History
A GTEM cell is a design variation of the TEM Cell (by Crawford) that allows the cell to operate in the Gigahertz frequency range (from here the suffix “G”). The external enclosure forms a long rectangular base pyramid. His inventor was Prof.Hansen Diethard Who patented with ABB Brown Bovery the device. GTEM is terminated on a lined surface made of radiation-absorbent material such as carbon-loaded foam, and absorbers line the side walls. A slightly spherical wave propagates from the source to the tapered waveguide, and since the solid opening angle is small, the undistorted spherical wave can be considered a plane wave. The termination load section uses absorbing material for an electromagnetic wave and a distributed resistive load for current termination. At low frequencies, it operates as a circuit 50 Ohm load; at high frequencies, the absorbers attenuate the incident waves as in an anechoic chamber, In this way, a termination from DC to several Gigahertz is achieved. Some restrictions and compromises limit the applications such as:
- The DUT dimension is limited to 1/3 – 1/2 of the internal cell height to maintain a safe distance that doesn’t interfere with measurements of immunity and emissions ensuring uniformity of the EM-Field volume within +/-3dB or +/-5dB respectively.
- The surface resistivity of the conductive metal adopted in the GTEM case stainless steel it is not suitable in the GHz range due to its surface resistivity being too high!
- Choice and quality of the type of absorber material (carbon foam is inefficient under 80MHz, while a combination of hybrid absorbers is able to cover the frequency range from DC to over 20GHz)
- Increasing the operating frequency of the cell over GHz increases the percentage of non-transverse mode components (Not TEM Mode) that decrease the quality of the measure introducing complex and cross-polarization modes.
