Well Stirred is Half Measured - EMC Tests in Reverberation Chambers

Overview and Motivation
Reverberation Chambers
Well Stirred is Half Measured
EMC Tests in Reverberation Chambers
Mathias Magdowski
Chair for Electromagnetic Compatibility
Institute for Medical Engineering
Otto von Guericke University Magdeburg, Germany
October 11, 2023
Mathias Magdowski (Otto von Guericke University Magdeburg, Germany) Well Stirred is Half Measured – EMC Tests in Reverberation Chambers

Overview Over Other Test Environments
Motivation
Table of Contents
(a) Open Area Test Site (Cambridge) (b) Semi-Anechoic Chamber (Magdeburg)
(c) Fully-Anechoic Room (Gent) (d) GTEM Cell (Magdeburg)

Motivation
Table of Contents
Motivation
Carl Edward Baum in Microwave Memo No. 3:
“The Microwave Oven Theorem
– All Power to the Chicken”
What is the difference between a
microwave oven and a mode stirred
chamber?
The former cooks chicken, the later
cooks electronics.
Figure: Carl Edward Baum (1940 –
2010)
Source: http://www.ece.unm.edu/summa/

Motivation
Table of Contents
1 Reverberation Chambers
Overview
Theoretical Fundamentals
Normative Validation and Measurements
Comparison With Other Test Environments

Overview
Setup
Figure: Schematic setup of a reverberation chamber (top view)

Overview

Overview
Draw a Good Stirrer Design!

Overview
Pros and Cons
Pros:
high Q −→ high E field strength with low input P
relatively low costs
statistical uniform field −→ robust tests

Overview
Pros and Cons
Pros:
high Q −→ high E field strength with low input P
relatively low costs
statistical uniform field −→ robust tests
Cons:
statistics necessary
no statement about directivity and polarization possible
EUT loads the chamber and lowers Q
comparison with established test environments

Overview
Setup
Figure: Typical Reverberation Chamber (IEC 61000-4-21)

Overview
Reverberation Chambers in Magdeburg
Figure: Large reverberation chamber in Magdeburg
Key figures:
built in 1998, dimensions of 7.9 m × 6.5 m × 3.5 m
first cavity resonance at 30 MHz
lowest usable frequency at 250 MHz

Overview
Figure: Small reverberation chamber in Magdeburg
Key figures:
built in 2003, dimensions of 1.5 m × 1.2 m × 0.9 m
lowest usable frequency at 1 GHz

Overview
Figure: Tiny reverberation chamber in Magdeburg
Key figures:
built in 2006, dimensions of 60 cm × 58 cm × 56 cm
lowest usable frequency at 2 GHz

Overview
Literature
Generic technical standard IEC 61000-4-21:
1. Edition from 2003, 2. Edition from 2011
explains the validation and measurements

Overview
Literature
Book:
John M. Ladbury, Galen H. Koepke, Dennis G. Camell: Evaluation of the NASA
Langley Research Center Mode-Stirred Chamber Facility

Overview
Literature
Book:
John M. Ladbury, Galen H. Koepke, Dennis G. Camell: Evaluation of the NASA
Langley Research Center Mode-Stirred Chamber Facility
Other standards:
ISO 11452-11:2010 (automotive)
RTCA DO-160 (aircraft)

Overview
Intermediate Overview
1 Reverberation Chambers
Overview

Overview
Survey: Resonances
https://strawpoll.de/
ybe3xcz
What happens if the frequency of the excitation
exactly coincides with some eigenfrequency of
the chamber (i. e. a resonance)?
1 There will be an incredibly high field
strength!
2 This is not possible, as the frequencies
never exactly match.
3 Nothing spectacular, this is the usual
operation of a reverberation chamber.
4 There will be a large reflected power going
back from the TX antenna to the power
amplifier.

