC 60169-1:1987, Radio-frequency connectors – Part 1: General requirements and measuring
methods
IEC 60169-1-3:1988, Radio frequency connectors – Part 1: General requirements and
measuring methods – Section 3: Electrical tests and measuring procedures – Screening
effectiveness
IEC 61196-1:1995, Radio-frequency cables – Part 1: Generic specification – General,
definitions, requirements and test methods

IEC 61726:1995, Cable assemblies, cables, connectors and passive microwave components –
Screening attenuation measurement by the reverberation chamber method
3 Electromagnetic phenomena
It is assumed that if an electromagnetic field is incident on a screened cable, there is only weak
coupling between the external field and that inside, and that the cable diameter is very small
compared with both the cable length and the wavelength of the incident field. The superposition
of the external incident field and the field scattered by the cable yields the total electromagnetic
field (E , H , in figure 1). The total field at the screen's surface may be considered as the
t t
source of the coupling: electric field penetrates through apertures by electric or capacitive
coupling; also magnetic fields penetrate through apertures by inductive or magnetic coupling.
Additionally, the induced current in the screen results in conductive or resistive coupling.

(E ,H )
i i
– 6 – 61917 © IEC:1998(E)
E
t
H
t
n
σ
J
X
(E ,H ) = (E ,H ) + (E ,H) (1)
t t i i s s
= n (2)
J • H
t
σ = n • E ε ε (3)
t 0 r
n: unit vector normal to surface
Figure 1 – Incident (i), scattered (s) and resulting total electromagnetic fields (E , H ) with
t t
induced surface current- and surface charge-densities J (A/m) and σ (C/m )
.
As the field at the surface of the screen is directly related to density of surface current and
surface charge, the coupling may be assigned either to the total field (E , H ) or to the surface
t t
current- and charge- densities (J and σ). Consequently, we may simulate the coupling into the
cable by reproducing through any means the surface currents and charges on the screen.
Because we assume a cable of a small diameter, we may neglect higher modes and can use

an additional coaxial conductor as our injection structure, as shown in figure 2.
l Concept of a triaxial set-up
+
1) outer circuit, formed by injection
E
1 cylinder and screen, characteristic
U impedance Z ,
Z
1n
U
1f
Z
1f
2) inner circuit, formed by a screen,
Z U
I
2n 2n U Z
1 (1)
2f and centre conductor, characteristic
2f
impedance Z ; screening at the ends
D
(2)
1 not shown.
Z
Z
Observe the conditions Z , Z , Z and λ in figure 3a and figure 3b.
1f 2n 2f
NOTE 1 – D << l.
NOTE 2 – Both ends of circuit (2) must be well screened.
Figure 2 – Defining and measuring screening parameters – A triaxial set-up
(E ,H )
s s
61917  IEC:1998(E) – 7 –
4 The intrinsic screening parameters of short cables

The intrinsic parameters refer to an infinitesimal length of cable, like the inductance or

capacitance per unit length of transmission lines. Assuming electrically short cables, with l << λ

which will always apply at low frequencies, the intrinsic screening parameters are defined and

