New Modular RF EMI Enclosure Design

New, semi-permanent, large scale EMI / RF test enclosure system

Select Fabricators, Inc. was presented with a new design challenge in 2015, to build a test enclosure under Mil-Std. 461G-Appendix A that is moveable, lower in cost and weight than a permanent welded shielded chamber, and easy to install. Finished Assembly 38 Image 0002 (1) SFi succeeded in designing a semi-permanent system that provides over 70dB attenuation over the range of 20MHz to 18 GHz (IEEE™ 299). A new take on current hardware cloth tent designs it has both removable panels and an internal frame. SFI has since made the design both scalable and patent pending. There are five parts to the system, an internal frame, metal ground plane floor, removable fabric panels, collapsible anteroom and suspended anechoic foam panels.

Internal Frame
A free-standing, weight bearing aluminum channeled post and beam frame was designed to meet modular specifications. The frame supports the weight of port plates, multiple vents for both intake and exhaust, the modular fabric panels and RF anechoic foam panels. Internal frame members have channels which are used to create a clamping system with fabric overlaps which provides a RF tight seal. The patent pending design includes a method to conductively connect every horizontal and vertical frame member in the system. We worked with the manufacturer to specify parts which are or can be cleaned to ISO Class 7 (Class 10,000 –Fed. Std. 209).

Metal Ground Plan Floor
The usual tent design floor is conductive fabric encased in heavy duty vinyl. For very large and heavy EUTs, the new temporary screen room design includes an aluminum sheet ground plane system which is also conductively connected to the frame and modular panels while allowing heavy units to enter and exit without damaging the enclosure or the conductivity. The floor panels use a rabbet edge design and are connected together with an EMI gasket and flush-mount clamping bar. The perimeter floor plates contain hardware for clamping the tent’s modular fabric panels and establishes the RF tight seal.

Removable Fabric Panels
The removable panels of this design is made from NovaSelect™ conductive silver/copper/nickel fabric, externally coated with an acrylic that minimizes particle shedding and metal plating breakdown and is cleanroom Class 10,000 compatible. Each panel can be removed from the internal aluminum frame for access to the test space as well as accommodate the installation process. Every modular panel has stainless steel grommets installed around the outside perimeter to allow for stud/clamp connection to frame system and aluminum panel floor system.

Because each panel is conductively connected to each other, the equipment door does not need a threshold and heavy items can be easily rolled in and out. The large equipment single door / double row flexible magnet seal (using SFi’s patented door seal system, US Pat. No. 9,029,714) is a separate attachable panel including an aluminum bar clamping system to prevent unintentional opening during equipment testing.

Modular Anteroom
In order to enter or exit the screen room after the main equipment door has been secured, man-doors and/or an external anteroom can be added. The external anteroom is also a control room for equipment during testing procedures. The anteroom can be sewn onto and permanently attached to a modular panel connecting to modular panels, internal frame and aluminum ground plane floor. The anteroom frame is designed to be removable which allows the anteroom to be accordion folded towards main chamber which helps minimize the test enclosure’s footprint when not in use.
External anterooms include Select’s standard conductive fabric floor with heavy duty vinyl protective layering on top and bottom. All four walls and ceiling inside the anteroom can be covered with lightweight fire barrier material.

Anechoic Foam Panels
Encased to prevent shedding and comply with ISO Class 7 (Class 10,000 –Fed. Std. 209) cleanroom specifications, the anechoic foam panels are designed into the modular system at discrete places to assist in absorbing unwanted reflection in the paths of the test antennas. These RF absorbent foam panels are installed using the prepositioned horizontal chair rails in the frame system.

The new, semi-permanent system design cost was five times less than the bids received for a permanent welded chamber. Installation took one week with another week for testing, well within the time constraints specified and less than the one month typical for permanent welded chambers. At a 10:1 weight ratio to a welded screen room, the ability to move the RF shielding fabric hardware enclosure to a new location is both cost and time efficient.

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