Reverberation (Mode-Stirred) Chambers for Electromagnetic Compatibility
J F Dawson, University of York, UK; M O Hatfield, Naval Surface Warfare Center, USA
L Arnaut, National Physical Laboratory, UK; N Eulig, Institut für EMV, Braunschweig, Germany
Fig 1: Car under test in a reverberation chamber – the stirrer can be seen along with a range of antennas for different frequencies.
This article intended to serve as an introduction to Reverberation Chambers and their applications in the field of Electromagnetic Compatibility (EMC). I hope it is a useful resource to both practising EMC engineers considering whether to use reverberation chambers, and practising researchers. I am assuming that readers already have a basic knowledge of EMC and its associated test and measurement standards.
Introduction to reverberation (stirred mode) chambers
Benefits of reverberation chambers
Measurements above 1 GHz
Whilst a range of EMC measurement techniques exist for use in the frequency range up to 1GHz, new measurement techniques are required above 1GHz. The reverberation chamber is one such technique – emission testing at frequencies above 1 GHz is described in more detail in [Rowell 2000]. A reverberation chamber can be used below 1GHz too.
Fig 2: Radiation patterns for small and large, dipole and loop radiators
At low frequencies, when an equipment under test (EUT) is small compared with the wavelength, it tends to radiate power in a broad, dipole pattern. It is relatively easy to find the direction of maximum radiation (emissions measurement). As frequency increases and the equipment under test becomes comparable in size with the wavelength, the radiation pattern becomes more irregular. By rotating the object and scanning the antenna height it is feasible to find the maximum radiation with an OATS or anechoic chamber measurement.
Fig 3: Radiation pattern for an electrically large object (3D view)
When the object is several wavelengths in extent its radiation pattern can be very irregular and determining the direction and magnitude of the maximum radiation then becomes very difficult using open-area test-site (OATS) or anechoic chamber measurements. A reverberation chamber measures the total radiated power, regardless of the radiation pattern.
In the case of immunity measurements a similar argument can be made, on account of reciprocity. An EUT with a complex radiation pattern will have a similarly complex pattern in reception. In order to find the most sensitive direction it would have to be rotated to a large number of positions about two axes. A reverberation chamber provides illumination from all directions and so must find the most sensitive direction without the need to rotate the EUT.
The reverberation chamber can be used for:
· Radiated emissions
· Radiated Immunity
· Enclosure Shielding
· Cable shielding
· Absorber characterization
· A screened environment – no ambient signals
· Wide frequency range
· Measures total radiated power (emissions)
· Illuminates with all directions and polarisations (immunity)
· Large working volume in chamber
· Needs measurement averaged over many stirrer positions
· Not in many current standards
· No simple method to compare emissions/immunity measurements with other techniques
What is a reverberation chamber ?
Fig 4: A typical reverberation chamber facility
An electromagnetic reverberation chamber is an electrically large, highly conductive closed cavity or chamber used to perform Electromagnetic (EM) measurements (both emissions and immunity) on electronic equipment. Any facility that fits this description can be considered a reverberation chamber (also called a mode-stirred chamber).
Fig 5:The Stirrer in one of QinetiQ’s reverberation chambers at Farnborough
A rotating paddle or other means of altering the geometry of the room is almost always used in a reverberation chamber for “mode-stirring” or “mode-tuning”. Sometimes a frequency-modulated source is used to achieve “electronic mode-stirring”.
Principles of operation
A closed cavity has many propagating modes which form 3-dimensional standing wave patterns with a large number of resonant modes. This gives rise to regions where the field is small and others where it is large. Typical variations are of the order of 40 dB, making the perceived test field very strongly dependent on the exact locations inside the cavity At sufficiently high frequencies, the coupling between an equipment and an antenna varies rapidly with position and frequency.
Fig 6: The standing wave pattern (E-field magnitude)for a TEX45 mode in a rectangular chamber.
When a large number of modes are present (as in a reverberation chamber) the field pattern becomes highly detailed (although regular), but there are still large and rapid variations in field with position and frequency (Fig. 7). The mode stirrer or tuner alters the boundary conditions, thus moving the position of the maxima and minima of the field magnitude (Fig. 8).
