ETS-Lindgren 5411 GTEM! Test Cell - Manual (Page 34)

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Measured GTEM! Performance
T
IME
D
OMAIN
R
EFLECTOMETER
The Time Domain Reflectometer (TDR) Measurement is an impedance
measurement performed in the time domain. The advantage here is that distance
information can be derived from the test and used to confirm the correct
alignment of the septum and other cell components along the length of the cell.
The TDR test is generally performed as standard at the factory on assembled
cells.
GTEM! Shielding Effectiveness
The shielding effectiveness (SE) of a GTEM! is difficult to measure by
conventional IEEE-299-type methods due to GTEM! size and flared walls.
Alternative methods have been developed, for example: The primary technical
issue is the choice of reference signal for the SE tests. Due to size constraints,
normal antennas cannot be used to generate the signal inside the GTEM!.
In addition, electric field intensity along the GTEM! can vary more than 20 dB
from apex to load. For these reasons, a modified two-antenna swept-frequency
procedure was used to measure SE of the Model 5400 Series. In this method,
power is input into the GTEM! as in a normal immunity test, and the receive
readings of two loop probe antennas, one inside and the other outside the
GTEM! walls, are compared. For the Model 5407, a >80 dB specification SE was
verified with this method.
GTEM! Emissions Testing Characterization
A representative radiated emissions comparison of a GTEM! measurement to an
Open Air Test Site (OATS) measurement is provided in detail in Appendix C,
Radiated Emissions Test Performance of GTEM! on page 71.