ETS-Lindgren 5411 GTEM! Test Cell - Manual (Page 69)

Emission Measurements in TEM Cells and TEM Lines
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Appendix B: Emission Measurements in TEM Cells and TEM Lines
Multipole Model
Any finite-sized radiation source may be replaced by an equivalent multipole
expansion that gives the same radiation pattern outside some volume bounding
the source. If the source is electrically small, then the initial multipole moments,
electric and magnetic dipoles, will yield an accurate representation. This holds for
an arbitrary source. If the source itself consists of electric and magnetic dipole
like elements only, then the electrically small restriction may be relaxed.
The basic approach of transverse electromagnetic mode (TEM) cell or TEM line
to Open Area Test Site (OATS) correlation routines is to use a set TEM cell
measurement to determine the multipole moments. Once the multipole moments
are known, radiation either in free space or over an infinite ground plane may be
simulated numerically. In this way it is possible to simulate the various
source-to-receiver antenna configurations required by OATS emission standards.
For two-port TEM cells or TEM lines, measurements at both ports yield amplitude
and relative phase information. In this way both the magnitude and phase of the
multipole moments may be determined and the radiation pattern accurately
simulated, including possible nulls due to phase cancellation. For one-port
TEM cells or TEM lines no relative phase information is available; therefore, it is
only possible to determine the magnitudes of the multipole moments. Such a
representation well simulates emission maximums, which are of primary interest
in electromagnetic compatibility (EMC) measurements; however, radiation
pattern nulls may not be well simulated.
Two-Port TEM Cell or TEM Line Correlation Routine
See References on page 89.
One-Port TEM Cell Correlation Routine
The most time-efficient one-port correlation routine is based on the assumption
that the Equipment Under Test (EUT) may be reasonably represented by the
initial multipole moments, namely the electric and magnetic dipole moments.
Three voltage measurements are then made in a TEM cell from which the EUT
total radiated power may be calculated. The individual dipole moments are not
separately determined. The total radiated power is then used to simulate the
maximum EUT fields over a ground plane based on a model of parallel dipoles,
either horizontal or vertical, radiating the same total power.