The following is a document received from the FCC concerning measurements
above a gig. Just thought it may be of interest to some.
Bob Heller
3M EMC Laboratory, 76-1-01
St. Paul, MN 55107-1208
Tel: 651- 778-6336
Fax: 651-778-6252
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Federal Communications Commission
Office of Engineering and Technology
Laboratory Division
Electric Field Radiated Emission Measurements above 1 GHz
March 9, 2007
The measurement method referenced in CISPR 22,1 for making electric field
radiated emission measurements above 1 GHz, does not satisfy the FCC
requirement to maximize the radiated emission, and may not be used for
making radiated emission measurements above 1 GHz. The measurement method
in ANSI C63.4-2003,2 as clarified below is to be used in making this
measurement.
Measurement procedures for electric field radiated emissions above 1 GHz
are covered in Clause 8 of ANSI C63.4-2003. The C63.4-2003 measurement
procedure consists of both an exploratory test and a final measurement.
The exploratory test is critical to determine the frequency of all
significant emissions. For each mode of operation required to be tested,
the frequency spectrum is monitored. Variations in antenna height, antenna
orientation, antenna polarization, EUT azimuth, and cable or wire placement
is explored to produce the emission that has the highest amplitude relative
to the limit.
The final measurements are made based on the findings in the exploratory
testing. When making exploratory and final measurements it is necessary to
maximize the measured radiated emission. Subclause 8.3.1.2 of C63.4-2003
states that the measurement is to be made “while keeping the antenna in the
‘cone of radiation’ from that area and pointed at the area both in azimuth
and elevation, with polarization oriented for maximum response.” We
consider the “cone of radiation” to be the 3 dB beamwidth of the
measurement antenna.
While the “bore-sighting” technique is not explicitly mentioned in
C63.4-2003, it is a useful technique for measurements using a directional
antenna, such as a double-ridged waveguide antenna. Several precautions
must be observed, including: knowledge of the beamwidth of the antenna and
the resulting illumination area relative to the size of the EUT, estimation
for source of the emission and general location within larger EUTS,
measuring system sensitivity, etc.
1 CISPR 22 (2005), Amendment 1 (2005-07), Information technology equipment
– Radio disturbance characteristics – Limits and methods of measurement,
Clause 10.6.
2 47 CFR Part 15.31(a)(3) specifies the use of the measurement procedure
ANSI C63.4-2003, American National Standard for Methods of Measurement of
Radio-Noise Emissions from Low-Voltage Electrical and Electronic Equipment
in the Range of 9 kHz to 40 GHz.
C63.4-2003 requires that the measurement antenna is kept pointed at the
source of the emission both in azimuth and elevation, with the polarization
of the antenna oriented for maximum response. That means that if the
directional radiation pattern of the EUT results in a maximum emission at
an upwards angle from the EUT, when a directional antenna is used to make
the measurement it will be necessary for it to be pointed towards the
source of the emission within the EUT. This can be done by either pointing
the antenna at an angle towards the source of the emission, or by rotating
the EUT, in both height and polarization, to maximize the measured
emission. The emission must be kept within the illumination area of the 3
dB beamwidth of the antenna so that the maximum emission from the EUT is
measured.
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