Experimental Results Obtained in the
Vibrating Intrinsic Reverberation Chamber
Frank Leferink
Hollandse Signaalapparaten B.V.
Research, Development &
Technology
Environmental Test Laboratory
P.O. Box 42, 7550 GD Hengelo
The Netherlands
leferink@signaal.nl
University of Twente
Faculty of Electrical Engineering
Department of
Telecommunication Engineering
P.O. Box 217, 7500 AE Enschede
The Netherlands
leferink@cs.utwente.nl
Abstract: Measurements have been performed in a Vibrating
Intrinsic Reverberation Chamber (VIRC). This chamber has
varying angles between wall, floor and ceiling. Inside the
VIRC a diffuse, statistically uniform electromagnetic field is
created without the use of a mechanical rotating mode stirrer.
In comparison to other reverberation chambers the VIRC
displays an improved low frequency behavior, enabling faster,
cost effective testing and allows for in-situ measurements.
Several measurement results obtained in the VIRC are
presented in this paper, such as the change in resonance
frequenccy, the stirring ratio and the power density function.
Introduction
A reverberation chamber generally consists of a rectangular
test room with metal walls and a mode stirrer, usually in the
form of a large paddle, near the ceiling of the chamber. The
EUT is placed in the chamber and exposed to an
electromagnetic field while the stirrer slowly revolves. The
average response of the EUT to the field is found by
integrating the response over the time period of one revolution
of the stirrer. The metal walls of the chamber allow a large
field to be built up inside the chamber. The EUT is therefore
exposed to a high field level consisting of several different
polarizations [1,2,3,4].
The electromagnetic field inside a reverberation chamber is
not time invariant as in most classic test methods utilizing free
space set-ups, but provides a periodic electromagnetic
environment, which is
• randomly polarized, i.e. the phase between all waves is
random,
• spatialy uniform, i.e. the energy density in the chamber is
uniform everywhere and
• isotropic, i.e. the energy flow in all directions is the same.
It has been shown that in a Mode Stirred Chamber (MSC) at
least 200 modes should be available [3] in order to fulfill these
requirements. In [6] several mode modification techniques,
such as irregular walls, a moving wall and the Vibrating
Intrinsic Reverberation Chamber (VIRC) have been discussed.
The VIRC has vibrating walls in order to change the boundary
conditions with large amplitude and short time variation. The
VIRC is decribed in the next paragraph.
Several tests have been carried out using the VIRC. The test
set-ups and results are discussed showing the advantages of
the VIRC compared to other techniques.
Jean-Claude Boudenot
Wim van Etten
Thomson-CSF Comsys
Hardening, Safe Instumentation
and Systems Unit
BP 82, 92704 Colombes
France
jean-claude.boudenot@
comsys.thomson-csf.com
University of Twente
Faculty of Electrical Engineering
Department of
Telecommunication Engineering
P.O. Box 217, 7500 AE Enschede
The Netherlands
etten@cs.utwente.nl
The Vibrating Intrinsic Reverberation Chamber
The VIRC is a reverberation chamber with walls made of
flexible conducting material. It is mounted in a rigid structure
and connected to that structure via flexible rubber strings, as
shown in Figure 1, and drawn schematically in Figure 2. By
moving one or more ridges or one or more walls the modal
behavior of the field inside the chamber is changed, and thus
the resonance frequencies are changed. Because this frequency
shift is much larger compared to what is possible with a
conventional mode stirrer, the frequency range of the chamber
is extended to lower frequencies compared to a reverberation
chambers with equal dimensions. Note the natural corrugation
of the flexible walls in Figure 1, which is beneficial for the
spatial uniformity too. Another advantage is that the flexible
chamber can be erected inside a standard anechoic chamber
where the EUT has been installed for standard EMI tests, thus
reducing measurement time and cost. Furthermore the VIRC
does not need extra space inside the laboratory: it can be
folded and put away fast. The most important advantage of the
flexible structure of the VIRC is that it can be installed in-situ.
Figure 1: The Vibrating Intrinsic Reverberation Chamber
hanging in strings
y
•
h
x
l
z
w
•
Figure 2: The VIRC: a flexible tent with irregularly
shaped walls and varied angles of the wall-floor-ceiling
interface
Experimental characterisation of a reverberation chamber
Numerous experiments could be carried out to characterize a
reverberation chamber. Some of them are:
• Resonance frequency variation
By measuring the field strength as function of the
frequency for several (fixed) boundary conditions, i.e.
