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 MIL-HDBK-419A
Figure 8-37. Effectiveness of Circumferential Spring Fingers
for Improving the Shielding of a Connector (8-18)
8.9.1 Demountable (Modular) Enclosures.
The basic construction of a demountable enclosure might be a 1.27 cm (1/2 inch) thick plywood panel faced on
both sides with an electro-galvanized steel sheet of nominal 0.56 mm (0.022") thickness. For non-isolated
double shields, the double facing of the walls makes panel-to-panel joining a considerably more certain process
as each bonding joint is duplicated. The joining between wall panels is effected by a specially formed metal
section, and the design of this requires a fairly precisely controlled blend of resilience and rigidity to establish
continuous contact without gaps throughout the length of each bonding member.
The most critical part of any shielded enclosure is the door; with some modern installations doors sizes of 1.86
square meters (20 square feet) and above are required. In general, two types of door bonds are used; these are
referred to as the "wedge" and the "knife edge" design. The most commonly used is the wedge door (Figure
8-38) which takes the form of a standard casement type hinged opening leaf or leaves with the frame and the
door leaf edges shaped to form a wedge entry, and beryllium copper finger stock affixed in a double layer
around the complete periphery of the door leaf. The reason for adopting the wedge design is that, by correctly
choosing the angle of wedge, contact pressure on the finger stock can be made high without the risk of tearing
and breaking of the spring fingers when the door is opened; it has been found that this type of construction can
achieve an overall performance on the order of 125 dB attenuation. The second type of door which has been
used for special applications is the knife edge design in which the door leaf is provided with a flanged edge
made to enter between two sets of finger stock contacts, enclosed within a channel section fixed upon the door
frame. An advantage of this construction is that finger stock is completely protected and the performance is
better than obtainable with a wedge door, especially at low frequencies.
8-66
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