Screening Attenuation of Long Cables
Carl W. Dole, John W. Kincaid
Belden Electronics Division
Richmond, Indiana
Abstract
corresponds to about one-half wavelength electrical length in the
sample under test.
The characteristics of a triaxial test fixture, which has been
developed for screening attenuation measurements on long
shielded cables, are described. Screening attenuation performance
of a selection of coaxial CATV cable braid and multi-foil/braid
shield designs are presented. Measured screening attenuation
results for 30 meter long cables covering the frequency range 1100 MHz are compared with results from measurements on 1.5
meter long cables that were measured over the frequency range of
1-1000 MHz. The comparison shows that the estimation or
deduction of long length, low frequency performance from high
frequency performance can have significant error. The paper
concludes that worse case or minimum screening attenuation
performance should be assessed for cable lengths and frequencies
that are pertinent to usage of the cable.
Section 2 of this paper presents the mechanical, electrical and
installation aspects of the triaxial fixture. Sections 3 and 4 cover
the test procedure and the test program, including samples tested
and results. The conclusions are given in section 5.
2. Triaxial Test Fixture
2.1 Overview
The triaxial test fixture is sketched in Figure 1. The main
components are five sections of six-meter length, 76.5 mm
diameter rigid transmission line segments (without center
conductor and end disc supports) that are cascaded together with
gas tight coupling flanges. The segment joints are fitted with
metallic spacer rings to maintain 50 ohm impedance across the
joint. A continuous length of low loss dielectric tubing material is
used to support the sample under test from end to end. The tubing
is centered with disc spacers that are randomly located along the
30 meter length. An end plate closes the housing at one end and
an end cap transition encloses the other. A feed-through connector
is mounted in the center of the end plate.
Keywords
Attenuation; braid; CATV; coaxial; effectiveness; foil; IEC; screen;
SCTE; shield; test
1. Introduction
Screening attenuation is a practical shield effectiveness parameter,
and the test method is being standardized in IEC 61196-1 [1],
prEN 50289-6 and SCTE test method IPS-TP-403B2 [2]. The test
was originally developed for short length copper coaxial cables
involving a resonant triaxial test fixture length of 1-2 meters [3],
and the frequency range of interest was 100 MHz to 3 GHz. The
minimum screening attenuation corresponds to the condition of
maximum power transfer that occurs at resonant frequencies
within the test fixture. With a 1.5 meter long fixture, the lowest
resonant frequency is about 100 MHz. Fixture length and the
fixture TEM mode cut-off frequency primarily determine the
frequency range. A complete test system is available commercially
[4], and test fixture implementations involving standard
commercially available components for fixture lengths of 1.5 and
6.7 meters have been reported [5,6]. Preliminary results obtained
with a 30 meter long fixture have also been reported [7].
The end cap is fitted with a 50-ohm type N jack connector. The
center pin of the N jack connects to an anchor connector socket.
The sample termination shield is made from a split anchor
connector that is mated to the end cap anchor connector socket.
One side of the split anchor connector is fitted with a feedthrough connector, and the mated pair encloses and shields the
cable sample termination. This arrangement provides for
terminating the cable sample under test with a shielded resistive
load and for connecting the shield to the center pin of the N
connector. The cable sample under test is fitted with a plug type
connector at each end and is connected between the feed-through
connectors, respectively located in the end plate and the split
anchor connector.
2.2 Electrical Characteristics
The shield of the cable under test (CUT) defines the boundary
between the two electrical regions within the fixture. As shown in
Figure 2, these are (1) the resonant region, which is located
between the rigid metallic cylindrical housing and the cable
sample shield, and (2) the cable sample (CUT). The diameter
ratio of the housing and sample shield diameters is important for
determining the impedance and percent velocity of the resonant
region, which are approximately 150Ω and 90% respectively.
