A COMPARISON OF BULK CABLE CURRENT INJECTION AND
RADIATED FIELD SUSCEPTIBILITY TEST METHODS
Joseph M. Harris
Georgia Tech Research Institute
Georgia Institute of Technology
Electromagnetic Environmental Effects Laboratory
Atlanta, Georgia 3 0 3 3 2
ABSTRACT
previously determined, through laboratory
tests, to be EM1 coupled into the system
through the interconnect cabling. The Radar
Altimeter system evaluated here consists of a
Indicator/Receiver/Transmitter (IRT) and a
Remote Indicator (RI) interconnected with a
cable harness made up of two shielded twisted
pairs, four shielded single wires, and six
unshielded wires.
Bulk cable current injection of modulated
radio frequency (RF) signals will be included
in MIL-STD-461D and MIL-STD-462A as conducted
susceptibility Test Method CS114. Test Method
to
evaluate
the
CS114
is
designed
susceptibility of electronic equipment to RF
signals coupled into the equipment through the
cabling. This paper presents an examination
of cable current produced by bulk cable
current injection and by a radiated field for
two test cases; a simple multi-wire cable
mounted above a ground plane with each wire
terminated with resistors at both ends, and an
operational Radar Altimeter system mounted on
a mockup of an aircraft instrument panel. For
the simple multi-wire test case, the bulk
cable current induced on the cable by a
current injection probe and by a radiated
field is examined for two cable shield
termination configurations and two wire
termination resistances. The Radar Altimeter,
with a known cable-coupled digital altitude
susceptibility, is evaluated for several cable
shield terminations configurations.
The purpose of this paper is to examine
the characteristics of the bulk cable current
produced by the two test methods to determine
the advantages or limitations that one method
may have over the other. All tests reported
in this paper were performed at a single
frequency in the UHF band at which the Radar
Altimeter is susceptible. All distances in
this report are normalized to the free-space
wavelength of the test frequency. All data
points presented in this paper represent the
average value after five test repetitions.
GENERIC TEST SETUP
Because of the simplicity, a multiconductor cable mounted over a ground plane
and terminated with fixed resistors at both
ends was chosen as a general, or generic, case
for comparing the BCCI and radiated field
methods.
This test case is designed to
minimize test uncertainty such that insight
can be gained into the consequences of
variations in the test item.
INTRODUCTION
Electromagnetic Interference (EMI) caused
by radiated signals in the HF, VHF, and UHF
bands is often caused by the signals coupling
to the system
interconnect cables and
conducting into an electronic unit to upset
the system response. The effectiveness of the
shielding of system interconnect cables to
prevent EM1 can be evaluated either by
exposing the cable to a radiated field from an
antenna or by injecting RF signals on the
cable bundle using a current injection probe.
This paper compares these two test techniques
applied to two test cases.
Two identical length multi-wire cables
were constructed for the generic tests. The
length of the cables was chosen to be the same
as the Radar Altimeter interconnect cable.
The first cable consists of eight unshielded
20 gauge wires attached to circular military
connectors at each end.
The second cable
consists of eight shielded 22 gauge wires
attached to identical circular military
connectors at each end.
The individual
shields of this cable assembly are “daisychained” and terminated to the EM1 backshells
on the connectors at each end of the cable.
Both cable assemblies were constructed such
that a constant cross-section exists along the
length of the cable. This is to say that all
wires remain parallel in the same relative
location in the bundle with no twist along the
length of the cable. This was done such that
any
correlation
between
variations
in
individual wire currents and wire location in
the bundle could be evaluated.
The first test case is a simple multiwire cable mounted above a ground plane and
terminated at both ends by resistors. This
test case serves as a controlled experiment in
which
various
cable shield
termination
configurations and termination resistance
values can be evaluated using bulk cable
current injection (BCCI) and radiated fields
to induce current on the wiring.
The second test case is a helicopter
Radar
Altimeter
system
with
a
known
susceptibility to radiated RF signals in the
UHF band. The susceptibility is evidenced by
variations in the digital altitude. The cause
of the ‘Radar Altimeter susceptibility has been
CH3310-0/93/0000-0013 $3.00 a 1993 IEEE
Two identical cable termination boxes
were constructed €or the generic tests. These
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boxes contain the mates to the circular
military connectors on the cable assemblies
and a means of terminating each wire passing
through the connector with a resistor grounded
to the metallic box.
The two shielded
termination boxes were electrically bonded to
the test ground plane with one of the test
cables
connected
between
them
in
the
configuration shown in Figure 1. This setup
allows the two cable configurations and cable
termination resistors to be interchanged
easily. The 90 degree elbow in the cable was
necessary to obtain sufficient isolation
between the current sensing probe and the
radiated field.
