ETS-Lindgren 5411 GTEM! Test Cell - Manual (Page 73)

Radiated Emissions Test Performance of GTEM!
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 Develop a simple, direct comparison of the results of these diverse tests
 Provide analytical result of the difference in the GTEM! and OATS
performance that is capable of being succinctly stated, yet remains complete
 Provide a supplement to the direct comparison by a statistical analysis that
provides a more sophisticated and meaningful comparison of the
measurements
 Provide a statistical measure of quality of the compared measurements other
than a direct statement of difference of the measurements.
Design of Testing
The development of the specific test approaches was based on the requirements
of ANSI 63.4. Since this document will, in time, become the test requirements
document for commercial EMC testing in the United States, it was selected as
the basis for testing.
G
ENERAL
ANSI 63.4 provides a provision for the development of data for the qualification of
alternate test sites. While this requirement is directed to the qualification of
semi-anechoic chambers for radiated emissions testing, it was felt that there
were technical features in this approach usable for GTEM! comparison
measurements. ANSI 63.4 requires the development of normalized site
attenuation data, in both vertical and horizontal polarizations, from a number of
specified locations on the included turntable at the EUT location in the
semi-anechoic chamber. In the ANSI documents, these measurements are made
with broadband antennas such as biconical and log periodic antennas.
A test object produces a certain amount of radiated energy. Some of this energy
may be directed up or down depending on the sources of radiation and the
coupling among the cables interconnecting the personal computer system
components. In customary traditional EMI tests on OATS, individual units and
cables are moved to try to cause as much of the energy as possible to be
radiated at the height around the computer such that it can be picked up in the
height scan of the measurement antenna, 1 to 4 meters.
In the GTEM!, three orientations of the test object are needed, so that all
three components of the total radiation vector can be picked up. It was
hypothesized that the GTEM! should predict the maximum field strength that
could be measured on an OATS since it picks up the total radiation vector from
the test object.
To accommodate the size of the planned EUT, a Model 5317 was used. The
external dimensions of this device are provided in the following illustration.