2. Operation
18
2.1.2 Radiation Power Measurement (Ref.2, Figure 4)
When a radiating object is placed inside the TEM cell, the radiated power Po (Watt)
travels equally to both ends of TEM cell. The voltage measured, Vm (Volt) at one end
can be expressed as follows:
Vm (Volt) = SQRT (Po x Zo/2) ……………....................... (3) or
Po (Watt) = 2 x (Vm) 2/Zo ..........………………................ (4)
Here, Zo is 50 Ohm, the characteristic impedance of the TEM cell.
[Figure 4] Radiation Power Measurement
Now, we can calculate the E-field radiation from the object, assuming the object is in a
free space and has antenna gain G. We also assume that in our measurement the
radiator size is sufficiently small compared to the TEM cell such that there is no
significant effect on radiation due to the TEM Cell. Then the hypothetical maximum
EMI field Er at distance D (meter) from the radiator would be:
Er (Volt/meter)= 60 x (G /Zo) x (Vm/D)………….....................(5)
Vm is the maximum measured voltage which is the vector sum of the 3 voltages
measured by rotating DUT in x, y, and z-axis inside the TEM Cell.
(Ref.1) NBS Technical Note 1319, US Department of Commerce, NBS
(Ref.2) NBS Technical Note 1013, US Department of Commerce, NBS
L (meter)
Spectrum
Analyzer
V (volt)
Z
DUT
DUT
Y
X
Top View
深圳德标仪器
135-1095-0799
