ETS-Lindgren 5411 GTEM! Test Cell - Manual (Page 12)

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Introduction
Basic Theory of RF Operation
The GTEM! is essentially a section of asymmetric rectangular transmission line
with a unique flared geometry and a hybrid termination. There is a 50-ohm
resistor termination for currents flowing on the septum, and an RF foam absorber
termination for electromagnetic fields generated in the GTEM! that propagate
towards the back wall. An RF signal applied to the center conductor will result in
the generation of a predominantly vertical Ē field above and below the septum
with radiating components toward the outer conductor, as is the case in a coaxial
transmission line.
Ideally, the center one-third of the volume below the septum, both vertically and
horizontally, is of sufficient uniform distribution to allow the use of the GTEM! for
immunity testing. In actuality, a test volume producing accurate results for
radiated emissions testing may be as large as two-thirds of the vertical and
horizontal dimensions (depending on the type of equipment under test). Under
ideal circumstances, the magnitude of the field changes gradually from a
maximum at the septum, to zero at the outer cell wall (conductor). The uniform
area, therefore, lies in the region where this transition in field is within the limits of
the specified measurement uncertainty.
The frequency of the transverse electromagnetic mode (TEM) mode supported in
the cell is a function of the distance between the center conductor (septum) and
the outer conductor. Above frequencies where this distance is greater than a
wavelength, higher order modes may also be supported.
The presence of the Equipment Under Test (EUT) may also affect the
performance of the GTEM! since to the advancing wave, it would appear as a
change in the impedance of the incident field.
The GTEM! provides a matched termination to input signals; there are no severe
VSWR problems as usually experienced with low-frequency biconical class
antennas. It is relatively easy to produce low frequency intense electromagnetic
fields with the GTEM!. The capability of a GTEM! to operate without size or
scaling problems well into the GHz frequency range allows the testing of items
without the need for the antenna changes associated with other test sites.