ETS-Lindgren 5411 GTEM! Test Cell - Manual (Page 71)

Radiated Emissions Test Performance of GTEM!
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Appendix C: Radiated Emissions Test Performance of GTEM!
Abstract
The Gigahertz Transverse Electromagnetic (GTEM!™) Cell has recently been
recognized as a competing technology for the accomplishment of
electromagnetic compatibility (EMC) radiated emissions testing for the
demonstration of compliance with commercial specifications. As a competing
technology, the direct comparison of the performance of the GTEM! for such
compliance measurements to the results obtained from an Open Area Test
Site (OATS), is an obvious and necessary step. This section describes the direct
comparison of results of two series of tests. Two differing comparisons are
described; three separate sets of reference dipole comparison measurements,
and two full personal computer systems tested per the requirements of
ANSI 63.4, Draft 11.4 on an OATS and in the GTEM!. In addition to a direct
comparison, the resultant data has also been subjected to statistical analysis.
The statistical analysis was performed on maximum electric field strength data
comparing GTEM! calculated levels with OATS measured levels. Pearson’s
correlation coefficient and Student’s-t distribution were used to analyze data.
Good agreement of results, both by direct comparison and by the statistical
analysis have been found, and are described.
Introduction
The GTEM! as shown on page 72 has existed in conceptual form and as a
practical device for some time. Only recently has the use of this device, as an
alternative to radiated emissions measurements on an OATS, been as a practical
choice. This change has been brought about by additional developments of the
theory of this device, such that a direct comparison can be made to the results
obtained from an OATS. The main contribution that has brought forward the
GTEM! as a practical radiated emissions device has been in the theoretical
development of a mathematical model allowing the direct comparison of data
taken in a GTEM! to data acquired on an OATS. The software implementation for
the GTEM! accomplishes the following, given three voltage versus frequency
measurements for three orthogonal orientations of the Equipment Under
Test (EUT) in the GTEM!.
At each frequency:
 Performs a vector summation of the three orthogonal voltages
 Computes the total power emitted by the EUT as determined from the
summation of the three voltages and the transverse electromagnetic
mode (TEM) equations for the GTEM!
 Computes the current excitation of an equivalent tuned, resonant dipole
when excited with that input power