Applications such as CATV return path and data I/O have
stimulated interest in screening attenuation performance where the
frequency may be as low as 5 MHz and cable lengths are tens of
meters long. However the chief obstacle to implementing the
screening attenuation test at these frequencies is the required
fixture length. A 6.7 meter length is limited to about 20 MHz,
and a 30 meter length is required to arrive at about 4 MHz, which
1
End Plate
0
Decibels
End Cap
Cascaded Test Fixture
30 m (5 x 6m)
2
0
20
40
60
80
100
Frequency - MHz
Dielectric tube support
measured
theoretical
Figure 3. RL of test chamber:
measured and theoretical
Dielectric
disk supports
(random)
In the resonant region of the test chamber, the termination at the
end plate (left hand side in Figure 2) is a short circuit, while, at
the end cap (right hand side in Figure 2), the termination is a 50
ohm coaxial interconnect. Thus the chamber is mismatched at
both ends with respect to 150 ohms. The return loss (RL)
measured at the end cap (shown in Figure 3) is a result of the
termination mismatches and the uniformity of the 30 meter long
coaxial test chamber. Here the center conductor is the shield of
the sample under test, and the dielectric is made up of the sample
jacket, dielectric support material, and air. The measured RL is in
close agreement with the theoretical values shown in Figure 3.
Termination shield
Cable Under Test
F Plug
Feed through
Figure 1. Triaxial test fixture overview
P1
~ (1) Resonant test chamber~
(2) CUT
shield
~
~
RL
load
~
P2
Installation of 30 meter test fixture
Metallic cylinder
Figure 4. Open corridor installation of 30 meter test
fixture
Figure 2. Electrical regions of triaxial test fixture
2
2.3 Installation
3.2 Normalization
The fixture was installed, as shown in Figure 4, above an open
corridor that was located on a mezzanine in the Belden
Engineering Center. Individual transmission line segments were
suspended 2.2 meters above the floor and aligned with
commercial “Clevis” hangars. This arrangement provided
relatively easy access to both ends of the fixture; nonetheless it
was necessary to run approximately 35 meters of low loss 50 Ohm
coax to provide the input power P1 at the end plate. The network
analyzer, amplifier, and data acquisition equipment were located
at the far end in Figure 4. This provided for direct connection of
the low-level power output P2 to either a preamplifier or the
network analyzer input.
Screening attenuation is derived from the difference in power
levels (insertion loss) between the end plate P1, (energized sample
input) and the end cap P2, (test chamber output). The measured
power ratio is normalized [1] with the following formula. The
screening attenuation curve is then obtained by drawing the
envelope (not shown) formed by connecting the resonant peaks in
the power ratio versus frequency plot.
a n = a meas + 10 log 10
3. Test Procedure
2Z S
Z1

ε2

1−

ε1
+ 2 0 log 10  2
ε3

1−

ε1








3.1 Equipment setup
Where:
The equipment setup is given in Figure 5, and the equipment used
is listed in Table 1.
an =
normalized screening attenuation (decibels).
ameas =
measured screening attenuation of sample in normalized
and calibrated setup (decibels).
30 m Test Fixture (5 x 6m)
Matching
Pad
Network analyzer
Amplifier
Z1 =
impedance of cable under test (ohms).
ZS =
normalized impedance of screening attenuation fixture
(150 ohms).
ε1 =
relative dielectric permittivity of cable under test.
ε2 =
relative dielectric permittivity of the environment of the
cable.
ε3 =
relative dielectric permittivity of screening attenuation
fixture outer circuit with respect to a 10% velocity
difference.
Splitter
4. Test program
4.1 Samples tested
Figure 5. Equipment setup for screening
attenuation measurement
Several constructions of RG-6 type and RG-59 type coaxial cable
were tested and are designated 1-4. (1 is RG-59 type and 2-4 are
RG-6 type cables). The details are given in Tables 2 and 3.
Table 2. Cable shield design data
Table 1. Test equipment
#
Network Analyzer:
Foil
Braid/
Foil
Braid
Shield
(inner)
Angle
(outer)
Angle
DCR
(outer)
mΩ/m
_
95% b. c.
