at the closure. With newly brightened fingers and their mating surfaces,
proper adjustment of hinges and cams to provide even compression around the
closures at the outer edges, most available types of doors will meet the
requirements of MIL-STD-285, Method of Attenuation Measurements for
Enclosures, Electromagnetic Shielding, for Electronic Test Purposes, and NSA
65-6, National Security Agency Specification for RF Shielded Enclosures for
Communications Equipment: General Specification, especially if the plane wave
measurements are made with the receiving antenna one foot from the surface.
Knife edge types require a maintenance schedule on the order of monthly, and
compression of fingerstrip types typically bi-annually require some cleaning
and replacement. The pneumatic expansion sliding door can be obtained with
automatic opening and closing, or simply with manual operation and automatic
pressurization of the bladder when the door engages the stop. Release of the
bladder pressure is accomplished upon the actuation of a door release valve.
The pneumatic door provides the highest level of SE of all the door types,
utilizes no fingerstrips or gaskets, but occasionally requires replacement of
the pneumatic bladder.
Effect of Construction on Door Shielding. Some shielding
manufacturers refuse to guarantee performance of their door and door frames in
shielding provided by others, unless they are in control of the door instal-
lation. In the construction of welded enclosures, the welding of the door
frame into the parent shielding can easily result in warping and twisting of
the door frame with a resulting misfit and failure of the door to provide the
required shielding performance. In both bolted and welded shielding construc-
tion the installation of the doors and frames prior to completion of the
remainder of the construction usually results in damage to fingerstock, door
mating surfaces, and can also result in the bending and denting of the door
sills. Protective coverings must be provided to protect delicate door sur-
faces and fingers during construction. Other hazards degrading closing
surfaces during construction are uncontrolled welding splatter, any mis-
applied paint finishes, and surface preparation and conditioning materials.