\

/'

•"•tAU O? * NBS TECHNICAL NOTE 1013

U.S. DEPARTMENT OF COMMERCE/ National Bureau of Standards
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress March 3, 1901. The

Bureau's overall goal is to strengthen and advance the Nation's science and technology and

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and the Institute for Computer Sciences and Technology.

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THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts

research and provides scientific and technical services to aid Federal Agencies in the selection, acquisition, application, and use of computer technology to improve effectiveness and

economy in Government operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this mission by managing the Federal Information Processing Standards Program, developing Federal ADP standards

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The National Bureau of Standards was reorganized, effective April 9, 1978.
Neflenal Baresjoi ef Sfandartfl J UN 4 1979

Using a TEM Cell for EMC Measurements of

Electronic Equipment

M. L Crawford

J. L. Workman

Electromagnetic Fields Division

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

^ kw \

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued April 1979
NATIONAL BUREAU OF STANDARDS TECHNICAL NOTE 1013

Nat. Bur. Stand. (U.S.), Tech. Note 1013, 72 pages (April 1979)

CODEN: NBTNAE

U.S GOVERNMENT PRINTING OFFICE WASHINGTON: 1979

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

Stock No 003-003-02053-2 Price $2.40 (Add 25 percent additional for other than U.S. mailing)
CONTENTS

Page

ABSTRACT 1

1. INTRODUCTION 1

2. PHYSICAL DESCRIPTION OF A TEM CELL 1

3. ELECTRICAL DESCRIPTION . . 2

4. RADIATED SUSCEPTIBILITY MEASUREMENTS 5

4.1 Using the TEM Cell for Susceptibility Measurements 5

4.2 Measurement Plan 6

4.2.1 Determine the Test Frequencies and Measurement Approach 6

4.2.2 Establish EUT Failure or Performance Degradation Criteria ..... 7

4.2.3 Establishing the Exposure Field Levels as Required for the Tests . . 7

4.3 Susceptibility Measurement Set-Up , 11

4.4 Radiated Susceptibility Measurement Procedures 11

5.0 RADIATED EMANATIONS MEASUREMENTS , 14

5.1 Using the Cell for Radiated Emanations Measurements ,,,,,,,,,,,, 14

5.2 Determining the Free-Space Equivalent Radiated Field Strength from the EUT Based on Measurements Made with the EUT Inside the TEM Cell ..... . 14

5.3 Formulating the Measurement Plan , 15

5.4 Measurement Set-Up , , , 17

5.5 Radiated Emanations Measurement Procedures , , . . 18

6. REFERENCES ....... 20

Appendix A , ...,,. 21

Appendix B 25

Tables 27

Figures 31

111

USING A TEM CELL FOR EMC MEASUREMENTS OF ELECTRONIC EQUIPMENT

M. L. Crawford and J. L. Workman

This publication describes the physical design and electrical evaluation of

pertinent parameters which influence the use and operation of a transverse

electromagnetic (TEM) cell. Detailed, step by step procedures are given for

using a TEM cell for performing either radiated electromagnetic (EM) susceptibility testing or for measuring radiated EM emissions from electronic/

electro-mechanical equipment. These measurement procedures provide guidelines

to potential users and also indicate precautions to observe to minimize problems

often encountered when performing EMC measurements and hence to enhance the

cell's usefulness. Where available, a brief error analysis associated with the

measurement technique is included.

Key Words: Measurement procedures; susceptibility and emission measurements;

transverse electromagnetic cell.

1. INTRODUCTION

Transverse electromagnetic (TEM) cells have shown great potential for performing

electromagnetic interference/electromagnetic compatibility (EMI/EMC) measurements with

substantially improved ease, versatility, and accuracy [1,2]. The TEM cell consists of a section of rectangular, coaxial line that serves as a broadband, linear phase and amplitude

transducer (in the sense that it converts field strength to rf voltage or rf voltage to field strength) either to establish standard electromagnetic (EM) fields for susceptibility

testing of electronic equipment or to detect radiated emanations from electronic

equipment [3]. The cell is also a shielded enclosure, thus providing electrical isolation

for the tests being performed; and, since it is a transducer, it eliminates the use of

conventional antennas with their inherent measurement limitations (i.e., bandwidth,

nonlinear phase, directivity, polarization, etc.). The newness of the TEM cell measurement

technique and its increasing use, and sometimes misuse, have motivated the writing of this

paper. This technical note gives physical and electrical descriptions of some TEM cells and

provides a detailed step-by-step procedure for their use for both radiated susceptibility

and emanation measurements. These measurement procedures are intended to provide guidelines

for the potential user to minimize problems encountered with this technique, enhancing the

cell's usefulness, and to insure that the results obtained are as meaningful as possible.

Where available, a brief error analysis associated with the measurement techniques is included.

2. PHYSICAL DESCRIPTION OF A TEM CELL

The TEM cell is a rectangular, coaxial, 50-ohm transmission line, tapered at each end

to adapt to conventional 50-ohm coaxial connectors. The center conductor, as shown in

figure 1, is a flat sheet of metal supported by dielectric rods in the center of the cell.

Access into the cell is by side doors located as shown in the figure. Conductive finger

stock material is used around the edges of the doors and around cover panels for

input/output connectors and AC power leads to insure the shielding integrity of the cell.

Fabrication details for constructing a large cell, similar to the 3 m x 3 m x 6 m eel 1 shown in figure 2, are contained in reference [4]. A cell of any size could be fabricated

using dimensional scaling and the procedures outlined in [4] with, of course, appropriate,

minor modifications as required. The design of the cell, as a 50-ohm impedance transmission

line, is derived approximately from the expression [5]:
z = 376.7

- 2

77

In /sinh tt^ 2b

AC

e

(i:

where a, b, and g are as shown in figure 3. The term AC/eq relates to the fringe

capacitance between the edges of the septum and the side walls. For large gaps

(i.e., w/g £ 5) typically used in cell designs, this term is negligible and can be

neglected. However, an exact expression for determining Zq can be obtained from

reference [5] if required. Curves for designing a cell with arbitrary cross section and

impedance are shown in figure 4. These curves are based on formulation developed by

Tippet [6]. Final line impedance adjustments, to compensate for the cell's septum

dielectric supports and transition angle impedance discontinuities, are made with the aid of

a time-domain reflectometer by trimming the septum.

Cross-sectional drawings and a photograph of a 1.2 m x 1.2 m x 2.4 m cell are shown in

figures 5 and 6. This cell is equipped with line filters to provide filtered ac power for

use by equipment under test (EUT) inside the cell. The cell also has a bulkhead panel for

feed-through connectors and rf filters to access the EUT via its input/output leads. If required, a shielded box can be installed over the bulkhead panel with the rf filters. This

box may be necessary to prevent rf conducted on the EUT input/output leads, from radiating

outside the cell or from being conducted into the cell.

3. ELECTRICAL DESCRIPTION

Electrically the TEM cell, as referred to earlier, is an expanded coaxial transmission line which propagates a TEM wave. This wave is characterized by orthogonal electric

(E) and magnetic (H) fields which are perpendicular to the direction of propagation along

the length of the cell or transmission line. Figure 7 shows the TEM mode field structure in

the cell. The E and H field components are quite uniform over much of the volume between

the septum and outer conductor, and simulate a planar field in free space (i.e., a wave impedance of 377 ohms). The TEM mode has no low frequency cut-off. This allows the cell to be used at frequencies as low as desired, limited only by the cell's magnetic shielding

effectiveness which is determined by the material from which it is made. This TEM mode has

the important linear phase and constant amplitude response as a function of frequency

referred to earlier. This makes it possible to use the cell for broadband cw sweep testing

as well as for complex wave-form simulation or detection without signal distortion; that is,

as long as single TEM mode operation is maintained.

The upper useful frequency for a cell is limited by the distortion of the test signal

caused by resonances and multimoding that occur within the cell at frequencies given by

eqs (2) and (3).

res \fri? cell resonant frequency in MHz. (2)

where f^ is the cell cut-off frequency for the TEj^g mode, c is the wave propagation

velocity ( 3.8 x lO^m/sec) and £ is the resonant length of the cell. The cell cut-off

frequency f^- is given as:
f \^~^ - — /i 4ab

c

'

a V 1Tb, b^ iln /8a\ . ''^'

The parameters a, b-j , b2, and g are as defined earlier for equation 1 and as shown in figure 9 and 13. Progress has been made toward suppressing these adverse

high-frequency effects by using rf absorber material placed in the cell as shown in

figure 8 [7].

Many different cell shapes or form factors, which influence the field distribution of

the cell, could be used for designing a cell. Two are shown in figures 9 and 10 with their

electric field distributions. These field distributions were obtained using formulation

derived by Tippet and Chang [5,6]. Tables 1 through 8 give the normalized field quantities

at various transverse position locations in the cell as shown in figure 11. The normalized

field, Ex y, was obtained from the expression:

E

E = ^^^ (4)

x,y V ' ^^'

h

where Ex « is the magnitude of either the vertical or horizontal component of the

electric field at the location of interest in the cell, V is the voltage between the septum

of the cell, and b is the separation distance between the septum and cell upper or lower

walls.

Various approaches have been investigated to find ways by which to increase the test volume available in a cell without reducing its upper useful frequency. One is to design

the cell with an off-set septum. An asymmetric or offset septum cell was fabricated as shown in the cross-sectional drawings of figure 12. The E-field distributions above and

below the septum of this cell are shown in figure 13. This cell offers approximately 25%

greater test volume in its lower half space than a symmetric cell of similar size with an equivalent high-frequency cutoff and resonance limitation. This additional test space,

however, comes at the price of less test field uniformity and greater difficulty in using

the cell at multimode frequencies. A previous publication comparing the characteristics of

this cell with a symmetric cell of similar size is available for further study, if

desired [8].

A comparison of figures 9, 10, and 13 gives an indication of the electric field strength

gradients in the area within which the EUT would be placed. For example, in the shaded area centered between the septum and lower or upper walls equal to 2a/3 x b/3, the cell of

figure 9 has less than +1.4 dB E-field gradient, the cell of figure 10 has less than

+0.6 dB E-field gradient and the cell of figure 13 has less than ±0.b dB above the septum

and less than +2.6 dB below the septum. The cell of figure 10 or the upper half of the cell

of figure 13 would be used for testing small items or calibrating small probes where good

field uniformity was required. The cell of figure 9 or the lower half of the cell of

figure 13 would be acceptable for testing larger objects at a maximum upper useful frequency

where a loss (compromise) in the test-field uniformity could be tolerated. If a metal case (simulating an EUT) is placed in the center of one half of the cell, the field distribution

is changed. The metal case disrupts the field and has a capacitive loading effect at that

point in the cell. Figure 14 shows the relative field distribution with a metal case occupying one third of the volume between the septum and outer wall of a cell having the cross section of figure 10. Five transverse scans, across the width of the cell, are shown.

The location of the scans are numbered on the side view of the cell and are indicated on the

graph.

k
The field strength has increased by 3 dB and 6 dB, respectively, in the regions directly

above and below the case. This increase in field strength must be taken into

account as indicated in section 4.2.3 when determining the absolute test field, or the

absolute level of the emitted field, if a significant cross section within the cell is

occupied by the EUT.

The electrical characteristics of a cell as a transmission line can be determined

simply by measuring the voltage standing wave ratios (VSWR) at the cell's measurement ports

and by measuring its insertion loss. A typical empty cell's input VSWR, both with and

without absorber loading, is shown in figure 15. These data were obtained by measuring the

return loss (i.e., the ratio of the power incident to the cell's input port to the power

reflected from the input port) of the cell shown in figure 4. The output port of the cell was terminated into a matched 50-ohm load. Resonant and multimode frequencies apparently

start at approximately 220 MHz. Note how the VSWR is greatly reduced at these

multimode/resonant frequencies by the rf absorber loading.

The insertion loss of a cell is obtained by measuring the difference in the net power

applied at the cell's input port and the power available at the cell's output port.

Inserting absorbers into the cell increases the loss substantially and must be limited to

prevent excessive signal loss for both susceptibility and radiated emissions testing. The

number, type, and placement location of absorbers inside the cell were experimentally

determined to minimize the cell's insertion loss without significantly altering the TEM mode

operation. The insertion loss of a typical cell (figure 5) is shown in figure 16. A loss

of a few dB (_< 3 dB) in signal strength is reasonable for most applications; an upper

frequency test limit for this particular cell, from figures 15 and 16, would be

approximately 220 MHz.

Measurements can be made to determine the E-field characteristics within the test

volume of the cell again with and without absorber loading for comparison, especially at high (multimode) frequencies. These measurements are made using an isotropic probe [9,10]

located at the center of the test zones in the cell. The probe measures, as a function of

frequency, the variation in the vertical Ey, transverse E^, longitudinal E^, and

Hermitian magnitude S* of the electric field relative to vertical E-field component of the TEM electric field. Examples of the results of these measurements obtained in the test zones of the cell of figure 4 are shown in figures 17-20. The subscripts m and c in the

ordinate of figures 17-20 refer to the measured as compared to the computed values of the

amplitude of the appropriate field component in the cell. These results obviously will be

influenced by inserting the EUT, especially at frequencies above the cell's resonances and

multimode, and will require further evaluation if the cell is to be used above

resonant/multimode frequencies. Ideally, the transverse, figure 18, and longitudinal,

figure 19, components of the electric field measured at the center of the test zone should

be zero. At multimode frequencies, the longitudinal and transverse components become

significant, modifying the coupling and test-field characteristics of each cell and

increasing the complexity of measuring the field patterns. This results in a measurement

uncertainty that is difficult to define, and hence, limits the upper useful frequency range

of the cell. Note that the variation in the normalized Hermitian magnitude, S, of the field

ratios at multimode frequencies (above approximately 220 MHz for this cell) even for the

absorber loaded case, would be quite large, resulting in large measurement uncertainties if

the cell were used at these frequencies.

*The Hermitian magnitude S is given as S = VEx "*" ^y • ^z where E^ is the

transverse component, Ey is the the vertical component, and £2 ""S the longitudinal

component, of the E-field strength inside the cell. For the TEM mode. Ex = E^ = at the center of the cell's test zone.
The data shown for al.2mxl.2mx2.4m symmetric cell are typical of any cell with

the exceptions that larger cells would have proportionally lower multimode/resonant

frequencies and hence lower upper useful frequency limits and that small cells would have

proportionally higher upper useful frequency limits.

The upper useful frequency of any particular cell may be expanded by using a matrix of

isotropic probes to map the complex field around an EUT inside the cell at multimode

frequencies or by using metal blade tuners in the cell similar to what is used in

reverberating enclosures [11], for mode stirring multimoded test fields. Each of these

ideas is under evaluation at NBS with the additional objectives of reducing the measurement

uncertainty, especially at frequencies above the cell multimode/resonant frequencies, and of

increasing the size of the EUT that could be tested in a particular cell.

4. RADIATED SUSCEPTIBILITY MEASUREMENTS

One definition of susceptibility is the characteristic of being influenced or affected by electromagnetic energy. Radiated susceptibility measurements are performed by

subjecting the EUT to a known test field while evaluating its performance. To make these

measurements meaningful, they must be repeatable and relate to how the equipment will

respond in its operational environment. The TEM cell was developed initially at the NBS as

a device for accurately generating known fields for calibration purposes [12]. Because of

its ability to excite a uniform, calculable, repeatable test field that can be related to free space, the cell has demonstrated that it is also a useful tool for susceptibility

measurements. If a piece of equipment is placed in the cell, it can be subjected to fields

of varying levels and waveforms, thus allowing a great deal of flexibility in the

measurement plan.

4.1 Using the TEM Cell for Susceptibility Measurements

Both top and bottom halves of the cell can be used for making measurements, but

generally the lower half is used. This allows the EUT to be supported on the bottom of

the cell instead of on the center conductor. The EUT typically is placed so that its center

is midway between the septum and the bottom of the cell and midway between the two side

walls as shown in figure 21. This is the region where the TEM field is the most uniform.

However, if required or desired, it can be placed almost at any location in the cell. For example, it may be advantageous to place it close to the bottom of the cell, as will be seen

later. For best results, the size of the EUT should not exceed more than one third of the

distance between the septum and the bottom or one third of the distance between the sides or

length of the main body of the cell. This prevents excessive capacitive loading which

shorts out part of the vertical test field and distorts the test-field pattern. The

one-third criteria were selected because this corresponds to approximately a 2-ohm change in

the cell's transmission line impedance and a 3- to 6-dB increase in the susceptibility test

field, as shown earlier. This increase in level can be corrected as indicated in

section 4.2.3 and Appendix A or with the use of calibrated probes. If a large EUT is

inserted into the cell, the cell VSWR increases rapidly, as does the field distortion,

making the correction progressively more difficult and less accurate. Not only should the

EUT be placed in the cell in a way that does not exceed the size limits for the cell, but it should also be positioned to simulate, as nearly as possible, actual field exposure expected

in the EUT's operational environment. This may require orienting the EUT in the cell's

vertical E-field to obtain the desired polarization exposure. For example, if the maximum

exposure is required and the EUT is rectangular, the edges of the EUT should be positioned

vertically to give maximum coupling to the vertical field. Thus, the EUT would have to be

repositioned or rotated along each orthogonal axis to assure that maximum coupling is acquired for each possible path of leakage. More details on placements of the EUT are contained in section 4.4, Measurement Procedures.
4.2 Measurement Plan '

The test plan normally will be based on the EM environment and EUT operational

requirements. The test plan will include selecting the measurement frequencies,

characteristic and signal waveform levels, EUT exposure aspect angles, EUT input/output

lead complement, and the measurement approach (swept, pulsed or discrete frequency

measurements). The plan should also include defining EUT susceptibility criteria and

requirements for monitoring failure modes.

