Shall I muddy the issue further...let me address an interesting
section of EN 55011:1991.
But first; for reference; Clause 5 of EN 55011 lists the limits and
measuring distance for Class B at 10 meters, Class A at 30 meters.
Section 8.1.3 entitled Radiation Measurements (9 kHz to GHz) speaks
to the issue of high ambients and moving to a shorter distance to
take measurements. However, the wording (IMO) indicates that the 20*
Log(D/d) correction is not allowed. Let me quote:
"Note - Due to the unpredictability of the relationship between
the measured values of the electromagnetic radiation disturbance
and the distance from the equipment under test no correction is
allowed for in the limits when measurements are made at a closer
distance."
"no correction"
The key implication (IMO again) is that running Class A measurements
at 10 meters must meet the limits specified at 30 meters! Please
correct me (pun) if I'm wrong!
To my knowledge, many labs are doing Class A measurements at 10
meters. (IMO, 10 meters is better because most sites need an
additional RF preamp for 30 meter sensitivity; if not handled
carefully, measurement uncertainty is worsened. Of course, EUT size
and other factors exist.)
CISPR-11 is due for an overhaul (and for other interesting problems
too) and I hope they fix this one while they're at it.
Regards,
Eric Lifsey
National Instruments
