ETS-Lindgren 5411 GTEM! Test Cell - Manual (Page 70)

70 |
Emission Measurements in TEM Cells and TEM Lines
TEM C
ELL
V
OLTAGE
M
EASUREMENTS
: T
HREE
P
OSITIONS
The EUT emissions are measured in three positions that are determined as
follows:
Assign an axis system (x, y, z) to the TEM cell and to the EUT. A standard choice
is to align the z-axis in the direction of propagation, the y-axis with the E-field
(vertical), and the x-axis with the H-field. A local coordinate primed system is
assigned to the EUT. Position one aligns x with x, y with y, and z with z.
Position two is obtained by simply permuting the primed EUT axes. This is
equivalent to two 90-degree rotations of the EUT. Position three is obtained by
further permutation. Denoting the three voltage measurements by V-V, it may be
shown that the total radiated power due to EUT is then given by the equation.
EUT emissions over a ground screen are simulated by assuming the same total
radiated power is emitted by a short dipole (replacing the EUT). The equations
for the fields from a dipole are well known and the ground screen may be
accounted for by introducing an image dipole. The fields may be calculated over
the equivalent sweep path of receiving antenna if required and the maximum
determined for both polarizations. The maximum for the two polarizations would
then give the maximum possible field. The geometry factor due to sweeping the
observation point is designated as f, and the maximum field E is given by the
equation.