can be measured as follows:
4.1 Surface transfer impedance, Z
T
As shown in figure 2 and figure 3a (where Z and Z are zero):
1f 2f
ZU=⋅/(Il) (Ω/ m) (4)
T 21
The dependence of Z on frequency is not simple and is often shown by plotting log Z against
T T
log frequency. Note that the phase of Z may have any value, depending on braid construction
T
and frequency range.
NOTE – In circuit 2 of figure 3a the voltmeter and short circuit can be interchanged.
4.2 Capacitive coupling admittance, Y
c
As shown in figure 2 and figure 3b (where Z and Z are open circuit):
1f 2f
YC==jω I /(U⋅l) (mho / m) (5)
CT
The through capacitance (C ) is a real capacitance and has usually a constant value up to
T
λ
1 GHz and higher (with aperture a << ).
While Z is independent of the characteristics of the coaxial circuits, C is dependent on those
T T
characteristics. There are two ways of overcoming this dependence:
a) The normalized through elastance K derived from C is independent of the size of the
T T
outer coaxial circuit, but it depends on its permittivity:
=⋅ εε+
KC /(CC ) (mF/ ) K ~ 1/( ) (6) (7)
TT 12 T r1 r2
where C and C are the capacitance per unit length of the two coaxial circuits.
1 2
b) The capacitive coupling impedance Z again derived from C is also independent of the
F T
size of the outer coaxial circuit and, for practical values of ε , is only slightly dependent on
r1
its permittivity:
ZZ==ZY ZZ jωεC (/Ω m)Z ~ (⋅ε )/(ε+ε ) (8) (9)
FC12 12 T F r1r2 r1r2
Compared with Z , Z is usually negligible, except for open weave braids. It may, however, be
T F
significant when Z and Z >> Z (audio circuits).
2n 2f 2
– 8 – 61917 © IEC:1998(E)
Injection cylinder
+
E
(1)
U
Z = 0
1f
Z • l
I T
1 Shield
(2)
U
T
V
U
Z = 0
2f
=
Z ∞
2n
Center conductor
l << λ
Figure 3a – Equivalent circuit for the definition and possible testing of Z
Injection cylinder
+
E
(1)
U
Z = ∞
1f
Shield with apertures
(2)
C • l
T
A
Z = 0
2n
Z =
∞
I 2f
Y • = j C •
l ω l
Center conductor
C T
l << λ
Figure 3b – Equivalent circuit for the definition and possible testing of Y = j C
ωω
c T
Z
U
+
E U U (x) Z , β U Z (1)
1 1 1 1f
1 1 1
Z
T
(x)
I
C
T
Z Z (2)
2 U U (x) U
Z , β 2
2n 2 2 2 2f
x
l
l : arbitrary
U U
2n 2f
NOTE – Z and C are distributed (not correctly shown here). The loads Z at the ends may represent matched
T T 2
receivers.
Figure 3c – Definition of electrical quantities in a set-up that is matched at all ends
Figure 3 – Defining and measuring screen parameters – Equivalent circuits

61917  IEC:1998(E) – 9 –
4.3 Injecting with arbitrary cross-sections

A coaxial outer circuit has been assumed so far in this report, but it is not essential because of

the invariance of Z and Z . Using a wire in place of the outer cylinder, the injection circuit
T F
becomes two-wire with the return via the screen of the cable under test. Obviously the charge

and current distribution become non-uniform, but the results are equivalent to coaxial injection,
especially if two injection lines are used opposite to each other, and may be justified for worst-
case testing. Note that the IEC line injection test uses a wire.

4.4 Reciprocity and symmetry
Assuming linear shield materials, the measured Z and Z values will not change when
T F
interchanging injection (1) and measuring (2) circuits. Each of the two conductors of the two-
line circuit can be interchanged, but in practice the set-up will have to take into account
possible ground loops and coupling to the environment.
4.5 Arbitrary load conditions
When the circuit ends of figure 3a and figure 3b are not ideally short or open circuit, Z and Z
T F
will act simultaneously. The superposition is noticeable in the low frequency coupling of the
matched circuits (figure 3c and table 1).
5 Long cables – coupled transmission lines
The coupling over the whole length of the cable is obtained by summing up (integrating) the
infinitesimal coupling contributions along the cable while observing the correct phase. It is
expedient to make the following assumptions and conventions:
– matched circuits considered with the voltage waves U , U , U , see figure 3c,
1 2n 2f
– representation of the coupling, using the normalized wave amplitudes UZ Watt ,
[]
instead of voltage waves. i.e. the coupling transfer function, in the following denoted by
"coupling function", will be defined as
UZ
UZ/ /
2n 2 f 2
T==, T (10) (11)
n f
UZ/ UZ/
1 1 1 1
NOTE 1 – T is the ratio of the power waves travelling in circuits (2) and (1). Due to reciprocity and assuming
T
linear screen (shield) materials, is reciprocal, i.e. invariant with respect to the interchange of injection and
measuring circuits (1) and (2).
NOTE 2 – The quantity 1/ T , or in logarithmic quantities
=−
AT20 log , (12)
S 10
may be considered as the "screening attenuation" of the cable, specific to the set-up.
Performing the straight forward calculations of coupled transmission line theory, the coupling
function T, given in table 1, is obtained. The term Slf is the "summing function" S, being
{}
dependent on l and f. (The wavy bracket just indicates that the product lf⋅ is the argument
of the function S and not a factor to S). S represents the phase effect, when summing up the
infinitesimal couplings along the line, and is:
βl ±
sin
 β l + 
Sl f = − j (13)
{}  exp
 
n
βl ±
f  
– 10 – 61917 © IEC:1998(E)
with
ββll±=()±β ⋅ =21πlf/v ±1/v
{}
21 2 1
(14a) (14b) (14c)
=±2πεlf()ε/c
rr21
subscript ± refers to near/far end respectively

