EMC Question of the Week: December 6, 2021
The EM test environment depicted in this image is
- an anechoic chamber
- a semi-anechoic chamber
- a TEM cell
- a GTEM cell
Answer
The correct answer is “d.” We can rule out the first two options, because the floor and at least one wall are metal with no absorber. We can rule out the TEM cell, because there are absorbing cones on one wall. The only remaining option is the GTEM cell.
GTEM is an acronym for Gigahertz Transverse ElectroMagnetic cell (or Gigahertz TEM cell). Traditional TEM cells have a maximum height and width that is less than half a wavelength at the highest measurement frequency. TEM cells that work at GHz frequencies have a maximum cross-section that is less than 15 cm. This is too small to measure most products. The GTEM cell is a modification of a TEM cell that attempts to maintain TEM wave propagation while greatly expanding the interior volume of the cell.
GTEM cells resemble a traditional TEM cell on one end with walls and a septum that increase in size maintaining a controlled characteristic impedance. The far-end, however, is terminated with resistors and material meant to absorb the incident plane wave. This photo shows the interior of a very large GTEM cell near the absorbing termination. The septum is at the top of the photo.
The degree to which plane-wave propagation is actually maintained depends on many factors, and GTEM measurements cannot be directly correlated with measurements made in other environments. Nevertheless, in some situations, GTEM cells can be an effective low-cost option for emissions and immunity testing.
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