Fig 7: Field distribution with 4 modes present
Fig 8: Field distribution with 4 modes after a change in boundary conditions (e.g. the movement of a stirrer in a chamber) which changes the relative phase of the modes.
The resonant frequencies (modal resonance frequencies) of a rectangular cavity (in hertz) are given by:
where a, b, and c are the dimensions of the enclosure (in metres); m, n, and p are integers, only one of which may be zero and c0 is the propagation velocity of waves in the cavity ( m/s in free-space).
Modal density and Minimum operating frequency
At frequencies above approximately 3 times the first (lowest) resonance frequency (where 0 is the value corresponds to the index of the smallest chamber dimension) more than 60 propagating modes are possible – this is used as a guide to obtain the lowest frequency of operation of a stirred-mode measurement chamber. The actual lowest usable frequency will depend on many factors, including what the user considers an acceptable performance or whether it meets the requirements (field uncertainty) of a particular standard. The number of possible resonant modes below the frequency f is given approximately by:
Fig 9 : Number of modes possible in a room of dimensions 2.3´2.3´4.5m
Stirring and Field uniformity
When a large number of modes exist (typically >60) and measurements are averaged over a number of stirrer positions, the field in a reverberation chamber is, on average, uniform throughout much of the volume (the field uniformity deteriorates near the walls and any other conducting structure). . It has equal energy propagating in every direction and polarisation. This is described in detail by Kostas and Boverie [Kostas 1991].
Fig 10: The field uniformity depends on the number of statistically independent stirrer (tuner) positions over which an average is taken – more positions are needed at lower frequencies to obtain a given degree of uniformity.
Mode tuned operation
In mode tuned operation the field or power is measured at different discrete frequencies for discrete positions of the tuner (Note: IEC 61000-4-21-B recommends stepping the frequency after stepping the tuner) . The tuner is moved to the next position and the measurement is repeated. The set of measurements are then averaged over all stirrer positions for each measurement frequency.
Mode stirred operation
In mode-stirred operation the stirrer rotates continuously, at constant speed, and a set of measurements are taken and averaged at each frequency, before moving on to a new frequency.
Quality factor is used to describe the ability of a chamber or cavity to store energy. The ability of a chamber to store energy is determined by the losses present in the chamber. The dominant loss in an empty chamber is due to the chamber walls. Additional losses due to items such as antennas, apertures, support structures, and the Equipment Under Test (EUT) also can affect the overall Q of the chamber. The Q-factor described here is often referred to as a composite or average Q-factor as it considers the composite (average) effect of a large number of modes.
At sufficiently high frequencies, the Q-factor of a reverberation chamber is approximately:
where V is the volume of the enclosure, S is the surface area of the internal walls, m is the permeability of the walls, d is the skin depth of the walls:
where s is the conductivity and is the permeability of the walls.
The Q-factor can be measured if the input power to the room and average field are measured:
where :V is the chamber volume, is the wavelength, (<PAveRec /Pinput>) is the ratio of the average received power to the input power over one complete tuner/stirrer sequence, and hTx and hRx are the antenna efficiency factors for the Transmit (Tx) and Receive (Rx) antennas, respectively. (If manufacturer’s data is not available then the efficiency is approximately 0.75 for log periodic antennas and 0.9 for horn antennas.)
The Q-factor determines the field strength in the chamber for a given level of radiated power. This determines:
· The sensitivity of the chamber for emissions measurements;
· The maximum field strength that can be achieved for a given amplifier in immunity tests.
The time constant, , of the chamber at sufficiently high frequencies is, on average:
The time constant determines the decay time of the field when a pulsed source is used. It is sometimes necessary to reduce the Q-factor of a chamber by adding additional radio absorptive material (RAM) to reduce the chamber time constant when pulsed waveforms are used, in order to allow the field to decay between pulses.
Fig 11: York University chamber: Q-factors: Unloaded Theoretical, Unloaded Measured, and when loaded with RAM
Although the Q-factor of a chamber can be calculated theoretically, measured Q values are usually around an order of magnitude lower (around 1/50 at low frequencies to about 1/5 at high frequencies). This is partly due to the imperfections in the construction of the room.
