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PUBLICATIONS

R All 102703727 - SUREMENT OF SHIELDING

EFFECTIVENESS OF DIFFERENT CABLE

AND SHIELDING CONFIGURATIONS BY

MODE-STIRRED TECHNIQUES

Ramon L. Jesch

National Bureau of Standards

U.S. Department of Commerce

Boulder, Colorado 80303-3328

October 1987

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87-3076

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MEASUREMENT OF SHIELDING ZV'^

EFFECTIVENESS OF DIFFERENT CABLE

AND SHIELDING CONFIGURATIONS BY

MODE-STIRRED TECHNIQUES

Ramon L. Jesch

Electromagnetic Fields Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303-3328

October 1987

Sponsored by

U.S. Army Aviation Systems Command

St. Louis, Missouri 63120

U.S. DEPARTMENT OF COMMERCE, Clarence J. Brown, Acting Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

CONTENTS

Page

1. INTRODUCTION . 1

2. CABLE AND SHIELDING CONFIGURATIONS....... 2 2.1 Cable Configurations 2

2.2 Shielding Conf igurations 3

3. MEASUREMENT APPROACH 6

3.1 Measurement System 6

3.2 Shielding Effectiveness 11

4. MEASUREMENTS RESULTS * 12

14.1 Cable Configurations 12

4.2 Shielding Configurations 15

5. SUMMARY AND CONCLUSION 17

6 . ACKNOWLEDGMENTS 18

7. REFERENCES 18

MEASUREMENT OF SHIELDING EFFECTIVENESS OF DIFFERENT CABLE

AND SHIELDING CONFIGURATIONS BY MODE-STIRRED TECHNIQUES

Ramon L. Jesch

Electromagnetic Fields Division

National Bureau of Standards

Boulder, Colorado 80303

The shielding effectiveness of cable configurations having

different shielding arrangements and of shielding configurations

used to terminate cable shields for helicopter wiring were

measured by mode-stirred techniques. The mode-stirred measurements were taken at discrete frequencies between 200 MHz and 6

GHz. In addition, shielding effectiveness data on the shielding

configurations were also obtained in a TEM cell down to 1 MHz.

A description of the cable and shielding configurations is given

along with plots of the measured shielding effectiveness data as

a function of frequency.

Key words: cables; connectors; mode-stirred chamber; relative

leakage power; shielding effectiveness.

1 . INTRODUCTION

In recent years, one major concern with the design and use of coaxial

and multicontact connectors and accompanying cable assemblies has been the

amount of electromagnetic (EM) energy or radio frequency (rf) leakage entering into the connector/cables that can be tolerated in today’s advanced

electronic systems. Moreover, these systems have to be designed to

withstand the effects of electromagnetic interference (EMI) at operating

frequencies from a few megahertz to many gigahertz. To facilitate system

design, shielding effectiveness (SE) data are required for cables and connectors that interconnect various components, subassemblies, equipment, and

subsystems. The evaluation of SE measurements of cables and connectors has

been the subject of a great deal of analysis and research for some time.

To determine the SE of cable configurations having different shielding

arrangements, the National Bureau of Standards (NBS) , in 1985, undertook a program to conduct a study on standard practices of terminating cable

shields for helicopter wiring for the U.S. Army Aviation Systems Command

1
( AVSCOM ) . One of the objectives of this program was to determine a suitable

technique for measuring the SE of terminated cable shields in order to

compare the EMI effects between different cable arrangements . To accomplish

this task, an extensive study of SE measurement techniques was conducted in

the area of cables and connectors. Prominent among the measurement techniques being surveyed were the triaxial technique [1] which configures the

test sample (cable/connector under test) as a segment of transmission line,

and the mode-stirred technique [2] which uses a test chamber to expose the

test sample to random incident fields. From the study taken [ 3 ], the modestirred technique was chosen as the most convenient technique to measure the

SE of cables and connectors over a wide frequency range above a few hundred

megahertz. Toward this end, another mode-stirred chamber was designed and

fabricated specifically to accommodate SE measurements of cables and connectors, and cable shield terminations for the AVSCOM task at NBS.

