Well Stirred is Half Measured - EMC Tests in Reverberation Chambers - Distinguished Lecturer Talk at the EMV-Fachtagung at TU Graz

The talk explains basic properties of reverberation chambers and presents some exemplary practical chambers with their parameters. The normative chamber validation as well as emission measurements and immunity tests are briefly explained. Finally, advantages and disadvantages in comparison with other EMC test environments are discussed.

This talk was given for the Austrian Chapter of the Electromagnetic Compatibility Society of the Institute of Electrical and Electronics Engineers (IEEE) during the annual "EMV-Fachtagung" symposium in Graz in Austria.

1. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
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
September 19, 2024

2. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
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:
Source:http://www.ece.unm.edu/summa/

3. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Change my mind!
Source:https://imgflip.com/i/6sa4e8

4. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Initial Question
All Power to the Chicken?
PInput=PWall+P
Stirrer+P
Chicken
PWall=0P
Stirrer=0PInput=P
Chicken“How can anyone seriously consider
such a test procedure?”

5. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Initial Question
All Power to the Chicken?
PInput=PWall+P
Stirrer+P
Chicken
PWall=0P
Stirrer=0PInput=P
Chicken“How can anyone seriously consider
such a test procedure?”

6. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Initial Question
All Power to the Chicken?
PInput=PWall+P
Stirrer+P
Chicken
PWall=0P
Stirrer=0PInput=P
Chicken“How can anyone seriously consider
such a test procedure?”

7. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Initial Question
All Power to the Chicken?
PInput=PWall+P
Stirrer+P
Chicken
PWall=0P
Stirrer=0PInput=P
Chicken“How can anyone seriously consider
such a test procedure?”

8. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Initial Question
All Power to the Chicken?
PInput=PWall+P
Stirrer+P
Chicken
PWall=0P
Stirrer=0PInput=P
Chicken“How can anyone seriously consider
such a test procedure?”

9. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Initial Question
All Power to the Chicken?
PInput=PWall+P
Stirrer+P
Chicken
PWall=0P
Stirrer=0PInput=P
Chicken“How can anyone seriously consider
such a test procedure?”

10. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Survey: Experience
https://particify.esalsa.
de/p/45311101
How much experience do you have with
EMC measurements in reverberation
chambers?
1.
chambers before.
2.
exist, but never used it.
3.
4.
measurements.
5.
experience.

11. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Overview and Motivation
Examples of Reverberation Chambers
Theoretical Fundamentals
Normative Validation and Measurements
Comparison With Other Test Environments

12. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Setup
Figure:

13. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Setup
Figure:

14. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Reverberation Chambers in Magdeburg
Figure: Key figures:
▶built in 1998, dimensions of7.9m×6.5m×3.5m
▶first cavity resonance at30MHz
▶lowest usable frequency at200MHz

15. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Reverberation Chambers in Magdeburg
Figure: Key figures:
▶built in 2003, dimensions of1.5m×1.2m×0.9m
▶first cavity resonance at160MHz
▶lowest usable frequency at1GHz

16. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Reverberation Chambers in Magdeburg
Figure: Key figures:
▶built in 2006, dimensions of60cm×58cm×56cm
▶first cavity resonance at360MHz
▶lowest usable frequency at2GHz

17. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Empty Rectangular Cavity Resonator
abcxyz
Cavity resonances:
f=
c0
2
s
ȷ
l
a
ff
2
+
ȷ
m
b
ff
2
+
ȷ
n
c
ff
2
l,mandnare non-negative integers, of which no more than one may be zero

18. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Resonant Frequencies
fin MHzl 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: 10resonant frequencies of the large reverberation chamber in Magdeburg with the
dimensions of7.9m×6.5m×3.5m

19. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Number of Resonant Frequencies
02040608010012014016018020022024026028030010
0
10
1
10
2
10
3
Frequency,f(in MHz)Cumulated number of modes
Figure:

20. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Number of Resonant Frequencies
02040608010012014016018020022024026028030010
0
10
1
10
2
10
3
Frequency,f(in MHz)Cumulated number of modes
Figure:

21. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Think about it!
Source:https://imgflip.com/i/6sa6a6

22. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Survey: Resonances
https://particify.esalsa.
de/p/45311101
What happens if the frequency of the
excitation exactly coincides with some
eigenfrequency of the chamber (i. e. a
resonance)?
1.
strength!
2.
never exactly match.
3.
operation of a reverberation chamber.
4.
going back from the TX antenna to the
power amplifier.

23. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Quality Factor and Modal Bandwidth Quality factor:
Q=
ω·stored energy
average power loss
Losses caused by: antennas, walls, EUT, stirrer, other scatterersRelation between bandwidth and quality factor:
Q=
resonant frequency
bandwidth
=
1
relative bandwidth

24. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Quality Factor and Modal Bandwidth Quality factor:
Q=
ω·stored energy
average power loss
Losses caused by: antennas, walls, EUT, stirrer, other scatterersRelation between bandwidth and quality factor:
Q=
resonant frequency
bandwidth
=
1
relative bandwidth

25. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Quality Factor and Modal Bandwidth Quality factor:
Q=
ω·stored energy
average power loss
Losses caused by: antennas, walls, EUT, stirrer, other scatterersRelation between bandwidth and quality factor:
Q=
resonant frequency
bandwidth
=
1
relative bandwidth

26. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Undermoding at “Low” Frequencies
124124.2124.4124.6124.8125125.2125.4125.6125.812610
−2
10
−1
10
0
10
1
Frequency,f(in MHz)Field strength (normalized)Superposition
Figure:
125MHz at a quality factor ofQ=1000

27. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Overmoding at “High” Frequencies
249249.2249.4249.6249.8250250.2250.4250.6250.825110
−2
10
−1
10
0
10
1
Frequency,f(in MHz)Field strength (normalized)Superposition
Figure:
250MHz at a quality factor ofQ=1000

28. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Undermoding Due to a High Quality Factor
249249.2249.4249.6249.8250250.2250.4250.6250.825110
−2
10
−1
10
0
10
1
Frequency,f(in MHz)Field strength (normalized)Superposition
Figure:
250MHz at a quality factor ofQ=10 000

29. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Cavity Resonator−→Reverberation Chamber
How to stir the field?
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

30. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Cavity Resonator−→Reverberation Chamber
How to stir the field?
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

31. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Cavity Resonator−→Reverberation Chamber
How to stir the field?
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

32. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Vibrating Intrinsic Reverberation Chamber
(a)(b)
Source: Prof. Leferink, University of Twente and THALES, Netherlands

33. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Oscillating Wall Stirrer
Figure:
Compatibility, School of Mechanical Engineering, Southeast University, Nanjing, China
Source:https://dx.doi.org/10.1109/TEMC.2020.2983981

34. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Statistical Properties of the Field Homogeneity:
▶independence of location
▶free placement of the EUT in the working volume
Isotropy:
▶independence of direction
▶free alignment of the EUT and attached cables
Validity:
▶only in the working volume
▶minimum distance to walls (>
λ/4)

35. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Statistical Properties of the Field Homogeneity:
▶independence of location
▶free placement of the EUT in the working volume
Isotropy:
▶independence of direction
▶free alignment of the EUT and attached cables
Validity:
▶only in the working volume
▶minimum distance to walls (>
λ/4)

36. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Statistical Properties of the Field Homogeneity:
▶independence of location
▶free placement of the EUT in the working volume
Isotropy:
▶independence of direction
▶free alignment of the EUT and attached cables
Validity:
▶only in the working volume
▶minimum distance to walls (>
λ/4)

37. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Chamber Validation Field uniformity and maximum loading validation:
▶check field homogeneity in the working volume
▶determine the lowest usable frequency (LUF)
▶obtain ratio between input power and field strength
▶analyze coupling between antennas
▶for the empty and maximum loaded chamber
Chamber validation with the EUT in place:
▶analyze the loading by the EUT (≤maximum loading)

38. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Chamber Validation Field uniformity and maximum loading validation:
▶check field homogeneity in the working volume
▶determine the lowest usable frequency (LUF)
▶obtain ratio between input power and field strength
▶analyze coupling between antennas
▶for the empty and maximum loaded chamber
Chamber validation with the EUT in place:
▶analyze the loading by the EUT (≤maximum loading)

39. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Radiated Immunity and Emission Immunity tests:
PInput∼E
2
Test
Proportionality factor results from:
▶empty chamber validation
▶validation with the EUT in place
Emission measurement based on the average power at the reference antenna:
Prad=
ηTX·PAveRec
CVF
Variables:
ηTX efficiency of the transmitting antenna (75%to90%)
CVF chamber validation factor from the validation with the EUT (switched off) in place

40. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Radiated Immunity and Emission Immunity tests:
PInput∼E
2
Test
Proportionality factor results from:
▶empty chamber validation
▶validation with the EUT in place
Emission measurement based on the average power at the reference antenna:
Prad=
ηTX·PAveRec
CVF
Variables:
ηTX efficiency of the transmitting antenna (75%to90%)
CVF chamber validation factor from the validation with the EUT (switched off) in place

41. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Radiated Immunity and Emission Immunity tests:
PInput∼E
2
Test
Proportionality factor results from:
▶empty chamber validation
▶validation with the EUT in place
Emission measurement based on the average power at the reference antenna:
Prad=
ηTX·PAveRec
CVF
Variables:
ηTX efficiency of the transmitting antenna (75%to90%)
CVF chamber validation factor from the validation with the EUT (switched off) in place

42. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Survey: Other Measurements
https://particify.esalsa.
de/p/45311101
What is difficult to be measured in a
reverberation chamber?
1.
connectors, waveguides and passive
microwave components
2.
materials
3.
wireless devices
4.
5.

43. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Electrical Size of an Equipment Under Test
a
Definition ask·a:
k: k=
2πf
c
=
2π
λ
a:
Questions:
▶What belongs to the EUT (case, cables, . . . )?
▶Which line length has to be considered?

44. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Electrical Size of an Equipment Under Test
a
Definition ask·a:
k: k=
2πf
c
=
2π
λ
a:
Questions:
▶What belongs to the EUT (case, cables, . . . )?
▶Which line length has to be considered?

45. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Electrical Small EUTs
Condition:k·a≤1
Figure:

46. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Electrical Large EUTs
Condition:k·a>1
Figure:

47. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Which field would you like to have? Environment:
EUT:
Environment:
EUT:

48. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Which field would you like to have? Environment:
EUT:
Environment:
EUT:

49. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Which field would you like to have? Environment:
EUT:
Environment:
EUT:

50. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Which field would you like to have? Environment:
EUT:
Environment:
EUT:

51. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Which field would you like to have? Environment:
EUT:
Environment:
EUT:

52. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Which field would you like to have? Environment:
EUT:
Environment:
EUT:

53. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Catch my radiation, if you can!
Source:https://imgflip.com/i/6sae2qSource:https://imgflip.com/i/6sagu4

54. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Catch my radiation, if you can!
Source:https://imgflip.com/i/6sae2qSource:https://imgflip.com/i/6sagu4

55. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Measurements With a Practical EUT
(a)(b)
Figure:

56. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Measurements With a Practical EUT
2002503003504004505005506006507007508008509009501 0005560657075Frequency in MHzField strength in dB
µV m
−1Measurement in the semi-anechoic chamberMeasurement in the reverberation chamber
Figure: =1
(Source: Matthias Hirte, OVGU)

57. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Measurements With a Practical EUT
2002503003504004505005506006507007508008509009501 0005560657075Frequency in MHzField strength in dB
µV m
−110
◦
steps90
◦
steps
Figure:
Hirte, OVGU)

58. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
There is a solution, wait, not.
Source:https://imgflip.com/i/6sahm3

59. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
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

60. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
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

61. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Don’t be that kind of test engineer!
Source:https://imgflip.com/i/8h538q

62. MotivationExample ChambersSome TheoryValidation and MeasurementsComparison
Source:https://twitter.com/MarkusRidderbu8/status/
1523708966039351297
Thank you very much for
your attention!
What questions do you
have?