Anechoic Chambers, Past And Present

This 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 e

This 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

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

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

Brian F Lawrence

Anechoic Chambers, Past And Present

2 CONFORMITY ® FEBRUARY 2005

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

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 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 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

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

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

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

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 a compact 7m x 3m x 3m

pre-compliance 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 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 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.