How Are Radiated Emissions Measured
Methods for measuring vary somewhat between regulatory agencies, but the general
approach is similar. The device under test is placed a specified distance
from a measurement antenna, usually 1 meter, 3 meters, 10 meters or 30 meters,
as shown in the figure below. The device is exercised through its normal
modes of operation and the electric fields picked up by the measurement antenna
are measured over the frequency range of interest using a spectrum analyzer or
EMI receiver. Measured field strength is compared to a predefined radiated
emissions limit and if measured levels are under the limit the unit is
considered to have passed its radiated emissions requirement.
The device under test is usually placed on a test bench, as
shown in the following figure from MIL-STD-461E, if it is relatively small or is
a piece of portable equipment. Larger devices may be free-standing.
Power leads and interconnecting cables are arranged on the test bench in a
standard fashion defined by the applicable EMI requirements document, i.e.
MIL-STD-461, DO-160, EN55022, etc. The test bench is covered with copper
if required. Antenna placement is also carefully controlled to facilitate
standardized and repeatable test results. For physically large devices or
systems more than one antenna position may be required.
With regard to the RE Analyst radiated emissions model, the
EUTs in the figure above may contain Sources and Filters, and because they are
each connected to another EUT they may also contain Loads. Each EUT may
have any number of Sources or Filters. For the EUT connected to the LISNs
the and would be
in the EUT, the LISNs would be the and the wires
between the EUT and LISNs would be the Conductors. For the two connected EUTs, the Source
and Filter would be in one EUT, the Load would be in the other EUT, and the
wires between the EUTs would be the Conductors.
If the system contains more than one Source, multiple
radiated emissions analyses are required, one for each Source/Filter/Conductor/Load circuit. More:
 How Are Radiated Emissions Controlled
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