2. CABLE AND SHIELDING CONFIGURATIONS

2.1 Cable Configurations

Figure 1 shows the four different cable configurations that were used

to determine and compare the shielding effects of different shielding

arrangements . The configurations were fabricated from 1.5 m (4.93 ft)

lengths of RG9/U coaxial cable, which has two shields. The cable was

modified to produce:

Configuration 1, consisting of the cable center conductor only, with

the vinyl jacket and two shields removed from the

cable;

Configuration 2, consisting of RG9/U cable retaining one shield only,

with the other shield removed;

Configuration 3» consisting of the regular RG9/U cable with two

shields

;

and

Configuration 4, consisting of the regular RG9/U cable with one more

shield added over the cable to give triple shielding.

All four cable configurations were equipped with Type N coaxial connectors.

2
Figure 1. Four different cable configurations.

2.2 Shielding Configurations

General types of shielding configurations that are used for helicopter

wiring were discussed with AVSCOM and other knowledgable personnel. It was decided to narrow down to three basic shielding configurations that should

be measured for SE by NBS for the AVSCOM program. These included:

3
1 . Single Shielded Wire. The shield is stripped back from one end of

the wire approximately 15 cm (6 in) and terminated by means of a solder sleeve and wire that is attached to the connector ground as shown in figure 2.

2. Twisted Shielded Pair. The shield is terminated in the same

manner as (1) above with one of the conductors attached to the

center conductor of the coaxial connector while the other

conductor is attached to the connector ground as shown in figure

3.

3. Multipin Connector /Cable Arrangement. This arrangement is the

most difficult to configure for SE measurements in the modestirred chamber. Two harness arrangements were used. One used

a single shielded wire arrangement in the harness while the other

used a twisted shielded pair arrangement in the harness. Figure 4 shows the single shielded wire arrangement that was measured which

is an actual harness assembly from an Army helicopter.

All three shielding configurations were equipped with Type N connectors.

Figure 2. Single shielded wire.

4
Figure 3. Twisted shielded pair.

Figure 4. Multipin connector/cable arrangement.

5
3. MEASUREMENT APPROACH

3.1 Measurement System

Two different size reverberation chambers were used for the SE measure- ments between 200 MHz and 6 GHz. Figure 5 shows a block diagram of the

small mode-stirred chamber with shielding configuration and test system that was used for the SE measurements between 1 and 6 GHz. Measurements of the

SE of the cable and shielding configurations were made in the small chamber

in accordance with Method 3008 of MIL-STD-1 3 ^ ^ A [4] as required by MIL-C38999H [5]. The wire antennas were later replaced with ridged-horn antennas

and the measurements repeated. The small chamber which has dimensions of

1.165 m x 1.928 m x 1.987 m (3-82 ft x 9.68 ft x 9.88 ft) is essentially a low-loss shielded enclosure that includes an input antenna, a reference

antenna and a mode stirrer. The antennas were placed in the chamber in such

Figure 5. Diagram of small mode-stirred chamber and test system.

6
a way as to minimize direct coupling from the input (excitation) antenna to

the reference (receiving) antenna and from the input antenna to the device

under test (DUT). Testing is conducted inside the test chamber whose smallest dimension is at least three wavelengths at the lowest test frequency.

This is to assure an ample mode density which is a necessary condition for

the validity for the mode-stirred technique [6]. In addition, the minimum

distance between the DUT and the chamber walls is at least one wavelength at

the lowest test frequency to maintain a uniform electric field throughout

the chamber. In order to insure that the shielding configurations (DUT)

meet this criterion, each end of the DUT was connected directly to a short

section of semi-rigid cable that extended inside the chamber as shown in

figure 6. The semi-rigid cables in turn were connected to the bulkhead

connectors on the chamber wall.

Figure 6. DUT connected to semi-rigid cables inside the chamber.

