ElectromagneticCompatibility Analysis Using TEM Cells
ABOUT THE AUTHOR
Kresimir Malaric is an
assistant at the Faculty of Electrical Engineering and Computing,
University of Zagreb, Croatia. His research interest area is
electromagnetic compatibility and TEMcell development.


This article describes various types of Transverse ElectroMagnetic (TEM) cells, including TEM, GTEM, WTEM and EUROTEM. Included are the design of and differences between the different cells, along with numerical modeling using Finite Element Method (FEM) techniques. Cell comparison was made with respect to electric and magneticfield distribution as well as electric potential.
Since 1974 and the introduction of TEM cells, there have been many improvements and different variations of transmissionline cells for electromagnetic interference (EMI) and electromagnetic compatibility (EMC) testing. GTEM, WTEM and EUROTEM are examples of available TEM cells. The TEM and GTEM cells described in this work were designed at FER, Zagreb.
Transverse electromagnetic (TEM) transmissionline cells are devices used for establishing standard electromagnetic (EM) fields in a shielded environment. The cell consists of a section of rectangular coaxial transmission line tapered at each end to adapt to standard coaxial connectors (Figure 1). You use TEM cells for emission testing of small equipment, for calibration of RF probes, and for biomedical experiments. The wave travelling through the cell has essentially a freespace impedance (377 W), thus providing a close approximation to a farfield plane propagating in freespace. The cell also has limitations, among which is that the upper useful frequency is bound by its physical dimensions which, in turn, constrain the size of an item you can test with the cell.
Figure 1: TEMcell diagram
The GTEMcell (Figure 2) is a transmission structure using a TEMcell approach. A slightly spherical wave propagates from the source into a 50W rectangular coaxial transmission line and its distributed hybrid termination without geometrical distortion of the TEM wave. Since the opening angle of the waveguide is small, you can consider the undistorted spherical wave to be a plane wave. The GTEMcell (the G stands for GHz) is a tapered section of rectangular 50W transmission line. The GTEM cell begins with a precisioncraft apex, where the transition from the standard 50W coaxial connector (or the Ntype connector) to the asymmetric rectangular waveguide is made. The distributedload section uses absorbing material for EMwave termination and a distributed resistive load for current termination. At low frequencies, the GTEM cell operates as a circuitelement 50W load. At high frequencies, the absorber attenuates the incident wave similar to what happens in an anechoic chamber. The broadband performance provided by the termination acts to suppress the creation of higherorder frequency modes. The TEM mode excited by either a CW source or a pulse generator simulates an incident plane wave for susceptibility and emission tests.
Figure 2: GTEMcell diagram
The WTEM cell (Figure 3) is essentially a transmissionline device well suited for generating EM fields and estimating radiated emissions (the W stands for wire). The WTEM cell's inner conductor is a wire array, while an outer conductor is a rectangular waveguide tapered at the input end, which acts as a shield. The line is terminated in a resistor array at low frequencies and in pyramidal absorbers at high frequencies.
Figure 3: WTEMcell diagram (EUT is equipment under test)
The EUROTEM (Figure 4) is a new type of symmetrical TEM cell. EUROTEM 2 is for sensor calibrations and compliance measurements for small objects. You can expand the cell into a larger version—EUROTEM 3 (basically a EUROTEM 2 cell with an extension on the right side)which can take larger objects. This is done using a modular approach, where the termination section of the EUROTEM 2 cell is removed and a EUROTEM 3 section added.
Figure 4: EUROTEMcell diagram
Figure 5: TEMcell line crosssection
The characteristics of the GTEM cell (Figure 6) are: 50W impedance, inner conductor at 3/4 height, inner heighttowidth ratio of twotothree, angle between septum and the bottom plate equals 15°, and angle between septum and top plate equals 5°. The septum as well as the cell's coating is copper. The Ntype connector is at the end of the tapered section. The septum is supported with dielectric material. The cell can test an object of approximately 400 cm².
Figure 6: GTEMcell line crosssection
The WTEM cell (Figure 7) is differentiated from standard striplines in the wire system in that it is curved rather than planar. This curvature provides better field uniformity. With the WTEM cell, the EUT can be about 1/2 of the distance between wire and floor compared to standard TEM cells that offer a distance of around 1/3 of the distance between wire and floor.
Figure 7: WTEM line crosssection
The EUROTEM cell (Figure 8) has two adjacent electrodes charged to the same polarity while the other two electrodes have an opposite polarity. All the walls are absorberlined. You can use a EUROTEM cell to test equipment from 30 MHz to 1 GHz.
Figure 8: EUROTEM line crosssection
Figure 9: An FEM example for a TEM cell
FEM solves for the unknown field quantities by minimizing an energy function. The energy function is an expression describing all the energy associated with the configuration under analysis. For 3D analysis, timeharmonic problems for this function may be represented as,
The first two terms in the integrand represent the energy stored in the magnetic and electric fields and the third term is the energy dissipated (or supplied) by conduction currents. Expressing H in terms of E and setting the derivative of this function with respect to E equal to zero, you get an equation of the form f(J,E) = 0. A kthorder approximation of the function f is then applied at each of the N nodes and boundary conditions are enforced, resulting in the system of equations:
The J values on the lefthand side of this equation are the source terms. They represent the known excitations. The elements of the Ymatrix are functions of the problem's geometry and boundary constraints. Since each element only interacts with elements in its own neighborhood, the Ymatrix is sparse. The terms of the vector on the right side represent the unknown electric field at each node. You obtain these values by solving the system of equations. You get other parameters, such as the magnetic field, induced currents, and power loss, from the electricfield values. In order to have a unique solution, it is necessary to constrain the values of the field at all boundary nodes. The metal box of the model in Figure 9 constrains the tangential electric field at all boundary nodes to be zero. The electrical and geometric properties of each element are defined independently. Figures 1013 show the results of numerical modeling analysis (brighter colors represent higher values).
Figure 10: TEM intensity with E and H vectors
Figure 11: GTEM intensity with E and H vectors
Figure 12: WTEM intensity with E and H vectors
Figure 13: EUROTEM intensity with E and H vectors