5
Subject to change without notice
Next, the same measurement is performed with
the additional shielding around the EUT. Again the
EUT must be rotated in azimuth until the maximum
RFI is observed. The direction may be different
than in the non-shielded equipment.
EUT RFI Characteristics with Additional Shiel-
ding
Figure 10
Figure 10 shows the data with the additional shield.
By comparing Figure 10 with Figure 9, it is obser-
ved that the entire spectrum is lower. The shielding
attenuation can be determined from these two
figures. For the marker frequency of 75MHz a
signal of -55.9 dBm gives a shielding effectiveness
of [-.8(-)-55.9 dBm] of 1.1 dB. For the frequency
of 800MHz, the shielding effectiveness is only 9
dB. Shielding effectiveness of this magnitude are
hardly worth the additional sheet metal. But such
results are quite common. These measurements
were performed on an EUT which was low-price
frequency counter. There is other equipment
where similar results are obtained because the
radiation may also occur from windows and other
openings in the housing or cables connected to
the EUT. However, it is cost-effective to measure
before spending money for sheet metal.
M e a s u r e m e n t o f S h i e l d i n g A t t e n u a t i o n o f S h i e l d e d H o u s i n g s
