Anechoic Chambers, Past And Present

Anechoic Chambers

4/11/05

11:08 AM

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Feature Article
Brian F. Lawrence

Anechoic Chambers,
Past And Present
his article will review the historical
development of absorber materials
and anechoic chambers, which play
an important role in the work of today’s
EMC test engineer. We will discuss
early attempts to achieve correlation
between Anechoic Chambers and Open
Area Test Sites and trace the
improvements made through the years
that resulted in current industry
practice.

T

Different industries and regulatory
authorities place different priorities on
emissions in comparison with
susceptibility. If a home computer

malfunctions it can be inconvenient,
but if an automobile, or worse, an
aircraft were to malfunction it could be
disastrous. On the other hand, if a
mass-produced item must be removed
from store shelves as a result of
regulatory spot checks, this could be a
different kind of disaster — an
economic one — for the manufacturer
and retailer. Responsible companies
and independent test laboratories,
therefore, responded to the emerging
EMC Regulations by developing their
own EMC testing capabilities,
including the construction of OATS
facilities and RF Shielded chambers.
The ideal OATS, as defined in the
standards, is practically impossible to
create, although with the right location
and careful design, there are now a
number of near perfect OATS facilities
in operation. Typical problems
associated with the use of an OATS
could include; ambient RF interference,
poor grounding conditions, inclement
weather, remote locations and testing
time limited to the daylight hours. If
weather protection is provided,
dielectric reflection from wood or
plastic walls, as well as reflection from

wiring and lighting, are also of
concern. While theoretically an RF

shielded chamber could solve some of
these problems, its imperfections will
result in internal surface reflections,
cavity resonances, yielding poor site
attenuation (in the case of emissions
tests) and non-uniform field conditions
(in the case of susceptibility testing).
Lining the internal surfaces of RF
Shielded chambers with an ideal
absorbing material would have the
effect of simulating OATS conditions
within a convenient, indoor, weather
protected test chamber. An RF anechoic
chamber could become an ideal EMC
test site, useful for both emissions and
susceptibility tests, if the absorber
materials can adequately eliminate
internal surface reflections over the test
frequency range. Producing such
absorber material was the challenge
presented to the anechoic chamber
industry during the 1970’s.
Absorber materials were not new. They
had been used in anechoic chambers
for many years to create test facilities
for radar and microwave antenna
evaluation. Absorbers were typically

manufactured by impregnating
conductive carbon into a foamed plastic
medium, such as polyurethane or
polystyrene. These carbon-impregnated
materials were fashioned into tapering
wedge and pyramid shapes to provide a
suitable impedance match between free
space and the resistive absorber
medium. Balancing the carbon content
with the shape of the tapering material
provided efficient and predictable

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As international regulatory agencies
introduced RF emission and
susceptibility requirements and
standards in the 1970’s and 1980’s, the
need to make accurate EMC tests
gained increasing importance.
Regulatory standards define not only
the permitted characteristics of the
equipment under test (EUT), but also
the test procedures and the calibration
of the test equipment and test facility.
Only by addressing all of these points

can the standards foster correlation
between measurements made at
different locations and times by
different engineers using different
instrumentation. In general, standards
on the measurement of radiated
electromagnetic emissions prescribe the
use of open area test site (OATS),
while those concerned with RF
susceptibility define an RF Shielded
environment in which a uniform field
can be established. Surrounding the
susceptibility test area with an RF
shield is necessary to prevent the test,
which deliberately creates strong
radiated signals, from interfering with

communications outside of the test
area.


Anechoic Chambers

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Feature Article

absorption of RF energy from
Microwave frequencies to below 500
MHz, where the tapered length of the
absorber would be greater than one
wavelength. The lower frequency of
good absorption performance was
strongly related to the length of the
absorber (and still is, for conductive
foam pyramidal units).
Extrapolating this established
technology down to 30 MHz and below
was the basis of many early EMC
Anechoic Chambers of the 1980’s.
Pyramidal absorbers six feet, eight feet,
and even up to twelve feet long were
produced and installed in large RF
shielded chambers with mixed success.
Not only were there physical problems
in manufacturing, handling and
installing these materials, but new
methods of factory quality testing also
had to be developed in order to make
meaningful and reliable production
tests on such large pyramidal shapes
and at frequencies down to 30MHz.

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Multi-national corporations such as
IBM and Hewlett Packard were

particularly interested in taking
advantage of anechoic chambers for
their EMC Test Programs. Available
OATS facilities, often remote from
their manufacturing plants, could not
keep up with their demanding test
schedules. The RF chamber industry
responded and in1982 the first full size,

3-meter range, EMC Anechoic
Chamber was built for IBM in Boca
Raton, Florida.
This chamber’s site attenuation was
tested according to the site attenuation
methods developed for ANSI C63.4
and accepted by the FCC as modeling
open area test site performance and
suitable for testing to the FCC Part 15
Rules. The chamber was designed and
built by Ray Proof at a cost of almost
$2M (two million dollars) and required
Ray Proof’s Absorber Division to
install a 50 foot long, walk-in
waveguide to test the 8 foot long
pyramids of foam that lined the walls
and ceiling of the IBM Chamber.
More 3m range anechoic chamber
installations followed the success at
IBM, but this chamber performance
and Absorber technology did not

conveniently scale up to a 10m range
length, desirable for testing Class A
computing devices. Something
different was needed.
The next step in chamber development
came as the result of a partnership
between customers, industry, and
academia. Funding provided to the
University of Colorado at Boulder by
IBM and Ray Proof resulted in the
development of a numerical model for
absorber materials. The model used a
homogenization principle to simulate a

