The GTEM is an alternative EMC site to perform emission and immunity test in "Far-field" conditions according to the requirements of the EMC measurements. A GTEM cell enclosure is made of conductive material such as metal, in the shape of a long, rectangular base pyramid. The pyramid is normally laid flat on the major side, although occasionally it may be stood on its base. The base is typically lined internally with radiation-absorbent material such as carbon foam loaded; hybrid absorbers with ferrite tiles, but the side walls are left bare to act as a waveguide. A stripline tapered conductor, known as the septum, connects internally the input/output port apex connector and a hybrid termination resistive and RAM absorbers. Under test operation, the D.U.T. is placed in the test volume in between the septum and the bottom chassis. Usually, the test volume size is considered as one cube of 1/3 up to 1/2height of the septum to the bottom distance: Within the test volume, the TEM field is considered uniform within an uncertainty of +/- 3dB or +/-5dB. Theoretically, the GTEM is a broadband device able to operate from DC to Microwaves with some restrictions and compromises that conditioning the applications such as surface resistivity of the conductive metal adopted; quality of the RAM, Distortion and not TEM mode that could appear increasing the frequency.
Principles of operation
The GTEM cell forms an enclosed TEM stripline, which acts as receivingemissions 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.
When performing radiated immunity, one end of the stripline is connected to a source of radiation. The other end is terminated with an RF load.
In operation, emitted radiation 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 crosspolar coupling of the cell have to be within certain limits set by IEC 61000-4-20.
