Reverberation Chambers
for EM Applications
Christopher L. Holloway
John Ladbury, Galen Koepke, and Dave Hill
National Institute of Standards and Technology
Electromagnetics Division
Boulder, Colorado
303-497-6184, email: holloway@boulder.nist.gov
OPEN AREA TEST SITES (OATS)
Problems:
Ambients
Reflections
Scanning
Interference
Positioning
TEM
Problems:
High Frequencies
Reflections
Test Volume
Positioning
GTEM
Problems:
Test Volume
Uniformity Along Cell
Positioning
ANECHOIC CHAMBER
Problems:
Low Frequencies
Reflections
Positioning
REVERBERATION CHAMBER
Why use reverberation chamber?
The Classical OATS Measurements
The classic emissions test and standard limits (i.e, testing a
product above a ground plane at a specified antenna separation
and height) have their origins in interference problems with TV
reception.
Emissions Test Standard Problem
One problem with the emissions test standard is that it is based on an interference
paradigm (interference to terrestrial broadcast TV) that is, in general, no longer
valid nor realistic today. In a recent report, the FCC indicated that 85 % of
US households receive their TV service from either cable, direct broadcast
satellite (DBS), or other multichannel video programming distribution service,
and that only a small fraction of US households receive their TV via direct
terrestrial broadcast.
Coupling to TV antennas designed to receive terrestrial broadcast may no longer
be an issue.
EMC Environment Today
•
In recent years, a proliferation of communication devices that are subject to
interference have been introduced into the marketplace.
•
Today, cell phones and pagers are used in confined offices containing personal
computers (PCs). Many different products containing microprocessors (e.g.,
TVs, VCRs, PCs, microwave ovens, cell phones, etc.) may be operating in the
same room.
•
Different electronic products may also be operating within metallic enclosures
(e.g., cars and airplanes). The walls, ceiling, and floor of an office, a room, a
car, or an airplane may or may not be highly conducting.
•
Hence, emissions from electric devices in these types of enclosures will likely
be quite different from emissions at an OATS. In fact, the environment may
more likely behave as either a reverberation chamber or a free space
environment.
Where Should We Test?
•
Thus, would it not be better to perform tests more appropriate to
today's electromagnetic environment?
•
Tests should be Shielded, Repeatable, Simple, Inexpensive, Fast,
Thorough, …
Commercial Solutions…
• Stirrer
• Turntable
Reverberation Chamber
Probe(s)
Receive Antenna
D-U-T
Transmit Antenna
Motor Control
Signal Generator
Amplifier
Directional coupler
Power Meter(s)
etc.
Control and Monitoring
Instrumentation for
Device-under-test
Receive Instrumentation:
Spectrum Analyzer
Receiver, Scopes
Probe System
etc.
Fields in a Metal Box (A Shielded Room)
Frozen Food
•In a metal box, the fields have well defined modal field distributions.
Locations in the chamber with very high field values
Locations in the chamber with very low field values
Fields in a Metal Box with Small Scatterer
In a metal box, the fields have well defined modal field distributions.
Small changes in locations where very high field values occur
Small changes in locations where very low field values occur
Fields in a Metal Box with Large Scatterer (Paddle)
Large changes in locations where very high field values occur
Large changes in locations where very low field values occur
In fact, after one fan rotation, all locations in the chamber will have the same
maxima and minima fields.
Stirring Method
TIME DOMAIN
Paddle
Click to play paddle
rotation
750MHz
Field Variations with Rotating Stirrer
Reverberation Chamber: All Shape and Sizes
Small Chamber
Large Chamber
Moving walls
Reverberation Chamber with Moving Wall
Original Applications
• Radiated Immunity
ƒ components
ƒ large systems
• Antenna efficiency
• Calibrate rf probes
• RF/MW Spectrograph
• Radiated Emissions
ƒ absorption properties
• Shielding
• Material heating
ƒ cables
• Biological effects
ƒ connectors
•
Conductivity and
ƒ materials
material properties
Wireless Applications
•
Radiated power of mobile phones
•
Gain obtained by using diversity antennas in fading environments
•
Antenna efficiency measurements
•
Measurements on multiple-input multiple-output (MIMO) systems
•
Emulated channel testing in Rayleigh multipath environments
•
Emulated channel testing in Rician multipath environments
•
Measurements of receiver sensitivity of mobile terminals
•
Investigating biological effects of cell-phone base-station RF
exposure
Fundamentals
• A Reverberation Chamber is an electrically large,
multi-moded, high-Q (reflective) cavity or room.