VIRC positions, the resonance frequencies and change in
resonance frequencies can be observed. The change in
resonance frequency provides an indication of the ‘stirring
efficiency’ of the VIRC
• Voltage Standing Wave Ratio
By measuring the incident and reflected power as function
of the frequency the voltage standing wave ratio (VSWR)
can be obtained. The VSWR indicates at what frequency
the maximum energy transfer can be achieved.
• Stirring Ratio
By recording the field strength as a function of time
during a periodic or sufficiently random change in
boundary conditions the so-called stirring ratio (SR) is
obtained. This figure is defined as the ratio between the
maximum and minimum field strength at a specific
position and frequency:
SR( f ) =
•
Emax
Emin
The stirring ratio indicates the effectiveness of a stirring
mechanism. If the stirring ratio would be small then the
boundary conditions are changed only slightly, and, as a
result, the spatial uniformity could be low. In [8] a stirring
ratio of at least 20 dB is considered as sufficient.
The SR is an assess of the stirring efficiency in terms of
field strength amplitude, which is an amplitude
modulation. The change in resonance frequency due to a
change in boundary condition is also an assess of the
stirring efficiency, but now in terms of frequency
modulation.
Power Density Function
The experimental power density function (PDF) is
obtained by normalizing the measured absolute values of
the electric field strength (in three directions) by their
mean, and sorting the samples of total field strength from
•
the minimum field strength to the maximum field
strength. By comparison of the experimental PDF with the
theoretical PDF the quality of the chamber can be found:
if the experimental PDF is equal to the theoretical PDF
then the accuracy of a prediction is very high.
Cumulative Density Function
The cumulative probability density function (CDF) is
obtained by integrating the PDF. Via the CDF a
comparison of PDFs for different frequencies, or a
comparison of chambers is facilitated.
Spatial Field Uniformity
Another important parameter is the spatial field
uniformity (SFU). The SFU gives the ability to generate
an isotropic, randomly polarised field which is stochastic
equal in the whole volume of the chamber, exept near the
walls. The SFU has not been measured yet.
Quality Factor
The chamber quality factor Q determines the efficiency of
the chamber in relation to the power needed for
generating a specific field strength or power density. The
quality factor Q is an important parameter. It should be
very high so that the generated field strength at a specific
net input power is high, and thus a high efficiency. On the
other hand, a low quality factor results in a higher mode
overlap. The quality factor has not been measured yet.
Measurement results
The measurements were carried out using an experimental
VIRC, as shown in Figure 1. The size of the VIRC was
approximately 1.5 x 1.7 x 1.9 m. The VIRC was not vibrating
for the measurement of the resonance frequency variation and
the VSWR. For the other measurements (SR, PDF and CDF)
the VIRC was randomly excited by three independent persons
creating an amplitude variation of at least 10% of the length,
width and heigth of the VIRC.
Because the field strength inside the VIRC changes rapidly
compared to the field inside the mode stirred chamber the
standard field strength sensors, as used for EMI testing, are
too slow. Therefore a very fast 3-axis sensor was used for the
field strength measurements: the ET2003 [7]. The advantages
of this sensor are the
• low loading of the chamber, thus not decreasing the
quality factor of the chamber,
• small dimensions (60x60x60 mm),
• large bandwidth with flat frequency response
(10 kHz - 2.5 GHz),
• large dynamic range (nearly 80 dB) and
• short risetime (200 ps)
making this sensor very useful for reverberation chamber
testing.
Resonance frequency variation
The field strength in the three orthogonal directions was
measured inside the VIRC as function of the frequency using
the set-up as drawn in Figure 3.
MHz and 200 MHz has been drawn in Figure 6. The
resonance frequency variation in the VIRC is quite large, upto
10 MHz for the f011 mode, while the resonance frequency
variation in the MSC is only a few MHz.
Field strength [dBV/m/dBm]
20
optical
coaxial
ESS
measuring
receiver
BR2000-3
optical
receiver
f101
f110
10
f011
0
Etotal 2
-10
Etotal 3
Etotal 4
-20
50
100
150
200
Frequency [MHz]
Field strength [dBV/m/dBm]
An electromagnetic field was generated inside the VIRC by a
log-spiral antenna (150 MHz-1 GHz). The antenna was
activated by the output of the tracking generator of the
Rohde&Schwarz ESS measuring receiver. The field strength
inside the VIRC has been measured using the Thomson-CSF
ET2003 three-dimensional E-field sensor. The output signal of
the sensor was fed to the optical receiver via an optical fiber.
