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Intelligent
Representation of Semi-Anechoic Chamber Wall Cuts Electromagnetic
Simulation Time 95%
By Gwenaël Dun, R&D Engineer, SIEPEL, La Trinité
sur Mer, FRANCE and Paul Duxbury, Senior EM Engineer, Flomerics
Ltd.
Electromagnetic simulation of semi-anechoic
chambers is a very difficult task. A very fine mesh is normally
required in the wall area to model the performance of absorbers
that are used to make the chamber act as if it were an Open
Area Test Site (OATS). The fineness of the mesh typically
results in very long simulation times, such as the 15 weeks
that could be needed on a desktop computer in the past to
model chambers to evaluate the effect of the qualification
antennas. Simulation is critical in the design process to
capture the near-field effects in the 30 to 200 MHz frequency
range, which cannot be determined by theoretical methods.
Gwenaël Dun, R&D Engineer for Siepel,
used a variety of different electromagnetic simulation tools
to address this challenge in the past but ran into problems
with both poor accuracy and long compute times. He then
worked with Flomerics, the developer of MicroStripes electromagnetic
simulation software, to implement a feature that makes it
possible to model the ferrite absorbers used in the chamber
as a boundary condition rather than part of the computational
domain. This change made it possible to increase mesh size
by a factor of 15, reducing compute time by 95%. The simulation
results provided a near-perfect match to physical testing.
Development of semi-anechoic chambers
International regulatory agencies have greatly increased
radio frequency (RF) emissions and susceptibility requirements
since they were first introduced in the 1970s. Generally,
the standards on RF emissions are based on tests performed
outside on an OATS, but these suffer from the effects of
weather conditions and ambient noise.
To overcome the problem of weather conditions and ambient
noise, semi-anechoic chambers have been developed. The chamber
is a RF shielded box with the walls and ceiling lined with
materials that are highly absorbent of RF waves in order
to provide conditions similar to an OATS. Siepel has been
manufacturing semi-anechoic chambers since 1986. Today,
regulatory agencies allow most products to be tested for
EMC in semi-anechoic chambers rather than OATS. They require,
however, that these chambers behave in a way that closely
corresponds to OATS. The American ANSI C63-4 and the European
EN50147-2 standards require that EMC testing be performed
in a chamber where the Normalised Site Attenuation (NSA)
deviates from an OATS by no more than ±4 dB.
The design challenge
Companies that build semi-anechoic chambers must be certain
that their products meet this specification. Physical testing
provides a poor solution because it is very expensive to
build a prototype chamber and the physical testing required
to evaluate the performance of the chamber over the full
range of required frequencies and in all areas of the chamber
would cost too much and take too long. Theoretical approaches
provide good results for certain subsets of the problem
but do not work for others. For example, at very high frequencies,
typically above 1 GHz, the antenna geometry is not important,
so the electromagnetic field can be calculated based on
the antenna radiation pattern and on the reflectivity of
the wall. But this approximation does not apply to lower
frequencies, where the geometry of the antenna is very important
due to the near-field effect and simulation is a must.
Dun felt that improving the simulation process was critical
to optimizing the performance of Siepel’s chamber,
so he decided to carefully evaluate the leading electromagnetic
simulation methods in terms of their ability in this area.
“Frequency methods such as Method of Moments (MoM)
do a good job of simulating the wire antennas used for the
qualification of semi-anechoic chambers but cannot accurately
simulate the walls of the chamber,” Dun said. “On
the other hand, finite difference time domain (FDTD) methods
work well for the walls but have difficulty in modeling
wire antennas, which typically require a mesh of 1 mm or
less. Models with meshes this small typically have solution
times measured in months, which is far too long to have
a positive impact on the design process.”
TLM Method Provides Accuracy and Speed
Dun had better luck with the MicroStripes implementation
of the transmission line method (TLM) from Flomerics. The
TLM method for solving Maxwell’s equations solves
for all frequencies of interest in a single calculation
and therefore, captures the full broadband response of the
system in one simulation cycle. The solver tolerates rapid
changes in grid density, large aspect ratios of grid cells
and localized gridding, enabling the mesh requirements to
be kept to an absolute minimum. An intuitive easy-to-use
graphical user interface, optimized meshing algorithm and
parallel processing for increased speed make the software
suitable for solving extremely complex and electrically
large problems.