If the Q factor is reduced too far by the presence of loss in the chamber then the field uniformity may be adversely affected. For CW operation, the minimum Q is determined by the operating frequency, as the excitation field needs to be sufficiently rapidly ‘refreshed’ relative to the rate of decay of energy inside the chamber. Thus, the minimum permissible Q is lower at high frequencies than at low frequencies.
This is a very brief summary of the available commercial standards.
· Intended for 500 MHz to 2 GHz, with planned expansion for 400 MHz to 11 GHz.
· Requires the use of mode-stirring technique.
n Gives typical rotation rates as 3-6 rpm.
· Recommends minimum size room 16’ x 12’ x 10’.
· Gives examples of tuner design.
· Performance based specification:
n Chamber must demonstrate field uniformity as a function of frequency.
n Uniformity is based on measurements using a field probe (total) and only two locations.
· Similar to GM-9120P.
· Currently being revised.
RTCA DO-160D(Change Notice 1 Nov 2000)
· Intended for 100 MHz to 18 GHz.
· Allows the use of mode-tuned or mode-stirred techniques.
n Optimized number of tuner steps for mode-tuned operation.
· Recommends minimum operating frequency based on 3 to 4 times first chamber resonance.
· Recommends method for determining tuner effectiveness via correlation coefficient.
· Performance based specification: requires field uniformity map at nine locations within the chamber using e-field probes. Probes must allow access to the rectangular components. Field uniformity must be within limits given in terms of standard deviation.
· Uses total field (root of sum of squares) to determine field levels.
· Follows closely the procedures of the proposed RTCA DO-160:
n Uses 9 locations to map the chamber uniformity.
n Field uniformity is given in terms of standard deviation.
n Uses total field (RSS) to determine field levels.
· Some differences:
n ED90 allows use of probes on the walls for reference, DO-160 does not.
n ED90 calls for a “check” of field uniformity when loading is present, DO-160 checks for loading effects during the initial chamber calibration.
n ED90 allows stirring. Must test to upset or ensure that DUT is exposed to peak level for at least 1 second.
n ED90 requires testing for windowing effects when using tuning.
EUROCAE WG 14/33 (Modified ED90)
This new standard is currently under development. It contains a wealth of information on setting up, testing, and using a reverberation chamber for Radiated Emissions, Radiated Immunity, and Shielding Effectiveness (Enclosures, Materials, Gaskets, Cables, Connectors etc.).
· Intended to be a comprehensive document covering reverberation chambers.
n Covers chamber calibration, immunity, emissions, and shielding effectiveness measurements.
n Currently being developed by a Joint Task Force (JTF) between SC77B and CISPR/A.
· At present follows closely the procedures of RTCA DO-160D Change Notice 1 - procedure for radiated immunity.
· Allows for Mode-tuned and Mode-stirred procedures.
n Mode-tuned procedures are emphasised for chamber calibration, radiated immunity and radiated emissions testing.
n Mode-stirred procedures are emphasised for shielding effectiveness testing.
This is a very brief summary of the available military standard.
MIL-STD-461E (issued 20 August 1999)
· Allows for mode-tuned operation only
· “Rule of thumb” based specification:
nRequires chamber to have 100 modes as lowest operating frequency.
ntuner performance based on correlation coefficient.
· Allows the procedures of RTCA DO-160 to be used for determining lowest useable frequency.
nUses 9 locations to map the chamber uniformity.
nField uniformity is given in terms of standard deviation.
· Allows for using both probes and antennas for determining the fields in the chamber.
· Uses rectangular field component to determine field levels.
Building a Reverberation chamber
Dimensions – what size and shape should it be
Room sized reverberation chambers (e.g., volumes of between 75 to 100 m3) are typically operated above 200 MHz . Operations below 200 MHz require chambers that are larger than the typical shielded room. Operation above 1 GHz allows for smaller chambers (cavities) to be used (e.g. the NPL stadium chamber Fig. 13).
The shape of a reverberation chamber is by and large unimportant – very different shapes have shown to perform equally well. Instead, the volume of the chamber is the key factor for satisfactory performance. When choosing a rectangular room as a basis for building a reverberation chamber, ideally the dimensions should not be simple multiples or rational fractions of each other – this gives the largest number of modes with different resonance frequencies, and in principle improves the room performance, particularly at lower frequencies.