The output voltage level of the optical receiver was measured
by the ESS measuring receiver for the three axes of the field.
The total measured electric field strength between 900 MHz
and 1 GHz has been drawn in Figure 4. During the
measurements the VIRC was at two fixed positions (position 3
and 4), i.e. not vibrating. In this frequency range, 900 MHz – 1
GHz, numerous resonance frequencies are available and a
distinctive resonance frequency can thus not be observed.
10
Etotal 3
Etotal 4
Figure 5: Total electric field strength as function of the
frequency for VIRC position 2, 3 and 4
60
Field strength [dBuV/m]
Figure 3: Measurement set-up for field modification
50
40
30
MSC, Stirrer pos. 1
MSC, Stirrer pos. 2
MSC, Stirrer pos. 3 (stirrer moved)
20
10
50
100
150
200
Frequency [MHz]
Figure 6: Total electric field strength as function of the
frequency for a MSC with 3 stirrer positions
Voltage standing wave ratio
0
-10
900
950
1000
Frequency [MHz]
Figure 4: Total electric field strength for VIRC position 3
and 4 between 900 MHz and 1 GHz
In Figure 5 the total electric field strength between 50 MHz
and 200 MHz has been drawn for the situation that the VIRC
was at three fixed positions (position 2, 3 and 4). The
resonance frequencies are evident. The resonance frequency
variation due to the changed VIRC positions is also
noticeable. A similar measurement has been carried out in a
MSC with dimensions of approximately 2x2x2 m and for three
stirrer positions. The total electric field strength between 50
The standing wave ratio (SWR) is defined as the ratio between
the incident and reflected power at a specific position and
frequency. If the measurement impedances are equal (50 Ω)
then the voltage SWR (VSWR) in terms of incident Ui and
reflected voltage Ur is:
U
VSWR = r
Ui
If the VSWR equals 1 then all incident energy is absorbed in
the reverberation chamber. If the VSWR is high then a large
portion of the energy is reflected.
The VSWR for VIRC position 3 has been drawn in Figure 7,
using the measurement set-up as drawn in Figure 3. From the
VSWR we can calculate the net power injected into the VIRC.
This net power has been drawn in Figure 8. The electric field
strength for VIRC position 3 has been drawn in Figure 9.
By comparison of Figure 8 and 9 the resemblance between net
power and resonance frequencies is high, as could be
expected.
5
11, 12 and 13 respectively. The measurement period was 4 s,
with 8000 samples and thus 0.5 ms/sample.
It can be observed in the figures that the SR for 150 MHz was
less than 20dB. For 400MHz and 1 GHz the SR satisfies the
rule SR>20dB, as stated in [8].
VSWR
4
3
2
10
50
1 00
150
2 00
F re q u en c y [M H z ]
Figure 7: VSWR for VIRC position 3
Net Power [dBm]
-1 8
-2 0
Field strength [dBV/m]
1
0
-1 0
-2 0
Ex
Ez
-2 2
-3 0
0
2000
4000
6000
8000
S am p le
-2 4
50
100
150
Figure 11: Field strength as function of time or stirring
ratio, for 150 MHz
200
F re q u e n c y [M H z ]
10
70
60
50
V IR C
V IR C
V IR C
E to tal
40
Ex 3
Ey 3
Ez 3
3
Field strength [dBV/m]
Figure 8: Net power for VIRC position 3
80
Field strength [dBuV/m]
Ey
E tot
30
0
-10
-20
Ex
Ey
Ez
Etot
-30
50
10 0
150
2 00
0
2000
F re qu en c y [M H z ]
4000
6000
8000
Sample
Figure 12: Field strength as function of time, 400 MHz
Figure 9: Electric field strength for VIRC position 3
10
The stirring ratio has been obtained by measuring the field
strength as a function of the time at a fixed frequency. In the
measurement period the VIRC was randomly excited by three
independent persons creating an amplitude variation of at least
10% in width, heigth and length of the VIRC. The test setup
has been drawn in Figure 10.
The electromagnetic field inside the VIRC was generated by a
R&S SMS-2 generator, amplified by a Kalmus amplifier and
injected by the log-spiral antenna. The power level was
adjusted to 30 dBm incident power. The field strength inside
the VIRC has been measured using the Thomson-CSF ET2003
three-dimensional E-field sensor. The output signal of the
sensor was fed via an optical fiber to the optical receiver. The
output voltage levels, for every axis (x,y,z) one channel, was
detected by a logarithmic detector. This detector makes use of
an AD 8313 integrated circuit, which has a bandwidth of
2.5 GHz, a dynamic range of 70 dB, an amplitude flatness of
±1 dB, a noise level of less than –73 dBm and a response time
of 40 ns.
