Anechoic Chambers_ Past and Present

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his article will review the historical

development of absorber materials

and anechoic chambers, which playan important role in the work of todays

EMC test engineer. We will discuss

early attempts to achieve correlationbetween Anechoic Chambers and Open

Area Test Sites and trace the

improvements made through the yearsthat resulted in current industry

practice.

As international regulatory agencies

introduced RF emission and

susceptibility requirements and

standards in the 1970s and 1980s, the

need to make accurate EMC testsgained increasing importance.

Regulatory standards define not onlythe permitted characteristics of the

equipment under test (EUT), but also

the test procedures and the calibrationof the test equipment and test facility.

Only by addressing all of these points

can the standards foster correlationbetween measurements made at

different locations and times by

different engineers using differentinstrumentation. In general, standards

on the measurement of radiated

electromagnetic emissions prescribe theuse of open area test site (OATS),while those concerned with RF

susceptibility define an RF Shielded

environment in which a uniform fieldcan 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 testarea.

Different industries and regulatoryauthorities 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 amass-produced item must be removed

from store shelves as a result of

regulatory spot checks, this could be adifferent kind of disaster an

economic one for the manufacturer

and retailer. Responsible companiesand independent test laboratories,

therefore, responded to the emerging

EMC Regulations by developing theirown 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 locationand careful design, there are now a

number of near perfect OATS facilities

in operation. Typical problems

associated with the use of an OATScould include; ambient RF interference,

poor grounding conditions, inclementweather, 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 ofthese problems, its imperfections will

result in internal surface reflections,cavity resonances, yielding poor siteattenuation (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 theeffect of simulating OATS conditions

within a convenient, indoor, weather

protected test chamber. An RF anechoicchamber could become an ideal EMC

test site, useful for both emissions and

susceptibility tests, if the absorbermaterials can adequately eliminate

internal surface reflections over the test

frequency range. Producing suchabsorber material was the challenge

presented to the anechoic chamber

industry during the 1970s.

Absorber materials were not new. They

had been used in anechoic chambers

for many years to create test facilitiesfor radar and microwave antenna

evaluation. Absorbers were typically

manufactured by impregnating

conductive carbon into a foamed plasticmedium, such as polyurethane or

polystyrene. These carbon-impregnatedmaterials were fashioned into tapering

wedge and pyramid shapes to provide a

suitable impedance match between freespace and the resistive absorber

medium. Balancing the carbon content

with the shape of the tapering material

provided efficient and predictable

Brian F. Lawrence

Anechoic Chambers,Past And Present

Anechoic Chambers 4/11/05 11:08 AM Page 1


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absorption of RF energy from

Microwave frequencies to below 500

MHz, where the tapered length of theabsorber would be greater than one

wavelength. The lower frequency of

good absorption performance was

strongly related to the length of theabsorber (and still is, for conductive

foam pyramidal units).

Extrapolating this established

technology down to 30 MHz and belowwas the basis of many early EMC

Anechoic Chambers of the 1980s.

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 problemsin manufacturing, handling and

installing these materials, but newmethods of factory quality testing alsohad to be developed in order to make

meaningful and reliable production

tests on such large pyramidal shapesand at frequencies down to 30MHz.

Multi-national corporations such asIBM and Hewlett Packard were

particularly interested in taking

advantage of anechoic chambers fortheir EMC Test Programs. Available

OATS facilities, often remote from

their manufacturing plants, could not

keep up with their demanding testschedules. 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 chambers site attenuation was

tested according to the site attenuation

methods developed for ANSI C63.4and accepted by the FCC as modeling

open area test site performance andsuitable 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 Proofs Absorber Division to

install a 50 foot long, walk-inwaveguide 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 atIBM, but this chamber performanceand Absorber technology did not

conveniently scale up to a 10m range

length, desirable for testing Class Acomputing devices. Something

different was needed.

The next step in chamber development

came as the result of a partnership

between customers, industry, andacademia. Funding provided to the

University of Colorado at Boulder by

IBM and Ray Proof resulted in the

development of a numerical model forabsorber 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 gaveindustry the capability to design and

build Absorber Materials with much

improved performance in the VHF

band.

A chamber simulation program wasalso developed that imported the

material performance files from the

absorber models to predict the fieldconditions that would exist in the final

chamber construction. This new tool

allowed design engineers to optimizechamber shaping and absorber layout

to provide the desired OATS

equivalence.