• Electromagnetic theory using mode or plane-wave
integral techniques provides several cavity field
properties (mode density, Q and losses, E2, etc.)
• Changing boundary conditions, antenna or probe
location, or frequency will introduce a new cavity
environment to measure (sample)
• The composite effects of multiple cavity
electromagnetic environments are best described
by statistical models
Sampling Considerations
• How many samples do we need?
ƒ time / budget constraints
ƒ acceptable uncertainties
ƒ standards may dictate
• Sampling rates may not be identical
ƒ equipment-under-test
ƒ instrumentation
ƒ probes
• How long to dwell at each sample?
Sampling Considerations
• How are samples generated?
ƒ change boundary conditions
ƒ change device-under-test and antenna locations
ƒ change frequency (bandwidth limits)
• Samples must be independent
ƒ Changes are ‘large enough’
• Measurement time proportional to number of
samples (time = $$)
Sampling Considerations
Techniques to Generate Samples
•
Mechanical techniques
ƒ Paddle(s) or Tuner(s)
• stepped (tuned)
• continuous (stirred)
•
•
ƒ Device-under-test and antenna position
ƒ Moving walls (conductive fabric, etc.)
Electrical techniques (immunity tests)
ƒ Frequency stirring
• stepped or swept
• random (noise modulation)
Hybrid techniques
Measurement Procedures
Calibrate and Initialize system
Generate new environment
i.e. move paddle
move antennas
change frequency, etc.
Establish Fields in Chamber
Measure response of EUT,
probes, receive antenna, etc.
<N
=N
END
EM Applications
HIDING Emission Problems
YOU CANNOT HIDE IN
Reverberation Chamber
In reverberation chambers you cannot hide emission problems.
Reverberation chambers will find problems.
Unethical Practices
• The current measurement methodologies of a product can
easily miss radiation problem of a products.
• That is, energy propagation in a direction that a received
antenna would miss.
• Not to suggest that companies would be unethical, but we
have been told the individuals have setup products for
emission measurements in such a manner to ensure that
emission problems would not be detected.
Unethical Practices
• Secondly, not to suggest companies are unethical once can,
but we have also been told that some individual have lists
of OATS around the world with their corresponding
ambient noise sources.
• Thus, if one had a product that has an emission problem at
frequency “x”, and it is known that one particular OATS at
some site in the world as a ambient noise problem at the
same frequency “x”, then a product could be tested and
possible certified on the OATS, in which the product
emission problems would not showing up.
One Possible Solution
• These two possible ways of hiding emission problems
cannot be accomplished in a reverberation chamber.
• In reverberation chambers you cannot hide emission
problems.
• Reverberation chambers will find problems.
• If reverberation chambers are to be used, new standards are
needed.
EMISSION LIMITS
•Devices and/or products are tested for emissions to ensure that electromagnetic field strengths emitted
by the device and/or product are below a maximum specified electric (E) field strength over the
frequency range of 30 MHz to 1 GHz.
•These products are tested either on an open area test site (OATS) or in a semi-anechoic chamber.
•Products are tested for either Class A (commercial electronics) or Class B (consumer electronics) limits,
Class A equipment have protection limits at 10 m, and Class B equipment have protection limits at 3 m.