The stirring ratio, or the field strength as function of the time,
for 150 MHz, 400 MHz and 1 GHz has been drawn in Figures
Field strength [dBV/m]
Stirring ratio
0
-10
-20
Ex
Ez
Ey
E to t
-30
0
2000
4000
6000
8000
Sam ple
Figure 13: Field strength as function of time, 1 GHz
Probability density function
The electric field strength has been measured in the three
ortogonal directions while the boundary conditions were
changing; i.e. the VIRC was vibrating. The movements were
random: three persons were independently moving the VIRC.
The test set-up was as drawn in Figure 10. Measurements have
been performed using a sample time of 1 ms for a signal
frequency of 150 MHz decreasing to 0.1 ms for a signal
frequency of 1 GHz. For every signal frequency (150 MHz,
500
450
SMS-2
M ea sured , V IR C , M P 1 , 30 0 M H z
Number of Samples
400
amplifier
dir. coupler
splitter
T h e o re ti c a l P D F , C h i (6 d o f)
350
300
250
200
150
100
50
optical
0
0
0 .5
1
1 .5
2
2 .5
3
3 .5
4
F ie ld S tr e n g th [V /m ]
coaxial
5-channel
logarithmic
detector
Figure 16: PDF for 300 MHz
BR2000-3
optical receiver
400
350
AR1100 5-ch.
analysing
recorder
T h e o r e ti c a l P D F , C h i( 6 d o f )
250
200
150
100
Figure 10: Test set-up for measuring the field strength
inside the VIRC as function of change in boundary
conditions
200 MHz, 400 MHz, 600 MHz, 800 MHz and 1 GHz) 4000
samples have been recorded. The samples were recorded
simultaneously for the incident and reflected power and the
field strength for every axis, giving a total of 5 channels with
4000 samples. To obtain the probability density function the
measured absolute value of the electric field strengths in the
three directions were normalized. The samples of the total
electric field strength have been sorted from the minimum
field strength to the maximum field strength. This yields the
experimental probability density function (PDF). The PDFs
are shown for the frequencies 150 MHz, 200 MHz, 300 MHz,
400 MHz, 600 MHz, 800 MHz and 1 GHz in Figures 14
through 20.
50
0
0
0 .5
1
1 .5
2
2 .5
3
3 .5
4
F ie ld S tre n gth [ V /m ]
Figure 17: PDF for 400 MHz
350
300
M e a s u re d , V I R C , M P 1 , 6 0 0 M H z
Number of Samples
T h e o r e ti c a l P D F , C h i( 6 d o f )
250
200
150
100
50
0
0
0 .5
1
1 .5
2
2 .5
3
3 .5
4
F ie ld S tre n gth [ V /m ]
Figure 18: PDF for 600 MHz
300
250
250
M ea sured , V IR C , M P 1 , 1 5 0M H z
T h e o r e ti c a l P D F , C h i ( 6 d o f )
M ea sured , 8 0 0M H z, V IR C 1
T h e o r e ti c a l P D F , C h i ( 6 d o f )
200
Number of Samples
Number of Samples
M e a s u re d , V I R C , M P 1 , 4 0 0 M H z
300
Number of Samples
power
meter
200
150
100
150
100
50
50
0
0
0 .5
1
1 .5
2
2 .5
3
0
3 .5
0
0 .5
F ie ld S tre n g th [ V /m ]
1
1 .5
2
2 .5
F ie ld S tre n g th [V /m ]
Figure 14: PDF for 150 MHz
Figure 19: PDF for 800 MHz
200
400
180
M easu red , 1G H z , V IR C 1
T h e o r e t ic a l P D F , C h i (6 d o f)
T h e o r e ti c a l P D F , C h i( 6 d o f )
300
140
Number of Samples
Number of Samples
350
M e asure d, V IR C , M P 1 , 2 0 0M H z
160
120
100
80
60
250
200
150
40
100
20
50
0
0
0
0 .5
1
1 .5
2
2 .5
F ie ld S tre n gth [ V /m ]
Figure 15: PDF for 200 MHz
3
3 .5
0
0 .5
1
F ie ld S tre n g th [ V /m ]
Figure 20: PDF for 1 GHz
1 .5
The theory of the stochastic behaviour of reverberation
chambers is described in detail in [4]. In [4, 9] it is shown that
the magnitude of the electric field strength in any direction is
Χ (Chi) distributed with two degrees of freedom, or Rayleigh
distributed. This is based on the assumption that there is no
direct path between the transmitting antenna and receiving
sensor. If there is a direct path between the antenna and sensor
then the Rayleigh distribution has to be modified resulting in
the Rayleigh-Rice distribution. This distribution results in a
longer tail for the total electric field strength distribution.