HP8753ES, +10dBm, 10 Hz res. bandwidth
(inner)
Power Splitter:
1
HP11850C, 9.5 dB loss nominal, DC-3 GHz
2
a
3
a
4
a
Power Amplifier:
HP8347A, 25 dB gain nominal, 100 kHz-3 GHz
Resistive Termination on sample under test:
75Ω type F for CATV applications
3
_
_
_
/23°
Matching Pad:
HP11852B, 5.7 dB loss nominal, DC-3 GHz
_
60% Al
27°
80% Al
/27°
60% Al
/27°
b
a
_
40% Al
/20°
9
31
15
17
40
Table 3. Shielding tape design data
1
Layer Thickness (mm)
Al Foil
Polyester
a
.00889
.02286
b
.0254
.02286
Al Foil
Width
Decibels
Foil
Type
60
(mm)
.00889
19.05
25.4
2
80
100
4
120
140
3
4.2 1.5 meter fixture test results
160
Test results are plotted versus frequency in Figures 6 and 7 for the
100-1000 MHz and 1-100 MHz frequency ranges respectively.
The test program involved testing multiple samples of the designs
given above. However, only single sample results are shown for
clarity. The statistical characteristics of screening attenuation are
beyond the scope of this paper.
1
100
Frequency - MHz
Figure 7. Screening attenuation
versus frequency
4.3 30 meter fixture test results
In Figure 6 the lowest frequency resonance peak (not shown) is
just below 100 MHz. Resonance peaks out to 1000 MHz are
connected by envelopes, as shown. The 1.5 meter fixture length is
long enough to produce response peaks, which correspond to
maximum power transfer as well as minimum screening
attenuation performance at specific frequencies. The exact
frequency location of the peaks will vary as the length is varied,
but the peak amplitude will follow the envelope. Envelopes for
designs 1 and 3 are approximately flat, whereas envelopes for
designs 2 and 4 show an upward slope. At these frequencies the
cable length under test is electrically long.
Figures 8-11 present screening attenuation results for 30 meter
lengths of designs 1-4 respectively. Results for 1.5 meter are also
shown.
40
Decibels
60
80
100
120
30 m
140
40
1.5 m
envelope
160
1
60
Decibels
10
1
10
envelope
80
Frequency - MHz
2
100
100
Figure 8. Screening attenuation
versus frequency – Design 1
4
120
40
140
3
160
60
.
Frequency - MHz
1000
Decibels
100
Figure 6. Screening attenuation
versus frequency
80
100
120
30 m
140
envelope
1.5 m
160
In Figure 7 only one resonance peak is shown, which is at about
95 MHz. The fixture length of 1.5 meters is too short to allow
resonance to occur at lower frequencies. Therefore, power transfer
does not reach a maximum and the measured screening attenuation
performance is consequently not minimal. Designs 1-4 show a
tendency to converge below 1 MHz. At these frequencies the cable
length under test is electrically short.
1
10
Frequency - MHz
Figure 9. Screening attenuation
versus frequency – Design 2
4
100
For each design the 30 meter length produces minimal screening
attenuation, as shown by the envelope curve.
40
envelope
Decibels
60
Table 4 summarizes the relationship between 1.5 meter and 30
meter lengths for 10 MHz and 100 MHz results. For example, there
is approximately a 20 decibel difference in performance at 10 MHz
between the 1.5 meter and 30 meter sample lengths.
80
100
120
Alternatively, it can be seen that the difference in performance
(error) is approximately 40 decibels (designs 3 and 4) if it is
assumed the 30 meter performance at 10 MHz should be
approximately equal to that of the 1.5-meter performance at 100
MHz. For design 1 the 30 meter performance at 10 MHz is about
the same as the 1.5 meter performance at 100 MHz. For design 2 the
difference is about 30 decibels.
30 m
140
1.5 m
160
1
10
100
Frequency - MHz
5. Conclusions
Figure 10. Screening attenuation
versus frequency – Design 3
The paper has described a test methodology that is in the process of
being standardized internationally. The test has been applied to 30
meter and 1.5 meter long cable samples, and a comparison of the
measured screening attenuation performance of a selection of braid
and multi-foil/braid shield designs has been presented. The
performance of 30 meter and 1.5 meter long samples has been
compared and differences as high as 40 decibels were noted
depending on frequency.