Overview
Empty Rectangular Cavity Resonator
a
b
c
x
y
z
Cavity resonances:
f =
c0
2
s
l
a
2
+

m
b
2
+

n
c
2
l, m and n are non-negative integers, of which no more than one may be zero

Overview
Resonant Frequencies
f in MHz l m n
29.8635 1 1 0
44.4060 2 1 0
46.8424 1 0 1
48.6416 0 1 1
49.8724 1 2 0
52.2113 1 1 1
57.2213 2 0 1
59.7270 2 2 0
61.4165 3 1 0
61.6935 2 1 1
Table: First 10 resonant frequencies of the large reverberation chamber in Magdeburg with the
dimensions of 7.9 m × 6.5 m × 3.5 m

Overview
Number of Resonant Frequencies
0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300
100
101
102
103
Frequency, f (in MHz)
Cumulated
number
of
modes
Figure: Cumulated number of modes for the large reverberation chamber in Magdeburg

Overview
Quality Factor
Q =
ω · stored energy
average power loss

Overview
Quality Factor
Q =
ω · stored energy
average power loss
What will cause losses?

Overview
Quality Factor
Q =
ω · stored energy
average power loss
What will cause losses?
Losses:
within the dielectric (very small in air)
at the walls (material: copper, aluminum, steel)
by loading the chamber (scatterers as the stirrer, antennas, cables, EUT, monitoring
equipment, . . . )

Overview
Bandwidth of the Resonant Frequencies
Relation between bandwidth and quality factor:
Q =
Resonant Frequency
Bandwidth
=
1
relative Bandwidth

Overview
Bandwidth of the Resonant Frequencies
Relation between bandwidth and quality factor:
Q =
Resonant Frequency
Bandwidth
=
1
relative Bandwidth
Consequences of a finite quality factor:
finite resonance magnification
field can be excited outside of the resonant frequencies
modes can overlap

Overview
Undermoding at “Low” Frequencies
124 124.2 124.4 124.6 124.8 125 125.2 125.4 125.6 125.8 126
10−2
10−1
100
101
Field
strength
(normalized)
Superposition
Figure: Schematic modal structure in the large reverberation chamber in Magdeburg around
125 MHz at a quality factor of Q = 1000

Overview
Overmoding at “High” Frequencies
249 249.2 249.4 249.6 249.8 250 250.2 250.4 250.6 250.8 251
10−2
10−1
100
101
Field
strength
(normalized)
Superposition
250 MHz at a quality factor of Q = 1000

Overview
Undermoding Due to a High Quality Factor
249 249.2 249.4 249.6 249.8 250 250.2 250.4 250.6 250.8 251
10−2
10−1
100
101
Field
strength
(normalized)
Superposition
250 MHz at a quality factor of Q = 10 000

Overview
Cavity Resonator −→ Reverberation Chamber
How to stir the field?

Overview
Changes of the electromagnetic boundary conditions:
mechanical stirrer
moving walls
replacing the transmitting antenna
switching between several antennas

Overview
Changes of the electromagnetic boundary conditions:
mechanical stirrer
moving walls
replacing the transmitting antenna
switching between several antennas
Narrow band frequency changes:
only for immunity testing

Overview
Vibrating Intrinsic Reverberation Chamber
(a) Demonstration with neon tubes (b) In-situ test on a ship
Source: Prof. Leferink, University of Twente and THALES, Netherlands

Overview
Oscillating Wall Stirrer
Figure: Reverberation chamber with an oscillating wall stirrer at the Laboratory of Electromagnetic
Compatibility, School of Mechanical Engineering, Southeast University, Nanjing, China
Source: https://dx.doi.org/10.1109/TEMC.2020.2983981

Overview
Terminology
Reverberation chamber =
mode-stirred chamber
mode-tuned chamber
overmoded cavity
reverberating enclosure
electrically large, complex cavity
Modenverwirbelungskammer
Feldvariable Kammer
. . .