distribution of pyramidal shapes as a
series of layers having different
impedances. This absorber model gave
industry the capability to design and
build Absorber Materials with much
improved performance in the VHF
band.
A chamber simulation program was
also developed that imported the
material performance files from the
absorber models to predict the field
conditions that would exist in the final
chamber construction. This new tool
allowed design engineers to optimize
chamber shaping and absorber layout
to provide the desired OATS

equivalence.
In parallel, the chamber industry had to
design and install more sophisticated
and accurate test equipment able to
verify the actual performance of the
optimized absorber designs. A huge 6 ft
square coaxial line, having a 2 ft square
center conductor was installed at Ray
Proof, able to measurement the low
frequency reflectivity of absorbers in
groups of 8 units. This original Ray
Proof test system, together with an
array of more modern systems
instrumented with network analyzers is
installed at the ETS-Lindgren absorber
plant in Durant, Oklahoma.
Anechoic Chambers that could meet
both the 3m and the 10m range OATS
characteristics of site attenuation
according to such standards as ANCIC63.4 and CISPR16 became available
by 1990. However, they were monsters,
large and expensive, and outside the
economic range of the majority of
potential customers.
In 1969 the University of Tokyo
patented the use of ferrite tiles in EMC
Anechoic Chambers. Sintered ferrite
tiles, only a few millimeters thick, can
exhibit excellent absorption properties
at frequencies below 100 MHz. By the

late 1980’s many ferrite tile lined
chambers were being used in Japan as
EMC Test Sites. The great advantage of
the ferrite tile technology was that
chambers could be dramatically
reduced in size. The surrounding shield
did not have to be sized to

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Feature Article
accommodate a large volume of thick
absorber lining in addition to the active
test volume. However, ferrite was still a
very expensive material to produce,
and was even more expensive when the
tiles were packed and shipped to sites
outside Japan.
When the original ferrite tile patent

expired in the mid 1980’s, competitive
pressures reduced the cost of a ferrite
lined chamber. Again, it was IBM who
in 1986 became the first company in
the U.S.A. to install a high
performance 10m range chamber using
ferrite tile technology at their facility in
Austin, Texas.
By the 1990’s ferrite tile suitable for
EMC test chambers were being
produced by several companies in Asia,
the U.S.A. and Europe. The original
absorber numerical modeling programs
and their later derivatives had been
modified to include ferrite parameters
together with dielectric matching
layers. As a result, a new generation of
optimized, hybrid absorbers combining
the best features of ferrite and
conductive foam could be designed and
applied to EMC chambers.

Having reached this point of
development with the EMC Test
Chamber, the focus on improving site
attenuation correlation to the
Normalized Site Attenuation, NSA, of
an ideal OATS has moved on from the
absorber and chamber to the calibration

As the EMC practice has evolved, so
too has chamber test site design. For
less than a $100K investment,
companies can now own and operate a
compact 7m x 3m x 3m precompliance EMC Chamber. Facilities
like this demonstrate +/- 6dB
correlation to NSA at low frequencies
and within +/- 4dB at frequencies from
100 MHz to millimeter wave
frequencies. Slightly larger chambers
can be designed to demonstrate +/-4dB
correlation to NSA for smaller EUTs
and with reduced scanning height of
the antenna. For Engineers who are
evaluating product susceptibility and
who self certify product emissions,
such pre-compliance chambers provide
ideal indoor test site convenience.
The next step up from the precompliance chamber is the fullcompliance, 3m-range facility. Such a
chamber which cost IBM $2M in 1982
is available today in a much reduced
9m x 6m x 6m size, or even smaller,
offering exceptional performance, for
around $300K. At the higher end, base
models of 10m range anechoic
chambers start below $1M.
Emerging test requirements have lead
to chamber designs that offer
specialized or combination test
capabilities.

These include,
for example,
EMC and
wireless testing
in a 3m to 5m
range length
according to
ETSI standards
and special
chambers for
automotive test
applications
according to
CISPR-25. For
the ultimate
susceptibility
testing of
complex and
large EUTs
there are also

fully qualified, non-anechoic
reverberation chambers. Today there is
a test chamber for almost every EMC
test requirement, making the emulation
of a wide variety of test conditions
possible.
EMC test engineers can now take
chamber anechoic performance for
granted and concentrate on selecting

other chamber features and accessories.
The optimal choice of antenna
frequency ranges, antenna patterns,
equipment handling ramps, hoists,
towers, turntables, automated sliding
doors, and other accessories will
improve the ease of use, lower the cost
of ownership, and allow the chamber
user to take full advantage of the
performance that the modern chamber
can deliver.
About The Author

Brian Lawrence is the Director of
Sales & Marketing for ETS-Lindgren,
Europe. Prior to the sale of Lindgren
RF Enclosures, Inc. to ESCO
Technologies Corporation in March of
2000, Brian Lawrence was responsible
for Lindgren’s EMC Test Chamber
business worldwide. Brian Lawrence
has over 40 years experience in
Anechoic Chamber and Absorber
Material development and has worked
for Ray Proof USA and Ray Proof UK
during his career.

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Chamber simulation programs,
incorporating hybrid absorber models
have been responsible for the modern
generation of EMC Anechoic
chambers. These chambers cannot only
meet OATS standards but will beat
almost every actual OATS site in terms
of correlation to the theoretical ideal
site model across the entire test
frequency range. Typical regulatory
standards require an acceptable OATS
test site to demonstrate site attenuation
correlation to the ideal model within
+/- 4dB. Modern 10m and 3m range
chambers available from ETS-Lindgren
are guaranteed to correlate to within +/3 dB of the ideal standard, using
optimized hybrid absorber technology.

and design of the Antennas that will be
used in the chambers.


Anechoic Chambers

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