4.2.1 Determine the Test Frequencies and Measurement Approach

If the operational environment of the EUT is known, then the selection of test

frequencies simply becomes one of choosing frequencies that correspond with significant

characteristics of the environment. Often, however, the environment is not sufficiently

characterized such that specifications could be chosen, or the EUT may be mobile so that the

environment is changing. Then more comprehensive testing may be required. Equipment

susceptibility to EMI is primarily determined by the degree to which the interference field

couples into and interacts with the equipment's components. This undesired coupling is influenced by a number of equipment parameters such as: input/output, power line, and

circuit lead impedances and lead lengths; impedances of circuit components (especially

those terminating lead wires); type of circuit components (particularly active

components); and amount and type of EMI shielding and filtering used. The susceptibility

of any particular equipment is frequently a function of frequency, suggesting resonance effects within the equipment with its input/output leads and other interconnected equipment.

These resonance cases may be caused, for example, by the reactance of the connecting leads

acting as an antenna, coupling with the input impedance of the terminating circuit

components. The quality factor (Q) of such a resonant circuit determines the maximum

spacing or increments between frequencies at which susceptibility tests must be performed.

For example, a circuit Q of 40 (typical in many electronic systems) requires tests at increments of 18% in frequency between 0.5 to 10 MHz, 4.8% between 10 and 20 MHz, 2.4%

between 20 and 50 MHz, 1.6% between 50 and 150 MHz, and 1.2% between 150 and 1000 MHz to characterize the circuit resonances. This calls for 310 measurements, perhaps not excessive

for prototype evaluations, but certainly too many for production testing unless the total

measurement process is automated.

Automated systems using minicomputer control are (by their digital nature) discrete

frequency systems. The number of test points one can obtain using such a system is limited

by the memory/storage capacity of the computer and the measurement system bandwidth. These

limitations can be partially overcome by choosing frequency and amplitude measurement

intervals compatible with the test system and the number of test points required.

Computer-automated systems obviously are very efficient, effective tools for obtaining

equipment susceptibility data; however, they can also be very costly.

Swept frequency susceptibility testing systems are also very useful in obtaining test

data. These systems have the advantage of complete frequency coverage within the systems'

bandwidths, but they lack computation capabilities for data analysis that are possible with

computer-controlled systems.

An important precaution that must be exercised when using swept frequency testing is to insure that the frequency sweep rate is slow enough to allow adequate time for the EUT to respond, if susceptible, to the test field.

Some EUT's have components with relatively slow thermal time constants. This equipment

must be exposed to test fields with sufficient duration to allow reaction/interaction;

otherwise susceptibility tests may not be true indicators of the equipment's vulnerability.

For example, some electronic systems require one to two seconds' exposure time for maximum
response. Swept testing, however, has some obvious advantages over stepped or discrete

frequency testing as demonstrated in figure 22. The solid line in the figure shows a typical plot of an EUT resonance response using swept measurements. The dashed line

represents results of the the same tests performed using the stepped frequency approach.

The stepped approach shows a peak at 102 MHz but has missed the resonant peak at 101.6 MHz.

If the stepped approach were used and greater resolution were required, further testing with

smaller frequency increments would have to be done in the frequency range 101 to 103 MHz.

Once again, determining the type of exposure fields to which an EUT should be subjected

is very EUT dependent. Such determinations should be made for each type of equipment,

considering its application and its planned operational environment. The waveform to

which an EUT is exposed can have a pronounced effect upon its susceptibility. Some EUT's,

for example, are not bothered by relatively strong cw fields but can be interfered with if

the signal is modulated. An example of this type of interference is the demodulation of an amplitude modulated (AM) signal by a solid-state device. If the EUT uses AM and

incorporates solid-state switching, the AM on the cw carrier can be demodulated by the

switches and coupled into the audio circuits of the EUT. In this case, the cw carrier is not the cause of the interference, but rather the interference is a result of the

interaction of the switches with the AM signal.

Another extremely important common source of interference is transient spikes on impulsive signals that get coupled into an EUT. These spikes can have considerable energy over large frequency bandwidths and are difficult to accurately simulate. TEM cells offer a useful tool for this type of testing.

4.2.2 Establish EUT Failure or Performance Degradation Criteria

Performance degradation or failure in an EUT typically relates to its compliance

with a required performance specification. If the EUT's performance deviates outside

these specification limits (for example, in its output or its ability to respond properly to

input) when subjected to an interfering field, then it is termed susceptible. Susceptible

modes for each EUT must be defined relative to a measurable response that can be monitored

in a repeatable, meaningful way for a practical range of test conditions representative of

typical operational environmental conditions.

4.2.3 Establishing the Exposure Field Levels as Required for the Tests

Normally, the electric field strength, E, is determined relative to the reference

position at the center of the test zone in the cell. This determination is made with

the cell empty. However, they represent the field exposure to which the EUT is subjected if

the EUT is small (less than 2a/5, b/5, L/5).

The electric field strength, Eq, at the reference point halfway between the septum

and floor in the center of the cell can be determined simply by measuring the voltage

potential, V, between the center plate and the cell's outer wall. The electric field is given then as:

b

where b is the separation distance between the center plate and the lower or upper wall.

If the EUT is not small, the EUT will effectively short out part of the vertical separation,

b, between the plates resulting in an increase in the exposure field. This new Eg

(with the EUT in the cell) can be found by determining the new effective separation

distance, b', given as: b' = b - h, where h is the effective width of the EUT between the
plates. Eq then is equal to V/b'. Actually, unless the EUT's length and width

occupied the full length and width dimensions of the cell, and the EUT case was metal

(highly conductive), the effective height of the EUT is not as large as its physical height.

The effective separation distance b' is then determined by measuring the distributed

impedance of the TEM cell as a transmission line after inserting the EUT inside. This

measurement is made using a time domain refl ectometer (TDR). The value of the cell

transmission line impedance in the section occupied the EUT is then used with eq (1) to compute b' and hence Eq. An, example, computation of b and Eg based on TDR

measurements is contained in Appendix A. If a TDR is not available or it is not convenient

to make this impedance measurement, b' can be estimated by measuring the exposure field

level at the center of the cell test zone, without the EUT in the cell, using a small dipole

probe [10]. The probe's output voltage is then recorded and used as a reference to compare

measurements of the exposure field made using the probe with the EUT in the cell similar to

the example shown in figure 14.

If the test frequency is high enough that the wavelength, X, is not significantly

greater than the cell length (X _< lOL), the voltage, V, measured at the input or output port

of the cell may be significantly different than the voltage potential at the center of the

cell based upon the cell's VSWR. This would result in an error proportional to the VSWR or

cell mismatch impedance. At frequencies for which X <_ lOL, a more accurate determination of

the E-field reference to the center of the test zone can be made by measuring the net input

power to the cell, Pp,, and its complex impedance. The E field is then given as: ^

.....I

IP R' E ^ ^-~- , or E' =^-^

(6)

b b

where Rq and R^ are the real part of the cell's characteristic impedance referenced to

the center of the cell with or without the EUT inserted in the cell.

Ppi is determined from the power meter readings on the sidearms of a calibrated

bi-directional coupler using the following equation

P = CR^ . P. - CR^ . P (7)

n f 1 R r

where CRf and CRr are the forward and reverse coupling ratios of the bi-directional

coupler and P-j and P^ are the indicated incident and reflected coupler sidearm power

meter readings.

The electric field, E^ y at any cross sectional location inside the cell can be

determined, relative to the field, Eq, at the center of the test zone by the expression;

JP R f n c

E = ^-—^ E or E' = '-Vr E (8)

x,y b x,y x,y b x,y

where P^, Re and b are as previously defined and Ex y can be obtained from

tables 1 to 8. These tables can be used to calculate the transverse and vertical components

of the electric field, as well as the magnitude and polarization angle of the electric field

defined by:
VT 2 E

E + E , and e = arctan -1. (9)

X

For example, from table 3, E^^y is equal to 0.853 for the center of the EUT

located at position A as shown in figure 27 inside a square cell. If the EUT is moved to position B, in figure 27, the correction factor is 1.157.

The absolute accuracy with which E^^y can be established is defined by the simple

relationship

E " = E + AE (10) x,y x,y - x,y

where AEy w is the error in determining E • x,y a x,y

The val^ue of AE^^y can be found by evaluating each independent variable, Pp,

Rq, b, and E^^y in expression (8) as follows:

d£ dE dE dE AE =^r^AP +^f^AR + -4;^ Ab + -~^^ AE . (11)

x,y 9P n 9R c 8b aE x,y

n c x,y

where,

aE .

^ 9E E x.y ^ 1 r 1. x,y ^ x,y

aP 2x,yP' 9b "b n

-^ n

9E E E x>y - 1 n i_ x,y _ j<^

3 R "2 x,y R ' E " E

^ ' c c x,y x,y

Substituting eq (12) into (11) and assuming each independent variable could have a positive or negative value, gives the maximum fractional error in determining E^

y

or:

where

^E

AE

X

E X ^ =

,y

1 /

n^p ^ 'r y %

.e~

^P =

AP

n

P ' n ^R

AR

c

Ab

b

', and

'e

AE

_ x,y

E x,y

(13:

Higher order terms contributing to small erorrs were dropped in the derivation of

eq (13).

The error ep, in determining Pp is due to uncertainties in coupler calibration,

absolute measurement of rf power on the side arm of the coupler, and impedance mismatch

between the cell, coupler, rf source, and cell termination. If a precision calibrated

coupler and power meter are used and the cell and its termination are impedance matched

(VSWR < 1.05), £p should be less than +5%.
The error, e\^, in determining R(~ is a function of the measurement accuracy of the

TDR, the impedance loading of the EUT inside the cell, and the test frequency. Obviously,

if the test frequency is low enough that the electrical length of the cell is small, R^. will be the real part of the cell's termination impedance. As the frequency increases and

the length of the cell becomes electrically significant, the impedance at the location of

the EUT, as measured by the TDR, becomes significant. If the EUT occupies a small portion

(<^ one fifth) of the cross section of the cell, er is small (<_ 3%) and is typically

neglected in the calculation of E^^y For larger EUT's (occupying up to one-third of

the cross section of the cell) and at frequencies where the cell length is less than A/10,

the impedance loading effect must be determined with the TDR and used to correct Re when

using eq (6) to calculate Eq- gr for these cases can be much larger but typically would

be less than 10% if the EUT were centered inside the cell. Exceeding the one-third load

factor is not recommended . The error, e^, is proportional to the accuracy in physically

measuring b or for significantly large EUT, in determining the effective separation

distance, b'. This is estimated at approximately +1%. Error in excess of this is reflected

in the estimate of e^.

Determining e^ can be difficult. Introducing the EUT inside the cell perturbs the

electric field distribution as described in the section on field mapping. This loading

factor (increase in E^) can be determined using small calibrated probes, to measure the

increase in the field around the EUT relative to the undisturbed field, or it can be

approximated by estimating a new value for b' as discussed earlier.

The value of e^ is also influenced considerably by the form factor (ratio of cell

width to the separation distance between the septum and its outside, parallel walls) and by

the size of the EUT. If the EUT is small (less than b/5, 2W/5 and L/5; see figure 5 or

12), the cell's test-field distortion will be small, and the main contribution to eg will

be due to the test-field gradient between the septum and parallel wall. For this case, the

sources of errors are summarized in table 9 for both asymmetric and symmetric cells. If the

EUT exceeds the l/5th factor, measurements of the field distribution around the EUT must be

made in order to estimate e^. When making these measurements, one must realize that the

EUT always interacts somewhat with the exposure field in any environment. Sorting out this

open-field scattering effect, caused by the EUT/field interaction, from the perturbation,

effect of the EUT/field/cell interaction is difficult and results in higher estimates of

ef. These estimates, of course, are unique to each EUT and are not included in table 9.

The problem of EUT/test field/transducer interaction is not unique, of course, to using

TEM cells for susceptibility testing. The problem exists to some degree (great or small)

depending on how and in what environment test fields are created and thus, in which the EUT

is tested. Susceptibility tests performed, for example, in conventional shielded enclosures

using antennas to irradiate the EUT suffer serious errors from this effect, as mentioned in

the introduction.

Equation (8) is applicable only to single-mode (TEM-mode) operation of the cell. If the cells are used with absorber loading at multimode frequencies, additional errors will

exist that must be evaluated. Then, the most likely approach is to use calibrated isotropic

probes to measure the field distribution in the cell rather than to calculate the field

level from measured input parameters. This would be done in much the same manner as probing

the field around the EUT to determine field perturbation for estimating e^. A careful

evaluation of uncertainties involved in using a matrix of calibrated probes to determine the

test field distribution around the EUT in the cell's test zone is needed and is anticipated

in the near future.

The sources of errors for the three different cell geometries are summarized in

table 9.

10
4.3 Susceptibility Measurement Set-Up

Figures 23 and 24 show two block diagrams of systems using the TEM cell for

susceptibility measurements. Figure 23 is used for frequencies below 1 MHz and

figure 24 is used for frequencies above 1 MHz. Both systems are configured for swept

frequency testing; however, either system could be used for discrete frequency testing

without the use of the xy recorder or a sweep-type generator.

The high-power generator generally consists of a variable frequency generator and

high-power linear amplifier. Most of today's linear amplifiers have built-in protection

circuits which protect them in the event the cell system becomes impedance mismatched. The

low-pass filter is needed to keep unwanted higher frequency components (second, third, etc.,

harmonics) from being introduced into the cell. This is especially important if the

harmonics or spurious frequencies are above the multimode or resonant frequency limit of the

cell. A frequency counter is used to tell precisely at what frequency a failure occurs.

This can be important if the signal generator frequency calibration is not sufficiently

accurate since EUT susceptibility often is characterized by high Q-resonances which have

very narrow frequency responses. The dual coupler and power meters are used to measure the

incident and reflected power at the input of the cell. A test monitor is shown with the

EUT and is used to determine failure of the EUT. A 50-ohm, high-power load is used to terminate the system.

The main difference between figures 23 and 25 is the use of an rf voltmeter with

monitor tee in place of the directional coupler and power meters. This is required at frequencies below 1 MHz because of the lack of availability of low-frequency,

high-directivity (below 1 MHz), bidirectional couplers. At frequencies below 1 MHz, the

cell is electrically small (i.e., much less than one wavelength), and accurate voltmeter

measurements, which can be referred to the EUT location at the center of the cell, can be

made.

Automated, interactive, susceptibility measurements can be made by using a minicomputer

controlled system as shown in figure 25. This system places the testing operation under

control of the computer which progressively increases the test-field level in the cell at selected test frequencies while monitoring the test-field level around the EUT and the

performance of the EUT. If degradation occurs as determined from preestabl ished, programmed

criteria, the computer can respond by limiting the test-field level to prevent damage to the

EUT. The computer, also according to software instruction and format, will print out the

susecptibil ity information. Such a system using a large 3mx3mx6m TEM cell is under

development at NBS and will be operational in the near future.

4.4 Radiated Susceptibility Measurement Procedures

The following measurement procedures are suggested as a systematic approach for

evaluating the EM radiated susceptibility of equipment using a TEM cell.

Step 1. Place the EUT inside the cell.

Normally the cell is placed in one of two locations in the lower half space

(below the septum) centered in the cell. The first location (location 1) is halfway between the septum and the cell bottom as was shown in figure 21. The EUT

is supported on dielectric material with as low a dielectric constant as possible. For example, plastic foams with a dielectric constant of

approximately 1.04 to 1.08 are almost invisible electrically and make good support

material

.

11
The second common location (location 2) for placing the EUT is on the floor of the

cell (fig. 26), but insulated from it (unless grounding the EUT case to the cell is desired). This position is used to minimize exposure of the EUT's input/output

leads and monitor leads to the test field as explained in step 5. If the EUT is placed near the floor of the cell, the test field will be lower, relative to the

field at the center (midway between the septum and floor) of the cell test zone. A correction for determining this field can easily be made as discussed in section 4.2.3.

Place the EUT in either location 1 or 2 and orient the EUT as desired relative to the cell's TEM field. Normally, the first orientation position is with the EUT

lying flat as in normal use.

Step 2. Access the EUT as required for operation and performance monitoring.

The EUT input/output and ac power leads should be as nearly the same as in its

anticipated use. Leads should be the same length, if possible, and be terminated

into their equivalent operational impedances so as to stimulate the EUT in its

operational configuration. Care must be taken in routing the leads including

monitor leads (if nontransparent to the rf field) inside the cell for the most

meaningful, repeatable results (i.e., desired lead exposure, minimum field

perturbation/interaction, etc.). To minimize exposure of the leads to the test

field, the EUT should be lowered as close to the cell floor as possible and its

leads should be routed along the floor of the cell in appropriate holders, to the

bulkhead panel and ac receptacle, as shown in figure 26. Shield the leads by

either taping the leads to the cell floor with 2" wide conductive tape or by using

braided wire slipped over the lead. Note, keep the leads separate; do not bundle

input/output, monitor, and ac power line leads together. Twist the leads if they

cannot be kept separated. If braided wire is used be sure braid is in electrical

contact with the cell floor. However, care must be taken to prevent the braided

shield or tape from contacting the case of the EUT unless, once again, a common

ground between the EUT and cell is required. Grounding the two together will

obviously influence the results of the susceptibility measurements. Connect the

EUT input/output and monitor leads to appropriate feed-through, filtered connectors

for accessing and operating the EUT. Input/output leads should be filtered to prevent rf leakage into or out of the cell, which would reduce the shielding

integrity of the measurement system. The monitor leads must also be accessed

through the bulkhead as appropriate for the measurement requirement. These leads,

which are used for sensing and telemetering the performance of the EUT while

exposed to the test fields, may require special "invisible," high-resistance leads

made of carbon-impregnated plastic [10] or fiber optics lines to prevent

perturbation of or interaction with the test environment. DC signals or signals

with frequency components below 1 KHz may be monitored via the high-resistance

lines. RF signals should be monitored via fiber optic lines.

If the monitor signal is at a frequency or frequencies sufficiently different from

the susceptibility test frequency or frequencies, conductive (hard wire) leads may

be used with appropriate filtering (high-pass, low-pass, band-pass, etc.) at the

bulkhead. Note that a separate, shielded filter compartment should be provided on the outside of the cell for housing the filters as shown in figures 21 and 26.

Step 3. Connect up the measurement system as shown in figure 23 or figure 24.

Step 4. With the cell rf input source turned off and the EUT turned on in the desired

operational mode, record the EUT monitor response and initialize (zero) the

test-field measurement instrumentation.