Figure 2
Generic Test Cross-sectional View.
F i g u r e 3 . Photograph of BCCI T e s t Sotup
Fwre 1. Generic Radiated Test Sew.
Figure 2 shows a cross-sectional view of
the generic setup showing the individual wire
positions in the cable bundle and the cable
position relative to the radiating antenna.
The generic BCCI tests use the same ground
plane setup as the radiated field tests except
with a current injection probe clamped around
the test cable. Figure 3 is a photograph of
the setup used for the generic BCCI tests.
Four
different
generic
test
configurations were evaluated using the
radiated field and BCCI test methods: (1) the
shielded wire cable with 50 R terminating
resistors, (2) the shielded wire cable with 1
kn terminating resistors, ( 3 ) the unshielded
wire cable with 50 R terminating resistors,
and ( 4 ) the unshielded wire cable with 1 kR
terminating resistors. The bulk cable current
produced by the two test methods was measured
for each cableftermination configuration.
GENERIC RADIATED FIELD TESTS
As seen in Figure 1, the radiated field
generic tests were performed in an anechoic
chamber. The radiated field was generated by
a dual ridge horn antenna connected to a UHF
power amplifier and signal source.
The
amplifier output power was held constant at 4 7
Watts for all generic radiated field tests.
The bulk cable current produced on the test
cable by the radiated field was measured using
a current sensing probe connected to a
spectrum analyzer. As mentioned previously,
significant care was taken to ensure that the
current sensing probe was isolated from the
radiated field so that the probe output
accurately reflected the current on the wire
being monitored.
The
measured
bulk
cable
current ,
corrected for the probe calibration factor
and cable loss, produced by the radiated field
is presented in Figure 4 as a function of
current sensing probe location. The current
sensing probe location is the distance from
termination box 1 normalized to the free space
wavelength of the radiated field. It should
be noted that the curves for the shielded and
unshielded cable cases start at different
locations due to the extended EM1 backshell on
the shielded cable. An inspection of Figure 4
yields the following observations:
(1) the
bulk cable current on the shielded cable
exhibits a more pronounced standing wave
characteristic than the unshielded cable, (2)
the magnitude of the bulk cable current on the
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50
40
30
+Shielded j l k
+Unshielded/50
20
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Sense Probe Location (Wavelengths)
20
0.8
1
1.2
1.4
1.6
1.8
2
Injection Probe Location (Wavelengths)
Figure 4. Generic Test: Bulk Current Induced
by Radiation
shielded cable is significantly affected by
the termination resistances on the shielded
wires, and ( 3 ) the bulk cable current varies
as much as 12 dB along the cable depending on
the shielding and termination configuration.
The current produced by the radiated
field on each wire in the bundle was also
measured at a fixed current sensing probe
location approximately 0.3 wavelengths from
the termination box 1. Table 1 presents the
individual wire current, averaged over five
test repetitions, normalized to the bulk cable
current measured at that current sensing probe
location. An inspection of the radiated test
columns in Table 1 yields the following
observations: (1) the normalized per wire
current on the shielded cable is larger for
the lower terminating resistance than for the
(2)
the
higher
terminating
resistance,
normalized per wire current on the unshielded
cable is larger for the higher terminating
resistance than for the lower terminating
resistance, and ( 3 ) no obvious correlation
exists between wire number (location in a
bundle) and per wire current.
GENERIC BULK CABLE CURRENT INJECTION TEST
The generic BCCI tests were performed by
clamping a current injection probe around the
test cable.
RF power was supplied to the
injection probe from a UHF power amplifier'and
signal generator.
The input power to the
probe remained at 10 Watts for all generic
BCCI tests. The sensitivity of the measured
bulk cable current to the location of the
injection probe on the cable is shown in
Figure 5.
These curves were produced by
measuring the bulk cable current at a fixed
location 0.3 wavelengths from termination box
1 while varying the injection probe location.
The injection probe location is the distance
along the test cable from termination box 1.
An inspection of Figure 5 shows that the bulk
cable current at a given location can vary up
to 20 dB depending on the location of the
Figure 5. Generic Test: Injection Probe Location
Sensitivity
injection probe.
The location of the peak
bulk cable current repeats at approximately
every one half wavelength along the cable just
as a standing wave.
A graph of the measured bulk cable
current produced by the injection probe for
each CableJtermination configuration is given
in Figure 6. For these tests, the injection
probe remained at a fixed location 1.5
wavelengths from termination box 1.
