Quantcast Low Impedance Magnetic Field Testing Using Small Loops

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MIL-HDBK-419A
A number of the above tests are very similar to tests designed to measure equipment and system EMC in
accordance with MIL-STD-462 (8-23). They also are similar to tests performed to evaluate EM effectiveness of
shielded enclosures used for testing purpose in accordance with MIL-STD-285 (8-24). One who is concerned
with the measurement of shielding properties should become familiar with both of these standards.
The MIL-STD-1377 tests represent procedures for evaluating the shielding (and filtering) effectiveness of
systems.  The specification contains a unique approach to shielding measurements; its cable effectiveness
evaluation methods are good illustrations of how cable and connector performance tests should be performed.
It should be pointed out that a high degree of measurement accuracy cannot generally be expected for shielding
tests. Typically, wave impedances are not established when the tests are performed, antenna correction factors
used for calibration purposes are based on plane-wave assumptions even though the test condition may not
warrant this assumption, the degree of radiated field distortion by proximal structures is not known, and other
factors limit the accuracy of the measurement. However, the tests can be expected to provide guidance on the
shielding design approaches and the general effectiveness to be expected of those approaches.
8.10.1 Low Impedance Magnetic Field Testing Using Small Loops.
This test is designed to indicate the shield's effectiveness in reducing the intensity of predominantly magnetic
field radiation. It employs two small loop antennas and evaluates loop coupling with and without an intervening
shield. MIL-STD-285 incorporates a similar magnetic field small loop measurement procedure to evaluate the
shielding effectiveness of shielded enclosures used for electronic testing purposes.
In this test, a pair of identical small loop antennas are used, one on one side of the shield and one on the other,
spaced equidistant from the shield. If an enclosure is being tested, the usual practice is to have the test signal
source within the enclosure and the receiving loop and detector outside the enclosure.
Figures 8-39 and 8-40 show the two basic loop orientations. In Figure 8-39 the loops are coaxial, that is, both
loops are normal to a common loop axis. In Figure 8-40 the loops are coplanar, that is, the loop surfaces lie on
the same plane. Tests using at least these two orientations should be employed, but orientations that may
result in a lower effectiveness figure should not be ignored. Both the loop diameters and the loop separations
should be significantly less than the shortest dimension of the box, container, or enclosure being tested. Since
this will result in only a small section of the shield being illuminated at one time, it will be necessary to move
the loop over the entire surface of the shield to establish the effectiveness of the shield.
The frequency range over which this test can be performed is a function of the level of shielding effectiveness
that must be measured (measurement system dynamic range), the sensitivity of the test equipment, the
available power to drive the test transmitting loop, and the loop-to-shield separations. The limiting factors are
usually the areas of the loops and the number of turns in the loops, since these establish the self-resonance
frequency of the loop. Loop-to-loop separation should not be closer than the loop diameter.
The small loop-to-loop setup specified in MIL-STD-285 is shown in Figure 8-40 with the following parameter
values employed:
8-73





 


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