12
step 5. Select the first test frequency, modulation rate, test waveform, etc., turn the

cell rf source on, and slowly bring up the field level inside the cell until:

a) the EUT response monitor(s) indicate susceptibilityO or b) the maximum required

test level is obtained. Note: Do not increase test level too fast. Sufficient

time must be spent at each test level to allow the EUT to respond. Record the

monitored response.

Step 6. Select the next test frequency, modulation rate or test waveform, etc., and

repeat procedures of step 5 until all frequencies, modulation levels and

rates, and waveforms required by the test plan are complete. Note: It may be

desirable to select specific test levels and sweep the frequency range of interest

at these levels while monitoring the EUT response. If this procedure is used, the

following precautions must be exercised: a) the sweep rate must be slow enough to allow the EUT to respond (remember, susceptibility often is due to resonance in the EUT circuits, leads, or apertures in its case); and b) the selected test

level must not be too high or damage may occur to the EUT.

Step 7. If exposure to interference fields of the EUT input/output and ac power line leads occurs in actual use, these leads should be raised from the floor of the cell and

extended inside the cell, matching polarization as much as allowed. Care must be

taken to clamp the leads with dielectric supports so they can be placed in the same location (exposure) if the EUT is taken out and then returned to the cell.

Both the EUT placement locations and orientation, and the input/output ac power

line leads placement locations and orientations must be carefully recorded in order to obtain repeatability when reevaluating the susceptibility characteristics

of an EUT. This is true no matter what the test environment (e.g., shielded

enclosure, TEM cell, anechoic chamber, etc.) since EUT susceptibility obviously is

a function of how the interference is coupled into the EUT (e.g., EUT exposure

aspect angle relative to interference polarization, etc.). With the EUT leads now

exposed to the test field, steps 4 through 6 should be repeated. NOTE: Performing

step 7 as a separate procedure provides information on how the interference fields

are coupled into the EUT.

Step 8. Next, the EUT should be reoriented from position 1 (flat, normal operating

position) by laying it on its side or on its end. All three orthogonal

orientations of the EUT may need to be tested in the cell. This is required to expose each surface of the EUT, matching polarization to the TEM field of the cell.

After changing the EUT orientation, steps 2, 4, 5, 6, and 7 should be repeated.

Note: If the field close to the EUT is monitored, using, for example, small

calibrated electric and/or magnetic field probes, the measured results must be

interpreted carefully. This is because such measurements are made in the near field of the scattered field from the EUT and its leads. This field can be

stronger than the test field (TEM field) launched inside the cell, and erroneous

results or conclusions may result.

Not all the tests outlined in this measurement procedure may be required, and only

those required by the test plan should be performed. For example, if the objective of the

measurement program is to reduce the vulnerability (susceptibility) of the EUT, one EUT

orientation with one input/output lead configuration could be tested in one particular

operational mode to a preselected susceptibility test-field waveform and amplitude. Then,

if corrective measures were made to the EUT and placement of the EUT and its leads inside

the cell were carefully duplicated, repeat measurements could be made. These measurements

could then be compared to determine degree of improvement.

13
5.0 RADIATED EMANATIONS MEASUREMENTS

Electronic or electromechanical equipment or components may emit energy which

interferes or interacts with the normal operation of either the system and/or other

receptors. To insure the electromagnetic compatibility (EMC) of such systems, it is

important to determine the amplitude levels of these emanations and to characterize their

waveform, polarization, etc. This is apparent since equipment performance degradation or failure is often dependent upon the interference signal waveform and the signal's

ampl itude.

A TEM cell is especially useful for emitted signal waveform characterization because of

its characteristic as a TEM transducer which permits detection of the signal with little or

no distortion in the signal waveform.

5.1 Using the Cell for Radiated Emanations Measurements

TEM cells are reciprocal devices (i.e., can receive or detect radiated fields from

equipment as well as establishing fields for testing). Thus, energy radiated from an EUT placed inside the cell will couple via the TEM mode characteristics to the cell's

output/input ports. If both cell ports are terminated into 50-ohm matched impedances

(detectors or terminations) and if the cell is symmetric and has negligible insertion loss, one half of the EUT's radiated energy will appear at each port. This energy can be

measured, for example, by using a spectrum analyzer or receiver (frequency-domain

measurements) on one port and an oscilloscope (time-domain measurements) on the other port.

The voltage measured at the ports, Vm, is then given as:

V = IK R m:i4:

where Pf^ is the power radiated by the EUT inside the cell, and Zq is the cell

characteristic impedance.

5.2 Determining the Free-space Equivalent Radiated Field Strength

from the EUT Based on Measurements Made with the EUT Inside

the TEM Cell.

The radiation characteristics of an EUT mounted inside a shielded, reflective

environment will change relative to the EUT's free-space radiation characteristics,

change can be determined approximately when using a TEM cell as follows:

This

Experience with measuring the directivity of radiation sources (EUT's) indicates that

they can be approximated reasonably well as electrically small dipole sources for

frequencies at which the cell is normally used. With this assumption, the change in radiation resistance due to inserting the EUT onto the cell becomes [5]:

2

R'a 3]T 2

E cos e K b

:i5:

14
where:

Rg is the radiation resistance of the EUT when mounted inside the TEM cell;

Rg is the radiation resistance of the EUT in a free-space environment;

Zq is the cell characteristic impedance;

i1q is the intrinsic wave impedance = 120tt ohms in the cell;

E is the normalized electric field inside the cell relative to the field strength at the center of the cell test region, (i.e., E = (EQ)/(V/b);

cos e corresponds to the polarization match between the radiated field from the EUT and the TEM mode field characteristics of the cell (maximum coupling occurs if the

radiated energy from the EUT is polarization matched to the TEM field of the cell, or

cos 6=1);

Kg is equal to Ztt/Xq where Xq is the wavelength of the radiated signal; and

b = separation distance between septum and lower or upper wall.

If Pr is the power radiated by the EUT in a free-space environment and Pr' is the

power radiated by the EUT in the TEM cell, then:

R' P-P,K^I)/ (16)

a

where

Ir K(I) =f, (17)

R

and corresponds to the change in the EUT radiation current caused by enclosing the

EUT inside the confining environment of the TEM cell. The value of K(I) is affected by the

size of the EUT relative to the size of the cell. If the radiation aperture is small, the

EUT radiation current distribution will be essentially the same as for open-space conditions

and K will be approximately equal to one. One can determine the open-space radiated

electric field strength, Eq, at a distance, d, from the EUT, assuming far-field

conditions, from the expressions:

(18)

P^

^

(19)

2 ' 4TTd

where:

Pq is the radiated power density at the distance, d;

Pr is the power radiated from the EUT;

15
G is the gain characteristics of the EUT; and

again is the instrinsic wave impedance.

Combining eqs (14 through (19) gives:

2b ^ V

lo m

E

~= ---= "^

(20)

D X dZ K(I) E cose 3

^ '

This equation then gives the field intensity in the direction of maximum radiation

(i.e., maximum Eq) assuming far-field conditions . An example computation of Eq based on the measurement of emissions from a spherical dipole emitter placed in a TEM cell is

contained in Appendix B. The radiation intensity very close to the EUT (small d) is

influenced by the reactive near-field components; hence, Ep will be in error. Equation (20), however, should be valid for separation distances as small as X/2, since the

EUT must be electrically small (longest dimension less than X/6) in order to satisfy the

criteria that the EUT dimension be less than b/3, 2a/3 and L/3, when placed inside a TEM

cell at frequencies below the cell's multimode cutoff. Some work has been done to test experimentally the validity of eq (20) and the limitations of the assumptions applicable to

its use for correlating TEM cell measurements to open-space measurements [13]. However,

additional work is needed. Work is progressing to develop a "standard radiator" that can be

analytically characterized and used to empirically determine the change of its radiation

characteristics (relative to open space) by inserting it into a cell. This radiator is

completely self-contained (i.e., housing its own power supply, rf source, amplifier, and

impedance matching network inside its radiating structure). Results of this development

should be available in the near future.

It should also be noted that eq (20) is not valid at frequencies above the first

resonant frequency of the cell. At these higher frequencies, distortion in the TEM field characteristics exists. This distortion, however, can be minimized by using rf

absorber to suppress the cell resonance and higher order multimoding.

Additional work is also needed to determine analytically the change in the current

distribution of the radiator (EUT structure, wiring harness, etc.) as a function of its

electrical size caused by confining it inside the TEM cell. This problem is a very complex

one and will require simplifying approximations, at least initially. A second effort will

be to evaluate this change experimentally. This will require the use of special current and

field measuring probes to determine the current distribution of the complex, radiated near field of the radiator, before and after it is confined in the cell, for comparison. Once

this effort is completed, an error analysis will be performed to establish the absolute

measurement uncertainties of the TEM cell radiated emanation technique.

5.3 Formulating the Measurement Plan

Formulating the measurement plan requires determining the EUT's operational modes and

orientations for which tests are to be performed. It also presumes anticipating the

types of tests to be performed (frequency domain; spectrum, or time-domain waveform

analyses) and the detection system requirements such as bandwidth, sensitivity, etc.

If the type of emanation from an EUT is known, the measurements can be simplified, and the

selection of an rf detector that will give the most accurate, meaningful information can be

made.

16
There are, in general, two types of interference emanations, narrowband and broadband.

There are a great many definitions for each of these terms. However, for our purposes,

narrowband means that the emanations are contained in a limited portion of the frequency

spectrum, normally within the bandwidth of the detection system. Broadband emanations are

produced over a large portion of the spectrum and may have signficant amounts of energy

extending beyond the detection system's bandwidth. For example, the signal shown in figure 28 would be classified as a narrowband signal if the difference in frequency between

fl and f2 were only a few KHz. This type of signal could be the carrier of an AM broadcast station or the emanation from a local oscillator in a receiver.

Examples of broadband emanations are shown in figure 29. These emanations are impulsive signals which may be periodic or random in time with varying or constant

amplitude. Some impulsive emitters are: the brushes of AC motors, switching transients

created by turning equipment on or off, keying transients in digital systems, or transmit

keying of tranceivers. Such emanations may cover a frequency spectrum from a few KHz to over 1 GHz.

In general, the detector and transducer selected to comply with the measurement

plan requirements must have sufficient bandwidths and sensitivities to measure the

amplitude and frequency or frequencies of the emanations and also must be capable of

preserving the waveform characteristics of the emanations. If only the amplitude and

spectrum of the emanations are required and the bandwidth of the emissions is less than

3 MHz, spectrum analyzers can be used. If additional detection bandwidth or sensitivity is required, broadband tunable receivers (field intensity meters) may be

required. If the emanations are periodic, a real-time oscilloscope can be used either

connected directly to the cell to measure the time-domain response or connected to the

predetection or postdetection outputs of the broadband tunable detectors, which are connected to the cell, to obtain a real-time display of the emanation. If, however,

the emanations are random, special sample and hold detection or statistical sampling

may be required to obtain meaningful information [14].

5.4 Measurement Set-Up

The block diagram of a typical measurement system is shown in figure 30. As mentioned

earlier, the choice of detectors connected to the cell measurement port is determined by

the measurement requirements such as bandwidth, frequency range, and/or time-domain

response, sensitivity, etc. If the detector is not sufficiently well matched to 50 ohms, an attenuator can be inserted as shown to provide the equivalent matched termination required

by the cell

.

Equipment required to operate the EUT inside the cell or to which the EUT must be

connected for proper operation is obviously dependent upon the particular EUT and its test

requirements. Access to the EUT's input/output leads is by rf-filtered, fed-through

connectors located on the outer wall of the cell near the floor as shown in figure 26. (See

sec. 4.4, step 2.) Again, this lead complement is EUT dependent and must be established in

the measurement plan. Filtered, ac power to operate the EUT (if required) is provided at a shielded receptacle recessed into the outer wall near the bottom of the cell.

17
5.5 Radiated Emanations Measurement Procedures

The following measurement procedures are suggested for making radiated emanations

measurements.

Step 1. Select type of test or measurements to perform (e.g., measurement plan). This

includes selection of EUT operational modes for testing, lead complement, and

signal detection requirements such as bandwidth, sensitivity, waveform analysis,

time domain, etc., and hence detector requirements.

Step 2. Place the EUT inside the cell with desired location and orientation to the TEM

field. Instructions for placing the EUT inside the cell are the same as those used

for susceptibility testing. See section 4.4, step 1 for details.

Step 3. Access the EUT as required for operation and monitoring. Again, this is similar to

instructions described in section 4.4, step 2, for susceptibility testing. The

only exceptions would be due to operational requirements of the EUT. For example,

waveform analysis of the emitted signal from the EUT may require hardline

synchronizing of the detected signal with the input signal to the EUT or to

internal signals being processed by the EUT. This requires additional leads to the

EUT which must be filtered and routed to minimize their interaction with the EUT

and its emitted signal. Routing of these leads should be along the floor of the

cell with minimum exposure (polarization mismatched) to the cell's TEM field

characteristic. This would be the same as discussed in section 4.4 for placement

of the EUT leads for minimum interaction with the test field.

Step 4. Connect the measurement system as shown in figure 30. The detectors (receivers,

spectrum analyzer, power meters) should be calibrated as per their instruction

manuals and impedance matched to the TEM cell. The 10 dB attenuator shown in figure 30 is connected between the cell and the detector to improve the impedance

match.

Step 5. With the EUT switched off, determine the ambient signal level in the cell. This

procedure verifies the shielding and filtering integrity of the cell and the EUT

input/output leads. It also provides baseline data for the measurement system to establish minimum, detectable signal levels (sensitivity), common mode or grounding

problems, and spurious signal determination, which is characteristic of the

measurement (detection) equipment. If ground problems or undesired signals exist,

efforts should be made to correct for or minimize them. This may require, for

example, additional filtering, ac line isolators or conditioners, common or single

point grounding, etc.

Step 6. Turn the EUT on in the desired operational mode and measure its radiated emanations

as follows.

a) First, determine the amplitude and spectrum of the radiated emanations from the

EUT. This can be done with either a spectrum analyzer or a tuned receiver (field

intensity meter) connected to one port of the cell with the cell's opposite port

terminated into 50 ohms.

• b) Using the data obtained in a) perform the required analysis of the emanations

at specific frequencies of interest. This can be done, for example, by selecting

an emanation of interest, tuning the receiver or spectrum analyzer to it, and then

adjusting the measurement bandwidth as required for signal definition and

18
resolution, and recording the measurement results.

c) If time-domain analysis or emanation characterization is required, use the

block diagram shown in either figure 31 with the oscilloscope connected directly to the cell measurement port, or the block diagram shown in figure 32, with the

oscilloscope connected to the predetection or postdetection outputs of the

receiving instrument. The arrangement shown in figure 32 provides greater

measurement sensitivity. In both measurement systems (fig. 31 or fig. 32), the

oscilloscope must be synchronized with either the periodic detected signal from the

cell or with an EUT-monitored periodic signal represented by the dashed line from

the EUT. The measurement results can then be recorded by photographing the

oscilloscope display.)

d) If the emanation is random, the oscilloscope cannot be synchronized properly.

Then the detected signal must be either: 1) recorded with a video disk or tape

recorder and played back frame by frame to analyze the emanation; or 2) analyzed

statistically using amplitude probability distribution analyzers, etc.

Complete step 6 for each EUT operational mode required.

Step 7. Reorient the EUT as required by the test plan and repeat steps 3, 5, and 6.

Step 8. If required, change orientation of EUT input/output and ac power cords used in the

cell and repeat step 3, 5, and 6. Note, keep in mind the importance of careful

placement of the lead to assure repeatability of measurement results.

19
6. REFERENCES

Crawford, M. L., Generation of standard EM fields using TEM transmission cells,

IEEE Trans, on EMC. EMC-16 , No. 4, 189-195 (Nov. 1974).

Crawford, M. L., Measurement of electromagnetic radiation from electronic equipment

using TEM transmission cells, (Feb. 1973), Unpublished report.

Crawford, M. L., Techniques for measurement of electromagnetic radiation and

susceptibility of electronic equipment, 1st International Symposium and Technical

Exhibition on EMC (Montreux), May 20-22, 1975, pp. 38-44, (IEEE 75CH1012-4 Mont).

Decker, W. T., Crawford, M. L., and Wilson, W. A., Construction of a transverse

electromagnetic cell, NBS Technical Note 1011 (Nov. 1978).

Tippet, J. C, and Chang D. C, Radiation characteristics of dipole sources(located

inside a rectangular coaxial transmission line, NBSIR 75-829 (Jan. 1976).

Tippet, J. C, Modal characteristics of rectangular coaxial transmission line, Doctorial

thesis submitted to faculty of Graduate School, Univ. of Colo. (1978).

Crawford, M. L., Workman, J. L., and Thomas, C. L., Expanding the bandwidth of TEM cells

for EMC measurements, IEEE Trans, on EMC EMC-20 , No. 3 (Aug. 1978).

Crawford, M. L., and Workman, J. L., Asymmetric versus symmetric TEM cell for EMC

measurements, IEEE 1978 International Symposium on EMC, Atlanta, Ga., June 20-22, 1978

(78-CH-1304-5 EMC).

Belsher, D. R., Development of near-field electric energy density meter model EDM-2, HEW Publication No. (NIOSH) 75-140 (March 1975).

Greene, F. M., Development of electric and magnetic near-field probes, NBS Technical

Note 658, 38-42 (Jan. 1975).

Corona, P., et. al.. Use of a Reverberating enclosure for measurement of radiated power

in the microwave range, IEEE Trans, on EMC Vol. EMC-18 , 54-59 (May 1976).

Crawford, M. L., Generation of standard EM fields for calibration of power density

meters, 20 KHz to 1000 MHz, NBSIR 75-804 (Jan. 1975).

Crawford, M. L., Experimental evaluation of the radiation characteristics of dipole

sources enclosed in a TEM transmission cell. Conference on Precision EM Meas.,

Boulder, Colo., June 28-July 1, 1976, 57-59 (IEEE Cat. No. 76CH1099-1IM)

.

Bensema, W. D., Amplitude, time, and frequency statistics of quasi-impulsive noise,

2nd Symposium and Technical Exhibition on EMC, Montreux June 28-30, 1977.