7
Figure 7 is a block diagram of the larger mode-stirred reverberation

chamber and measurement system that was used for the SE measurements between

200 MHz and 1 GHz [7]. The dimensions of the chamber are 2.74 m x 3.05 m x 4.57 m (8.99 ft x 10 ft x 15 ft), which allow a lower frequency of approx imately 200 MHz. Figure 8 shows the placement of the DUT and the logperiodic antennas inside the large mode-stirred chamber.

TUNER CONTROL

Figure 7. Block diagram of the large mode-stirred chamber

and measurement system.

8
Figure 8. Placement of the DUT and the log periodic antennas inside

the large mode-stirred chamber.

Figure 9 is a block diagram of a transverse electromagnetic (TEM) cell

that was used for the SE measurements between 1 MHz and 200 MHz. The TEM

cell [8] is well known for making susceptibility measurements of electronic

equipment for frequencies from dc to 500 MHz but not for SE measurements

which require more study and investigation. Some SE measurements of

materials; e.g., plastics, conductive laminates, or composites have been

conducted with a dual TEM cell [9]. However, for purposes of obtaining

several discrete frequencies on the shielding configurations for the AVSCOM

program below 200 MHz, it became necessary to use the TEM cell. The design

of the TEM cell is based on the concept of an expanded transmission line

operated in a TEM mode. The DUT is oriented along the longitudinal axis of

the TEM cell to maximize coupling between the DUT and the E and H fields in

the region where the DUT is connected to the wall of the TEM cell as shown

in figure 10. Moreover, the area of interest for the shielding configurations (DUT) is the region from the connectors back approximately 15 cm (6

in) where the E and H fields are uniform.

9
Figure 10. Placement of the DUT inside the TEM cell.

10
3.2 Shielding Effectiveness

The relative leakage power for determining SE is the of power which

leaks into a cable, connector, or other device at a given frequency and is

defined by the relationship,

SE - 10 HogjJP^Pj) dB, (1)

where

P, - total power received by the reference antenna for a given power

applied to the input antenna, and

P 2 - total power leaked into the DUT for the same power applied to the

input antenna.

To measure the SE of the cable or shielding configurations (DUT) in the

mode-stirred chamber, reference conditions are established with the DUT

inside the chamber. Energy is injected inside the chamber from a source

into the input (excitation) antenna to set up an electric field. The

electric field in turn is received and coupled into the reference

(receiving) antenna to establish a reference level which is measured as P t by the detection system. The objective is to obtain a time average for Pj

due to variations of field strength or power density inside the chamber.

Next with the input antenna receiving the same amount of power as before,

the detection system is disconnected from the reference antenna and connected directly to the DUT. The output power leaking into the DUT is

measured as P a

. The SE of the DUT is determined using eq (1) by taking the

ratio of the averages (Pj and P 2 ) over one rotation of the mode stirrer.

The mode-stirred technique uses two operational approaches depending

upon the Standing Wave Ratio (SWR) for performing SE measurements inside the

chamber. The first approach, mode tuned (used when the input SWR is relatively large in the order of 1.5 or more), steps the mode tuner at selected

uniform increments (typically 200 steps or more), permitting measurements of

the net input power, power received by the reference antenna, and the
monitored DUT response at each tuner position. This allows corrections to

be made for the variations in the chamber's test field resulting from

changes in the SWR of the transmitting antenna as a function of the mode

tuner position. The second approach, mode stirred (used when the input SWR

is below 1.5), was used for the SE measurements which rotates the tuner

continuously while sampling the reference antenna received power and DUT

response at rates much faster than the tuner revolution rate. Large data

samples up to 9,999 can be obtained for a single tuner revolution which

takes approximately 3 to 6 minutes per revolution.

For the measurement of SE of the shielding configurations in the TEM

cell, energy is fed from a signal source into one port of the TEM cell with

the other port terminated into 50 ohms. The electric field is received and

coupled into a 1.5 m single conductor wire to establish a reference level

which is measured as Pj by the spectrum analyzer. Next the single conductor

wire is replaced by the DUT and with the TEM cell receiving the same amount

of power as before, the output power leaking into the DUT is measured as P 2

. The SE of the DUT is determined in the same manner as before by taking the

ratio of P l to P 2 in eq (1).

