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 MIL-HDDK-419A
Use of screens over meter faces and other large apertures has often been employed for shielding purposes. A
typical screen introduces a minimum of 15%-20% optical loss which can create difficulties in reading meters.
If the device being shielded has a scale (such as an oscilloscope graticule), bothersome zoning patterns can
result. However, these potential deficiencies are counterbalanced by good shielding efficiencies at a fairly low
cost.
Glass coated with conducting material such as silver can provide shielding across viewing surfaces with some
loss in light transmission. Conductive glass is commercially available from a number of glass manufacturers.
Figure 8-28 provides shielding effectiveness data on 50 and 200 ohms per square silver-impregnated glass
against electric arc discharges. Figure 8-29 indicates shielding effectiveness as a function of surface
resistance for plane waves in the frequency range from 0.25 to 350 MHz. The light transmission characteristics
of this type of glass as a function of surface resistance is presented in Figure 8-30. For effective shielding,
good contact to the conducting surface of the glass must be maintained around its periphery.
8.6 SELECTION OF SHIELDING MATERIALS.
The selection of the material should be based on its ability to drain off induced electrical charges and to carry
sufficient out-of-phase currents to cancel the effects of the interfering field. The inherent characteristics of
the metal to consider are its relative conductivity, and its relative permeability,
The thickness of the
shield and the frequency of the signal to be attenuated are also important.
The selection of proper materials for shielding should be made in accordance with the following basic rules:
At low frequencies (LF), only magnetic materials can furnish appreciable shielding against magnetic
a.
fields.
b.
For a given material, magnetic fields require a greater shield thickness than do electric fields.
At higher frequencies, smaller shield thickness is required for a given material.
c.
d . At sufficiently high frequencies, nonferrous materials such as copper and aluminum will give
adequate shielding for either electric or magnetic fields.
e . The electric field component for frequencies from 60 to 800 Hz (i.e., ac power) can readily be
shielded with thin sheets of conducting materials such as iron, copper, aluminum, and brass.
For a detailed description of the procedure for selecting a shield material for a facility, see Volume II. Care
must be used when adding a shield to a subsystem. For example, a shield placed too close to a circuit in which
the circuit Q is a critical factor can cause degradation of performance because the losses in the shield will
appear as an effective resistance in the critical circuit, thereby lowering the circuit Q.
8-53
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