SEMI-ANECHOIC CHAMBERS - WILL THEY REPLACE OATS
A STUDY ON THE CORRELATION OF A SEMI-ANECHOIC
ROOM WITH AN OPEN AIR TEST SITE (OATS)
James Press
Chief Scientist
National Technical Systems
1146 Massachusetts Ave.
Boxborough, MA 01719
Mark Betts
EMC Engineer
National Technical Systems
36 Gilbert St. South
Tinton Falls, NJ 07701
John Ngo
Compliance Manager
National Technical Systems
1536 E Valencia Dr.
Fullerton, CA 92831
INTRODUCTION
As anyone who has ever performed an Open Area Test Site (OATS) test finding a site
without high ambients is becoming extremely difficult. Most major metropolitan areas
have many sources of electromagnetic radiation. And with the upswing in OATS testing
to meet the requirements from the European and United States markets along with the
telecommunication testing standards (Bellcore), these problems associated with OATS
testing are once again a major topic of conversation among EMC professionals.
These problems are not new and are well documented. Besides the weather they
include, but not limited to, high ambients, field uniformity, test time lost in dismantling
setups in the EMI chamber to move the equipment-under-test to the OATS, the
requirement for pre-scans in an EMI chamber, and the cost for construction of the site.
With the difficulties associated with OATS it is easy to conclude that a semi-anechoic
chamber is a cost-effective solution. The only other option would be to place the OATS
in a secluded area far away from potential ambients. This option is typically rejected due
to travel costs and weather-related concerns.
This paper explores the advantages of testing in a semi-anechoic chamber and a
method_ for correlating emission measurements from an OATS with those in a 3-meter
semi-anechoic chamber. There are situations were a 10-meter OATS is required and
they will be discussed.
REQUIREMENTS
The international committees (IEC, ANSI, CISPR, etc.) dictated many years ago that the
OATS measurement was the standard. In fairness to those committee members who
have worked hard over the years, the recent developments in ferrite technology has
made 3-meter chambers cost-effective. Even the 10-meter chamber is now viable for
larger companies with deep pockets and a few are presently in operation.
Like many other testing companies, National Technical Systems (NTS) has recently
decided to expand their existing EMC/EMI test capabilities at the Boxborough,
Fullerton, and New Jersey facilities. After many discussions with the technical staff,
the European Notified Body, and others involved with EMI/EMC testing, NTS concluded
that the construction of a 3-meter semi-anechoic chamber along with a simple but
functional 10-meter OATS was the best solution. The OATS was designed to meet the
10-meter site attenuation as required by ANSI C63.4 and the chamber was configured to
meet the site attenuation for a 3-meter alternate test site per C63.4. Surprisingly the cost
for constructing a 3-meter semi-anechoic chamber with a functional OATS is about the
same as building and maintaining a fully climate-controlled and automated OATS that
addresses the ambient issue.
The advantages of a semi-anechoic chamber are numerous. Time after time our
customers have complained about ambient problems and the time-consuming task of
tearing down the equipment in order to re-assemble it on the OATS. In addition, with the
European Notified Body placing more faith in the semi-anechoic measurement than any
OATS and the telecommunication requirements specified in both 3 and 10-meter limits,
the semi-anechoic chamber was definitely the most cost-effective facility.
Like many others, NTS will also perform Radiated Immunity in the chamber. A small
penalty must be paid in that the floor of the chamber will require ferrite material to meet
the EN61000-4-3 field uniformity requirements. However, NTS solved this problem by
mounting the ferrite on plywood panels with steel backing which can be placed on the
floor between the transmit antenna and the turntable. This change-out of panels can be
quickly performed and both the technician and customers definitely preferred removing a
few ferrite panels rather than tearing down the equipment-under-test and moving it to an
outside location. This was especially true for our telecommunications customers who
typically have special test support equipment. The majority of these test sets are unique
and were specifically designed and built to exercise the telecom unit. These test sets
normally are not designed to be moved around and do not travel well.
NOT DEAD YET
After the discussion above, one may conclude that an OATS is obsolete. Although the
authors would not mourn the death of OATS, its demise is far from near. An OATS with
a climate-controlled turntable and support equipment will complement a semi-anechoic
chamber for the special cases were the OATS is necessary. In addition, until the cost of
10-meter chambers becomes cost-effective, the OATS will be a necessary evil. In
particular, when a 3-meter measurement is very close to the limit (1 to 4 dB) the OATS
will be the definitive measurement. Note that this does not mean that the chamber
measurement is not correct – in fact the chamber measurement will be identifiable and
accurate. The OATS measurement may even require the antenna to be moved closer
than 10-meters (3-meters!) if the signal is lost in the ambient.