40
envelope
Decibels
60
80
The worse case or minimum screening attenuation performance of a
particular shield design can depend on the length of the cable and
the frequencies involved. In this work the test fixture has been
normalized to 150 Ω and 90 %VP, but in actual usage the electrical
environment around the cable can also influence the actual
screening attenuation performance.
100
120
30 m
140
1.5 m
160
1
10
Cable specifiers and system designers should be sure to specify
screening attenuation requirements, which take into account the
frequency band of operation and the cable lengths utilized.
100
Frequency - MHz
6. Acknowledgements
Figure 11. Screening attenuation
versus frequency – Design 4
The authors are grateful to the Belden Electronics Division for the
support extended to develop screening attenuation measurement
technology. Thanks to Benjamin Willett for assistance with the
laboratory measurements.
7. References
Table 4. Screening attenuation estimates versus
test length
Design #
dB at 10 MHz
[1] 61196-1 Amendment 1 Radio-Frequency Cables – Part 1:
Generic Specification – General, definitions, requirements
and test methods.
dB at 100MHz
1.5 meter
30 meter
1.5 meter
1
60
41
41
2
73
52
82
3
106
82
135
4
104
82
120
[2] SCTE (Society of Cable Telecommunications Engineers,
Inc.) IPS-TP-403B2 (preliminary-2/18/2000), Test Method
for Shield Effectiveness: Screening Attenuation of Coaxial
Cable.
[3] O. Breitenbach, T. Hähner, and B. Mund, “Screening of
Cables in the MHz to GHz Frequency Range Extended
Application of a Simple Measuring Method”, IEE
Colloquium on Screening Effectiveness Measurements,
Savoy Place, London, May 6, 1998.
5
[4] bedea/Rosenberger, CoMeT Coupling Measuring Tube.
bedea BERKENHOFF & DREBES GMBH, Herborner
Straβe 100 • 35614 Aβlar • Germany
[5] J. Kincaid, and C. Dole, “Test Fixture Design and Shielded
Screening Attenuation Performance of CATV Coaxial
Cable”, IEE Colloquium on Screening Effectiveness
Measurements, Savoy Place, London, May 6, 1998.
[6] J. Kincaid, and C. Dole, “Implementation of IEC 61196-1
Shielded Screening Attenuation Test Method”, International
Wire and Cable Symposium, Philadelphia, Pennsylvania,
November 19, 1998.
[7] J. Kincaid, and C. Dole, “ Shielded Screening Attenuation
Test Method Down to 5 MHz”, International Wroclaw
Symposium and Exhibition on Electromagnetic
Compatibility, Wroclaw, Poland, June 27, 2000.
Carl Dole
John Kincaid
Carl is a Product Engineer and has been with the Belden
Engineering Center of Belden Electronics Division since 1990.
He currently holds one U.S. patent. His academic achievements
include a B.S. degree in Electrical Engineering Technology
(“With Highest Distinction”) from Purdue University. Prior to
joining Belden, Carl worked 10 years in television broadcast
engineering. He is a Certified Senior Broadcast Engineer and has
a lifetime FCC General Class Radiotelephone License. His areas
of responsibility include developing improved electrical test
methodologies, writing technical papers, and working on new
product development. He is a member of SMPTE, IEEE, and
SBE.
John is a Senior Product Engineer at the Belden Engineering
Center. He holds BSEE and MSEE degrees from the University of
Oklahoma and has over 25 years experience with Belden. His
experience encompasses engineering management and product
development positions in the USA as well as in Europe. He holds
nine patents. John is a member of the IEEE and is active in IEC
and TIA cable standardization activities. He is the US Technical
Advisor to IEC SC 46A on coaxial cables, and is Convenor of
IEC SC 46A/WG3 on data and CATV cable. He is also an expert
on working groups 5 and 7 dealing with shielding and premises
cabling issues.
Carl W. Dole originally presented this paper November 14, 2000 at IWCS 2000, Atlantic City, New Jersey, USA.
6