20
Appendix A

Sample calculations of b' and ^\^y for Susceptibility Testing

(correcting test field level for loading effect of EUT).

The following is a sample calculation for determining the corrected, effective

separation distance b' and hence the electric field, £\ „ inside a TEM cell after

inserting an EUT.

From equation (1) of this technical note, the empty cell's impedance was given as;

376.7

a^ 2 iln/ si nh tt^

b

" ^ \ 2h

Ac (A-1)

The equation parameters a, b, and g were defind in the main body of the text.

The cell's impedance after inserting an EUT with its image in the oposite half space of

the cell as shown, for example in figure A-1 is:

Z' = 376.7

4

ab''2 Jln(sinh irg)

" TT 2b '_

Ac

e

(A-2)

The parameter, b , is the effective separation distance between the cell septum and

outer wall, and is the parameter to be determined. Taking the ratio of IqIzo' ^"*^ assuming — is negligible gives the expression:

2 Jln /sinh irg \

' " TT \ 2b 7

- 1 iln /sinh TT_a\

TT V 2b/

(A-3)

To find b', measure the distributed impedance of both the empty cell and the cell with

the EUT and its image using a time domain reflectometer (TDR). Assume for the purpose of

this example that the EUT is to be susceptibility tested in the cell of figure 5. Assume

that the impedance of this cell, measured by the TDR in the section of the cell to be

occupied by the EUT, is 50.5 ohm with the cell empty and 48.3 ohms with the EUT and its

image inside the cell. We can compute the effective separation distance b' as follows:

For this cell, a = 0.6 m, b = 0.6 m, and g = 0.104 m.

Substituting these values along with the measured value for Zq and Z'q into

equation (A-3) we obtain:

50.5 ^ 48.3 b -.6 2 , / . ^ 0.104tt\ --I^smh-^^j

, 2 , / . . 0.104Tr\

1 --in^smh^;^

21
or

, „„„ 0.6 ,^^(^ . , 0.163\ ,. ,, 1.902 = —— - .637 lln sinh ——j (A-4)

Equation A-4 can now be solved numerically to obtain the value of b' , which is approximately 0.50.

If it is inconvient to place a second equivalent EUT inside the cell to serve as an image for the principle EUT when measuring Zq', the following approximation can be used.

Measure the distributed impedance Zq", with only the EUT (no image) inside the cell.

Then:

where

or

Zq " 2AZo -Zq

(A-5;

AZo = Zq -Zq"

7

' S 97 "_7

^0 '^^0 ^0

The approximate value, Zg', the symmetrically loaded impedance, can then be

calculated and used to obtain the corrected equivalent separation distance b'. For example,

Zq" measured, for the sample calcuation of b' given above was 49.4 ohms, or computing the

approximate value of Zq' gives

Zq' == 2(49.4) - 50.5 = 48.3 fi

If the EUT is centered in the test zone of the cell, midway between the septum and

floor, we can obtain the value of Ex^y from equation (8) as. JP R ' J48.3 P 13.9 Jp" volts/ , E' = ^^ E = ^-—^ = ^ " '"^^^^ (A-6)

x,y b' x,y 0.5

where E^^y' is equal to 1 at this location in the cell. For the empty cell,

Ex,y "is given as. JFr~ J50.5 P ll.aJp^volts/ ^ E

= V-n_^ E =V-—il = —JLjI "leter

_ x,y b x,y 0.6

the ratio of E

E^^= 1.18

x,y

This represents again for this particular EUT located at the center of the test zone, inside this particular cell, a 1.44 dB correction to the E-field, E^ y, computed from

the power measured at the cell's input port.

If the EUT was located in the center of the cell, but near the cell floor (position A indicated in figure 27) and if the impedance loading caused by the EUT inside the cell is

the same as the above example, the corrected electric field strength, E^ v is:

22
E' = 13.9 P (.853) = 11.86 P volts/ (A-8)

x,y n n meter

where E^ y is equal to 0.853 at this location in the cell. This value, for E^

y

was obtained from table 3 of this technical note which gives the magnitude of the normalized

electric field as a function of cross section location in a square TEM cell.

The value of the electric field strength, E^ v at postion A in the empty cell is:

E = 11.8 JP~(.853) = 10.07 Jp" volts/ (A-9)

x,y ' n ' n meter

The ratio, E' then is 11.86jP~ , . x,y V> n _ 1...18

x,y

E lO.O/JT"

y n

This correction factor, for the loading effect of the EUT, is the same for either test location of the EUT.

23

Appendix B

Evaluation of cell reciprocity and calculation of field intensity, Eq relative to open space for radiated emissions testing.

The following demonstrates the reciprocity of the TEM Cell for use in performing

radiated emissions measurements. It also gives a sample calculation for determining the

field intensity, Eq, relative to open space, of radiated emissions measurements made with

an EUT placed inside a TEM Cell. For this example, a 10 cm spherical dipole radiator was placed at four different locations inside the cell of figure 5 as shown in figure B-1. The

dipole radiator operates at a fundamental frequency of 30 MHz with harmonics up to approximately 500 MHz. Spectrum analyzer displays of the detected emissions from the

dipole, measured at one of the output ports of the cell are shown in figure B-2. The

opposite port of the cell was terminated into 50 ohms. Note, that the relative amplitudes

of the four traces are comparable at frequencies below the mul timode/resonance frequencies

(240 MHz) of this cell. The small differences in detected amplitude as a function of dipole

placement location in the cell, at frequencies below 240 MHz, corresponds to the TEM field

distribtuions shown for this cell in figure 9 and tables 1-4. This demonstrates the

reciprocity of the TEM cell.

To calculate the field intensity, Eq, relative to open-space, for the radiated

emissions from the EUT, we use equation (20) from section 5.2.

2b n V

r ~ ni D X d Z K(I) E cose'3

For measurements made using the cell of figure 5, equation (20) or (B-1) simplifies

to

(2) (0.6) (376.7) V —

50) X d K(I) E cose'3

"D ^-

For the spherical dipole radiator, the maximum directive gain, G, is 1.5. Also, if the polarization of the dipole is matched to the TEM field polarization in the cell, Cos 9 will be 1. In addition, since the dipole is small compared to the test volume inside the

cell, the dipole current distribution for the radiator will be essentially the same as for

open-space conditions, hence K(I) will be approximately 1.0. Then equation (B-1) becomes

(for this cell and EUT).

(1.2) (376.7) V

(50) X^ d (1) E(l)

6.39 V

jn X d E

1) ' 3

:b-2)

The detected, measured voltage, V^ at the cell port for the dipole centered in the

cell test zone (trace A, figure B-2) is -28 dBM at 30 MHz. This corresponds to a voltage

measured with a 50 ohm analyzer of 8.9 mV. At the center of the test zone, E equals 1.0 and

at 30 MHz, Xq is 10 meters.

25
This equation (B-2) again for this specific emission measured from the spherical dipole

ratio inside the 1.2 m x 2.4 m cell yields;

^

0.00569

D "

d

or for a separation distance or 10 meters

E^~0.569 mV/

D~ meter

(B-3)

This same 30 MHz emission measured with the dipole close to the floor the cell

(trace C) has a detected voltage of -30 dBM or 7.07 m/V. For this case E, interpolated from

table 2 of this report, is 0.839 and the calculated field intensity, Eq, relative to open-space is:

^D^

:6.39) (.00707)

(10) d(.839)

^ 0.00538

d

or for a separation distance of 10 meters.

E^ = 0.538 mV/

D meter

The value obtain for Eq (eqs B-3 and B-4) for the two locations of the dipole inside

the cell compares well within the measurement resolution and accuracy of this technique.

This demonstrate the independence of placement location of the EUT inside the cell on the

computed equivalent open-space radiated emissions results.

26
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LOWER

ACCESS

DOOR

UPPER

ACCESS

DOOR

OUTER SHIELD

DIELECTRIC

SEPTUM SUPPORTS

SEPTUM (INNER CONDUCTOR)

Figure 1. View of transverse electromagnetic (TEM) cell.

31
32
2b

Figure 3, Cross-section of a TEM cell

33
~\

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0.6

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1.0

Figure 4. Transverse electromagnetic (TEM) cell design curves showing

ratio of cell outer conductor dimensions to ratio of center plate width

to cell width for various transmission line characteristic impedances

and vertical center plate locations.

34
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35
14

36
E FIELDS

LDS

Figure 7, Trnsverse electromagnetic (TEM) field distribution

inside a TEM cell.

37

ttfiiiTimrTrin
5" EHP-CL Absorber

8" EHP-CL Absorber

Figure 8. Diagram for absorber loading symmetric TEM cell

(Shaded areas indicate placement of anechoic material.

38
2a/3 a ^ Distance Proir Dooir Side

Figure 9^ Relative electric field distribution inside symmetric cell,

Shaded area corresponds to 1/3 cross-section area shown in top view.

39
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Distance From Door Side

' a oar

. ; 'III

,

j

'

' ' '

! ' : i ^ 1 -4-^ ^^^_ ———>:' ^T^l ; ;. ;

;

Figure 10. Relative electric field distribution inside rectangular

(0.6 a = b) , symmetric TEM cell. Shaded area corresponds to 1/3

cross-section area shown in top view.

40
—'^;p^

center of

TEM cell

X X X ;l

Septum

4b/5

3b/5

2b/5

b/5

^ floor

center 2a 3a 4a wall

Figure 11, Field distribution locations shown

in tables 1-8.

41
T

a e c » • 9 -^ UK c a 'H'^ o

-H «> Oh e o 0- iH < <£ a> • ^ B h O K d J3 o> u a o "o • -H a • e rH ja o b O > -H B 9 't' n u c B O C O V

iH « -rt

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CO

(D •H>aaoHo

0)n

inouo

•- CSI

0)3

bO

•H

1^

42
Be^-ow

Septum]

ow

Figure 13. Relative electric field distribution inside asymmetric cell,

(h = b/3) Shaded area corresponds to 1/3 cross-section area shown in cross sectional view of cell. Figure A and B data corresponds to area labeled A and B in top view.

43
Figure 14. Relative electric field distribution inside cell with EUT

(metal case) occupying 1/3 vertical separation distance between septum

and floor. (Cross sectional cuts at center (L/2) and off end (L/6) of EUT

as indicated by numbers in ton view.

44
'''''-' i . . i I I lO W Oi «> rH iH HMSA

JJ i I I I I I i i

63CH o "O o oi o O

iH iH

;^

0) £

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46
(ap) i^^/^^) Vl^Td pszti^uiaoH

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m a O -H

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(ap) (°^a/«K3) piaTJ pazTTBuijoN

48
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rt s

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o

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49
tH

» bC

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with

o >

in o

C£) 3 r-{ 4J (U

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S c3 W

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>. a cj y pj -H o a •H iL, o Q) -(-> -p

in 3 H Q) o* B B

Q) u e U Q) >>

PL4 s: to a s

•H Tf c c>q

•H X o +J in rt s

CO •H CM

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N CM •H • iH iH o S CD o u -o CM O -H

:z: tcn

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o ©

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3 +J bD m

•H ©

[i( -p

J I—I—

I

o

CO

I (HP) (^S/TMS) PiaTii pazTX-BiuaoN

50
Box

Figure 21. Placement of EUT in cell for EMC measurements,

51
PT9IJ oia;jo-[3

<^ -p

I—

t

(1)

CQbO

•HP

OS

^< +J

(QS

tvj o O n '

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0>

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Q.

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em

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-55 3

en , q: q: «^«. d iJJ ^ i^ o < _i R 1-

SIGN AMP

uj 5

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54
0) <z a: O M

-I

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U

O

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ti. * F-

(- 2 s > cc a X < z sc O

IZ Kl

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0)

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ja p

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•Ham

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-MOwa

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03

0)

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55
Filter box Leads covered with 2 inch

conductive tape.

Fig. 26. Placement of EUT in cell for minimum exposure of leads to the TEM field.

56
Septum —

\

1 1] b

f

3b

>fb/5;;

[A 5

V 3a

5

Fig. 27. Determination of E field correction factor as a function of the EUT location inside the cell.

57
Figure 28. Example of narrowband emanation, (frequency domain)

58
2OH<2<Q2<

CQW<<H1—4 uQ

laao

•H

•Pa

+jn

0)

CO

<D

J

ha < 0) H f4

(—4

a 1—1

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ai

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60
o i w

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t2

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CO>> rHaaaG

•H

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+->

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at •

+-)

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>>

Q

ra

+j s G w

Q)

H S

cS ^3 M M G

gS •H

0) Cfl S S

tH Eh o ;dw s

OS s u ;^

bfi d CH

•H

T3 Cfi G ^O •H

4-) rH OJ CQ G-as • CD iHCO Ti

CD

CD -P U a P H

bj3T3 •H PJ

P14 Sh

61
50

Ohm

Tuned

u

'J

-JJ

c

Predetect

Postdetec

Real-time

Oscilloscope

^^ U

m o !_: CL u

Out ment

-4-» C

•r-4 - ^ d

^ 3 ^ ^ oCU ^ W H

0)h m •t-l ^

C! (1) O

•1-1 ^<u

:3 I—

1

o

+-> >

;3 M

a

T3 1—1

r-H

(U o; a o

(-> ^

^ w

H

fl

rt

<1) bo

4-) ti m •^^

CO :3 ^ 4-> H H

P P

a W W

(U a(Ua

rn ;h

(ti m CD

(U 01 m

a o

O

^4H

•r-l

a

CO

cti a0)

^H ti M

rt

(U •T-l .—

1

•1-1 T) T) '0

Q) ;h o

+->

•1-1

COU

too

62
1

1

1

1 EUT

1

1

1

1

EUT

Figure A-K Placement of EUT and its image ( second

identical EUT) in cell for measurement of loaded

impedance, Zq «

63
<- 0„6 m0,06m

lo 2m

]

i

0, 3m

®

-UrOo 24m-J

7r-(c ©

0, 6m

Figure B-l„ Placement locations of the spherical dipole

radiator inside the 1. 2m x K 2m x 2,4m symmetric

TEM cell for radiated emissions measurements.

64
240

Frequency (MHz)

240

Frequency (Ifflz)

*i \ iLL^i.^

"Sir

L

•

M

1 1 1 1 a ...

240

Frequency (MHz)

30 240

Frequency (MHz)

Figure B-2. Radiated emissions from 10cm spherical dipole measured inside a 1.2m X 1.2m x 2.4m TEM cell. Spectrum analyzer display (0-500) MHz. 50MHz/div,

Reference level = -lOdBM, 10 dB/div. Trace A obtained with dipole centered

in test zone midway between septum and outer walls. Trace B same as trace A,

except dipole was moved to 36cm from door side^ Trace C same as trace A ex- cept dipole lowered to 6cm from floor. Trace D obtained with dipole 6cm above

floor and 36cm from door side. (Location measurement referred to center of

spherical dipole.)

65
U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS TN-1013

2« Gov't Accession NoJ t,fkit\pimi!iAce^$i9a^tf^

4. TITLE AND SUBTITLE

Using a TEM Cell for EMC Measurements of Electronic

Equipment

7. AUTHOR(S)

M. L. Crawford and J. L. Workman

^^^^^^^^mrrtmmtmm

5. Publication Daie April 1979

S.Perife^Bg Organization Coiel

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

723.0

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

10. ffOjeiS/Task/Work Unit Ho.

ji«^-!^-»:sa!^.;F^-;;!^-i.N^5^v!TO.=«:.>»..^wr.^~~^^ 11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS fSfreef, City, state. ZIP) 13. Type of Report & Period Covered

X -N. .NN.NS...^. . ^^^ 15. SUPPLEMENTARY NOTES

I I Document descriJDes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or less tactual summary ot moat significant inlormation. If document includes a significant bibliography or

literature survey, mention it here.) This publication describes the physical design and electrical evaluation of

pertinent parameters which influence the use and operation of a transverse

electromagnetic (TEM) cell. Detailed, step by step procedures are given for using

a TEM cell for performing either radiated EM susceptibility testing or for

measuring radiated EM emissions from electronic/electro-mechanical equipment.

These measurement procedures provide guidelines to potential users and also

indicate precautions to observe to minimize problems often encountered when

performing EMC measurements and hence to enhance the cell's usefulness. Where

available, a brief error analysis associated with the measurement technique is

included.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name;

separated by semicolons)

Key Words: Measurement procedures; susceptibility and emission measurements;

transverse electromagnetic cell.

18. AVAILABILITY [x}Unlimited

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\ ,Av^A NBS TECHNICAL NOTE 1013

^'"fEAXi 0* "

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economy in Government operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this mission by managing the Federal Information Processing Standards Program, developing Federal ADP standards

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provides the technical foundation for computer-related policies of the Federal Government.

The Institute consists of the following centers:

Programming Science and Technology — Computer Systems Engineering.

