hundred megahertz; its shielding effectiveness increases with frequency since the bond between adjacent sheets
is primarily capacitive. The normal application for this type of shield is for the protection of computers and
data processing installations operating in the vicinity of high power radars. Where shields of this type are
intended to work only at very high frequencies, it may be possible to dispense with the shielding over part of the
central floor area in ground level installations.
8.10 TESTING OF SHIELDS.
Shield testing may be categorized as (1) the testing of shielding materials to determine their shielding
properties, and (2) the testing of shield designs (such as shielded enclosures) to determine whether or not the
design and construction are satisfactory. The first category of testing results in design data such as that
described earlier in this chapter, and is usually performed by the shielding material manufacturer rather than
the equipment designer or user. Methods for performing these and related tests can be found in Reference 8-21
and are not discussed further here. On the other hand, the second category (the testing of equipment shields
and shielded rooms for verification of sufficient shielding effectiveness) is a necessary part of equipment
development and/or acceptance and is therefore discussed in the following.
The testing of constructed shields is necessitated by the somewhat unpredictable effects of both intentional and
unintentional openings and seams in the shield. Localized testing can point out the location of electromagnetic
(EM) leaks such as those resulting from welding faults in seams and from poorly fitting gaskets. Such testing is
frequently necessary for the successful construction of shielded enclosures. Uniform field (as opposed to local)
testing is useful for acceptance testing of a shield. Methods have been developed for both localized and
uniform shielding tests for variable-frequency EM fields of low impedance (magnetic), high impedance
(electric), and plane waves.
The variety of test methods available for evaluating shielding effectiveness are due, at least in part, to the
many different factors that can affect material shielding capabilities. These factors include the configuration
of the shield (Is it a sheet of material, or is it a box?), the frequency range of concern, whether or not the
impinging wave is planar, the wave impedance, and others. This section will discuss some frequently employed
and generally applicable shielding effectiveness tests. Frequently employed tests include:
Low Impedance Magnetic Field Testing Using Small Loops,
Low Impedance Magnetic Field Testing Using a Helmholtz Coil,
High Impedance Electric Field Testing Using Rod Antennas,
High Impedance Electric Field Testing Using a Parallel Line Radiator,
Plane Wave Testing Using Antennas,
Plane Wave Testing Using a Parallel Plate Transmission Line, and
MIL-STD-1377 Testing (8-22).