Table 1: Volumes of different chambers and the associated LUFs
The above table gives the approximate values of lowest useable frequency (LUF) against volume for a range of chamber sizes. The frequency of the first resonance (f110) is also given.
Fig 12: View inside the Institut für EMV,Braunschweig reverberation chamber (11m x 7.6m x 7.1m, V=594m3)
Fig 13: NPL Stadium Reverberation chamber #1 (dia 70 cm, V=0.24m3) for high frequency (>1GHz) use.
Stirrer design - what size and shape, how to drive etc.
There seems to be very little published literature on the design of a suitable stirrer.
At the University of York we have carried out some tests using numerical electromagnetic models and measurements to determine the effectiveness of different stirrers. We compared the field uniformity in the room with different sized stirrers.
Fig 14: Field uniformity with different size simple stirrers, measured at 8 points in the room, compared with that required by IEC 61000-4-21. The stirrer height and diameter are shown on the graph. One polarisation only is shown.
Fig 15: The simple stirrer consists of four equal sized flat plates which rotate about the vertical axis.
We then took the 2x1.2m stirrer size and allowed each plate to bend in the centre with adjustable angles. We optimised the stirrer by varying the angles using a Genetic Algorithm.
Fig 16: The Bent Plate stirrer: TLM model and in the chamber at York
Fig 17 : Field uniformity with optimal bent plate stirrer, measured at 8 points in the room, compared with that required by IEC 61000-4-21. All 3 polarisations shown.
The best bent-plate stirrer had a better performance than the flat-plate stirrer over much of the frequency range. The field uniformity improvement is not great, however bending the plates gives better scattering of energy between different polarisations than with a simple flat plate stirrer. This is highly desirable.
Fig 18: NPL Rectangular reverberation chamber #2 (6.55m x 5.85m x 3.50m, V=134m3) showing the stirrer
Driving the stirrer
If mode tuned operation is required it must be possible to position the stirrer accurately in a number of different positions whilst a swept frequency measurement is taken. Often this is achieved by the use of a stepper motor with a suitable drive and controller.
If mode-stirred operation is required it must be possible to rotate the stirrer at a controlled speed.
In either case it is desirable to ensure that the motor and drive do not couple electromagnetically to the inside of the chamber.
Useful information on the mechanical design of a stirrer can be found in Stress Analyses of a Tuner for an Electromagnetic Reverberation Chamber, by Frank Weeks of the Australian Department of Defence (http://www.dsto.defence.gov.au/corporate/reports/DSTO-TN-0272.pdf).
Since a reverberation chamber may be used to perform EMC measurements over a large frequency range (typically 200 MHz to 18 GHz), the equipment required depends greatly on your measurement requirements. The aim of the following notes is to indicate the similarities and differences in equipment requirement compared other EMC test systems.
Signal generators and amplifiers
The signal generators and power amplifiers used for immunity testing in (semi-) anechoic chambers are likely to be more than adequate for use in a reverberation chamber if they extend to the required frequency range. However high reflections are often present in reverberation chamber tests and an amplifier with output stage protection is likely to be required.
Fig 19: Field strength in a 80m3 chamber with 10W input power and 75% antenna efficiency for a range of Q-factors (A loaded Q-factor of 1000 is a reasonable value).
The exact fields that can be achieved in a reverberation chamber depend on both the amplifier power and Q-factor of the room when fully loaded with the test object and any other equipment. The average electric field strength is:
For comparison an amplifier of 10W, in a typical anechoic chamber, can generate a field of 10V/m at 3m using a suitable antenna above 100 MHz (100W for 10m). Greater power is often required below 100 MHz depending on the antenna used.
The receiving equipment used for open area test-site and/or (semi-) anechoic chamber measurements is likely to be suitable for use in a reverberation chamber if it extends to the required frequency range.
Antennas and field probes
The antennas used for other EMC test purposes are likely to be suitable for use in a reverberation chamber of they cover the desired frequency range. Horn antennas are typically used at higher frequencies though antenna gain, and pattern are not important in a reverberation chamber.