'Headquarters and Laboratories at Gaithersburg, MD, unless otherwise noted; mailing address Washington, DC 20234. 'Some divisions within the center are located at Boulder, CO 80303.
Using a TEM Cell for ElVIC Measurements of

Electronic Equipment

Electromagnetic Fields Division

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

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MAVOWIAL BOBSAD or STAMDASDS

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AUG 1 1981

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M.L. Crawford

J.L. Workman /?0. /O/I

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Revised July 1981

mMM^Mmamimmmimii
NATIONAL BUREAU OF STANDARDS TECHNICAL NOTE 1013

Nat. Bur. Stand. (U.S.), Tech. Note 1013, 72 pages (Revised July 198i;

CODEN: NBTNAE

(Supersedes NBS Technical Note 1013 dated April 1979)

US. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1981 For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

Price $4.25 (Add 25 percent additional for other than U.S. mailing)
CONTENTS

Page

ABSTRACT ]

1. INTRODUCTION 1

2. PHYSICAL DESCRIPTION OF A TEM CELL 1

3. ELECTRICAL DESCRIPTION 2

4. RADIATED SUSCEPTIBILITY MEASUREMENTS 5

4.1 Using the TEM Cell for Susceptibility Measurements ...... 5

4.2 Measurement Plan 6

4.2.1 Determine the Test Frequencies and Measurement Approach 6

4.2.2 Establish EUT Failure or Performance Degradation Criteria 7

4.2.3 Establishing the Exposure Field Levels as Required for the Tests . . 7

4.3 Susceptibility Measurement Set-Up , . 11

4.4 Radiated Susceptibility Measurement Procedures 11

5.0 RADIATED EMANATIONS MEASUREMENTS 14

5.1 Using the Cell for Radiated Emanations Measurements 14

5.2 Determining the Free-Space Equivalent Radiated Field Strength from the

EUT Based on Measurements Made with the EUT Inside the TEM Cell .. ... . 14

5.3 Formulating the Measurement Plan 16

5.4 Measurement Set-Up 17

5.5 Radiated Emanations Measurement Procedures , ... 18

6. REFERENCES , 20

Appendix A , , . . 21

Appendix B 25

Tables 27

Figures 31

iii

USING A TEM CELL FOR EMC MEASUREMENTS OF ELECTRONIC EQUIPMENT

M. L. Crawford and J. L. Workman

This publication describes the physical design and electrical evaluation of

pertinent parameters which influence the use and operation of a transverse

electromagnetic (TEM) cell. Detailed, step by step procedures are given for

using a TEM cell for performing either radiated electromagnetic (EM) susceptibility testing or for measuring radiated EM emissions from electronic/

electro-mechanical equipment. These measurement procedures provide guidelines

to potential users and also indicate precautions to observe to minimize problems

often encountered when performing EMC measurements and hence to enhance the

cell's usefulness. Where available, a brief error analysis associated with the

measurement technique is included.

Key Words: Measurement procedures; susceptibility and emission measurements;

transverse electromagnetic cell.

1. INTRODUCTION

Transverse electromagnetic (TEM) cells have shown great potential for performing

electromagnetic interference/electromagnetic compatibility (EMI/EMC) measurements with

substantially improved ease, versatility, and accuracy [1,2]. The TEM cell consists of a section of rectangular, coaxial line that serves as a broadband, linear phase and amplitude

transducer (in the sense that it converts field strength to rf voltage or rf voltage to field strength) either to establish standard electromagnetic (EM) fields for susceptibility

testing of electronic equipment or to detect radiated emanations from electronic

equipment [3]. The cell is also a shielded enclosure, thus providing electrical isolation

for the tests being performed", and, since it is a transducer, it eliminates the use of

conventional antennas with their inherent measurement limitations (i.e., bandwidth,

nonlinear phase, directivity, polarization, etc.). The newness of the TEM cell measurement

technique and its increasing use, and sometimes misuse, have motivated the writing of this

paper. This technical note gives physical and electrical descriptions of some TEM cells and

provides a detailed step-by-step procedure for their use for both radiated susceptibility

and emanation measurements. These measurement procedures are intended to provide guidelines

for the potential user to minimize problems encountered with this technique, enhancing the

cell's usefulness, and to insure that the results obtained are as meaningful as possible.

Where available, a brief error analysis associated with the measurement techniques is included.

2. PHYSICAL DESCRIPTION OF A TEM CELL

The TEM cell is a rectangular, coaxial, 50-ohm transmission line, tapered at each end

to adapt to conventional 50-ohm coaxial connectors. The center conductor, as shown in figure 1, is a flat sheet of metal supported by dielectric rods in the center of the cell.

Access into the cell is by side doors located as shown in the figure. Conductive finger

stock material is used around the edges of the doors and around cover panels for

input/output connectors and AC power leads to insure the shielding integrity of the cell.

Fabrication details for constructing a large cell, similar to the 3mx3mx6mcell

shown in figure 2, are contained in reference [4]. A cell of any size could be fabricated

using dimensional scaling and the procedures outlined in [4] with, of course, appropriate,

minor modifications as required. The design of the cell, as a 50-ohm impedance transmission

line, is derived approximately from the expression [5]:
z 376.7 (1)

a^ - 2^ Jin /sinh tt£ b TT \ 2b- ^

e

where a, b, and g are as shown in figure 3. The term AC/eg relates to the fringe

capacitance between the edges of the septum and the side walls. For large gaps

(i.e., w/g <^ 5) typically used in cell designs, this term is negligible and can be

neglected. However, an exact expression for determining Zg can be obtained from

reference [5] if required. Curves for designing a cell with arbitrary cross section and

impedance are shown in figure 4. These curves are based on formulation developed by

Tippet [6]. Final line impedance adjustments, to compensate for the cell's septum

dielectric supports and transition angle impedance discontinuities, are made with the aid of

a time-domain reflectometer by trimming the septum.

Cross-sectional drawings and a photograph of a 1.2 m x 1.2 m x 2.4 m cell are shown in

figures 5 and 6. This cell is equipped with line filters to provide filtered ac power for

use by equipment under test (EUT) inside the cell. The cell also has a bulkhead panel for

feed-through connectors and rf filters to access the EUT via its input/output leads. If required, a shielded box can be installed over the bulkhead panel with the rf filters. This

box may be necessary to prevent rf conducted on the EUT input/output leads, from radiating

outside the cell or from being conducted into the cell.

3. ELECTRICAL DESCRIPTION

Electrically the TEM cell, as referred to earlier, is an expanded coaxial transmission line which propagates a TEM wave. This wave is characterized by orthogonal electric

(E) and magnetic (H) fields which are perpendicular to the direction of propagation along

the length of the cell or transmission line. Figure 7 shows the TEM mode field structure in

the cell. The E and H field components are quite uniform over much of the volume between

the septum and outer conductor, and simulate a planar field in free space (i.e., a wave impedance of 377 ohms). The TEM mode has no low frequency cut-off. This allows the cell to

be used at frequencies as low as desired, limited only by the cell's magnetic shielding

effectiveness which is determined by the material from which it is made. This TEM mode has

the important linear phase and constant amplitude response as a function of frequency

referred to earlier. This makes it possible to use the cell for broadband cw sweep testing

as well as for complex wave-form simulation or detection without signal distortion; that is,

as long as single TEM mode operation is maintained.

The upper useful frequency for a cell is limited by the distortion of the test signal

caused by resonances and multimoding that occur within the cell at frequencies given by

eqs (2) and (3).

/

f = Vf + {—] = cell resonant frequency in MHz, (2)

where f^; is the cell cut-off frequency for the TE;^o mode, c is the wave propagation

velocity ( 3.8 x lO^m/sec) and £ is the resonant length of the cell. The cell cut-off

frequency f^ is given as:
/^ f - — h

4ab

c

~

a V Tib^b^ in /8a\ ' ^^'

The parameters a, b-j , b2, and g are as defined earlier for equation 1 and as shown in figure 9 and 13. Progress has been made toward suppressing these adverse

high-frequency effects by using rf absorber material placed in the cell as shown in figure 8 [7].

Many different cell shapes or form factors, which influence the field distribution of

the cell, could be used for designing a cell. Two are shown in figures 9 and 10 with their

electric field distributions. These field distributions were obtained using formulation

derived by Tippet and Chang [5,6]. Tables 1 through 8 give the normalized field quantities

at various transverse position locations in the cell as shown in figure 11. The normalized

field. Ex u, was obtained from the expression:

x,y V ' ^^>

b

where Ex^y is the magnitude of either the vertical or horizontal component of the

electric field at the location of interest in the cell, V is the voltage between the septum

of the cell, and b is the separation distance between the septum and cell upper or lower

walls.

Various approaches have been investigated to find ways by which to increase the test volume available in a cell without reducing its upper useful frequency. One is to design

the cell with an off -set septum. An asymmetric or offset septum cell was fabricated as shown in the cross-sectional drawings of figure 12. The E-field distributions above and

below the septum of this cell are shown in figure 13. This cell offers approximately 25%

greater test volume in its lower half space than a symmetric cell of similar size with an equivalent high-frequency cutoff and resonance limitation. This additional test space,

however, comes at the price of less test field uniformity and greater difficulty in using

the cell at multimode frequencies. A previous publication comparing the characteristics of

this cell with a symmetric cell of similar size is available for further study, if

desired [8].

A comparison of figures 9, 10, and 13 gives an indication of the electric field strength

gradients in the area within which the EUT would be placed. For example, in the shaded area centered between the septum and lower or upper walls equal to 2a/3 x b/3, the cell of

figure 9 has less than +1.4 dB E-field gradient, the cell of figure 10 has less than

+0.6 dB E-field gradient and the cell of figure 13 has less than +0.5 dB above the septum

and less than +2.6 dB below the septum. The cell of figure 10 or the upper half of the cell

of figure 13 would be used for testing small items or calibrating small probes where good

field uniformity was required. The cell of figure 9 or the lower half of the cell of

figure 13 would be acceptable for testing larger objects at a maximum upper useful frequency

where a loss (compromise) in the test-field uniformity could be tolerated. If a metal case (simulating an EUT) is placed in the center of one half of the cell, the field distribution

is changed. The metal case disrupts the field and has a capacitive loading effect at that

point in the cell. Figure 14 shows the relative field distribution with a metal case occupying one third of the volume between the septum and outer wall of a cell having the cross section of figure 10. Five transverse scans, across the width of the cell, are shown.

The location of the scans are numbered on the side view of the cell and are indicated on the

graph.
The field strength has increased by 3 dB and 6 dB, respectively, in the regions directly

above and below the case. This increase in field strength must be taken into

account as indicated in section 4.2.3 when determining the absolute test field, or the

absolute level of the emitted field, if a significant cross section within the cell is

occupied by the EUT.

The electrical characteristics of a cell as a transmission line can be determined

simply by measuring the voltage standing wave ratios (VSWR) at the cell's measurement ports

and by measuring its insertion loss. A typical empty cell's input VSWR, both with and

without absorber loading, is shown in figure 15. These data were obtained by measuring the

return loss (i.e., the ratio of the power incident to the cell's input port to the power

reflected from the input port) of the cell shown in figure 4. The output port of the cell was terminated into a matched 50-ohm load. Resonant and multimode frequencies apparently

start at approximately 220 MHz. Note how the VSWR is greatly reduced at these

multimode/resonant frequencies by the rf absorber loading.

The insertion loss of a cell is obtained by measuring the difference in the net power

applied at the cell's input port and the power available at the cell's output port.

Inserting absorbers into the cell increases the loss substantially and must be limited to prevent excessive signal loss for both susceptibility and radiated emissions testing. The

number, type, and placement location of absorbers inside the cell were experimentally

determined to minimize the cell's insertion loss without significantly altering the TEM mode

operation. The insertion loss of a typical cell (figure 5) is shown in figure 16. A loss

of a few dB (_< 3 dB) in signal strength is reasonable for most applications; an upper

frequency test limit for this particular cell, from figures 15 and 16, would be

approximately 220 MHz.

Measurements can be made to determine the E-field characteristics within the test

volume of the cell again with and without absorber loading for comparison, especially at

high (multimode) frequencies. These measurements are made using an isotropic probe [9,10]

located at the center of the test zones in the cell. The probe measures, as a function of

frequency, the variation in the vertical Ey, transverse E^, longitudinal E^, and

Hermitian magnitude S* of the electric field relative to vertical E-field component of the

TEM electric field. Examples of the results of these measurements obtained in the test zones of the cell of figure 4 are shown in figures 17-20. The subscripts m and c in the

ordinate of figures 17-20 refer to the measured as compared to the computed values of the

amplitude of the appropriate field component in the cell. These results obviously will be

influenced by inserting the EUT, especially at frequencies above the cell's resonances and

multimode, and will require further evaluation if the cell is to be used above

resonant/multimode frequencies. Ideally, the transverse, figure 18, and longitudinal,

figure 19, components of the electric field measured at the center of the test zone should

be zero. At multimode frequencies, the longitudinal and transverse components become

significant, modifying the coupling and test-field characteristics of each cell and

increasing the complexity of measuring the field patterns. This results in a measurement

uncertainty that is difficult to define, and hence, limits the upper useful frequency range

of the cell. Note that the variation in the normalized Hermitian magnitude, S, of the field

ratios at multimode frequencies (above approximately 220 MHz for this cell) even for the

absorber loaded case, would be quite large, resulting in large measurement uncertainties if

the cell were used at these frequencies.

*The Hermitian magnitude S is given as S = VEx *" Ey + e| where E^ is the

transverse component, Ey is the the vertical component, and E^ is the longitudinal

component, of the E-field strength inside the cell. For the TEM mode. Ex = E^ = at the center of the cell's test zone.
The data shown for a 1.2 m x 1.2 m x 2.4 m symmetric cell are typical of any cell with

the exceptions that larger cells would have proportionally lower multimode/resonant

frequencies and hence lower upper useful frequency limits and that small cells would have

proportionally higher upper useful frequency limits.

The upper useful frequency of any particular cell may be expanded by using a matrix of

isotropic probes to map the complex field around an EUT inside the cell at multimode

frequencies or by using metal blade tuners in the cell similar to what is used in reverberating enclosures [11], for mode stirring multimoded test fields. Each of these

ideas is under evaluation at NBS with the additional objectives of reducing the measurement

uncertainty, especially at frequencies above the cell multimode/resonant frequencies, and of

increasing the size of the EUT that could be tested in a particular cell.

4. RADIATED SUSCEPTIBILITY MEASUREMENTS

One definition of susceptibility is the characteristic of being influenced or affected by electromagnetic energy. Radiated susceptibility measurements are performed by

subjecting the EUT to a known test field while evaluating its performance. To make these

measurements meaningful, they must be repeatable and relate to how the equipment will

respond in its operational environment. The TEM cell was developed initially at the NBS as

a device for accurately generating known fields for calibration purposes [12]. Because of

its ability to excite a uniform, calculable, repeatable test field that can be related to free space, the cell has demonstrated that it is also a useful tool for susceptibility

measurements. If a piece of equipment is placed in the cell, it can be subjected to fields

of varying levels and waveforms, thus allowing a great deal of flexibility in the

measurement plan.

4.1 Using the TEM Cell for Susceptibility Measurements

Both top and bottom halves of the cell can be used for making measurements, but

generally the lower half is used. This allows the EUT to be supported on the bottom of

the cell instead of on the center conductor. The EUT typically is placed so that its center

is midway between the septum and the bottom of the cell and midway between the two side

walls as shown in figure 21. This is the region where the TEM field is the most uniform.

However, if required or desired, it can be placed almost at any location in the cell. For example, it may be advantageous to place it close to the bottom of the cell, as will be seen later. For best results, the size of the EUT should not exceed more than one third of the

distance between the septum and the bottom or one third of the distance between the sides or

length of the main body of the cell. This prevents excessive capacitive loading which

shorts out part of the vertical test field and distorts the test-field pattern. The

one-third criteria were selected because this corresponds to approximately a 2-ohm change in the cell's transmission line impedance and a 3- to 6-dB increase in the susceptibility test

field, as shown earlier. This increase in level can be corrected as indicated in section 4.2.3 and Appendix A or with the use of calibrated probes. If a large EUT is

inserted into the cell, the cell VSWR increases rapidly, as does the field distortion,

making the correction progressively more difficult and less accurate. Not only should the

EUT be placed in the cell in a way that does not exceed the size limits for the cell, but it should also be positioned to simulate, as nearly as possible, actual field exposure expected

in the EUT's operational environment. This may require orienting the EUT in the cell's

vertical E-field to obtain the desired polarization exposure. For example, if the maximum

exposure is required and the EUT is rectangular, the edges of the EUT should be positioned

vertically to give maximum coupling to the vertical field. Thus, the EUT would have to be

repositioned or rotated along each orthogonal axis to assure that maximum coupling is acquired for each possible path of leakage. More details on placements of the EUT are contained in section 4.4, Measurement Procedures.
4.2 Measurement Plan

The test plan normally will be based on the EM environment and EUT operational

requirements. The test plan will include selecting the measurement frequencies,

characteristic and signal waveform levels, EUT exposure aspect angles, EUT input/output

lead complement, and the measurement approach (swept, pulsed or discrete frequency

measurements). The plan should also include defining EUT susceptibility criteria and

requirements for monitoring failure modes.

4.2.1 Determine the Test Frequencies and Measurement Approach

If the operational environment of the EUT is known, then the selection of test

frequencies simply becomes one of choosing frequencies that correspond with significant

characteristics of the environment. Often, however, the environment is not sufficiently

characterized such that specifications could be chosen, or the EUT may be mobile so that the

environment is changing. Then more comprehensive testing may be required. Equipment

susceptibility to EMI is primarily determined by the degree to which the interference field

couples into and interacts with the equipment's components. This undesired coupling is

influenced by a number of equipment parameters such as: input/output, power line, and

circuit lead impedances and lead lengths; impedances of circuit components (especially

those terminating lead wires); type of circuit components (particularly active

components); and amount and type of EMI shielding and filtering used. The susceptibility

of any particular equipment is frequently a function of frequency, suggesting resonance effects within the equipment with its input/output leads and other interconnected equipment.

These resonance cases may be caused, for example, by the reactance of the connecting leads

acting as an antenna, coupling with the input impedance of the terminating circuit

components. The quality factor (Q) of such a resonant circuit determines the maximum

spacing or increments between frequencies at which susceptibility tests must be performed.

For example, a circuit Q of 40 (typical in many electronic systems) requires tests at increments of 18% in frequency between 0.5 to 10 MHz, 4.8% between 10 and 20 MHz, 2.4%

between 20 and 50 MHz, 1.6% between 50 and 150 MHz, and 1.2% between 150 and 1000 MHz to characterize the circuit resonances. This calls for 310 measurements, perhaps not excessive

for prototype evaluations, but certainly too many for production testing unless the total

measurement process is automated.

Automated systems using minicomputer control are (by their digital nature) discrete

frequency systems. The number of test points one can obtain using such a system is limited

by the memory/storage capacity of the computer and the measurement system bandwidth. These

limitations can be partially overcome by choosing frequency and amplitude measurement

intervals compatible with the test system and the number of test points required.

Computer-automated systems obviously are very efficient, effective tools for obtaining

equipment susceptibility data; however, they can also be ^ery costly.

Swept frequency susceptibility testing systems are also very useful in obtaining test

data. These systems have the advantage of complete frequency coverage within the systems'

bandwidths, but they lack computation capabilities for data analysis that are possible with

computer-controlled systems.

