The ASTM D 4935-10 standard involves the measurement of shielding effectiveness of planar material (square) for plane far-field electromagnetic waves. An illustration of such is shown below.
The samples under test must be electrically thin for a near field magnetic field (H) calculation. Electric field shielding effectiveness can be calculated for far-field as well. Measurements are taken within a frequency range of 30 MHz – 1.5GHz and follow the typical setup of shielding effectiveness.
The standard states that the transition region between near field and far field is not abrupt, but occurs approximately λ/2π from a dipole source, where λ is the free-space wavelength of the frequency of the source. This concept of regions is further blurred by reradiating due to scattering by reflecting materials or objects that may be distant from the source. The interior of metallic structures often contains a mixture of near-field regions.
Scope of ASTM D 4935-99 Compliance
ASTM D 4935-99 applies to the measurement of shielding effectiveness of planar materials under normal incidence, far-field, plane-wave conditions. (electric and magnetic tangential to the surface of the material).
The uncertainty of the measured shielding effectiveness values is a function of material, mismatches throughout the transmission line path, dynamic range of the measurement system, and the accuracy of the ancillary equipment. An uncertainty analysis is given to illustrate the uncertainty that may be achieved by an experienced operator using good equipment. Deviations from the procedure in this test method will increase this uncertainty.
Approximate near-field values of shielding effectiveness may be calculated for both electric or magnetic sources by using measured values of far-field shielding effectiveness. A program may be generated from the source code that is suitable for use on a personal computer.
This standard test method measures the net shielding effectiveness caused by reflection and absorption. Separate measurement of reflected and absorbed power may be accomplished by the addition of a calibrated bidirectional coupler to the input of the holder.
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