How much does it cost – Courtesy of NPL
The NPL chamber (6.55m x 5.85m x 3.50m with a lowest usable frequency of 124 MHz ~ 3.6 fs) costs (1999) were approximately:
For the following specification:
· 2 access doors (1 single, 1 double); 2 access panels (1m x 0.3m, 0.3m x 0.3m)
· walls: spangle-galvanized mild stainless-steel
· (2.44m x 1.22m x 0.5mm modular panels)
· trunking and mesh (electrical protection)
· alu hat&batten clamps + fixings, alu inner door skins
Modelling Reverberation Chambers
Modelling the operation of a reverberation chamber is of interest because it allows the rapid comparison of different scenarios, and the determination of much greater information about the field structure than is easily possible with measurements. However modelling electrically large, high-Q structures, such as reverberation chambers, is not a trivial exercise. For this sort of model the Finite Difference Time-Domain (FDTD) and Transmission line matrix (TLM) methods of numerical electromagnetic modelling are more suitable than integral equation or finite element techniques. These methods allow the low frequency behaviour of reverberation chambers to be modelled. Ray tracing methods have also been used and are suitable for modelling the high frequency behaviour of reverberation chambers.
TLM modelling of the York Reverberation chamber
At the University of York we have undertaken some modelling of reverberation chambers to allow us to design/optimise the stirrer for our 4.7m x 3m x 2.37m chamber. We used the TLM method with a 5cm discretisation and were able to gain useful results up to a frequency of 1200MHz, over 50 stirrer positions in about four and a half days run time on a 450MHz Pentium PC. This is described in the section on stirrer design.
Reverberation Chambers Around the World
Here is a selection of reveberation chambers around the world.
In the UK
De Montfort University, http://www.eng.dmu.ac.uk/aeg/
QinetiQ, Farnborough, http://www.qinetiq.com/
MIRA, Nuneaton, http://www.mira.co.uk/
National Physical Laboratory, Teddington, http://www.npl.co.uk/
University of Warwick, Warwick Manufacturing Group, http://www.wmg.org.uk/
University of York, Applied Electromagnetics Research Group, http://www.emc.york.ac.uk/
TRW Conekt (TRW Automotive), Solihull, http://www.trwauto.com/productsandtechnologies/main/0,1247,9_32_88_222_279%5E5%5E279%5E279,FF.html
MBDA Filton, Bristol, http://www.mbda.net/
Istituto Universitario Navale, Naples
Institut für EMV,Braunschweig, Germany, http://loretta.emv.ing.tu-bs.de/institut/forschung/index.shtml
National Bureau of Standards (NBS), Boulder, Colorado, http://www.nist.gov/
Dahlgren, Navel Surface Warfare Center, http://www.nswcdc.navy.mil/
NASA Langley Research Center (photo here: http://ece.okstate.edu/cbunting/Reverberation.htm , and several publications available here: http://museum.nist.gov/exhibits/timeline/printerFriendly.cfm?itemId=23)
Resources on the Web
This section includes a range of other resources on the web that are not specifically mentioned elsewhere.
IEE Reverberation Chamber Web Site
The information in this article is also published on the IEE web site (www.iee.org) at http://www.iee.org/OnComms/PN/emc/Reverberation/Index.cfm. The web site will be updated periodically in response to any feedback and developments in standards.
Reports from the National Physical Laboratory (NPL)
The NPL publications website at http://www.npl.co.uk/npl/publications/ contains much useful material including:
Arnaut, L R and West, P D: Evaluation of the NPL Untuned Stadium Reverberation Chamber Using Mechanical and Electronic Stirring Techniques; NPL Report CEM 11 (Aug 1998), pp 1-176. (http://libsvr.npl.co.uk/npl_web/pdf/cem11.pdf)
Arnaut, L R: Ensemble decimation factors for reverberation chamber stirrer data; NPL Report CETM 12 (Aug 1999), pp 1-12. (http://libsvr.npl.co.uk/npl_web/pdf/CETM12.pdf).
Arnaut, L R: Electric Field Probe Measurements in the NPL Untuned Stadium Reverberation Chamber; NPL Report CETM 13 (Sep 1999), pp 1-304. (http://libsvr.npl.co.uk/npl_web/pdf/cetm13.pdf)
Arnaut, L R: Uncertainty Reduction and Decorrelation of Mode-Stirred Reverberation Chamber Data Using Transformation and Expansion Techniques; NPL Report CETM 21 (Jun 2000), pp 1-15. (http://publications.npl.co.uk/npl_web/pdf/cetm21.pdf)
EMC & Compliance Journal and European Compliance Club
This site (http://www.compliance-club.com/) has a range of EMC related articles and links to manufacturers sites. The following articles discuss EMC testing in general with a short mention of the role of reverberation chambers.
