8.13.1 Size. If a very sensitive piece of equipment or small system is to be located in a large structure,
shielding the entire structure to protect that one small element is probably not cost effective. The cost of
shielding is closely related to the size of the enclosed volume, assuming all other factors equal. Thus, a more
economical approach would perhaps be to shield only the room in which the equipment is to be located,
construct a shielded cage just for the susceptible (or offending) equipment, or upgrade the shielding of the
particular equipment cabinet or enclosure. If, on the other hand, the susceptible element is a fairly large
system, e.g., a communications center or a large scale computer, then incorporating appropriate shielding
materials into the walls, floor, and ceiling of the room or structure may be necessary. If this requirement is
recognized early in the design stage of the facility, the required shielding may be provided by properly-installed
conventional structural materials. Also, supplemental shields can frequently be installed with greater economy
if done during construction rather than later.
If a susceptible equipment or system is to be located in a building and some choice exists as to position, special
effort should be made to take advantage of the inherent shielding properties of the structure. The existence of
metal walls, decorative screens, and other conductive objects may provide all the shielding necessary. Further,
equipments frequently are more sensitive to radiated signals impinging from only one or two directions. Thus,
orienting the equipment such that the susceptible side is facing away from the incident signal can lessen the
Signal and control cables deserve special mention. Because the voltage (or current) in the receptor wire is
inversely dependent upon the distance from the source wire and directly proportional to the length of the path,
every effort should be made to avoid long runs in parallel.
8.13.3 Signal Properties.
The shielding effectiveness of practically all materials is frequency dependent. The type of shield which will
protect against an X-band radar signal will not necessarily be effective against a commercial broadcast
transmitter. In choosing a shield for a particular purpose, compare the attenuation properties of the material
with the frequency of the threat signal.
The amplitude of the signal to be shielded indicates the amount of field attenuation the shield must provide.
For most fields, the attenuation provided by the shield is not influenced by the magnitude of the field, i.e., a
shield which will attenuate a low level field 60 dB will likewise attenuate a high level field 60 dB. Very strong
magnetic fields, however, can cause saturation effects and the attenuation of the shield will generally decrease
under very strong magnetic fields. This phenomenon is very important in choosing shields to protect against
EMP for instance. Where saturation effects are likely, thicker shields are required in order to maintain the
attenuation needed to protect against the very strong fields.
The impact of size on cost was noted previously in Section 8.13.1 above. Other cost factors to consider include
those associated with providing input and output ports for wiring and cabling, ventilation, and physical and
visual access (doors, windows, meter openings, etc.) while maintaining the effectiveness of the shield.