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 MIL-HDBK-419A
Loop diameters (d): 12 inches (30.48 cm)
Loop-to-shield separations (r/2): 12 inches
Loops: One turn of No. 6 AWG Copper Wire
The test setup in this specification is intended to provide a minimum of 70 dB measurement range.
The corresponding test for a uniform magnetic field requires placing the test shielded enclosure within a
Helmholtz coil (large loop), with a small detection loop inside the test enclosure. The use of a Helmholtz coil
enables a large portion of the enclosure to be illuminated at one time. Various orientations of the sample
relative to the loop should be tried. The frequency range is limited by the test sample size, which affects the
size of the Helmholtz coil. Increasing coil size increases its inductance, reduces its self-resonant frequency,
and decreases the frequency range over which its magnetic field strength remains constant. The coil diameter
should be at least two and preferably three times the longest test sample dimension. The upper frequency limit
is typically 100-500 kHz.
8.10.2 Additional Test Methods. Although additional test methods for medium and high impedance incident
waves exist, they are less frequently applied since the most difficult problem is the shielding at low frequency,
magnetic fields.
8.11 PERSONNEL PROTECTION SHIELDS.
Shields for the reduction of EMI are also appropriate for protecting people from potentially hazardous radiation,
either ionized or nonionized in nature. For most situations where sensitive electronic apparatus is present, the
facility shielding required to prevent EMI is more than adequate to provide personnel protection. However, for
high level incident fields, the need for personnel protection alone should not be overlooked. Areas of particular
concern are those near high voltage vacuum devices which may emit X-rays, near high power rf sources or
emitters such as acquisition and search radars, or near other sources of potentially damaging emanations such
as laser emissions encountered during maintenance of fiber optics containing laser diodes.
Shields for protection against contact with hazardous voltages at very low frequencies, i.e., dc and 50/60 Hz,
are not generally of the same type as those which protect against radiated fields. Personnel protection may be
nonconducting and function more as a simple physical barrier which prevents accidental contact. Metal
electromagnetic shields may also establish a physical barrier; however, the barrier is an incidental byproduct
and should not be considered to be the primary purpose of the shield.
8.12 DETERMINATION OF SHIELDING REQUIREMENTS. Comprehensive shielding of a structure,
particularly a large one, can be very expensive. Fortunately, if the threat signal environment is known or can
be predicted, an appropriate choice of available or existing materials can accomplish the necessary shielding
with minimum costs. Methods available for establishing the amount of shielding required in a given location
include analyzing equipment malfunctions or disturbances, performing an electromagnetic site survey to obtain
power density levels, and performing electromagnetic susceptibility and emissions tests of the equipments
which are to be located in the facility. Shielding requirements can then be determined by comparing the
susceptibility levels of the equipment against the power density levels measured in the area where the
equipment is to be located.
8-74
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