An
inspection of Figure 6 results in the
following observations:
(1) the bulk cable
current
exhibits
a
standing
wave
characteristic along the cable for all cable
shield termination and terminating resistor
combinations, (2) the terminating resistance
on the shielded cable has little effect on the
bulk cable current, and ( 3 ) the bulk cable
current on the shielded cable is generally
larger than on the unshielded cable.
The current produced by the injection
probe on each wire was also measured at a
fixed location 0.3 wavelengths from the
termination box 1.
The injection probe
remained at a fixed location 1.5 wavelengths
from termination box 1. Table 1 presents the
individual wire current, averaged over five
test repetitions, normalized to the bulk cable
current measured at the same sensing probe
location.
An inspection of the per wire
currents from the BCCI test given in Table 1
does not yield any clear correlations between
the per wire current and wire number (location
in the bundle) or the cable shielding/
termination configuration.
COMPARISON OF THE GENERIC CASE BCCI AND
RADIATED FIELD TEST RESULTS
A comparison of the test results
presented in Figures 4 (radiated field) and 6
(BCCI) yields the following observations:
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Table 1.
Generic Test: Individual Wire Currents Normaliaed to the Bulk Cable Current
(1) Both
(2)
test methods tend to produce a
significant standing wave on the shielded
cable.
The radiation method does not
appear to produce as significant a
standing wave on the unshielded cable
compared to the BCCI method.
The termination resistances have less
effect on the shielded cable bulk current
using the BCCI method than the radiated
field method.
A comparison of the radiated field and
BCCI test results presented in Table 1
indicates that the normalized per wire current
is greater for the BCCI test than the radiated
field test. In addition, the per wire current
in the BCCI test is less sensitive to shield
and cable termination configurations than the
radiated field test.
RADAR ALTIMETER TEST SETUP
The Radar Altimeter tests were performed
with the system installed in a mockup of an
instrument panel designed to simulate the
actual grounding provisions and IndicatorReceiver-Transmitter (IRT)/Remote Indicator
(RI) spacing found on the helicopter.
The
interconnect harness between the IRT and RI
was constructed identically to the harness
used on t h e , helicopter with no particular
attention given
to the positioning of
individual wires within the bundle. Figure 7
shows the setup used in performing the
radiated susceptibility tests on the Radar
Altimeter.
The same setup was used for the
BCCI tests of the system except with a current
injection
probe
clamped
around
the
interconnect harness.
Figure 8 is a
photograph of the Radar Altimeter test setup.
Five
different
configurations
for
terminating the shields of the two shielded
twisted pairs and four shielded single wires
in the Radar Altimeter interconnect harness
were evaluated in the radiated field and BCCI
tests.
These configurations were; (1) all
shields floating at both ends, (2) all shields
terminated to the connector backshell at both
ends, (3) all shields terminated to the
T 'O
:
:L
i 4 0
30
0
0.3 0.4 0.5 0.6 0.7
Sense Probe Location (Wevelengths)
0.1
0.2
Figure 6. Generic Test: Bulk Current induced by Probe
connector backshell only at the IRT end of the
interconnect
harness,
(4)
all
shields
.terminated to the connector backshell only at
the RI end of the interconnect harness, and
(5) all shields connected to the signal ground
pin at the IRT and floating at the RI end of
the cable.
These five configurations were
tested in an attempt to determine if the Radar
Altimeter susceptibility could be eliminated
simply by changing the shield termination
configuration method.
Configuration (5)
represents the method used to terminate the
shields in the actual helicopter installation.
The radiated
field
for the Radar
Altimeter tests was generated identically to
the generic radiated tests except with higher
power levels. The bulk cable current produced
on the interconnect harness by the radiated
field was measured using a current sensing
probe connected to a spectrum analyzer. The
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I
lI
b
70
+Both Ends
+IRT Only
-RI Only
-Signal Gnd
I
40
,
0.2
'
'
0.3
I
,
0.4
,
,
,
)
,
0.5
0.6
0.7
0.8
0.9
Sense Probe Location (Wavelengths)
Figure 7. Radar Altimeter Radiated Test Setup.
Figure 9. Radar Altimeter Test: Bulk Current Induced
by Radiation to Cause Susceptibility
RADAR ALTIMETER BULK CURRENT INJECTION TESTS
Figure 8. Photograph of Radar Altimeter Teat
setup
radiated tests were performed by adjusting the
radiated field level until the susceptibility
threshold (lowest level at which the digital
altitude varied) of the Radar Altimeter was
attained.
At the susceptibility threshold,
the bulk cable current induced by the
radiation was measured for each shield
termination configuration.
