Anechoic Foam, and Ventilation Enhance Shielding Performance When Properly Specified
As budgets continue to be scrutinized, design engineers and managers are looking into semi-permanent, temporary or mobile EMI shielded enclosure options including hard-wall relocation and soft-sided tent enclosures. In my previous article 5 EMI RF Shielded Room Design Considerations I talked about five initial design considerations: existing space, design cycle, shielding effectiveness, controlling entry and exit, and size requirements. Here are three more considerations to keep in mind:
I/O plates and filters: The method of I/O plate installation and the selection of interface connectors are two key factors that determine the shielding performance of the entire system. When considering a portable RF shielded tent for your EMC testing, providing the required shielding performance is helpful in determining the proper filter and connector options. Select’s RF shielded enclosures are fabricated using a single layer (65 to 70 dB, 150 kHz to 18 GHz) or double layer (80 to 90 dB, 150 kHz to 18 GHz) conductive fabric construction. In most cases, standard performance filters are recommended for the single layer enclosures and high performance filters are recommended for the double layer.
One design consideration is how to properly support the weight of the filter. Floor mounted stands, structural framing and flanges are typically used to brace the I/O plate without having an impact on the rest of the structure. These I/O plates may house filters, ranging from simple power filtering with standard performance 0-250 VAC, 15 Amp IEC or filtered barrier strips with USA 120 VAC/15A 5-outlet power strips or 230 VAC international power strips, to large facility filters that provide single or three phase power, are key to RF isolation and high performance test solutions. Simple power filtering can be accommodated using a floor stand for support and ease of connection. Select has designed framing systems that can support IO plates bearing large filters that weigh 100 lbs. or more.
Often filter plates need to be configured with custom connectors including: SMA, BNC, Type-N, Ethernet, USB 2.0, USB3.0, HDMI, POTS, ISDN, HDMI, fiber optics, which continue to change as the electronics industry and its testing standards evolve.
Sourcing a portable RF tent enclosure without specifying the key filtering requirements, shielded connectors, and IO plate installation method can lead to additional costs before the system can be used.
Ventilation: of course everyone needs to breathe; creating an EMI shielded intake and exhaust system also keeps electronics cool, allows vehicles of all sizes to be tested, and when well designed, yields the shielding effectiveness that meets requirements.
- Design specifications for correct ventilation include:
- The minimum shielding requirements and frequency range.
- The number of people or the amount of machinery scheduled to be inside the mobile enclosure will help determine if air conditioning is required.
- The size and diameter of honeycomb intake and exhaust vents
- If the faraday cage tent is being used inside a clean room, specific metals need to be incorporated to meet the special requirements for non-contamination.
When testing requirements are in the 85 to 90 dB level, a high performance option is needed. Different honeycomb metal plating and thickness are design considerations that can be discussed.
Not only is air circulation a consideration, heating and cooling of personnel and equipment can also lead to the specification of a ventilation system that includes temperature control.
Anechoic Foam: Large, semi-permanent enclosures may need to minimize internal RF reflections during EMC pre-compliance testing. Anechoic foam panels can be incorporated into the design to improve the reflective attenuation performance of the structures. These panels are easily installed on internal semi-permanent frame systems constructed using aluminum profiles or traditional wooden frames. The walls can be rolled into position to allow for changes at each test requirement. The foam panels can be covered with ESD or compatible fabrics for use in large cleanrooms. Manufacturers of the foam can provide clean room acceptable coatings or covers that can be discussed at the requirements review stage. For example, Select Fabricators completed what we believe is the world’s largest RF shielded tent system earlier this year, and at a height of three stories it required some careful design considerations that included anechoic foam panels for performing the EMC test protocols.
These are just a few of the design considerations which may improve performance and lead to lower initial capital costs as well as on-going testing expenditures. Including and reviewing all the design needs necessary for the test standard to be performed is critical for success.
Continue Reading Shielded Room Design Considerations – Part 1