6.0E-4
5.0E-4
Class A: based on 10 m separation
4.0E-4
| Emax |
(V/ m)
Class B: based on 3 m separation
3.0E-4
2.0E-4
1.0E-4
0.0E+0
0
200
400
600
Frequency (MHz)
800
1000
Total Radiated Power for Reverberation Chambers
Holloway et al., IEEE EMC Symposium
-40
Ptotal (dBm)
-45
-50
-55
Class A: based on 10 m separation
Class B: based on 3 m separation
-60
Class A: average limit
Class B: average limit
-65
0
100
200
300
400
500
600
Frequency (MHz)
700
800
900
1000
Emission Measurements
Relate total radiated power in reverberation
chambers to measurements made on OATS
with dipole correlation algorithms.
Spherical Dipole
-44
Reverb Chamber
OATS
-48
Reverb Chamber
-52
Anechoic Chamber
-52
-56
E-field [dBV/ m]
E-field (dBV/ m)
-48
-60
200
300
400
500
600
700
Frequency (Hz)
800
900
-56
1000
-60
-64
200
400
600
Frequency (Hz)
800
1000
Emissions Measurements of Devices
Total Radiated Power Comparison
Max Field Comparison
Shielding Properties of Materials
The conventional methods uses
normal incident plane-waves, i.e.,
Coaxial TEM fixtures.
E
i
In most applications, materials are exposed
to complex EM environments where fields are
incident on the material with various
polarizations and angles of incidence.
E
t
⎛P ⎞
SE = −10 Log 10 ⎜⎜ t ⎟⎟
⎝ Pi ⎠
However, these approaches determine SE
for only a very limited set of incident
wave conditions.
Therefore, a test methodology that better
represents this type of environment would
be beneficial.
Nested Reverberation Chamber
Sample
Nested Reverberation Chamber
C.L. Holloway, D. Hill, J. Ladbury, G. Koepke, and R. Garzia, “Shielding
effectiveness measurements of materials in nested reverberation chambers,”
IEEE Trans. on Electromagnetic Compatibility, vol. 45, no. 2, pp. 350-356,
May, 2003.
⎛ Pr ,in , s Pr ,o ,ns PrQ ,in ,ns Ptx ,in , s ⎞
⎟
SE3 = −10 Log10 ⎜
⎟
⎜P
P
P
P
r
,
in
,
ns
r
,
o
,
s
rQ
,
in
,
s
tx
,
in
,
ns
⎠
⎝
Sample
Nested Reverberation Chamber
80
Material 1
70
Material 2
Material 3
60
Material 4
90
Material 2: with coating
80
Material 2: with no coating
40
70
30
60
a
SE (dB)
20
50
10
90
40
a
0
80
1
30
10
Frequency (GHz)
70
Different Materials
20
60
10
SE (dB)
SE (dB)
50
1
50
10
Frequency (GHz)
Edge Effects
40
30
SE3: Chamber A
a
SE3: Chamber B
20
SE1: Chamber A
10
SE1: Chamber B
0
1
10
Frequency (GHz)
Different Chamber Sizes
Loading Effects: Rat in a Cage
Research Goal
Descritize in to FDTD
Grid
Take Real System
•
To provide a way to gain
intuition into loaded
chamber responses to
better help measurements
•
Ultimately correlate
measured and modeled
data, and use models to
predict what we cant
measure.
•
Develop a fast and efficient
numerical code to explore
multiple chamber
topologies. (FDTD)
Simulate
Predict Stirred Fields Computationally to give us insight
into chamber measurements in a loaded environment
Measurements of Fake Rats
Lossy Body Configuration
Large Lossy Body
Distributed Lossy Bodies
Periodic Lossy Bodies
Measuring Shielding for “small” Enclosures
reverb chamber
Pin
Pout
“small” enclosure
SE= -10 Log(Pin/ Pout): with frequency stirring
Problem with “small” Enclosures:
Measuring the Fields Inside
•The “rule-of thumb” for antennas positioning in a chamber is ½ wavelength
from the wall. This is because the tangential component of the
E-field is zero are the wall.