The
measured
bulk
cable
current ,
corrected for probe calibration factor and
cable loss, present at the susceptibility
threshold of the Radar Altimeter is given in
Figure 9 . The current sensing probe location
is measured along the interconnect cable from
the RI. An inspection of Figure 9 yields the
following observations:
(1) the bulk cable
current does not exhibit a standing wave
characteristic, (2) less bulk cable current is
present on the harness when the interconnect
cable shields are floating (not grounded) at
both ends, and ( 3 ) the bulk cable current is
closely correlated for all shield terminations
in which at least one end is grounded to the
connector backshell or signal ground (which is
the same as chassis ground).
The BCCI tests on the Radar Altimeter
system were performed by clamping a current
injection probe around the interconnect
harness.
RF power was supplied to the
injection probe from a UHF power amplifier and
signal generator.
The sensitivity of the
Radar Altimeter susceptibility threshold to
the injection probe location was evaluated by
varying the probe location and adjusting the
probe input power until the Radar Altimeter
susceptibility threshold was reached. Figure
10 presents the minimum power input to the
current injection probe required to cause the
Radar Altimeter susceptibility. The injection
probe location is the distance along the cable
from the RI. Only shield configuration (5)
(same as the helicopter installation) was
evaluated
for
injection probe
location
sensitivity.
An inspection of Figure 10
indicates
that
the
Radar
Altimeter
susceptibility threshold is very sensitive to
the injection probe location and can cause
susceptibility level variations up to 5 dB.
The bulk cable current, present on the
Radar Altimeter interconnect harness at the
susceptibility threshold, produced by the
injection probe was measured for each shield
termination configuration. Figure 11 presents
the measured bulk cable current, corrected for
the probe calibration factor and cable loss.
The current sensing probe location is the
distance along the interconnect cable from the
RI. The injection probe remained at a fixed
location 1.25 wavelengths from the RI.
An
inspection of Figure 11 yields the following
observations: (1) the bulk cable current
exhibits a standing wave characteristic for
several
of
the
shield
termination
configurations with variations up to 8 dB
along the cable, and (2) the bulk current for
all shield termination configurations tend to
converge as the sensing probe is moved closer
to the RI.
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(3) The BCCI test results indicate the
existence of a standing wave on the cable
which is not indicated by the radiated
field tests.
COHCLUBIONS
The bulk cable current for two test cases
was examined
using
bulk
cable current
injection and radiated field test methods.
Based on the previous observations of the test
data and comparisons of the test methods for
each test
case,
the following general
conclusions can be made:
t
(1)
BCCI testing tends to produce a more
significant standing wave on the test
cable than radiated field testing. This
standing wave is heavily influenced by
the location of the current injection
probe. The injection probe can be moved
along the test cable a distance of one
half wavelength to determine the best
position for evaluating susceptibilities.
(2)
Radiated field testing produces greater
variation in the per wire current than
BCCI testing.
Therefore, if a system
susceptibility is strongly influenced by
the coupling to one particular wire, the
BCCI test would have a better chance of
identifying that susceptibility assuming
the same bulk current is produced on the
cable bundle by both methods. This may
explain why the bulk cable current for
Radar Altimeter BCCI and radiated field
tests was different
for the same
susceptibility threshold.
Figure 10. Radar Altimeter Test: Injection Probe
Location Sensitivity
+Floating
+
+-Both Ends
+IRT Only
-RI Only
-Signal Gnd
Figure 11. Radar Altimeter Test: Bulk Current Induced
by Probe to Cause Susceptibility
(3) BCCI testing is an effective technique
for evaluating system susceptibilities
caused by EM1 coupling into the system
through the cabling.
It offers the
benefit
of
lower
power
amplifier
requirements, better test repeatability,
and more uniform per wire current than
radiated field testing.
However, the
radiated field testing may have an
advantage when the system susceptibility
is a complex function of cable coupling,
box grounding, and box leakage. Careful
use of both techniques together can
effectively
identify susceptibilities
prior to fielding the system such that
modifications can be made to eliminate
any problems.
COMPARISON OF THE RADAR ALTIMETER BCCI
"
A comparison of the Radar Altimeter test
results presented in Figures 9 (radiated
field) and 11 (BCCI) yields the following
observations:
A portion of the work presented here was
performed under Contract No. F09603-91-G-00960010 funded by the Warner Robins Air Logistics
Center at Robins AFB, Georgia.
(1) The bulk current required to produce the
digital
altitude
susceptibility
is
greater for the radiated field test than
the BCCI test.
(2) The radiated field test results indicate
that less bulk current is required to
upset the digital altitude with the
harness shields floating. This effect is
not indicated by the BCCI test results.
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