4. MEASUREMENT RESULTS

4.1 Cable Configurations

Figures 11-14 show SE data that were obtained on cable configurations

1 , 2, 3 and 4 between 600 MHz and 6 GHz using the small mode-stirred

chamber. These data were obtained to establish reference conditions with

the cable configurations to use as a benchmark for comparing the EMI effectiveness of the AVSCOM shielding configurations. The SE varies considerably

between the different cable configurations. For example, the difference in

SE between cable configurations 1 and 4 is approximately 100 dB at 1 GHz

while the difference between configurations 3 and 4 is about 20 dB.
Figure 1

1

. SE of configuration 1 at selected frequencies between

600 and 6000 MHz using the small mode-stirred chamber

Figure 12. SE of configuration 2 at selected frequencies between

600 and 6000 MHz using the small mode-stirred chamber

13
Figure 13. SE of configuration 3 at selected frequencies between

600 and 6000 MHz using the small mode-stirred chamber

with both the long wire and ridged-horn antennas.

Figure 14. SE of configuration 4 at selected frequencies between

600 and 6000 MHz using the small mode-stirred chamber,

14
4.2 Shielding Configurations

Figures 1 5~ 1 8 show SE data that were obtained on the shielding configurations 1, 2 and 3 for the AVSCOM program between 1 MHz and 6 GHz using

the TEM cell and the mode-stirred chambers. The TEM cell was used for the

SE measurements at discrete frequencies of 1, 10, 100 and 200 MHz. The

large mode-stirred chamber was used for the SE measurements between 200 MHz

and 1 GHz while the small mode-stirred chamber was used for the SE measurements between 1 GHz and 6 GHz. On configuration 3 (multipin connector/cable

arrangement), two separate SE measurements were made with the harness as- sembly which included investigating the SE of the single shielded wire in

the harness and the twisted shielded wire in the harness.

Overall, the configurations offer very little shielding from EMI. It

appears that the twisted shielded pair offers a bit more shielding (~2 dB at

several frequencies) than the single shielded wire.

Figure 15. SE of shielding configuration 1 at selected frequencies between

1 MHz and 6 GHz using the TEM cell and the mode-stirred

chambers

.

15
SHIELDING

EFFECTIVNESS

Figure 16. SE of shielding

1 MHz and 6 GHz

chambers

.

configuration 2 at

using the TEM cell

selected frequencies

and the mode-stirred

between

Figure 17. SE of shielding configuration 3 with single shielded wire

at selected frequencies between 1 MHz and 6 GHz using the

TEM cell and the mode-stirred chambers.

16
0

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inu

z

>u

u

L.L

U

0

z

(-1 Q

_JuM

1

in

1 0 -

20

30 -

40

50-

G 0

MULTIPIN CONNECT OR/CPBLE RRRRNGEMENT

(TWISTED SHIELDED PRIR)

TEM Cell Data

I I 0 2 3 4

FREQUENCY (GHz)

Figure 18. SE of shielding configuration 3 with twisted shielded pair

at selected frequencies between 1 MHz and 6 GHz using the

TEM cell and the mode-stirred chambers.

5. SUMMARY AND CONCLUSION

Plots of SE were obtained for cable and shielding configurations which

allowed the EMI effectiveness of the shielding conf igurat ions for the AVSCOM

program to be compared with the reference conditions of the cable

configurations. The SE measurements of the cable and shielding configurations were measured by mode-stirred techniques. To obtain SE data of the

shielding configurations down to 1 MHz, it was necessary to use a TEM cell.

It was encouraging that the SE data points measured at 200 MHz using both

the TEM cell and the large mode-stirred chamber varied between the shielding

configurations by approximately 1 to 8 dB.

17
The estimated measurement uncertainties for SE of all cable and shielding configurations in the mode-stirred chamber is < ± 4 dB [7]. This value

is also a good estimate for the shielding configurations for the AVSCOM

program that were measured for SE in the TEM cell.