•However, Hill (IEEE Trans on EMC, 2005) has recently shown that the
statistics for the normal component of the E-field at the wall
are the same for the field in the center of the chamber.
Thus, small monopole (or loop) probes attached to the wall can be used to determine
the power in the center of a “small” enclosure.
Comparison with Different Reverberation
Chamber Approaches
port 3
port 2
port 2
port 1
port 1
port 4
Mode-Stirred with a Horn Antenna: SE => S31
Mode-Stirred with a Monopole Antenna: SE => S41
port 3
port 2
port 2
port 1
port 1
port 4
Frequency Stirring with a Horn Antenna: SE => S31
Frequency Stirring with a Monopole Antenna: SE => S41
Comparison with Different Approaches
20
20
18
18
mode_stirring_horn
16
freq_stirring_horn
14
mode_stirring_horn
16
freq_stirring_horn
14
mode_stirring_monopole
12
freq_stirring_monopole
SE (dB)
SE (dB)
mode_stirring_monopole
10
8
10
8
6
6
4
4
2
2
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
0
1000
10000
freq_stirring_monopole
12
2000
3000
4000
Frequency (MHz)
20
7000
8000
9000
10000
20
18
mode_stirring_horn
18
16
freq_stirring_horn
16
mode_stirring_monopole
14
14
12
SE (dB)
SE (dB)
6000
(b) half-filled aperture
(a) open aperture
10
8
mode_stirring_horn
6
freq_stirring_horn
4
mode_stirring_monopole
freq_stirring_monopole
12
10
8
6
4
freq_stirring_monopole
2
0
1000
5000
Frequency (MHz)
2
2000
3000
4000
5000
6000
7000
8000
9000
Frequency (MHz)
(c) narrow slot aperture
10000
0
1000
2000
3000
4000
5000
6000
7000
8000
Frequency (MHz)
(d) generic aperture
9000
10000
Different Probe Lengths and Locations
30
50
Position 1, probe length=1.3 cm
Position 2, probe length=1.3 cm
45
25
Position 1, probe length=2.5 cm
40
a=0.95 cm
35
20
SE (dB)
30
SE (dB)
probe location 1
probe location 2
probe location 3
probe location 4
Position 2, probe length=2.5 cm
a=0.5 cm
25
15
20
10
15
10
5
5
0
3000
0
4000
5000
6000
7000
8000
Frequency (MHz)
9000
10000
11000
12000
2
2.5
3
Frequency (GHz)
3.5
4
Antenna Measurement:
NIST Reverberation Chamber and Experimental
Set-up
Dual-Ridged Horn used as receive antenna
Antenna Under Test (AUT)
Two paddles used
Experimental Results to Compared to Horn Antenna
Results presented here are for relative total radiated power (RTRP)
referenced to either a Horn or to a small Loop. For a Horn we have:
PAUT
Total Radiated Power =
PHorn
where PAUT is the received power in the chamber when
transmitting on the antenna under test (AUT), and PHorn is the
received power in the chamber when transmitting on the horn.
All antennas were connected to a 50 ohm cable, so results
include both mismatch and efficiency (ohmic loss) effects.
If we assume the horn is the “best” antenna available, then these
measurements will show how “well” an antenna is performing.