An important precaution that must be exercised when using swept frequency testing is to insure that the frequency sweep rate is slow enough to allow adequate time for the EUT to respond, if susceptible, to the test field.

Some EUT's have components with relatively slow thermal time constants. This equipment

must be exposed to test fields with sufficient duration to allow reaction/interaction;

otherwise susceptibility tests may not be true indicators of the equipment's vulnerability.

For example, some electronic systems require one to two seconds' exposure time for maximum
response. Swept testing, however, has some obvious advantages over stepped or discrete

frequency testing as demonstrated in figure 22. The solid line in the figure shows a typical plot of an EUT resonance response using swept measurements. The dashed line

represents results of the the same tests performed using the stepped frequency approach.

The stepped approach shows a peak at 102 MHz but has missed the resonant peak at 101.6 MHz.

If the stepped approach were used and greater resolution were required, further testing with

smaller frequency increments would have to be done in the frequency range 101 to 103 MHz.

Once again, determining the type of exposure fields to which an EUT should be subjected

is very EUT dependent. Such determinations should be made for each type of equipment,

considering its application and its planned operational environment. The waveform to

which an EUT is exposed can have a pronounced effect upon its susceptibility. Some EUT's,

for example, are not bothered by relatively strong cw fields but can be interfered with if

the signal is modulated. An example of this type of interference is the demodulation of an amplitude modulated (AM) signal by a solid-state device. If the EUT uses AM and

incorporates solid-state switching, the AM on the cw carrier can be demodulated by the

switches and coupled into the audio circuits of the EUT. In this case, the cw carrier is not the cause of the interference, but rather the interference is a result of the

interaction of the switches with the AM signal.

Another extremely important common source of interference is transient spikes on impulsive signals that get coupled into an EUT. These spikes can have considerable energy

over large frequency bandwidths and are difficult to accurately simulate. TEM cells offer a useful tool for this type of testing.

4.2.2 Establish EUT Failure or Performance Degradation Criteria

Performance degradation or failure in an EUT typically relates to its compliance

with a required performance specification. If the EUT's performance deviates outside

these specification limits (for example, in its output or its ability to respond properly to

input) when subjected to an interfering field, then it is termed susceptible. Susceptible

modes for each EUT must be defined relative to a measurable response that can be monitored

in a repeatable, meaningful way for a practical range of test conditions representative of

typical operational environmental conditions.

4.2.3 Establishing the Exposure Field Levels as Required for the Tests

Normally, the electric field strength, E, is determined relative to the reference

position at the center of the test zone in the cell. This determination is made with

the cell empty. However, they represent the field exposure to which the EUT is subjected if

the EUT is small (less than 2a/5, b/5, L/5).

The electric field strength, Eq, at the reference point halfway between the septum

and floor in the center of the cell can be determined simply by measuring the voltage

potential, V, between the center plate and the cell's outer wall. The electric field is given then as:

b

where b is the separation distance between the center plate and the lower or upper wall.

If the EUT is not small, the EUT will effectively short out part of the vertical separation,

b, between the plates resulting in an increase in the exposure field. This new Eq

(with the EUT in the cell) can be found by determining the new effective separation

distance, b', given as: b' = b - h, where h is the effective width of the EUT between the
plates. Eq then is equal to V/b'. Actually, unless the EUT's length and width

occupied the full length and width dimensions of the cell, and the EUT case was metal

(highly conductive), the effective height of the EUT is not as large as its physical height.

The effective separation distance b' is then determined by measuring the distributed

impedance of the TEM cell as a transmission line after inserting the EUT inside. This

measurement is made using a time domain reflectometer (TDR). The value of the cell

transmission line impedance in the section occupied the EUT is then used with eq (1) to compute b' and hence Eq. An, example, computation of b and Eq based on TDR

measurements is contained in Appendix A. If a TDR is not available or it is not convenient

to make this impedance measurement, b' can be estimated by measuring the exposure field

level at the center of the cell test zone, without the EUT in the cell, using a small dipole

probe [10]. The probe's output voltage is then recorded and used as a reference to compare

measurements of the exposure field made using the probe with the EUT in the cell similar to the example shown in figure 14.

If the test frequency is high enough that the wavelength, X, is not significantly

greater than the cell length (A _< lOL), the voltage, V, measured at the input or output port

of the cell may be significantly different than the voltage potential at the center of the

cell based upon the cell's VSWR. This would result in an error proportional to the VSWR or

cell mismatch impedance. At frequencies for which X _< lOL, a more accurate determination of

the E-field reference to the center of the test zone can be made by measuring the net input

power to the cell, Pp, and its complex impedance. The E field is then given as: Ob Ob

where Gc and Gc are the real part of the cell's characteristic admittance referenced to

the center of the cell with or without the EUT inserted in the cell.

Pp is determined from the power meter readings on the sidearms of a calibrated

bi-directional coupler using the following equation

P = CR^ . P. - CR^ . P (7)

n f 1 R r

where CRf and CRr are the forward and reverse coupling ratios of the bi-directional

coupler and P-j and P^- are the indicated incident and reflected coupler sidearm power

meter readings.

The electric field, E^ y at any cross sectional location inside the cell can be determined, relative to the field. Eg, at the center of the test zone by the expression:

E = Mr E orE' = Mr E (8)

x,y b x,y x,y b x,y

where Pp, Gc and b are as previously defined and Ex y can be obtained from

tables 1 to 8. These tables can be used to calculate the transverse and vertical components

of the electric field, as well as the magnitude and polarization angle of the electric field

defined by:
VI 2 E

E + E , and 6 = arctan JL (9)

.. X y' E . ' X

For example, from table 3, E^ » is equal to 0.853 for the center of the EUT

located at position A as shown in figure 27 inside a square cell. If the EUT is moved to position B, in figure 27, the correction factor is 1.157.

The absolute accuracy with which Ex^y can be established is defined by the simple

relationship

E " = E + AE (10) x,y x,y - x,y

where AE^ y

is the error in determining E x,y

The val^ue of AEx^y can be found by evaluating each independent variable, Pp,

Gq, h, and E^^y in expression (8) as follows:

aE aE aE be AE =-Tf^AP +-^IG + -t;^ Ab + -~^^ AE . (11) x,y ap n ao c clb 8E x,y

n c x,y

•"

where,

8E , , BE E x.y ^ 1 ^ j_ x,y ^ x,y

ap 2x,yP' 9b 'b n n

9E ^

. E E x.y ,-1 r 1- x,y _ x,y

9G "

2 x,yG ' E " E ^''^

c c x,y x,y

Substituting eq (12) into (11) and assuming each independent variable could have a positive or negative value, gives the maximum fractional error in determining E^

y

or:

AE . . .

x,y

where „ ~ AP AG ,^ AE

n c Ab , x,y PPGoDD n c EEx,y

Higher order terms contributing to small erorrs were dropped in the derivation of

eq (13).

The error ep, in determining Pp is due to uncertainties in coupler calibration,

absolute measurement of rf power on the side arm of the coupler, and impedance mismatch

between the cell, coupler, rf source, and cell termination. If a precision calibrated

coupler and power meter are used and the cell and its termination are impedance matched

(VSWR _< 1.05), ep should be less than +5%.
The error, i„, in determining Gc ""s a function of the measurement accuracy of the

TDR, the impedance loading of the EUT inside the cell, and the test frequency. Obviously,

if the test frequency is low enough that the electrical length of the cell is small, Gc

will be the real part of the cell's termi nation admittance j\s the frequency increases and

the length of the cell becomes electrically significant, the impedance at the location of

the EUT, as measured by the TDR, becomes significant. If the EUT occupies a small portion

(_< one fifth) of the cross section of the cell, e- is small (£ 37o) and is typically

neglected in the calculation of E^ v- For larger tUT's (occupying up to one-third of

the cross section of the cell) and at frequencies where the cell length is less than X/10,

the impedance loading effect must be determined with the TDR and used to correct Gc ^^^n

using eq (6) to calculate Eg. 6q for these cases can be much larger but typically would

be less than 10% if the EUT were centered inside the cell. Exceeding the one-third load

factor is not recommended . The error, et,, is proportional to the accuracy in physically

measuring b or for significantly large EUT, in determining the effective separation

distance, b'. This is estimated at approximately +1%. Error in excess of this is reflected

in the estimate of e£.

Determining e£ can be difficult. Introducing the EUT inside the cell perturbs the

electric field distribution as described in the section on field mapping. This loading

factor (increase in Ey) can be determined using small calibrated probes, to measure the

increase in the field around the EUT relative to the undisturbed field, or it can be

approximated by estimating a new value for b' as discussed earlier.

The value of eg is also influenced considerably by the form factor (ratio of cell

width to the separation distance between the septum and its outside, parallel walls) and by

the size of the EUT. If the EUT is small (less than b/5, 2W/5 and L/5; see figure 5 or

12), the cell's test-field distortion will be small, and the main contribution to eg will

be due to the test-field gradient between the septum and parallel wall. For this case, the

sources of errors are summarized in table 9 for both asymmetric and symmetric cells. If the

EUT exceeds the l/5th factor, measurements of the field distribution around the EUT must be

made in order to estimate e^. When making these measurements, one must realize that the

EUT always interacts somewhat with the exposure field in any environment. Sorting out this

open-field scattering effect, caused by the EUT/field interaction, from the perturbation

effect of the EUT/field/cell interaction is difficult and results in higher estimates of

ef. These estimates, of course, are unique to each EUT and are not included in table 9.

The problem of EUT/test field/transducer interaction is not unique, of course, to using

TEM cells for susceptibility testing. The problem exists to some degree (great or small)

depending on how and in what environment test fields are created and thus, in which the EUT

is tested. Susceptibility tests performed, for example, in conventional shielded enclosures

using antennas to irradiate the EUT suffer serious errors from this effect, as mentioned in

the introduction.

Equation (8) is applicable only to single-mode (TEM-mode) operation of the cell. If the cells are used with absorber loading at multimode frequencies, additional errors will

exist that must be evaluated. Then, the most likely approach is to use calibrated isotropic

probes to measure the field distribution in the cell rather than to calculate the field

level from measured input parameters. This would be done in much the same manner as probing

the field around the EUT to determine field perturbation for estimating e£. A careful

evaluation of uncertainties involved in using a matrix of calibrated probes to determine the

test field distribution around the EUT in the cell's test zone is needed and is anticipated

in the near future.

The sources of errors for the three different cell geometries are summarized in table 9.

10
4.3 Susceptibility Measurement Set-Up

Figures 23 and 24 show two block diagrams of systems using the TEM cell for

susceptibility measurements. Figure 23 is used for frequencies below 1 MHz and

figure 24 is used for frequencies above 1 MHz. Both systems are configured for swept

frequency testing; however, either system could be used for discrete frequency testing

without the use of the xy recorder or a sweep-type generator.

The high-power generator generally consists of a variable frequency generator and

high-power linear amplifier. Most of today's linear amplifiers have built-in protection

circuits which protect them in the event the cell system becomes impedance mismatched. The

low-pass filter is needed to keep unwanted higher frequency components (second, third, etc.,

harmonics) from being introduced into the cell. This is especially important if the

harmonics or spurious frequencies are above the multimode or resonant frequency limit of the

cell. A frequency counter is used to tell precisely at what frequency a failure occurs.

This can be important if the signal generator frequency calibration is not sufficiently

accurate since EUT susceptibility often is characterized by high Q-resonances which have

very narrow frequency responses. The dual coupler and power meters are used to measure the

incident and reflected power at the input of the cell. A test monitor is shown with the

EUT and is used to determine failure of the EUT. A 50-ohm, high-power load is used to terminate the system.

The main difference between figures 23 and 25 is the use of an rf voltmeter with

monitor tee in place of the directional coupler and power meters. This is required at frequencies below 1 MHz because of the lack of availability of low-frequency,

high-directivity (below 1 MHz), bidirectional couplers. At frequencies below 1 MHz, the

cell is electrically small (i.e., much less than one wavelength), and accurate voltmeter

measurements, which can be referred to the EUT location at the center of the cell, can be

made.

Automated, interactive, susceptibility measurements can be made by using a minicomputer

controned system as shown in figure 25. This system places the testing operation under

control of the computer which progressively increases the test-field level in the cell at selected test frequencies while monitoring the test-field level around the EUT and the

performance of the EUT. If degradation occurs as determined from preestablished, programmed

criteria, the computer can respond by limiting the test-field level to prevent damage to the

EUT. The computer, also according to software instruction and format, will print out the

susecptibil ity information. Such a system using a large 3mx3mx6m TEM cell is under

development at NBS and will be operational in the near future.

4.4 Radiated Susceptibility Measurement Procedures

The following measurement procedures are suggested as a systematic approach for

evaluating the EM radiated susceptibility of equipment using a TEM cell.

Step 1. Place the EUT inside the cell.

Normally the cell is placed in one of two locations in the lower half space

(below the septum) centered in the cell. The first location (location 1) is halfway between the septum and the cell bottom as was shown in figure 21. The EUT

is supported on dielectric material with as low a dielectric constant as possible. For example, plastic foams with a dielectric constant of

approximately 1.04 to 1.08 are almost invisible electrically and make good support

material.

11
The second common location (location 2) for placing the EUT is on the floor of the

cell (fig. 26), but insulated from it (unless grounding the EUT case to the cell is

desired). This position is used to minimize exposure of the EUT's input/output

leads and monitor leads to the test field as explained in step 5. If the EUT is

placed near the floor of the cell, the test field will be lower, relative to the

field at the center (midway between the septum and floor) of the cell test zone. A correction for determining this field can easily be made as discussed in section 4.2.3.

Place the EUT in either location 1 or 2 and orient the EUT as desired relative to the cell's TEM field. Normally, the first orientation position is with the EUT lying flat as in normal use.

Step 2. Access the EUT as required for operation and performance monitoring.

The EUT input/output and ac power leads should be as nearly the same as in its

anticipated use. Leads should be the same length, if possible, and be terminated

into their equivalent operational impedances so as to stimulate the EUT in its

operational configuration. Care must be taken in routing the leads including

monitor leads (if nontransparent to the rf field) inside the cell for the most

meaningful, repeatable results (i.e., desired lead exposure, minimum field

perturbation/interaction, etc.). To minimize exposure of the leads to the test

field, the EUT should be lowered as close to the cell floor as possible and its

leads should be routed along the floor of the cell in appropriate holders, to the

bulkhead panel and ac receptacle, as shown in figure 26. Shield the leads by

either taping the leads to the cell floor with 2" wide conductive tape or by using

braided wire slipped over the lead. Note, keep the leads separate; do not bundle

input/output, monitor, and ac power line leads together. Twist the leads if they

cannot be kept separated. If braided wire is used be sure braid is in electrical

contact with the cell floor. However, care must be taken to prevent the braided

shield or tape from contacting the case of the EUT unless, once again, a common ground between the EUT and cell is required. Grounding the two together will

obviously influence the results of the susceptibility measurements. Connect the

EUT input/output and monitor leads to appropriate feed-through, filtered connectors

for accessing and operating the EUT. Input/output leads should be filtered to prevent rf leakage into or out of the cell, which would reduce the shielding

integrity of the measurement system. The monitor leads must also be accessed

through the bulkhead as appropriate for the measurement requirement. These leads,

which are used for sensing and telemetering the performance of the EUT while

exposed to the test fields, may require special "invisible," high-resistance leads

made of carbon-impregnated plastic [10] or fiber optics lines to prevent

perturbation of or interaction with the test environment. DC signals or signals

with frequency components below 1 KHz may be monitored via the high-resistance

lines. RF signals should be monitored via fiber optic lines.

If the monitor signal is at a frequency or frequencies sufficiently different from

the susceptibility test frequency or frequencies, conductive (hard wire) leads may

be used with appropriate filtering (high-pass, low-pass, band-pass, etc.) at the

bulkhead. Note that a separate, shielded filter compartment should be provided on the outside of the cell for housing the filters as shown in figures 21 and 26.

Step 3. Connect up the measurement system as shown in figure 23 or figure 24.

Step 4. With the cell rf input source turned off and the EUT turned on in the desired

operational mode, record the EUT monitor response and initialize (zero) the

test-field measurement instrumentation.

12
step 5. Select the first test frequency, modulation rate, test waveform, etc., turn the

cell rf source on, and slowly bring up the field level inside the cell until:

a) the EUT response monitor(s) indicate susceptibilityO or b) the maximum required

test level is obtained. Note: Do not increase test level too fast. Sufficient

time must be spent at each test level to allow the EUT to respond. Record the

monitored response.

Step 5. Select the next test frequency, modulation rate or test waveform, etc., and

repeat procedures of step 5 until all frequencies, modulation levels and

rates, and waveforms required by the test plan are complete. Note: It may be

desirable to select specific test levels and sweep the frequency range of interest

at these levels while monitoring the EUT response. If this procedure is used, the

following precautions must be exercised: a) the sweep rate must be slow enough to

allow the EUT to respond (remember, susceptibility often is due to resonance in

the EUT circuits, leads, or apertures in its case); and b) the selected test

level must not be too high or damage may occur to the EUT.

Step 7. If exposure to interference fields of the EUT input/output and ac power line leads occurs in actual use, these leads should be raised from the floor of the cell and

extended inside the cell, matching polarization as much as allowed. Care must be

taken to clamp the leads with dielectric supports so they can be placed in the same location (exposure) if the EUT is taken out and then returned to the cell.

Both the EUT placement locations and orientation, and the input/output ac power

line leads placement locations and orientations must be carefully recorded in order to obtain repeatability when reevaluating the susceptibility characteristics

of an EUT. This is true no matter what the test environment (e.g., shielded

enclosure, TEM cell, anechoic chamber, etc.) since EUT susceptibility obviously is

a function of how the interference is coupled into the EUT (e.g., EUT exposure

aspect angle relative to interference polarization, etc.). With the EUT leads now

exposed to the test field, steps 4 through 6 should be repeated. NOTE: Performing

step 7 as a separate procedure provides information on how the interference fields

are coupled into the EUT.

Step 8. Next, the EUT should be reoriented from position 1 (flat, normal operating

position) by laying it on its side or on its end. All three orthogonal

orientations of the EUT may need to be tested in the cell. This is required to expose each surface of the EUT, matching polarization to the TEM field of the cell.