Dun found that the TLM method successfully modeled both
the antennas and the chamber itself. Dun took advantage
of MicroStripes’ ability to create compact models
of antenna structures that reduce the size of the resulting
model while maintaining high levels of accuracy. He defined
the transmission parameters by the scattering parameters
of the balun and the simulation results of the wires. The
use of a compact model to represent the antenna meant that
the smallest element size required was 15 mm for the wire
connection.
Special boundary condition overcomes problem
But he ran into a problem in modeling the walls of the chamber.
The ferrite absorbers SIEPEL FE30Z used in the chamber are
only 6.7 mm thick, which meant that a mesh of 1 mm was needed.
Reducing the mesh size to this level would require a 15
week simulation time. This was much too high, so Dun spoke
to Flomerics to ask if there was a way around the problem.
He worked with them to develop a special boundary condition
that simulates the reflectivity of the ferrite absorbers,
eliminating the need to include them in the model. The boundary
condition was defined by the frequency dependent surface
impedance of a one dimensional TLM ladder network and defined
at the air-ferrite interface for the two polarizations of
the E field parallel and perpendicular to the air/ferrite
interface. This limit condition takes into account the incidence
angle and the polarization of the electromagnetic wave.
The key advantage of making the walls into boundary conditions
is the elimination of the need for the 1 mm mesh in this
area. This means that the most critical area is the antenna
connection, which only requires a 15 mm mesh. The resulting
increase in the mesh size reduced the computation time to
only 1 week on a desktop computer, which was fast enough
to serve as the primary evaluation tool during the design
process. The limit boundary condition had no effect on the
accuracy of the simulation. “To validate our model,
we compared simulation results and measurement results for
the two polarizations and two heights of the emission antenna,”
Dun said. “The deviation between the simulation and
the measurements was, in 99% of the cases, lower than +/-1dB
and in every case, lower than +/-1.5dB, which was sufficient
to optimize the performance of semi or full anechoic chambers.”
The Result is a Successful Product
The new SIEPEL HERMES 3, 3m EMC semi-anechoic chamber, developed
with the aid of the simulation methods described here, makes
it possible to perform full compliance radiated EMI and
EMS measurements at 3 meters distance, according to the
most commonly used international standards. The optimized
design saves space inside the chamber, providing a comfortable
work environment. In addition to the ferrite absorbers described
above, the semi-anechoic chamber also uses a low-carbon
loaded pyramidal absorber that is transparent in the low
frequency band but preponderant above 1 GHz. Since the reception
antenna is directional above 1GHz, the pyramidal absorber
only needs to cover the specular zone (optimized design).
Semi-anechoic chamber manufacturer Siepel has validated
the ability of MicroStripes software to meet its demanding
accuracy requirements while reducing compute time to only
6% of the time required by the software used in the past.
“The key to the outstanding performance of MicroStripes
in this application is the boundary condition for the modeling
of the ferrite tiles, which increases the time step that
can be used,” said Gwenaël Dun, Design Engineer
for Siepel. “We know that MicroStripes can predict
the performance of semi-anechoic chambers with excellent
precision, making it possible for us to evaluate many more
alternatives during the design process without physical
prototyping.”
Jean-François Rosnarho, R&D SIEPEL Manager, said
“The Flomerics’ software allows us to focus
on detailed semi-anechoic designs. Now, with MicroStripes,
it is possible to optimize the shielded room sizes, the
location of the absorbers, and the dimensions and location
of the quiet zone. It contributes to the design of the new
EMC chambers, increasing the chambers’ performances
and decreasing the cost.
For more information about MicroStripes, visit http://www.microstripes.com
For more information about Siepel, visit http://www.siepel.com
Flomerics
www.flomerics.com
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