Ten-Layer Spherical 612 MHz Structure
8 mm Loop for 612 MHz antenna
612 MHz antenna: loop with ten-layer spherical material
Ten-Layer Spherical 612 MHz Structure
-5
-10
20
-15
Ten-Layer Spherical Structure
-20
8 mm Loop for 612 MHz Antenna
32mm Loop Antenna
-25
500
550
600
650
700
750
800
850
Frequency (MHz)
Comparison to Horn
900
950
1000
Total Radiated Power Relative to Loops (dB)
Total Radiated Power Relative to Horn (dB)
0
Comparison to 8 mm Loop
15
Comparison to 32 mm Loop
10
5
0
-5
-10
500
550
600
650
700
750
800
850
900
Frequency (MHz)
Comparison to Loops
950
1000
Magnetic Antenna
Comparison to Horn
New Magnetic Antenna
Loop Antenna
Total Radiated Power Relative to Horn (dB)
5
0
-5
-10
-15
-20
-25
-30
-35
-40
-45
-50
200
Magnetic EZ-Antenna: Ground Plane-Center of Chamber
Magnetic EZ-Antenna: No Ground Plane-Center of Chamber
Magnetic EZ-Antenna: On Chamber Wall
3 cm Loop antenna
250
300
Frequency (MHz)
350
400
Can NOT Measurement Directivity
0
Near-Field Antenna Tests
nor-xscan w/o material
nor-xscan w metamaterial
-10
-20
P (dBm)
-30
-40
-50
-60
-70
0
20
40
60
80
100
Location
120
140
160
180
200
Multipath Environments
Multipath Environments
Extensive measurements have shown that when light of sight (LOS) path is present
the radio multipath environment is well approximated by a Ricean channel, and
when no LOS is present the channel is well approximated by a Rayleigh channel:
N
E = ALOS cos(2π f c t ) + ∑ An cos[2π ( f c + f n )t + φn ]
n =1
The Amplitude of E is either Rayleigh or Ricean depending if a LOS path is present.
Urban Environment
Rural Environment
Ricean K-factor
k-factor:
k=
direct component
scattered components
or
K = 10 Log (k )
K= - ∞ dB
(Rayleigh)
K=1 dB
K=4 dB
K=10 dB
Ricean K-factors and rms Delay Spreads
Besides chancing the K-factor, we need to vary its decay rate.
Standardization of Wireless Measurements
Can we use a reverberation chambers for a reliable and
repeatable test facilities that has the capability of simulating
any multipath environment for the testing of wireless
communications devices?
If so, such a test facility will be useful in wireless measurement
standards.
Reverberation Chambers are
Natural Multipath Environments
Typical Reverberation Chambers Set-up
Antenna pointing away from probe (DUT)
a
metallic walls
Paddle
DUT
Transmitting Antenna
a
A Rayleigh test environment
Can we generate a Ricean environment?
Chamber Set-up for Ricean Environment
Antenna pointing toward (DUT)
metallic walls
Paddle
DUT
Transmitting Antenna
We will show that by varying the characteristics of the reverberation
chamber and/or the antenna configurations in the chamber, any desired
Rician K-factor can be obtained.
Reverberation Chamber Ricean Environment
It can be shown: see Holloway et al, IEEE Trans on Antenna and Propag., 2006.
K=
3 V D
2 λ Q r2
Note:
•We see that K is proportional to D. This suggests that if an antenna with a well defined antenna
pattern is used, it can be rotated with respect to the DUT, thereby changing the K-factor.
•Secondly, we see that if r is large, K is small (approaching a Rayleigh environment);
if r is small, K is large. This suggests that if the separation distance between the antenna
and the DUT is varied, then the K-factor can also be changed to some desired value.
•Next we see that by varying Q (the chamber quality factor), the K-factor can be changed to some
desired value. The Q of the chamber can easily be varied by simply loading the chamber
with lossy materials.
Also, if K becomes small, the distribution approaches Rayleigh.
Thus, varying all these different quantities in a judicious manner can result in
controllable K-factor over a reasonably large range.
Measured K-factor for Different
Antenna Separation
1.0E+02
0.5 m
1 m separation
1.0E+01
2 m separation
K-factor
1.0E+00
1.0E-01
1.0E-02
1.0E-03
1.0E-04
1000
2000
3000
4000
5000
6000
7000
Frequency (MHz)
Each set of curves represents a different distance of separation. The thick black curve
running over each data set represents the K-factor obtained by using d determined in
the anechoic chamber.
Measured K-factor for
Chamber Loading
1.0E+03
1.0E+02
6 pcs absorber
2 pcs absorber
K-factor
0 pcs absorber
1.0E+01
1.0E+00
1.0E-01
1.0E-02
1000
2000
3000
4000
5000
6000
7000
Frequency (MHz)
The thick black curve running over each data set represents the K-factor obtained
by using d determined in the anechoic chamber.
