How is Shielding Tested to Determine Shielding Effectiveness?

No matter the standard for shielding effectiveness, the general procedure for testing shielding effective compliance follows the same process.

First, two antennas are set up on opposite sides of the sample for test. One antenna acts as a transmit antenna, connected to a signal generator to sweep through the required frequency range, and the second a receive antenna that measures the received field strength which can then be stored in data form. This data represents the signal attenuation, or loss of signal through the barrier.

As a verification of results, the prior step to the actual measurement of the sample under test for shielding effectiveness one must calculate the dynamic range such that accurate results are being made. Therefore, two measurements must be made: no barrier (nothing between the transmitting and receiving antenna) and with a barrier (typically a metallic surface that is the best under materials used in shielding, and the resulting signal should be the floor level noise of the antenna and receiver with additional safety margin, usually 6 dB, to take account for error.

Once these two verification measurements are taken, the dynamic range can be calculated, which is simply the difference between the maximum and minimum signals. The amount of dynamic range must be greater than the amount of desired signal attenuation from enclosure shielding from the sample. Otherwise, the unit outperforms the test chamber, and thus concludes the sample shields better than the chamber, which should never be the case.

Two kinds of measurements can be taken regarding an electric or magnetic field: 1) far field electromagnetic wave; 2) near field electromagnetic field. When considering a far field electromagnetic wave measurement, this is when the electric (sym. E) and magnetic field (sym. H) are orthogonal (perpendicular) to each other, and normal (along with) the direction of the energy generation (in the test setup, this would be the signal generator). For the opposite direction, this is just the opposite case, where there is no region where the electric and magnetic fields are related at all. In other words, considering the working of an antenna system, the near field is very close to the antenna, and far field means further away.

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