Shields terminated by means of a solder sleeve and wire, daisy chains,

and other noncont inuous shielding arrangements do not provide adequate

shielding from the surrounding EMI and they allow electronic systems to be

exposed to high-level fields. Even at 1 MHz, these shielding configurations

offer little protection from EMI. Other measurements of SE on other shielding arrangements and devices need to be determined and the results compared.

6. ACKNOWLEDGMENTS

The author wishes to thank M. Crawford for helpful discussions on measurement techniques and data analysis; and J. Workman for his role in

preparing the cable and shielding configurations and making the

measurements

.

7. REFERENCES

[1] Zorzy, J.; Muehlberger, R.F. RF leakage characteristics of popular

coaxial cables and connectors, 500 Me to 7.5 Gc. Microwave J., Vol. 4; 1961 November. 80-86.

[2] Bean, J.L.; Hall, R . A . Electromagnetic susceptibility measurements

using a mode-stirred chamber. Proc. IEEE International EMC Symposium;

1978 June 20-22; Atlanta, GA. 143-150.

18
[ 3 ] Jesch, R.L. A survey of triaxial and mode-stirred techniques for

measuring the shielding effectiveness of connectors and cables. Nat.

Bur. Stand. (U.S.) NBSIR 86-3060; 1986 October.

[4] MIL-STD- 1 344A Appendix A. Design of Mode-Stirred Test Chamber. Method

3008. Shielding Effectiveness of Multicontact Connectors. 1980

September

.

[5] MIL-C-38999H . Military Specification, Connectors, Electrical,

Circular, Miniature, High Density, Quick Disconnect, Environment

Resistant, Removable Crimp Contacts, and Hermetic Contacts. 1981

February

.

[6] Ma , M.T.; Kanda, M.; Crawford, M.L.; Larsen, E.B. A review of

electromagnetic compatibility/interf erence measurement methodologies.

Proc. of the IEEE; 73(3): 388-1(11; 1985 March.

[7] Crawford, M.L.; Koepke, G.H. Design, Evaluation, and Use of a Reverberation Chamber for Performing Electromagnetic Susceptibility/

Vulnerability Measurements. Nat. Bur. Stand. (U.S.) NBS Tech. Note

1092; 1986 April.

[8] Crawford, M.L. Generation of Standard EM fields using TEM

transmission cells. IEEE Trans, on EMC; EMC-16 ( 4 )

:

1 89-1 95; 1 9 7

November

.

[9] Wilson, P.F.; Ma, M.T. Shielding-effectiveness measurements with a dual TEM cell. IEEE Trans, on EMC; EMC-27 (3): 137-142; 1985 August.

19
NBS-M4A (rev. 2 -ec) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO. NBSIR 87-3076

2. Performing Organ. Report No. 3. Publication Date October 1987

4. TITLE AND SUBTITLE

Measurement of Shielding Effectiveness of Different Cable

and Shielding Configurations by Mode-Stirred Techniques

5. AUTHOR(S)

Ramon L. Jesch

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

I. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. Stote, ZIP)

U.S. Army Aviation Systems Command

St. Louis, Missouri 63120

10. SUPPLEMENTARY NOTES

|

'

| Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bliography or literature survey, mention it here)

The shielding effectiveness of cable configurations having

different shielding arrangements and of shielding configurations

used to terminate cable shields for helicopter wiring were

measured by mode-stirred techniques. The mode-stirred measurements were taken at discrete frequencies between 200 MHz and 6

GHz. In addition, shielding effectiveness data on the shielding

configurations were also obtained in a TEM cell down to 1 MHz.

A description of the cable and shielding configurations is given

along with plots of the measured shielding effectiveness data as

a function of frequency.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) cables; connectors; mode-stirred chamber; relative leakage power; shielding

effectiveness

13. AVAILABILITY

[ XI Uni imited

[ | For Official Distribution. Do Not Release to NTIS

Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

QXj Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

24

15. Price

tr U S. Government Printing Office: 1987-573-410/85003 USCOMM-DC 8043-P80