Overview
Statistical Properties of the Field
Homogeneity:
independence of location
free placement of the EUT in the working volume

Overview
Homogeneity:
Isotropy:
independence of direction
free alignment of the EUT and attached cables

Overview
Homogeneity:
Isotropy:
independence of direction
free alignment of the EUT and attached cables
Ergodicity:
interchangeability of different statistical ensembles
e. g. stirrer positions, spatial points, neighboring frequencies, etc.

Overview
Chamber Field Uniformity and Loading Validation
Goal:
verification of a sufficiently small field inhomogeneity in the working volume
determination of the lowest usable frequency (LUF)
for the empty and maximum loaded chamber
Maximum chamber loading verification:
simulate the chamber loading by the EUT
using a sufficient amount of absorbers
EUT loading ≤ maximum loading

Overview
Chamber Validation With the EUT in Place
Goal:
analyze the loading by the EUT
loading ≤ maximum loading
Same procedure as empty chamber validation with the following simplifications:
no field probe measurements
only one locations of the reference antenna

Overview
Radiated Immunity Testing
Determination of the chamber input power:
setting of the required field strength ETest
feeding of a certain forward power
PInput ∼ E2
Test (1)

Overview
Radiated Immunity Testing
Determination of the chamber input power:
setting of the required field strength ETest
feeding of a certain forward power
PInput ∼ E2
Test (1)
Proportionality factor results from:
empty chamber validation
validation with the EUT in place

Overview
Performing an Immunity Test
Preparation:
remove unnecessary absorbing materials → high Q
no wooden tables, carpets, wall and floor coverings

Overview
Performing an Immunity Test
Preparation:
remove unnecessary absorbing materials → high Q
no wooden tables, carpets, wall and floor coverings
Procedure:
logarithmic frequency spacing with 100 frequencies/decade
same minimum numbers of stirrer steps as during validation
appropriate dwell time per frequency and stirrer step
no stirring in conjunction with swept frequency testing

Overview
Radiated Emission Measurement
Based on the measurement of the average received power:
Prad =
ηTX · PAveRec
CVF
(2)
Variables:
ηTX efficiency of the transmitting antenna (75 % to 90 %)
PAveRec average received power at the reference antenna
CVF chamber validation factor from the validation with the EUT (switched off) in place

Overview
Survey: Other Measurements
https://strawpoll.de/
r27a16r
What is difficult to be measured in a
reverberation chamber?
1 Shielding effectiveness of cables,
connectors, waveguides and passive
microwave components
2 Shielding effectiveness of gaskets and
materials
3 Modulated-signal measurements of
wireless devices
4 Antenna efficiency
5 Antenna directivity

Overview
What is Meant by the “Electric Field”?
c
E
H

Overview
c
E
H
Linearly polarized plane wave with the amplitude E0:
amplitude of the Cartesian field component is E0
total electric field strength is E0
minimum, average and maximum field strength is E0
minimum, average and maximum squared magnitude is E2
0

Overview
c
E
H
Linearly polarized plane wave with the amplitude E0:
amplitude of the Cartesian field component is E0
total electric field strength is E0
minimum, average and maximum field strength is E0
minimum, average and maximum squared magnitude is E2
0
This is not valid for a reverberant field!

Overview
Electrical Size of an Equipment Under Test
a
Definition as k · a:
k: wave number, k = 2πf
c = 2π
λ
a: radius of the smallest sphere surrounding the
EUT

Overview
Electrical Size of an Equipment Under Test
a
Definition as k · a:
k: wave number, k = 2πf
c = 2π
λ
a: radius of the smallest sphere surrounding the
EUT
Questions:
What belongs to the EUT (case, cables, . . . )?
Which line length has to be considered?