As chamber technology becomes better and more cost effective, the OATS will be
retired. Until then, an OATS will be necessary for larger equipment (2 or more bays)
where the antenna distance of 3 meters is not practical and the units may be too large to
enter a 3-meter chamber. Obviously, this argument can be taken further for units that are
too large for OATS (5 or more bays). This is when engineering judgement and technical
analysis and reports (Technical Construction Files) are required.
Writers' note: With the signing of the Mutual Recognition Agreement in 1998 between
the U. S. and the European Union, U.S. laboratories will be allowed status of Competent
Bodies to make these judgements. National Technical Systems has received notice
from the National Institute of Standards and Technologies, Office of Standards
Services allowing East and West Coast facilities to operate as a Conformity
Assessment (CAB) as of April 8, 1999.
CORRELATION with an OATS
To ensure accuracy and site independence, the authors made measurements at two of
the three NTS sites with 3-meter semi-anechoic (Boxborough, MA and Tinton Falls, NJ).
Testing was performed in accordance with ANSI C63.4 from 30 MHz to 1 GHz for
alternative test sites. This requires the Normalized Site Attenuation (NSA) to be
performed at five positions (for larger test items) around the turntable (essentially the
four corners and the standard center position). In addition, the measurement must be
performed with the antenna placed at 1.5 meters and 2-meter height. Thus, a total of 20
measurement positions were required – 5 positions for vertical and horizontal with the
antenna at 1 meter, 5 position with the antenna at 1.5 meters in the vertical position, and
5 measurements with the antenna at 2-meter in the horizontal position. A measurement
data sheet is shown in Figure 1. A photograph of the Boxborough, MA 3-meter
chamber is shown in Figure 2. Note that the chamber is a hybrid containing both
anechoic absorbers for high-frequency performance and ferrite tiles for low-frequency
performance. The ferrite tiles are not visible since they are covered by the anechoic
materials.
Once the NSA is performed on the chamber and complies with the requirement of ANSI
C63.4, testing can begin immediately to the telecom and FCC requirements (assuming
there is no problems in the listing process). The chamber can also be used for emission
tests in compliance with MIL-STD-462D and RTCA DO-160. (NTS may be the first
laboratory to be accredited by the National Voluntary Laboratory Accreditation Program
(NVLAP) sponsored by the National Institute of Standards and Technology (NIST) for
MIL-STD-461D/462D testing.) The only issue to be considered is testing to the European
CE-Mark in accordance with EN55022 or other Product Specifications for the consumer
market. The European requirements specify a 10-meter emission measurement.
In order for the Notified Body to accept data at 3-meters for the 10-meter requirement, a
correlation with an OATS that has demonstrated compliance to the ANSI C63.4 NSA is
required. All OATS in the USA should be listed with the FCC, which requires the site to
demonstrate compliance to ANSI C63.4. The OATS at the NTS New Jersey facility is
shown in Figure 3. In order to satisfy the concern of the Notified Body, a calibrated
broadband noise generator was required to be used to identify emissions. The
generator is placed on the OATS as describe in EN55022 at a distance of 10 meters
from the receiving antenna. Measurements were performed at the bandwidths and
procedure in accordance with EN55022. This data was then stored and compared to the
class A and B limits. The generator is then taken into the chamber and the emission
measurement repeated at 3 meters. The data from the chamber was then compared to
the class A and B limits adjusted by the 10/3-meter ratio (approximately 10 dB). The
generator can be adjusted to emphasize any specific frequency in the 30 MHz to 1 GHz
frequency range if the data shows any possible trouble frequencies. Of course, the
OATS measurement near high-ambient frequencies requires special consideration
whereas the chamber ambient is well below the EN55022 limits. Data is shown in Figure
4 and is within 2 dB. Since the uncertainty in the measurement is above 2 dB this
correlation is excellent. It should be noted that the data in figure 4 did not need
correction due to perturbation of the ground plane.
This was somewhat
surprising since we expected a larger difference in the data. Further studies will
be performed in an attempt to determine when or if this perturbation becomes
significant i.e antenna height, polarization, off-axis measurements, etc.)
CONCLUSION
Utilizing today's state-of-the-art technology, we have concluded that a 3-meter semianechoic chamber along with a functional OATS is the most cost-effective means of
performing radiated emissions testing. The majority of consumer products can be tested
in the chamber for all tests without either dismantling the unit or taking the device
outdoors. Even single rack telecom equipment can be tested using this method. The
figures and results prove that a chamber can be correlated with an approved 10-meter
OATS that meets the requirement of C63.4. The measurements in the chamber are
repeatable and without ambient difficulties. In the opinion of the authors, it may be time
to bring OATS testing indoors.
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