In parallel, the chamber industry had to

design and install more sophisticatedand accurate test equipment able toverify the actual performance of the

optimized absorber designs. A huge 6 ft

square coaxial line, having a 2 ft squarecenter conductor was installed at Ray

Proof, able to measurement the low

frequency reflectivity of absorbers ingroups of 8 units. This original Ray

Proof test system, together with an

array of more modern systemsinstrumented 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 attenuationaccording to such standards as ANCI-

C63.4 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 EMCAnechoic Chambers. Sintered ferrite

tiles, only a few millimeters thick, canexhibit excellent absorption properties

at frequencies below 100 MHz. By thelate 1980s many ferrite tile lined

chambers were being used in Japan as

EMC Test Sites. The great advantage ofthe ferrite tile technology was that

chambers could be dramatically

reduced in size. The surrounding shielddid not have to be sized to

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accommodate a large volume of thick

absorber lining in addition to the active

test volume. However, ferrite was still avery 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 patentexpired in the mid 1980s, competitive

pressures reduced the cost of a ferrite

lined chamber. Again, it was IBM whoin 1986 became the first company in

the U.S.A. to install a high

performance 10m range chamber usingferrite tile technology at their facility in

Austin, Texas.

By the 1990s ferrite tile suitable forEMC test chambers were being

produced by several companies in Asia,the U.S.A. and Europe. The originalabsorber numerical modeling programs

and their later derivatives had been

modified to include ferrite parameterstogether with dielectric matching

layers. As a result, a new generation of

optimized, hybrid absorbers combiningthe best features of ferrite and

conductive foam could be designed and

applied to EMC chambers.

Chamber simulation programs,

incorporating hybrid absorber models

have been responsible for the moderngeneration of EMC Anechoic

chambers. These chambers cannot only

meet OATS standards but will beatalmost every actual OATS site in terms

of correlation to the theoretical ideal

site model across the entire testfrequency range. Typical regulatory

standards require an acceptable OATS

test site to demonstrate site attenuationcorrelation to the ideal model within

+/- 4dB. Modern 10m and 3m range

chambers available from ETS-Lindgrenare guaranteed to correlate to within +/-

3 dB of the ideal standard, usingoptimized hybrid absorber technology.

Having reached this point of

development with the EMC Test

Chamber, the focus on improving siteattenuation correlation to the

Normalized Site Attenuation, NSA, of

an ideal OATS has moved on from theabsorber and chamber to the calibration

and design of the Antennas that will be

used in the chambers.

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 acompact 7m x 3m x 3m pre-

compliance EMC Chamber. Facilitieslike this demonstrate +/- 6dB

correlation to NSA at low frequencies

and within +/- 4dB at frequencies from100 MHz to millimeter wave

frequencies. Slightly larger chambers

can be designed to demonstrate +/-4dBcorrelation to NSA for smaller EUTs

and with reduced scanning height of

the antenna. For Engineers who are

evaluating product susceptibility andwho self certify product emissions,

such pre-compliance chambers provideideal indoor test site convenience.

The next step up from the pre-

compliance chamber is the full-compliance, 3m-range facility. Such a

chamber which cost IBM $2M in 1982

is available today in a much reduced9m x 6m x 6m size, or even smaller,

offering exceptional performance, for

around $300K. At the higher end, basemodels of 10m range anechoic

chambers start below $1M.

Emerging test requirements have leadto chamber designs that offer

specialized or combination test

capabilities.These include,

for example,

EMC andwireless testing

in a 3m to 5m

range lengthaccording to

ETSI standards

and specialchambers for

automotive testapplications

according toCISPR-25. For

the ultimate

susceptibilitytesting of

complex and

large EUTsthere are also

fully qualified, non-anechoic

reverberation chambers. Today there is

a test chamber for almost every EMCtest requirement, making the emulation

of a wide variety of test conditions

possible.

EMC test engineers can now take

chamber anechoic performance forgranted and concentrate on selecting

other chamber features and accessories.

The optimal choice of antennafrequency ranges, antenna patterns,

equipment handling ramps, hoists,

towers, turntables, automated slidingdoors, and other accessories will

improve the ease of use, lower the cost

of ownership, and allow the chamber

user to take full advantage of theperformance 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 Lindgrens 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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