Overview
Electrical Small EUTs
Condition: k · a ≤ 1
Figure: Radiation pattern of a small dipole (Source: Wikipedia)

Overview
Electrical Large EUTs
Condition: k · a 1
Figure: Radiation pattern (planar cut) of a practical EUT (Source: Magnus Höijer, FOI)

Overview
Transition From Electrically Small to Electrically Large
Frequency f Wave Number k k · a Radius a Diameter d
in GHz in m−1 in cm in cm
0.03 0.6 1.0 159.2 318.3
0.10 2.1 1.0 47.8 95.5
0.20 4.2 1.0 23.9 47.7
0.50 10.5 1.0 9.6 19.1
1.0 20.9 1.0 4.8 9.6
2.0 41.9 1.0 2.4 4.8
5.0 104.7 1.0 0.9 1.9
10.0 209.4 1.0 0.5 1.0
Table: Example values for an electrical EUT size of k · a = 1 (Source: Perry Wilson, NIST)

Overview
Sampling
Frequency f Wave Number k Radius a k · a Orientations
in GHz in m−1 in cm
0.03 0.6 0.25 0.2 12
0.10 2.1 0.25 0.5 12
0.20 4.2 0.25 1.1 13
0.50 10.5 0.25 2.6 48
1.0 20.9 0.25 5.2 152
2.0 41.9 0.25 10.5 522
5.0 104.7 0.25 26.2 2951
10.0 209.4 0.25 52.4 11 385
Table: Number of independent orientation for an object with 50 cm diameter (Source: Perry
Wilson, NIST)

Overview
Comparison

Overview
Comparison
Environment: deterministic

Overview
Comparison
EUT: random

Overview
Comparison
EUT: random
Environment: random

Overview
Comparison
EUT: random
Environment: random
EUT: deterministic

Overview
Measurements With a Practical EUT
(a) in the reverberation chamber (b) in the semi-anechoic chamber
Figure: Utilized equipment under test (Source: Matthias Hirte, OVGU)

Overview
200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1 000
55
60
65
70
75
Frequency in MHz
Field
strength
in
dB
µV
m
−1
Measurement in the semi-anechoic chamber
Measurement in the reverberation chamber
Figure: Comparison of the emission measurement in different environments, directivity = 1
(Source: Matthias Hirte, OVGU)

Overview
200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1 000
55
60
65
70
75
Frequency in MHz
Field
strength
in
dB
µV
m
−1
10◦ steps
90◦ steps
Figure: Comparison of the emission measurement for different turntable steps (Source: Matthias
Hirte, OVGU)

Overview
Semi-Anechoic vs. Reverberation Chamber
Property Semi-Anechoic Reverberation
Chamber Chamber
Field plane wave multiple reflections
Polarization linear unknown
Correlation length very long short (λ
2 )
Incident direction known all directions
Field impedance 377 Ω unknown

Overview
SAC vs. GTEM vs. RVC
Property Semi-Anechoic
Chamber
GTEM Reverberation
Chamber
f Range above 30 MHz
(10 m test distance)
from 0 Hz above “Lowest usable
frequency” (LUF)
Problems test in the
near field
at low f
standing
waves at
medium f
no statistic
uniform field
a low f
Test Time increases
with f
increases
with f
stays constant
EUT V large small medium
Costs high low low

Overview
More Fundamental Question
What is a good measurand for emission?
field strength in V m−1 (in a certain distance)
power flux density in W m−2 (in a certain distance)
total radiated power W (independent of the distance)

Overview
More Fundamental Question
What is a good measurand for emission?
field strength in V m−1 (in a certain distance)
power flux density in W m−2 (in a certain distance)
total radiated power W (independent of the distance)
In which environment is the measurement performed?
reflection-free environment
environment with reflections
highly reflective environment

Overview
Final Question
All Power to the Chicken?
PInput = PWall + PStirrer + PChicken

Overview
Final Question
PWall = 0

Overview
Final Question
PWall = 0
PStirrer = 0

Overview
Final Question
PWall = 0
PStirrer = 0
PInput = PChicken

Overview
Thank you very much for your attention!
Are there more questions?

Well Stirred is Half Measured - EMC Tests in Reverberation Chambers

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