+ refers to both near/far ends

Note that weak coupling, i.e. T << 1, has been assumed. This case, including losses, is given

*
in [20 Halme, Szentkuti] .
NOTE – The equation (15) and representation in table 1 visualizes the contributions of the different parameters to
T
the coupling function :
1 l
TZ=±()Z⋅ ⋅⋅Sl⋅f,εε, (15)
{}
n FT n rr12
ZZ⋅ 2
f f
Note especially the following points:
a) There may be a directional effect (T ≠ T ) in the whole frequency range if Z is not
n f F
negligible. (But Z is usually negligible except with loose, single braid shields.)
F
b) Up to a constant factor, T is the quantity directly measured in a set-up.
c) For low frequencies, i.e. for short cables (l << λ), the trivial coupling formula is obtained
that is directly proportional to l :
1 l
TZ=±()Z⋅ ⋅, withZZ= ⋅Z  (16a) (16b)
n FT 12 1 2
Z 2
f
d) The summing function Sl⋅f is presented in figure 4. Note also that:
{}
e) Sl⋅f has a sin(x)/x behaviour. A cut-off point may be defined as ()lf⋅ :
{}
C
c
()lf⋅= (17)
C
n
πε ± ε
f
rr12
f) The exact envelope of Sl⋅f is
{}
Env Sl{}⋅=f (18)
n
()lf⋅
f
1+
()lf⋅
cn
f
___________
*
Numbers in square brackets refer to the bibliography (see annex B).

61917  IEC:1998(E) – 11 –
1)
Table 1 – The coupling transfer function T (coupling function)

2)
Set-up parameters
()Zl,,ε
11r
/\
--------------------- -----------------------

/ \
1 l
TZ=±()Z⋅ ⋅⋅Sl⋅f,εε,
{}
n FT n rr12
ZZ⋅
f f
\ / \ /
----- -------- ----------------- -----------------
\/ \/
2)
Intrinsic Cable parameters
ε
screen parameters (,Zl),
r
\ / \ /
---------------- ----------------- ----------- ------------
\/ \/
"Low-frequency coupling", "HF-effect",
3)
short cables cut-off ()lf⋅ .
C
\ /
-------------- ---------------
\/
Length + frequency effect
1)
T is the power coupling from circuit (1) to circuit (2).
n
The stacked subscripts are associated to the stacked operation symbols ± in
f
the obvious way: upper subscript → upper operation, lower subscript → lower
operation.
2)
ε and ε contained in S as parameters.
r1 r2
3)
for << λ : Sl f → 1 .
l {}
⋅
g) The first minimum (zero) of Sl{}f occurs at

()lf⋅= π(lf⋅) . (19)
min C
⋅
h) As seen from equations (13) and (18), below the cut-off points ()lf is Sl{}⋅f ≈ 1 and
cn
f
above them it starts to oscillate and its envelope drops asymptotically 20 dB/decade,
 
 
⋅
()lf
cn
 
 
 f 
Env Sl{}⋅≈f (20)
n
()lf⋅
f
– 12 – 61917 © IEC:1998(E)
i) S is symmetrical in l and f, i.e. l and f are interchangeable. For a fixed length a cut-off

frequency f and vice versa, for a fixed frequency a cut-off length l may be defined.
c c
Substituting c/λ for f, we obtain the cut-off length as
o
λ
o
l = (21)
C
n
πε ± ε
f
rr12
j) The effect of S in the frequency range ( l = constant) is illustrated in figure 5. The coupling

function is proportional to Z , only if f < f . Note also the typical values indicated for f .
T c c
k) The minima and maxima of S are not resonances, they are due to cancelling and additive