After changing the EUT orientation, steps 2, 4, 5, 6, and 7 should be repeated.

Note: If the field close to the EUT is monitored, using, for example, small

calibrated electric and/or magnetic field probes, the measured results must be

interpreted carefully. This is because such measurements are made in the near

field of the scattered field from the EUT and its leads. This field can be

stronger than the test field (TEM field) launched inside the cell, and erroneous

results or conclusions may result.

Not all the tests outlined in this measurement procedure may be required, and only

those required by the test plan should be performed. For example, if the objective of the

measurement program is to reduce the vulnerability (susceptibility) of the EUT, one EUT

orientation with one input/output lead configuration could be tested in one particular

operational mode to a preselected susceptibility test-field waveform and amplitude. Then,

if corrective measures were made to the EUT and placement of the EUT and its leads inside

the cell were carefully duplicated, repeat measurements could be made. These measurements

could then be compared to determine degree of improvement.

13
5.0 RADIATED EMANATIONS MEASUREMENTS

Electronic or electromechanical equipment or components may emit energy which

interferes or interacts with the normal operation of either the system and/or other

receptors. To insure the electromagnetic compatibility (EMC) of such systems, it is

important to determine the amplitude levels of these emanations and to characterize their

waveform, polarization, etc. This is apparent since equipment performance degradation or failure is often dependent upon the interference signal waveform and the signal's

ampl itude.

A TEM cell is especially useful for emitted signal waveform characterization because of

its characteristic as a TEM transducer which permits detection of the signal with little or

no distortion in the signal waveform.

5.1 Using the Cell for Radiated Emanations Measurements

TEM cells are reciprocal devices (i.e., can receive or detect radiated fields from

equipment as well as establishing fields for testing). Thus, energy radiated from an EUT placed inside the cell will couple via the TEM mode characteristics to the cell's

output/input ports. If both cell ports are terminated into 50-ohm matched impedances

(detectors or terminations) and if the cell is symmetric and has negligible insertion loss, one half of the EUT's radiated energy will appear at each port. This energy can be

measured, for example, by using a spectrum analyzer or receiver (frequency-domain

measurements) on one port and an oscilloscope (time-domain measurements) on the other port.

The voltage measured at the ports, Vf„, is then given as: ^1 • r. X Z V = If R m

(14)

where P^ is the power radiated by the EUT inside the cell, and Zq is the cell

characteristic impedance.

5.2 Determining the Free-space Equivalent Radiated Field Strength

from the EUT Based on Measurements Made with the EUT Inside

the TEM Cell.

The radiation characteristics of an EUT mounted inside a shielded, reflective

environment will change relative to the EUT's free-space radiation characteristics,

change can be determined approximately when using a TEM cell as follows:

This

Experience with measuring the directivity of radiation sources (EUT's) indicates that

they can be approximated reasonably well as electrically small dipole sources for

frequencies at which the cell is normally used. With this assumption, the change in radiation resistance due to inserting the EUT onto the cell becomes [5]:

2

R'

a 371 _0 E cos 6 K b (15)

14
where:

Rg is the radiation resistance of the EUT when mounted inside the TEM cell;

Rg is the radiation resistance of the EUT in a free-space environment;

1q is the cell characteristic impedance;

tIq is the intrinsic wave impedance = IZOtt ohms in the cell;

E is the normalized electric field inside the cell relative to the field strength at the center of the cell test region, (i.e., E = (Eo)/(V/b);

cos 6 corresponds to the polarization match between the radiated field from the EUT

and the TEM mode field characteristics of the cell (maximum coupling occurs if the

radiated energy from the EUT is polarization matched to the TEM field of the cell, or

cos e = 1)

;

Kq is equal to Ztt/Xq where Xq is the wavelength of the radiated signal; and

b = separation distance between septum and lower or upper wall.

If Pr is the power radiated by the EUT in a free-space environment and Pr' is the

power radiated by the EUT in the TEM cell, then:

. P'=P,k2(.)/ (16)

a

where

Ir K(I) =Y ' (1^)

R

and corresponds to the change in the EUT radiation current caused by enclosing the

EUT inside the confining environment of the TEM cell. The value of K(I) is affected by the

size of the EUT relative to the size of the cell. If the radiation aperture is small, the

EUT radiation current distribution will be essentially the same as for open-space conditions

and K will be approximately equal to one. One can determine the open-space radiated

electric field strength, Eq, at a distance, d, from the EUT, assuming far-field

conditions, from the expressions:

^n

- VV7. (18)

P !^ (19)

2 ' 47rd

where:

Pq is the radiated power density at the distance, d;

Pr is the power radiated from the EUT;

15
G is the gain characteristics of the EUT; and

again is the instrinsic wave impedance.

Combining eqs (14 through (19) gives:

^

b ^ V

E . 20L,, .^ (20'

D \ dZ K(I) E cose

^-^ ^

This equation then gives the field intensity in the direction of maximum radiation

(i.e., maximum Eq) assuming far-field conditions . An example computation of Eq based on the measurement of emissions from a spherical dipole emitter placed in a TEM cell is contained in Appendix B. The radiation intensity very close to the EUT (small d) is

influenced by the reactive near-field components; hence, Eq will be in error. Equation (20), however, should be valid for separation distances as small as X/Z, since the

EUT must be electrically small (longest dimension l(;ss than A/6) in order to satisfy the

criteria that the EUT dimension be less than b/3, 2a/3 and L/3, when placed inside a TEM

cell at frequencies below the cell's multimode cutoff. Some work has been done to test experimentally the validity of eq (20) and the limitations of the assumptions applicable to

its use for correlating TEM cell measurements to open-space measurements [13]. However,

additional work is needed. Work is progressing to develop a "standard radiator" that can be

analytically characterized and used to empirically determine the change of its radiation

characteristics (relative to open space) by inserting it into a cell. This radiator is

completely self-contained (i.e., housing its own power supply, rf source, amplifier, and

impedance matching network inside its radiating structure). Results of this development

should be available in the near future.

It should also be noted that eq (20) is not valid at frequencies above the first

resonant frequency of the cell. At these higher frequencies, distortion in the TEM field characteristics exists. This distortion, however, can be minimized by using rf

absorber to suppress the cell resonance and higher order multimoding.

Additional work is also needed to determine analytically the change in the current

distribution of the radiator (EUT structure, wiring harness, etc.) as a function of its

electrical size caused by confining it inside the TEM cell. This problem is a ^jery complex

one and will require simplifying approximations, at least initially. A second effort will

be to evaluate this change experimentally. This will require the use of special current and

field measuring probes to determine the current distribution of the complex, radiated near field of the radiator, before and after it is confined in the cell, for comparison. Once

this effort is completed, an error analysis will be performed to establish the absolute

measurement uncertainties of the TEM cell radiated emanation technique.

5.3 Formulating the Measurement Plan

Formulating the measurement plan requires determining the EUT's operational modes and

orientations for which tests are to be performed. It also presumes anticipating the

types of tests to be performed (frequency domain; spectrum, or time-domain waveform

analyses) and the detection system requirements such as bandwidth, sensitivity, etc.

If the type of emanation from an EUT is known, the measurements can be simplified, and the

selection of an rf detector that will give the most accurate, meaningful information can be

made.

16
There are, in general, two types of interference emanations, narrowband and broadband.

There are a great many definitions for each of these terms. However, for our purposes,

narrowband means that the emanations are contained in a limited portion of the frequency

spectrum, normally within the bandwidth of the detection system. Broadband emanations are

produced over a large portion of the spectrum and may have signficant amounts of energy

extending beyond the detection system's bandwidth. For example, the signal shown in

figure 28 would be classified as a narrowband signal if the difference in frequency between

fl and f2 were only a few KHz. This type of signal could be the carrier of an AM broadcast station or the emanation from a local oscillator in a receiver.

Examples of broadband emanations are shown in figure 29. These emanations are impulsive signals which may be periodic or random in time with varying or constant

amplitude. Some impulsive emitters are: the brushes of AC motors, switching transients

created by turning equipment on or off, keying transients in digital systems, or transmit

keying of tranceivers. Such emanations may cover a frequency spectrum from a few KHz to over 1 GHz.

In general, the detector and transducer selected to comply with the measurement

plan requirements must have sufficient bandwidths and sensitivities to measure the

amplitude and frequency or frequencies of the emanations and also must be capable of

preserving the waveform characteristics of the emanations. If only the amplitude and

spectrum of the emanations are required and the bandwidth of the emissions is less than

3 MHz, spectrum analyzers can be used. If additional detection bandwidth or sensitivity is required, broadband tunable receivers (field intensity meters) may be

required. If the emanations are periodic, a real-time oscilloscope can be used either

connected directly to the cell to measure the time-domain response or connected to the

predetection or postdetection outputs of the broadband tunable detectors, which are connected to the cell, to obtain a real-time display of the emanation. If, however,

the emanations are random, special sample and hold detection or statistical sampling

may be required to obtain meaningful information [14].

5.4 Measurement Set-Up

The block diagram of a typical measurement system is shown in figure 30. As mentioned

earlier, the choice of detectors connected to the cell measurement port is determined by

the measurement requirements such as bandwidth, frequency range, and/or time-domain

response, sensitivity, etc. If the detector is not sufficiently well matched to 50 ohms, an attenuator can be inserted as shown to provide the equivalent matched termination required

by the cell

.

Equipment required to operate the EUT inside the cell or to which the EUT must be

connected for proper operation is obviously dependent upon the particular EUT and its test

requirements. Access to the EUT's input/output leads is by rf-filtered, fed-through

connectors located on the outer wall of the cell near the floor as shown in figure 26. (See

sec. 4.4, step 2.) Again, this lead complement is EUT dependent and must be established in

the measurement plan. Filtered, ac power to operate the EUT (if required) is provided at a shielded receptacle recessed into the outer wall near the bottom of the cell.

17
5.5 Radiated Emanations Measurement Procedures

The following measurement procedures are suggested for making radiated emanations

measurements.

Step 1. Select type of test or measurements to perform (e.g., measurement plan). This

includes selection of EUT operational modes for testing, lead complement, and

signal detection requirements such as bandwidth, sensitivity, waveform analysis,

time domain, etc., and hence detector requirements.

Step 2. Place the EUT inside the cell with desired location and orientation to the TEM

field. Instructions for placing the EUT inside the cell are the same as those used

for susceptibility testing. See section 4.4, step 1 for details.

Step 3. Access the EUT as required for operation and monitoring. Again, this is similar to

instructions described in section 4.4, step 2, for susceptibility testing. The

only exceptions would be due to operational requirements of the EUT. For example,

waveform analysis of the emitted signal from the EUT may require hardline

synchronizing of the detected signal with the input signal to the EUT or to

internal signals being processed by the EUT. This requires additional leads to the

EUT which must be filtered and routed to minimize their interaction with the EUT

and its emitted signal. Routing of these leads should be along the floor of the

cell with minimum exposure (polarization mismatched) to the cell's TEM field

characteristic. This would be the same as discussed in section 4.4 for placement

of the EUT leads for minimum interaction with the test field.

Step 4. Connect the measurement system as shown in figure 30. The detectors (receivers,

spectrum analyzer, power meters) should be calibrated as per their instruction

manuals and impedance matched to the TEM cell. The 10 dB attenuator shown in figure 30 is connected between the cell and the detector to improve the impedance

match.

Step 5. With the EUT switched off, determine the ambient signal level in the cell. This

procedure verifies the shielding and filtering integrity of the cell and the EUT

input/output leads. It also provides baseline data for the measurement system to establish minimum, detectable signal levels (sensitivity), common mode or grounding

problems, and spurious signal determination, which is characteristic of the

measurement (detection) equipment. If ground problems or undesired signals exist,

efforts should be made to correct for or minimize them. This may require, for

example, additional filtering, ac line isolators or conditioners, common or single

point grounding, etc.

Step 6. Turn the EUT on in the desired operational mode and measure its radiated emanations

as follows.

a) First, determine the amplitude and spectrum of the radiated emanations from the

EUT. This can be done with either a spectrum analyzer or a tuned receiver (field

intensity meter) connected to one port of the cell with the cell's opposite port

terminated into 50 ohms.

b) Using the data obtained in a) perform the required analysis of the emanations

at specific frequencies of interest. This can be done, for example, by selecting

an emanation of interest, tuning the receiver or spectrum analyzer to it, and then

adjusting the measurement bandwidth as required for signal definition and

18
resolution, and recording the measurement results.

c) If time-domain analysis or emanation characterization is required, use the

block diagram shown in either figure 31 with the oscilloscope connected directly to the cell measurement port, or the block diagram shown in figure 32, with the

oscilloscope connected to the predetection or postdetection outputs of the

receiving instrument. The arrangement shown in figure 32 provides greater

measurement sensitivity. In both measurement systems (fig. 31 or fig. 32), the

oscilloscope must be synchronized with either the periodic detected signal from the

cell or with an EUT-monitored periodic signal represented by the dashed line from

the EUT. The measurement results can then be recorded by photographing the

oscilloscope display.)

d) If the emanation is random, the oscilloscope cannot be synchronized properly.

Then the detected signal must be either: 1) recorded with a video disk or tape

recorder and played back frame by frame to analyze the emanation; or 2) analyzed

statistically using amplitude probability distribution analyzers, etc.

Complete step 6 for each EUT operational mode required.

Step 7. Reorient the EUT as required by the test plan and repeat steps 3, 5, and 6.

Step 8. If required, change orientation of EUT input/output and ac power cords used in the

cell and repeat step 3, 5, and 6. Note, keep in mind the importance of careful

placement of the lead to assure repeatability of measurement results.

19
6. REFERENCES

Crawford, M. L., Generation of standard EM fields using TEM transmission cells,

IEEE Trans, on EMC. EMC-16 , No. 4, 189-195 (Nov. 1974).

Crawford, M. L., Measurement of electromagnetic radiation from electronic equipment

using TEM transmission cells, (Feb. 1973), Unpublished report.

Crawford, M. L., Techniques for measurement of electromagnetic radiation and

susceptibility of electronic equipment, 1st International Symposium and Technical

Exhibition on EMC (Montreux), May 20-22, 1975, pp. 38-44, (IEEE 75CH1012-4 Mont).

Decker, W. T., Crawford, M. L., and Wilson, W. A., Construction of a transverse

electromagnetic cell, NBS Technical Note 1011 (Nov, 1973).

Tippet, J. C, and Chang D. C, Radiation characteristics of dipole sources (located

inside a rectangular coaxial transmission line, NBSIR 75-829 (Jan. 1976).

Tippet, J. C, Modal characteristics of rectangular coaxial transmission line, Doctorial

thesis submitted to faculty of Graduate School, Univ. of Colo. (1978).

Crawford, M. L., Workman, J. L., and Thomas, C. L., Expanding the bandwidth of TEM cells

for EMC measurements, IEEE Trans, on EMC EMC-20 , No. 3 (Aug. 1978).

Crawford, M. L., and Workman, J. L., Asymmetric versus symmetric TEM cell for EMC

measurements, IEEE 1978 International Symposium on EMC, Atlanta, Ga., June 20-22, 1978

(78-CH-1304-5 EMC).

Belsher, D. R., Development of near-field electric energy density meter model EDM-2, HEW Publication No. (NIOSH) 75-140 (March 1975).

Greene, F. M., Development of electric and magnetic near-field probes, NBS Technical

Note 658, 38-42 (Jan. 1975).

Corona, P., et. al ., Use of a Reverberating enclosure for measurement of radiated power

in the microwave range, IEEE Trans, on EMC Vol. EMC-18 , 54-59 (May 1976).

Crawford, M. L., Generation of standard EM fields for calibration of power density

meters, 20 KHz to 1000 MHz, NBSIR 75-804 (Jan. 1975).

Crawford, M. L., Experimental evaluation of the radiation characteristics of dipole

sources enclosed in a TEM transmission cell. Conference on Precision EM Meas.,

Boulder, Colo., June 28-July 1, 1976, 57-59 (IEEE Cat. No. 76CH1099-1IM)

.

Bensema, W. D., Amplitude, time, and frequency statistics of quasi-impulsive noise,

2nd Symposium and Technical Exhibition on EMC, Montreux June 28-30, 1977.

20
Appendix A

Sample calculations of b' and E'^^y for Susceptibility Testing

(correcting test field level for loading effect of EUT).

The following is a sample calculation for determining the corrected, effective

separation distance b' and hence the electric field, E'^ y inside a TEM cell after

inserting an EUT.

From equation (1) of this technical note, the empty cell's impedance was given as:

Z

= 376.7

4

a^

.b

2 Jln/sinh -n^

-u \ 2b/

Ac

e

(A-1)

The equation parameters a, b, and g were defind in the main body of the text.

The cell's impedance after inserting an EUT with its image in the oposite half space of

the cell as shown, for example in figure A-1 is:

V-- 376.7

4

a^ 2 lr\ (sinh Trq)" Ac

b' ir Xb' £

(A-2)

The parameter, b , is the effective separation distance between the cell septum and

outer wall, and is the parameter to be determined. Taking the ratio of Iq/iq' and

assuming — is negligible gives the expression:

£o

2 Jin (sinh Trg \

2b'/

Z'

2 in (sinh T\q \

?.b)

(A-3)

To find b', measure the distributed impedance of both the empty cell and the cell with

the EUT and its image using a time domain reflectometer (TOR). Assume for the purpose of

this example that the EUT is to be susceptibility tested in the cell of figure 5. Assume

that the impedance of this cell, measured by the TDR in the section of the cell to be

occupied by the EUT, is 50.5 ohm with the cell empty and 48.3 ohms with the EUT and its

image inside the cell. We can compute the effective separation distance b' as follows:

For this cell, a = 0.6 m, b = 0.6 m, and g = 0.104 m.

Substituting these values along with the measured value for Zq and Z'q into

equation (A-3) we obtain:

50.5

48.3

.6 2 , / . , 0.104Tr\

l-^ln(s 0.104TT 1.2

21
or

.902 = ^- .637(ln sinh ^) {A-4;

Equation A-4 can now be solved numerically to obtain the value of b' , which is approximately 0.50.

