Electromagnetic simulation of 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 free space. 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 (before 2004) to model chambers to predict the performances. 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 (Ph. D.), 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 the developers of CST 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 minimum factor of 15, reducing compute time by more than 95%. The simulation results provided a near-perfect match to physical testing....
Development of 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 on an open area test site (OATS) which is equivalent to testing in free space defined by the absence of reflecting objects.
To overcome the problem of weather conditions and ambient noise, anechoic chambers have been developed. The chamber is an RF shielded box with walls lined with materials that are highly absorbent of RF waves in order to provide conditions similar to an OATS. Siepel has been manufacturing anechoic chambers since 1986. Today, regulatory agencies allow most products to be tested for EMC in anechoic chambers rather than OATS. They require, however, that anechoic chambers behave in a way that closely corresponds to OATS. The International CISPR 16-1-4 and the American ANSI C63-4 standards require that EMC testing be performed in a chamber that deviates from an open space by no more than ± 4 dB.
The design challenge
Companies that build 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 absorbers. 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.
Dr. 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 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
Dr. Dun had better luck with the CST MICROSTRIPES™ implementation of the transmission line method (TLM) from CST. 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. A further advantage is that the TLM method creates a matrix of equivalent transmission lines and solves for voltage and current on these lines directly. This uses less memory and CPU time than solving for E and H fields on a conventional computational grid. 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 graphical user interface, optimized meshing algorithm and parallel processing for increased speed, make the software suitable for solving extremely complex and electrically large problems.
Gwenaël Dun found that the TLM method successfully modeled both the antennas and the chamber itself. He took advantage of the CST 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 CST 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 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 anechoic chamber, developed with the aid of the simulation methods described here, makes it possible to perform full compliance radiated EMI and EMS measurements, according to the most commonly used international standards. The optimized design, with for example partial lining, saves space inside the chamber, providing a comfortable work environment. In addition to the ferrite absorbers described above, the 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).
Anechoic chamber manufacturer Siepel has validated the ability of CST MICROSTRIPES™ software to meet its demanding accuracy requirements while reducing compute time to less than 5 % of the time required by the software used in the past. “The key to the outstanding performance of CST 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 Micro-Stripes can predict the performance of 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 CST’ software allows us to focus on detail anechoic designs. Now with CST 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. ”