Keith Armstrong and Tim Williams, EMC testing, Part 1 - Radiated Emissions, http://www.compliance-club.com/archive1/010327.htm
Keith Armstrong and Tim Williams, EMC testing, Part 4 - Radiated immunity, http://www.compliance-club.com/archive1/010822.htm
Warwick Manufacturing group
A good overview of EMC test techniques can be found at: http://www.warwick.ac.uk/atc/electronics/emc/immu_pg.html
These pages have been produced from information published by many practising users and researchers of reverberation chambers whose contributions are gratefully acknowledged. In particular contributions of data and photographs from J Clegg, University of York, and photograph from Richard Hoad and Nigel Carter, QinetiQ, Farnborough, UK
This bibliography is by no means comprehensive, but aims to provide references to papers on a good range of topics related to mode-stirred chambers.
[Arnaut 2001] L. R. Arnaut, Operation of electromagnetic reverberation chambers and wave diffractors at relatively low frequencies; IEEE Transactions on Electromagnetic Compatibility, 43(4): 637-653, November 2001.
[Arnaut 2002] L. R. Arnaut, Compound exponential distributions for undermoded reverberation chambers; IEEE Transactions on Electromagnetic Compatibility, 44(3): 442-457, August 2002.
[Arnaut 2003] L. R. Arnaut, Statistics of the quality factor of a rectangular reverberation chamber, IEEE Transactions on Electromagnetic Compatibility, 45(1): 61-76, February 2003.
[Corona 2002] P. Corona, J. Ladbury, and G. Latmiral., Reverberation-chamber research-then and now: a review of early work and comparison with current understanding, IEEE Transactions on Electromagnetic Compatibility, 44(1):87-94, February 2002.
[Crawford 1986] M. L. Crawford and G. H. Koepke, Design, evaluation and use of a reverberation chamber for performing electromagnetic susceptibility/vulnerability measurements, Tech. Note 1092, National Bureau of Standards, 1986.
[Hill 2002] P. Hill, D. A. Corona and Ladbury, Spatial-correlation functions of fields and energy density in a reverberation chamber, IEEE Transactions on Electromagnetic Compatibility, 44(1):95-101, February 2002.
[IEC 2000] International Electrotechnical Commission, 61000-4-21 ELECTROMAGNETIC COMPATIBILITY (EMC), Part 4:Testing and Measurement Techniques, Section 21: Reverberation Chamber Test Methods
[Kostas 1991] J. G. Kostas and B Boverie, Statistical model for a mode-stirred chamber, IEEE Transactions on Electromagnetic Compatibility, 33(4):229-238, 1991.
[Ladbury 1999] J. H. Ladbury and G. H. Koepke, Reverberation chamber relationships: corrections and improvements or three wrongs can (almost) make a right, IEEE International Symposium on Electromagnetic Compatibility, 1-6, Seattle, Aug 1999.
[Ladbury 2000] J.M. Ladbury and K. Goldsmith, Reverberation chamber verification procedures, or, how to check if your chamber ain’t broke and suggestions on now to fix it if it is, IEEE International Symposium on Electromagnetic Compatibility, volume 1, pages 17-22, 2000.
[Ma 1983] M. Ma B. Liu, D. Chang, Eigenmodes and the composite quality factor of a reverberating chamber, Tech. Note 1066, National Bureau of Standards, 1983.
[Petirsch 1998] M. Petirsch and A. Schwab, Improving a mode-stirred chamber utilizing acoustic diffusers, IEEE International Symposium on Electromagnetic Compatibility, pages 39-43, Denver, CO, USA, 23-28 August 1998
[Rowell 2000] A. J. Rowell, D. W. Welsh, and A. D. Papatsoris, Practical limits for EMC emission testing at frequencies above 1 GHz Final report (AY3601) for the radiocommunications agency, York EMC Services Ltd., Univ. of York, Heslington, York, U.K., 2000, (http://www.yorkemc.co.uk/Research/Above_1GHz/Summary.htm)