If it is inconvient to place a second equivalent EUT inside the cell to serve as an image for the principle EUT when measuring Zg', the following approximation can be used.

Measure the distributed impedance Zq", with only the EUT (no image) inside the cell.

Then:

where

or

Zq - 2AZo Zq' ^^_5j

^^0 ~ ^0 "^0

7 ' = ?7 "_7

^0 ^'-0 '-0

The approximate value, Zq', the symmetrically loaded impedance, can then be

calculated and used to obtain the corrected equivalent separation distance b'. For example,

Zq" measured, for the sample calcuation of b' given above was 49.4 ohms, or computing the

approximate value of Zq' gives

Zq' = 2(49.4) - 50.5 = 48.3 fl

If the EUT is centered in the test zone of the cell, midway between the septum and

floor, we can obtain the value of Ex^y from equation (8) as. JP R ' J48.3 P 13.9 Jp" volts/ ^ E' = ^^ E = ^--—^ = ^ " "^^^^^ (A-6)

x,y b' x,y 0.5

where Ex,y' ""^ equal to 1 at this location in the cell. For the empty cell,

Ex,y is given as. JFT~ JbO.5 P 11.8Jp~"volts/ ^

E = V-^ E =V——a= 1j1 .-"ete^(A-7)

x,y b x,y 0.6

the ratio of E'

E x,y

1.18

This represents again for this particular EUT located at the center of the test zone, inside this particular cell, a 1.44 dB correction to the E-field, E^^y, computed from

the power measured at the cell's input port.

If the EUT was located in the center of the cell, but near the cell floor (position A indicated in figure 27) and if the impedance loading caused by the EUT inside the cell is

the same as the above example, the corrected electric field strength, E^ « is:

22
E' = 13.9 P (.853) = 11.86 P volts/ (A-8)

x,y n n meter

where E^ y is equal to 0.853 at this location in the cell. This value, for Ex y was obtained from table 3 of this technical note which gives the magnitude of the normalized

electric field as a function of cross section location in a square TEM cell.

The value of the electric field strength, E^ y at postion A in the empty cell is:

E = 11.8 Jp~(.853) = 10.07 -Jp" volts/ (A-9)

x,y ' n ^ n meter

The ratio, E' then is 11.86-/P~ , . x,y \V n 1...18

E 10.07JT~

y n

This correction factor, for the loading effect of the EUT, is the same for either test location of the EUT.

23

Appendix B

Evaluation of cell reciprocity and calculation of field intensity, Eq relative to open space for radiated emissions testing.

The following demonstrates the reciprocity of the TEM Cell for use in performing

radiated emissions measurements. It also gives a sample calculation for determining the

field intensity, Eq, relative to open space, of radiated emissions measurements made with

an EUT placed inside a TEM Cell. For this example, a 10 cm spherical dipole radiator was placed at four different locations inside the cell of figure 5 as shown in figure B-1. The

dipole radiator operates at a fundamental frequency of 30 MHz with harmonics up to approximately 500 MHz. Spectrum analyzer displays of the detected emissions from the

dipole, measured at one of the output ports of the cell are shown in figure B-2. The

opposite port of the cell was terminated into 50 ohms. Note, that the relative amplitudes

of the four traces are comparable at frequencies below the multimode/resonance frequencies

(240 MHz) of this cell. The small differences in detected amplitude as a function of dipole

placement location in the cell, at frequencies below 240 MHz, corresponds to the TEM field

distribtuions shown for this cell in figure 9 and tables 1-4. This demonstrates the

reciprocity of the TEM cell.

To calculate the field intensity, Eq, relative to open-space, for the radiated

emissions from the EUT, we use equation (20) from section 5.2.

b n Vm

D A d Z K(I) E cose'3 (B-1)

For measurements made using the cell of figure 5, equation (20) or (B-1) simplifies

to

(0.6) (376.7) V rEq s

(50) X d K(I)

-^

E cose'3

^

For the spherical dipole radiator, the maximum directive gain, G, is 1.5. Also, if

the polarization of the dipole is matched to the TEM field polarization in the cell, Cos 6 will be 1. In addition, since the dipole is small compared to the test volume inside the

cell, the dipole current distribution for the radiator will be essentially the same as for

open-space conditions, hence K(I) will be approximately 1.0. Then equation (B-1) becomes

(for this cell and EUT).

(0.6)(376.7)V^ ^g^^

(50) X d :i) E(i)^ 3

X d E [B-2]

The detected, measured voltage, Vpi at the cell port for the dipole centered in the

cell test zone (trace A, figure B-2) is -28 dBM at 30 MHz. This corresponds to a voltage

measured with a 50 ohm analyzer of 8.9 mV. At the center of the test zone, E equals 1.0 and

at 30 MHz, Xq is 10 meters.

25
This equation (B-2) again for this specific emission measured from the spherical dipole

ratio inside the 1.2 m x 2.4 m cell yields;

^

0.00402

D ~

d

or for a separation distance or 10 meters

E„^0.4O2mV/ ^ D meter

i-3:

This same 30 MHz emission measured with the dipole close to the floor the cell

(trace C) has a detected voltage of -30 dBM or 7.07 m/V. For this case E, interpolated from

table 2 of this report, is 0.839 and the calculated field intensity, Ep, relative to open-space is:

(6.39: .00707:

:iO) d(.839Xl.41)

0.0038O

or for a separation distance of 10 meters.

E^ = 0.38OmV/ ^ D meter

The value obtain for Eq (eqs B-3 and B-4) for the two locations of the dipole inside - the cell compares well within the measurement resolution and accuracy of this technique.

This demonstrate the independence of placement location of the EUT inside the cell on the

computed equivalent open-space radiated emissions results.

26
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29
I
LOWER

ACCESS

DOOR

UPPER

ACCESS

DOOR

OUTER SHIELD

DIELECTRIC

SEPTUM SUPPORTS

SEPTUM (INNER CONDUCTOR)

Figure 1. View of transverse electromagnetic (TEM) cell.

31
uH

tos

COXs

CO

bJD •H

32
2b

Figure 3, Cross-section of a TEM cell

33
2b

g

V L

1

2w

2a

> X

1.0

0.*

0.6

0.4

0.2 -

0.5 0.6 1.0

1.0

0,8

0.6

0.4

0.2

h/b = 4 1.0 r^

0.8

0.6

0.4

0.2

Figure 4. Transverse electromagnetic (TEM) cell design curves showing

ratio of cell outer conductor dimensions to ratio of center plate width

to cell width for various transmission line characteristic impedances

and vertical center plate locations.

34
I ooaHHH9

boa

ci +>ooo

•H

(4 +»

(D

(0 «HO

(0

a>

OSao

•Hu

(9

CO

(0

(0o

35
H

CMs

CM

r-lG

CD

<DU3

36
E FIELDS

Figure 7, Trnsverse electromagnetic (TEM) field distribution

inside a TEM cell.

37
5" EHP-CL Absorber

8" EHP-CL Absorber

Figure 8. Diagram for absorber loading symmetric TEM cell,

(Shaded areas indicate placement of anechoic material.

38
I

a/3 2a/3 a ^ 4a/3^

Distance Prom DooirSide

Figure 9^ Relative electric field distribution inside symmetric cell,

Shaded area corresponds to 1/3 cross-section area shown in top view.

39
[EpEJ .^-;_^ . . , -. ----*-----

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oe; /^ 2a/3 n 4 a/3

Distance From Door Side

a 5a/3 ; '111

III', ,

, i

. 1

i

- [

1

'

[ 1

• i

'

'

;

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'

'

' '

1

1 ' 1 1

'

' '

1 1 . 1 i 1

' ^

'

. :

. . : . 1 . , . !!'!;:;;

Figure 10. Relative electric field distribution inside rectangular

(0.6 a = b) , symmetric TEM cell. Shaded area corresponds to 1/3

cross-section area shown in top view.

40
—

center

z^

of

TEM cell

.^^ ^i K u Septum

4b/5

3b/5

2b/5

b/5

_^ .,t ^ jt floor

center 2a 3a 4a wall

Figure 11, Field distribution locations shown

in tables 1-8,

41
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>H

B

O -<

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0)

(Qn

10ouo•

CMu3

bO

42
I

^.stance—Be^.ow

ow

Figure 13, Relative electric field distribution inside asymmetric cell

(h = b/3) Shaded area corresponds to 1/3 cross-section area shown in cross sectional view of cell. Figure A and B data corresponds to area labeled A and B in top viev.

43
'- a/3. 2a/3 a 4a/3

- -!. Distance From Door Side

__^a/3 . .m: 1

-• --i- -.- -;

Figure 14. Relative electric field distribution inside cell with EUT

(metal case) occupying 1/3 vertical separation distance between septum

and floor. (Cross sectional cuts at center (L/2) and off end (L/6) of EUT

as indicated by numbers in too view44
lO '''''''''''

CM (Ji ''''•'''''-

<o CO dbJL

bOs

•H o 73 o a o o

rH iH

t)

(U XIu

o O m

rH r! Ol d

+-) 3

O^

+J o •H iH *

CO

-do

rt

X!

-P

•H o S

iHr* H

0)osH

o-^ H

iH N «5g

>M^ ;^

+-) >. <u O § c O 0) >> rH D M

in o"

(0u

f^ c^X o s

(M

•<1< .^

M

m

CO

O

>

+-> 3s

in

0)u

3

hJD •H

[14 HMSA

45
-o

0) T3 T3

0) cd •o o

a iH o a

.J P o

o

o

lO es) (ap) ssoi aox:}.a9sai

n

a

•H

Ido

u

o

(0

3

o

-p

c

£:p

•H

y

S

H

OH

U

+-> o

sc

>>

CQa

CM

CO

o

too

c

o

•H

;h

COa

U

3

•H

46
(HP) (3^/'"%) PieTJ pazTXBuiaON

47
•HT3O c

o O X3

•H 4J U -H o

iH a

0) X3 U -H >

C rH u oS

O Eh

(D O

a -H

4-> -P

fcl) s

c s

•H Tf< a M

o

•H X

d s

•H (N U • >

X

0) S

N N

•H »

r-l rH a

O -HG

00

3 4->

•H (U

(ap) (^'^a/^'^a) pt^tj pazxiBiujoM

48
(eP) (°'^3/"'^a) PISTJ P8ZXIT3U1JC02J

49
(aP) (^S/TMS) PISTii pazTi^uiaoN

50
Box

Figure 21. Placement of EUT in cell for EMC measurements,

51
in

bec

•H

4-> cS

5m P

COC

o

g

o

P c

S

^-^ o

;hP

W

0) o

•H

-Po

s

pt^TjI '^t-i;^»t3

o

>. p

•H O H U

•H SH X!

•H 0) -P -P

0< 0)

0) ;h o o P -H

O

CM

CM TiQ

O 5h p a

bo S

•H O

[X4 U

52
ao

•Ha

>.

0)mp

0)s

cu

•H3

0) <HO

bO

fl •HP

(0

>.

4J

•H

rH

•H^

•Ho

CO

COoe

<D •P

(0>»

c»OH

•Hoo

r-i

CO §

CSI iH 3 O

bO rH

•H

O

1^4 XI

53
a:5c!Q

Q. < , OO L in _i I is

ki

\- «

^^ a> / \ "O « / X c 2 /

UJo5 3c

a>E

Q. 'a

MO

TEST

ONITOR > /

UJ cE 5 UJ o- a. X /

UJ

II o

cr *5

x o- \/ 1-UJ X XI UJ

q:

I--

0) c

•s > o»

_ ^ _c P

_J

II

M 1

DUAL

DIRECTION/!

COUPLER o Q

UJ

Q q:1"

:9

X

Q. 5

a. « O

em

for

Su

MHz

to

5i

OAXIAL WITCH

-PWR

son LOAD o w I ^ 3

en , CE q: «^» d l^ t-m ^ o < _l R 1-

SIGN AMP

Ul ^ ^ o^^Eo

*/> o oo

to

u

0. UJ o

LOWFILT

m•ou

OS

p ^ Q. bO

ft

g •H

M <

Ph

M

J

Q

M tf

!z; o X CUW iz; W M M ftj ^ O o w

iH " >H

v/^

JH OJ ^

54
s

0) +J

rr, 1 ^ <O Kl f

RF POWER METER \ C|c / \ o < / \ -J / / ^ \/

A S

<D

;hw

CD p

•H

•H

'A

jja

a>

c;m

-J00

0)osoao3

Pi CMoa

ijw

•Hoo

inw

Si

•H

[i4

55
Filter box Leads covered with 2 inch

conductive tape.

Fig. Z6. Placement of EUT in cell for minimum exposure of leads to the TEM field.

56
Septum —

\

b

Aj

B

3b

5

3a

5

Fig. 27. Determination of E field correction factor as a function of the EUT location inside the cell.

57
Figure 28. Example of narrowband emanation, (frequency domain)

58
i i ((((

r-|H /

M|H ^ />-|H o<<Q<

CO<<HI—I Ut—f Q

(»so

•H

+J

at +>So

CO

0)

J< 0) H !h

1—1 O

a

1—1

•H

Q Ri >< §

CQ •o Q 0) W S H •H <

+J

J •o D 13 Q a

>,

:§ o

fl mh" a>3

C3"

^4 cd 2

Di

O < w • U 0}

,.

Q

flo §1

•HPq

ciu c4

p CO

g

CU < 0) 10 CQ T3C

X?' •o

cih

Si

«H

U)

0) iH

o.

cdXH• 0^CM

0)

;43

bO

•HP^
o^

G<O

HW

S-i

Xn

Qi o O

0.

w \

T'

MH > u

un 1—1

_j ^

c/:i ^ Z

cr

N

T3

H o H<

1:1: > , r >^

Tl.

b r-' ^ W U - H ^Z

C^ ^ <

•H

CQ>> iH

OSC

CJa

•H

oig

73

>>cp

o*

a>u

^-^U

«H

iH S iH

+J

(0>»s m w

Eh

pa

a s bO

<D c

;h •H P m

en

p

d

BW

«H s g u

CO «H

!^bo CO a s

•H

-a •H

t-> ^ Cjc

c^ iH g

03 <a -o » 0) p

CO ctf •H

73

;<

d P ;h be •H tH

pL4

60
1 WQ

PS I 1

hJ

O Di < < O

t3 Pi

U

Q

HW

SHW

•Hm

>»

rH

OSGas

•H

c^go

TS

0)

fi •H

-Pho

s-ls

0) • -p rH M rH

>>

O

m o

+-> s c w

Q) E-i s

cJ

;^p bO w C

cJ •H

CD

M s P

tH E-I o pH S

pj

g ^ ^H

bflO

CS tH

•H

t; Cfi c ^o •H

•p

rH cJ 03 G

ois • 0) tHCO -a

CD

Q -P

;h ri

13 •H bDTJ

•H PJ

Pt, Jh

61
50

Ohm

Tuned

u

'J>

-J J

"J c

-*—

-a

CD

ex c• I-t l->DX

Real-time

Oscilloscope

•^

<DU

<+H • 1-1 o 13 ao

•i-i

U

*->

u

1—1 oM >

(1) (U

ri *->

•r-4 4-> ni <: a

T3 1-H

i-H (U (U a u

•1-1 +-> ^ M WH a

nj

(1)

M

•4-> c!

to •1-1

CO

en

M

H

H

u

ao a

CD

u o

n) ^H CD

0) CO M-l a o O

•1-1

•4-1

a

CD

cti u

a

0) ^

(U • i-i 1—1 • 1-1 Tl T)

'U ;h

i->

•i-<

cti

T! f 1 1-H

(vi CO

0)u

7iDO h

62
1

1

1 1

' EUT [

EUT

Figure A-K Placement of EUT and its image ( second

identical EUT) in cell for measurement of loaded

impedance, Zq „

63
0„6 mlo 2m

]

1

0. 3m

® ®

Oo 06m

7R-CC ©

0. 6m

Figure B-1. Placement locations of the spherical dipole

radiator inside the 1. 2m x 1, 2m x 2,4m symmetric

TEM cell for radiated emissions measurements„

64
30 240

Frequency (lIHz)

3G 240

Frequency (MHz)

240

Frequency (MHz)

240

Frequency (?.CIz)

Figure B-2. Radiated emissions from 10cm spherical dipole measured inside a lo2m X 1.2m x 2.4m TEM cell. Spectrum analyzer display (0-500) MHz . 50MHz/div.

Reference level = -lOdBM, 10 dB/div. Trace A obtained with dipole centered

in test zone midway between septum and outer walls. Trace B same as trace A,

except dipole was moved to 36cm from door side^ Trace C same as trace A except dipole lowered to 6cm from floor. Trace D obtained with dipole 6cm above

floor and 36cm from door side. (Location measurement referred to center of

spherical dipole.)

65
NBS-114A (REV. 2-8C) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO. NBS TN-1013

2. Performing Organ. Report No. 3. Publication Date Revised

July 1981

4. TITLE AND SUBTITLE

Using a TEM Cell for EMC Measurements of Electronic Equipment

5. AUTHOR(S)

M. L. Crawford and J. L. Workman

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.

8. Type of Report & Period Covered

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

10. SUPPLEMENTARY NOTES

Supersedes NBS TN-1013 dated April 1979

[^^ Document describes a computer program; SF-185, PIPS Software Summary, is attached.

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

bi bliograptiy or literature survey, mention it here)

This publication describes the physical design and electrical evaluation of pertinent parameters which influence the use and operation of a transverse electromagnet!

(TEM) cell. Detailed, step-by-step procedures are given for using a TEM cell for

performing either radiated EM susceptibility testing or for measuring radiated EM

emissions from electronic/electro-mechanical equipment. These measurement procedure

provide guidelines to potential users and also indicate precautions to observe to

minimize problems often encountered when performing EMC measurements and hence to

enhance the cell's usefulness. Where available, a brief error analysis associated

with the measurement technique is included.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

measurement procedures; susceptibility and emission measurements; transverse

electromagnetic cell.

13. AVAILABILITY

^

Unlimited For Official Distribution. Do Not Release to NTIS ^ Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

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

14. NO, OF

PRINTED PAGES

72

15. Price

$4.25

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