MS84.OCT–Gore–SC
EMI/RFI shielding for military applications
Shielding requires a system approach, the right test method, and careful
selection of construction materials. One test method, mode-stirred, is
especially effective
BY VU ANH LAI W.L. Gore & Associates Manor, TX
The end of the Cold War era is driving the development of less
destructive, more surveillance-oriented electronics warfare applications.
Also, military spending cutbacks are forcing contractors to forsake new
systems and instead upgrade existing systems. Both factors stress the need
to effectively shield existing military systems from electromagnetic and
radio frequency interference. To accomplish this shielding, military
system designers must take three steps. They must learn to tackle the
shielding problem on a system and not individual component level. They
must use the right testing method. And they must carefully select
shielding construction materials. The basic consideration for military
system designers who design in shielding is that EMI must be solved with a
system approach. Each component in a system–down to cables and
connectors–can be a source of EMI problems. Moreover, how these
components connect to each other is critical in determining the overall
EMI/EMC level of that system. Often, either the individual cable or
connector meets EMI requirements, but the completed assemblies fail
because of poor termination techniques. Figure 1 shows a basic
interconnect system consisting of two electronic devices connected by a
cable assembly. Even if each component in the system meets EMI
requirements, it does not guarantee that the whole system will meet EMI
requirements. In addition, the interface of any two components must also
be carefully designed because, in many instances, they are the weakest
spots. As shown in Fig. 1, the electronic system and connector interface,
connector, connector and cable interface, and cable are possible problem
spots in a typical cable assembly system. Therefore component selection
and termination techniques must be selected carefully to ensure system
performance and to avoid costly repairs later. This system can be compared
to a water pipe system in a home, where all pipes and the joints must be
proofed against holes and gaps to prevent leakage. Testing for EMI
traditionally involves the use of shielded rooms, anechoic chambers, and
open-field or ground-plane test facilities. But another technique, the
mode-stirred chamber, provides more reliable, repeatable results. This
test method was originally developed by the National Institute of
Standards and Technology (NIST) and the Naval Surface Warfare Center for
military-related EMI testing a few years ago. It is now specified under
MIL-STD 1344A Method 3008. The mode-stirred chamber consists of a
shielded room containing an antenna that injects electromagnetic (EM)
fields and a reflector (tuner) to “scatter” the EM fields within the
shielded room. The chamber's tuner, a paddle-wheel model, has a relatively
long wavelength to efficiently obtain a uniform (time-averaged) field that
simulates a real-world environment. Gore has a representative
mode-stirred chamber in its Austin, TX, facility. This double-shielded 10
(L) x 10 (W) x 9 (H)-ft enclosure has a large reflector that is controlled
and monitored by a computer. This reflector can rotate continuously up to
5 revolutions per second or in steps as small as 0.028 degrees (12,800
steps per revolution). Supporting equipment includes a 10-MHz through
20-GHz signal synthesizer, 20-W power amplifiers (2 to 18 GHz), two power
meters, and a 20-GHz spectrum analyzer. Advantages of the mode-stirred
chamber over other test areas include: 1. Broader frequency range (200
MHz to more than 18 GHz). 2. Ability to generate electromagnetic (EM)
fields efficiently over large test volume. 3. Electrical isolation to and
from external environment. 4. Protection of test personnel from exposure
to rf field. 5. Accessibility (indoor test facility). 6. Device under
test (DUT) held stationary while EM fields are stirred. 7. Cost
effectiveness compared to echoic chamber and open-field test setups. 8.
Repeatability (less than 4 dB). The mode-stirred method can be used to
measure the shielding effectiveness of a cable assembly as follows. A
defined EM field is injected into the chamber via a double rid-horn
antenna. While the field is injected into the chamber, the reflector is
rotated at a constant speed to scatter the EM field within the chamber.
The maximum level of the field coupling into a second double rid-horn
antenna (equivalent to no shield condition) is recorded in dBm. With the
same EM field condition inside the chamber, the maximum coupled level
coupling into the DUT is recorded, also in dBm. The difference between the
maximum coupling into the reference antenna and the maximum coupling into
the DUT is the shielding effectiveness of the DUT in decibels. As with
other EMI test setups, correlations between similar test setups is very
important to compare data from different mode-stirred chambers. Figure 2
shows a typical correlation between data from cables tested by NIST and
Gore. Both test setups used a 2-ft, single-braided coaxial assembly with
an n-type connector.
Typical test results Various shield constructions can be tested with the
mode-stirred chamber. A typical test compares 50-ohm coaxial cables that
have 26 AWG center conductors. Constructions represented include single-
and double-braided shield Gore and industry-standard RG cables, as well as
Gore's Paratron cable, which combines a double-braided construction with
conductive polymer. Results from the mode-stirred chamber show that the
use of hybrid construction materials like those in the Paratron cable can
improve shielding performance, because of the addition of conductive
materials in the cable. Test results show that the Paratron cable
exhibited a shielding effectiveness of 90 dB, exceeding that of other
construction materials. Figure 3 compares the shielding effectiveness of
the Paratron cable with a Gore double-braided shielded cable.
CAPTIONS:
Fig. 1. Weak links in a typical electronic interconnection system–where
shielding needs must be carefully considered–include the electronic
system and connector interface, connector, connector and cable interface,
and cable.
Fig. 2. To compare test results from different mode-stirred chambers,
test setup data should be correlated, as is the case in this graph between
cables tested by the National Institute of Standards and Technology and
Gore.
Fig. 3. Shielding effectiveness for the Paratron cable from Gore (a),
which has additional conductive material, exceeds that of a conventional
double-braided cable (b).
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