effects of the coupling along the line.
l) The far end cut-off frequency is significantly influenced by the permittivity of the outer
ε εε→ ⋅→∞
system (). Selecting we obtain ()lf , i.e. no cut-off at the far end. Due
Cf
r1 rr12
to practical aspects (tolerances, homogeneity, etc.), an ideal phase matching ()εε≡ is
rr12
not feasible.
m) The total effect of l on the coupling is not contained in S alone, but in the product lS⋅⋅l f .
{}
The product lS⋅ is presented in figure 7 for f = constant. The coupling function T which can
be measured in a set-up, is proportional to < . However, for appropriately long cables
lif l l
C
>
()ll , the maximum coupling is independent of l and we obtain a length independent
C
⋅ ⋅
shielding attenuation above the cut-off point ()lf . But we should remember that ()lf
C C
ε
as well as A are still dependent on the set-up parameters (, Z) .
r11
s
S
Log scale
S S
n f
(l • f ) (l • f ) (l • f ) log (l • f )
cn min,n cf cn
NOTE – SS> above near end cut-off, yielding a directive effect.
fn
()lf⋅ : cut-off point
C
Figure 4 – The summing function Sl ⋅f for near (n) and far (f) end coupling
{}
61917  IEC:1998(E) – 13 –
log Z
T
log f
f
Zf== Ω
()10 MHz 20 m / m
T 1
Figure 5a – Transfer impedance of a typical single braid screen
Z  l  1
log T • •
T
2  Z
Env (T )
f
T
f
Env ( )
T
n
T
n
log f
f f f
r cn cf
Figure 5b – Coupling transfer function for the same cable with negligible ZZ < ()
FF T
frequency responses of figure 4 and figure 5a added on log scale

Note the cut-off effect for f > f .
c
Example: ε = 1 (set-up), ε =22. (cable),
r1 r2
=→ = =
lf1 m 40 MHz,f 200 MHz
Cn Cf
Figure 5 – The effect of the summing function
20 dB/dec.
– 14 – 61917 © IEC:1998(E)
-40
T [dB]
T
f
Figure 6a – Calculated coupling transfer

functions T and T for a single braided when
-60
T
fztdBk n f
Z = 0
T
n F
– In calculations the used parameters are:

Z (d.c.) =15 mΩ/m and Z (10 MHz) = 20 mΩ/m
T
T
increasing 20 dB/decade (see figure 5a), cable
-80
length 1 m, and velocities of the outer and inner
T
nztdBk
line: v = 200 Mm/s and v = 280 Mm/s

1 2
corresponding a velocity difference of 40 %.

5 6 7 8 9 10
1⋅10
1⋅10 1⋅10 1⋅10 1⋅10 1⋅10
f
k f [Hz]
-40
T [dB]
T
fztdBk
T
f Figure 6b – As figure 6a but Im(Z ) is positive
T
T
fdBk
and Z = +0,5*Im (Z ) at high frequencies:
-60
F T
– T is 3,5 dB higher and T 6 dB lower than in
Tn
n f
reference figure 6a because
T
ndBk
T ∼Z + Z = 1,5*Z and
n F T  T
-80 T ∼Z – Z = 0,5*Z .
f F T  T
T
nztdBk
5 6 7 8 9
1⋅10 1⋅10 1⋅10 1⋅10 1⋅10 1⋅10
f [Hz]
f
k
-40
T [dB]
T
f
TfdBk
Figure 6c – As figure 6a but Im(Z ) is negative
T
fztdBk
T
-60
and Z = –0,5*Im(Z ) at high frequencies:
F T
– T is 3,5 dB higher and T 6 dB lower than in
f n
T
nztdBk
reference figure 6a because
T T ∼Z – Z = 1,5* Z  and
ndBk
f F T T
T
n
-80
T ∼Z + Z = 0,5* Z 
n F T T
5 6 7 8 9 10
1⋅10 1⋅10 1⋅10 1⋅10 1⋅10 1⋅10
f
k
f [Hz]
NOTE 1 – T for near-end, T for far-end and dB means that T are calculated in dB ( 20 lg | T | )
n f n,f n,f
NOTE 2 – T dB: near-end whenZZ =⋅(1 / 2) and T dB: near-end when Z = 0.

n FT nzt F
NOTE 3 – T dB: far-end when ZZ =⋅(1 / 2) and T dB: far-end when Z = 0.

f FT fzt F
Figure 6 – The effects of the Z and Z to the coupling transfer functions T and T
T F n f
– In figure 6a, Z = 0.
F
– In figure 6b and figure 6c, Z is significant (ZZ =⋅(1 / 2) ).
F FT
– In figure 6b is positive and figure 6c negative at high frequencies.
Z
T
61917  IEC:1998(E) – 15 –
log l • S
Env (l • S )
f
f
Env ( • S )
l
n
n
= const.
f
log l
l l
cn c
...