From the comparison between measured and theoretical PDF
in figures 14 to 20 we can observe that the measured PDF has
a longer tail than the theoretical PDF. Because the dimension
of the VIRC was small with respect to the transmitting
antenna, the longer tail could be caused by a direct
transmission path between transmitting antenna and field
strength sensor.
Cumulative probability density function
The cumulative probability density function (CDF) for the
measurement frequencies 150 MHz, 200 MHz, 300 MHz, 400
MHz, 600 MHz, 800 MHz and 1 GHz are shown in Figure 21.
To estimate the quality of the chamber the experimental data
has been compared to the theoretical curve. A similar
measurement has been carried out in a MSC. The CDFs are
drawn in Figure 22.
The CDF of 200 MHz deviates considerably while the CDF of
150 MHz is nearly equal to the theoretical curve.
100%
200 MHz
Cumulative Probability
80%
Theoretical CDF
CHI (6DOF)
VIRC, MP1, 150MHz
60%
Conclusion
A new type of reverberation chamber to create a spatial
uniform and isotropic electromagnetic field is the Vibrating
Intrinsic Reverberation Chamber (VIRC).
Several measurements have been performed within the VIRC.
It was shown that the VIRC can even change the lowest
resonance frequency by more than 10 MHz, whereas the
conventional mode stirred chamber (MSC) resonance
frequency changes only a few MHz. The stirring ratio was
more than 40 dB for low frequencies, which proves that the
VIRC is capable to change the boundary conditions
adequately.
The field strength at several frequencies has been measured as
function of the time and the samples of the total electric field
strength have been sorted from the minimum field strength to
the maximum field strength, which yields the experimental
probability density function. The corresponding cumulative
probability density function (CDF) has been compared to the
theoretical CDF and shows good resemblance.
VIRC, MP1, 200MHz
1 GHz
References
VIRC, MP1, 300MHz
40%
[1]
VIRC, MP1, 400MHz
VIRC, MP1, 600MHz
20%
VIRC, MP1, 800MHz
150 MHz
VIRC, MP1, 1000MHz
0%
0
0.5
1
1.5
2
Field Strength w.r.t. Mean
Figure 21: CDF for the VIRC
100%
Cumulative Probability
Furthermore the CDFs for higher frequencies are lower than
the theoretical CDF, for both the VIRC and the MSC. This
could be caused by the direct ray path and corresponding
Rayleigh-Rice distribution as described in the previous
paragraph .
A comparison of the VIRC and MSC CDFs, as shown in
Figure 21 and 22, shows that the VIRC CDFs are approaching
the theoretical CDF better than the MSC CDF. Therefore we
can conclude that an estimation of the field strength in the
VIRC is much more reliable than an estimation of the field
strength in this MSC. We have to admit that the MSC used for
the experiments is not optimized as reverberation chamber,
and a properly operated MSC should result in a better CDF.
200 MHz
80%
1 GHz
Theory, Chi(6dof)
MSC 1, 200MHz
MSC 1, 400MHz
MSC 1, 600MHz
MSC 1, 800MHz
MSC 1, 1000MHz
60%
40%
Theoretical CDF
20%
0%
0
0.5
1
Field Strength w.r.t. Mean
Figure 22: CDF for a MSC
1.5
2
P. Corona, G. Latmiral, E. Paolini, L. Piccioli, Use of a reverberating
enclosure for measurements of radiated power in the microwave range,
IEEE Transactions on Electromagnetic Compatibility, vol.EMC-18,
no.2, May 1976, p.54-9
[2] M.L. Crawford, G.H. Koepke, Operational Considerations of a
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[3] M.L. Crawford, G.H. Koepke, Design, Evaluation and Use of a
Reverberation
Chamber
for
Performing
Electromagnetic
Susceptibility/Vulnerability Measurements, NBS Technical Note 1092,
April 1986.
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Chamber, IEEE Transactions on Electromagnetic Compatibility, 2000
[7] Mélopée-ET2003, 3-axis Electric Field Sensor, Thomson-CSF
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