Door Closure/Seal Comparisons

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Figure 1. RF Door and Wall Seam Type Cross-Section

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Radio Frequency Shielded Enclosures

Figure 2. Preferred Waveguide Air Vent Filter

2.6.2.2

Considerations for Fingerstock Replacement. Because of the delicate

nature of fingerstock materials, the door design should provide for rapid and

easy replacement of fingerstock without the requirement for special tools and

soldering. Pneumatic doors, which are all of the sliding type, must have

available a pocket with a removable cover so that the door can be serviced or

removed. The use of dissimilar metals or metal finishes should be minimized

in door design because of the possible battery action and corrosion which

occurs in the presence of any moisture.

2.6.2.3

Door Closure/Seal Comparisons. Door latching mechanisms must be

designed to provide uniform wiping and compression of the fingerstock with

enough force to provide the required shielding effectiveness. They must also

allow quick egress to satisfy the requirements of governing National Fire

Protection Association (NFPA) rules and regulations. The pneumatic type door,

even with automatic opening and closing, requires the longest opening time,

typically seven or more seconds. The Army Corps of Engineers Civil

Engineering Laboratory Technical Report M-313, April, 1983, Study of EMI/RFI

Seals on Shielded Enclosure Personnel Access Doors, provides an experimental

analysis of EMI door sealing mechanisms which indicates that wedge and knife

edge type doors degrade as much as 15 dB in as little as 4 months due to

routine use and exposure. The magnetic strip type of door is new, and little

long terms data is available. The brass shimstock material covering the

magnets tends to oxidize rapidly, and a maintenance schedule similar to knife

edge doors is projected. A further problem projected for the magnetic strip

doors is the work hardening and eventual cracking of the brass shimstock which

holds the individual magnets in place.

2.6.3

Air Duct Penetrations. The second largest openings in shielding are

intake and exhaust of air ducting in heating, ventilation, and air

conditioning (HVAC) of the enclosures. In order to maintain the required

shielding effectiveness through frequencies as high as 10 GHz, a waveguide

below cutoff air filter must be placed in the duct penetrations through the

shielding. The waveguide filter may be a standard shielding manufacturer

product, typically a brass or steel core, tin-dipped, which may be similar in

appearance to a honeycomb with the walls of each small cell continuously

welded or soldered to the next cell. Waveguide or honeycomb assemblies are

soldered in a parent framework, which is then welded or bolted into the

shield, see Figure 2. The attenuation characteristics for each individual

cell must exceed the shielding effectiveness requirements of the total

enclosure. The static pressure drop that the waveguide filter causes in the

airstream must be included in the HVAC calculations, as well as being

specified and controlled in the governing specifications. The use of radio

frequency (RF) gasketing materials in the assembly and closure of the air

filter should be expressly forbidden by the written specification.

Dissimilar, and electrochemically active combinations of metal surfaces should

also be prohibited in this location where severe corrosion can be accelerated

by the combination of mechanical vibration, air flow, possible moisture, and

changing temperatures at metal-to-metal interfaces. Filter honeycomb inserts

required for MIL-STD-285 and NSA 65-6 shielding requirements are typically one

inch in depth, with individual cell openings of 1/4 in. (6.3 mm) or less.

Custom made units may have clusters of larger diameter metal tubes,

continuously bonded at the end of a shielding plate, with hole sizes up to 1/2

in. (12.7 mm) diameter for MIL-STD-285 and NSA 65-6 required shielding

effectiveness at 10 GHz.