GTEM Over the Top, the strength made in Italy

The GTEM cell is, in principle, a tapered coaxial line (offset septum plate), which is terminated by a hybrid combination of discrete resistors and RF absorber to achieve a 50 Ohm broadband match. It is applied for the Measuring of Radiated Emission, radiofrequency field-immunity test.

The GTEM is an alternative EMC site to perform emission and immunity tests correlated at “Far-field” conditions according to the requirements of the standard EN 61000-4-20. A GTEM cell provides homogeneous electromagnetic fields with TEM (Transverse electromagnetic wave mode) distribution, similar to the free space. The electric and magnetic field inside the cell can be accurately predicted using numerical methods. A GTEM cell external enclosure is made of conductive material such as metal, in the shape of a long, rectangular base pyramid.

The GTEM cell forms an enclosed TEM (transverse electromagnetic mode) stripline, which acts as receiving emissions or transmitting emissions antenna:

• When measuring radiated emissions, one end of the stripline is connected to a spectrum analyzer. The other end is terminated with an RF load (e.g., 50Ω).

• When performing radiated immunity, one end of the stripline is connected to a source of radiation (signal generator). The other end is terminated with an RF load.

• Under test operation, the D.U.T. (Device Under Test) is placed in the test volume in between the septum and the cell floor. According to the standard EN61000-4-20, the test volume zone is considered as one cube of 1/3 up to 1/2 of the septum height than the floor of the GTEM. Within the test volume, the TEM (Transverse Electromagnetic) field is considered uniform with an uncertainty of +/- 3dB or +/-5dB respectively.

• In operation, emitted radiation (whether from an antenna or from the equipment under test) travels along the length of the chamber and is absorbed by the absorbent load at the end.

• For immunity tests, the field uniformity and the cross-polar coupling of the cell have to be within certain limits set by IEC 61000-4-20.