Measured K-factor for
Different Antenna Rotations
1.0E+02
0 degrees
30 degrees
K-factor
1.0E+01
1.0E+00
90 degrees
1.0E-01
1.0E-02
1.0E-03
1000
2000
3000
4000
5000
6000
7000
Frequeency (MHz)
The thick black curve running over each data set represents the K-factor obtained
by using d determined in the anechoic chamber. Each data set was taken at 1 m separation
and with 4 pieces of absorber in the chamber.
Measured K-factor for
Different Antenna Polarizations
1.0E+03
co-polarized
1.0E+02
45 degree polarization
K-factor
1.0E+01
1.0E+00
cross-polarized
1.0E-01
1.0E-02
1.0E-03
1.0E-04
1000
2000
3000
4000
5000
6000
7000
Frequency (MHz)
The thick black curve running over each data set represents the K-factor obtained
by using d determined in the anechoic chamber. Each data set was taken at 1 m separation
and with 4 pieces of absorber in the chamber.
Simulating Propagation Environments
with Different Impulse Responses and rms Delay Spreads
S21 Measurements: Loading the Chamber
Impulse Responses and Power Delay Profiles
Power Delay Profile:
PDP(τ ) = h(t )
2
where h(t) is the Fourier transform of S21(ω)
Loading the Chamber
rms Delay Spreads
One characteristic of the PDP that has been shown to be particularly important
in wireless systems that use digital modulation is the rms delay spread of the PDP:
∞
τ rms =
∫ (t − τ
o)
2
P(t ) dt
0
∞
∫ P(t ) dt
0
where τo is the mean delay of the propagation channel, given by
∞
τo =
∫ tP(t ) dt
0
∞
∫ P(t ) dt
0
rms Delay Spreads vs Threshold Levels
Impulse Responses and rms Delay Spreads
(200 MHz band filter on S21 data)
rms Delay Spreads from Q measurements
τ rms =
Q
ω
2α ln(α ) − ln 2 (α )ε − 2α + 2 (α ln(α ) + 1 − α ) 2
−
(1 − α ) + K
((1 − α ) + K ) 2
where K is the k-factor and α threshold.
Thus, once we have Q, we can estimate τrms
Impulse Responses and rms Delay Spreads
for Different Ricean K-factors
BER Measurements - setup
Agilent 4438C Vector Signal Generator
Reverberation chamber
External trigger
Agilent 89600 Vector Signal Analyzer
GPIB connection
to control VSG
Firewire connection
to control VSA
BER Measurements
BER for a 243 ksps BPSK signal
BER for a 786 ksps BPSK signal
BER Measurements
Demodulated 768ksps BPSK signal in I-Q diagram
How Well Can we Simulate a Real Environment?
Power Delay Profile in an oil refinery.
How Well Can we Simulate a Real Environment?
BER measurement in a laboratory.
Wireless Measurements
1. Reverberation chambers represent reliable and repeatable test
facilities that have the capability of simulating any multipath
environment for the testing of wireless communications devices.
2. Such a test facility will be useful in the testing of the operation and
functionality of the new emerging wireless devices in the future.
3. Such a test facility will be useful in wireless measurements
standards.
Summary
• Reverberation chamber measurements are
thorough and robust.
• Proper sampling techniques reduce
measurement uncertainties
• Statistical models help minimize the
number of samples required
Summary
• Reverberation chambers capture radiated
power (total within the measurement
bandwidth)
• Results are insensitive to EUT placement in
the chamber
• Results are independent of EUT or antenna
radiation pattern
• Enclosed system free from external
interference
Rome’s Old Chamber
Rome’s New Chamber
Rome’s New Chamber
Reverberation Chamber Standards
• International Standard IEC 61000-4-21:
Testing and measurement techniques –
Reverberation chamber test methods