8.8.2 Terminations and Connectors.
If the effectiveness of a shield is to be maintained, the cable shield must be properly terminated. In an
otherwise adequately shielded system, rf currents that are conducted along shields can be coupled to the system
wiring from the point of an improper cable termination. This is a particularly important consideration in the
case of cables exposed to high power rf fields.
In a properly terminated shield, the entire periphery of the shield is grounded to a low impedance reference,
minimizing any potentials at the surface of the termination. MIL-E-45782B (8-16) recommends against use of
soldering to terminate shields because of the danger of damaging conductor insulation, and suggests a variety of
termination methods, all involving crimping operations. The use of silver epoxy or other synthetic conducting
material has been found to be unacceptable for shield bonding because of lack of mechanical strength necessary
for this application.
Cable connectors are made in many styles for a multitude of power, signal, control, instrumentation,
transducer, audio, video, pulse, and rf applications. They are made to fulfill special functions and may be
required to be hermetically sealed, submersion proof, and weatherproof. They are manufactured in the straight
type, angle type, screw-on type, bayonet twist-and-lock type, bayonet screw-on-type, barrier type, straight
plug-in type, and push-on types (see Table 8-18).
Figure 8-35 illustrates the type of connector that should be used when a shielded cable assembly contains
individually shielded wires. The practice of pigtailing these shields and connecting them to one of the pins is
not recommended. The individual shields should be connected to coaxial pins specifically adapted for this
purpose, with the shields of the mating surfaces making contact before the pins.
When maintaining the shielding integrity of a connector pair (i.e., two interconnecting connectors), a good
method to employ (see Figure 8-36) is to place spring contacts inside one portion of one connector so that
positive contact is made along the circumference of the mating parts. These contacts are extended so that the
shell of the connector mates before the pins make contact on assembly of the connector and breaks after the
pins on disassembly. A connector which meets these requirements is available under MIL-C-27599 (8-17) and is
the preferred type to be used in rf-proof designs.
The advantages gained using circumferential spring fingers over bayonet coupling is dramatically illustrated in
Figure 8-37. In this case, the spring contacts were of silver-plated beryllium copper.
8.9 SHIELDED ENCLOSURES (SCREEN ROOMS). Screen rooms are specially constructed enclosures designed
to provide an electromagnetically quiet area. In very high level signal environments or where very sensitive
equipments must be protected, screen rooms may be necessary. Table 8-19 summarizes some of the more
significant features of twelve different types of screen rooms. These same rooms with carefully engineered
apertures and openings can be expected to provide at least 100 dB attenuation to electric and plane wave fields.
When the installation of a shielded room is required, a number of alternatives must be considered. The most
important of these alternatives is whether to shield an existing or future room or building, or whether to
provide a demountable enclosure which may be relocated quite simply when the need arises.