Current methods and techniques for measuring the shielding effectiveness (SE) of composites intended for shielding electromagnetic interference in home and business computing devices are examined. Emphasis is placed on those techniques that offer the best repeatability of SE data and aid the original equipment manufacturer in their proper evaluation and interpretation. Experimental results show that the NBS flanged coaxial fixture method demonstrated significant advantages over the other tests evaluated its dynamic range and was consistently higher than any of the other methods evaluated; it allows the evaluation of both near-and far field shielding (using theoretical calculations), so that both emission and susceptibility problems could be addressed; and there was less variability from location to location. It is concluded that this fixture is the most useful for repeatable material ranking. >

Test methods for electromagnetic shielding materials

A new measurement method for magnetic antenna factor, called the impedance method, is applied to the calibration of a shielded standard loop antenna. Generally, the magnetic antenna factor is defined in the far-field condition. However, it is difficult to realize the far-field condition when the electromagnetic wave is generated by electrically small antennas in a frequency range below 30 MHz. Although some methods have been proposed earlier, they present difficulties. It is possible to obtain the magnetic antenna factor of an electrically small loop antenna from Faraday's law and the input impedance of the antenna. In this paper, a comparison is made between a numerical simulation and experimental results. It shows that the impedance method is simple and efficient for calibrating a standard loop antenna

Impedance Method for a Shielded Standard Loop Antenna

It is shown that arbitrary electromagnetic fields and wave impedances can be generated inside a transverse electromagnetic (TEM) cell for RF susceptibility testing. This is achieved by simply exciting one port and terminating the other port with appropriate loads. Thus, TEM cells are not limited to producing a planar field environment with the free-space impedance of 377 Omega , but can also be used to generate high-impedance or low-impedance fields for special testing needs. Experimental results for only three load impedances are described. In principle, many other loads including reactive terminations could be used to create a particularly desired field and wave impedance inside the TEM cell. >

Electromagnetic fields with arbitrary wave impedances generated inside a TEM cell

The theoretical basis and test procedures for calibrating horizontally polarized dipole antennas at the National Bureau of Standards are discussed. Two different techniques and two different test sites are used. In the standard antenna method, a field-strength level is calculated from the response of a simple half-wave dipole. This method is used at an open-field site in the frequency range of 25 to 1000 MHz. In the standard-field method, the theoretical gain equations of waveguides and horn antennas are used to determine the field-strength level. This latter method is used in an anechoic chamber in the frequency range of 200 to 1000 MHz. Procedures for both techniques are explained and measurement setups are illustrated. Measurement uncertainties are discussed. >

/pdf/nbs-calibration-procedures-for-horizontal-dipole-antennas-25-1b7mb7m41l.pdf

NBS calibration procedures for horizontal dipole antennas (25 to 1000 MHz)

The author reports on the results of a correlation exercise comparing radiated emissions measurements obtained at three different kinds of facilities: a GHz transverse electromagnetic (G-TEM) cell, a semianechoic chamber (SAC), and an open area test site (OATS). A source with a data cable which must exit the cell is used. The test objects were digital phones. These telephones receive their power and communicate over an unshielded twisted pair cable. The cable must pass out of the cell and to a switch simulator, which maintains the telephone in its operation state. Several steps taken to improve the correlation between these various radiated emissions test facilities are reviewed. The correlation between the G-TEM and OATS is found to be generally better than 4.5 dB. For most frequencies the correlation is better than 2.5 dB. >

http://ieeexplore.ieee.org/iel2/3346/9997/00473726.pdf

Radiated emissions test correlation between G-TEM, SAC and OATS facilities using digital phones

/pdf/electromagnetic-compatibility-and-interference-metrology-24xb7e2sxk.pdf

M. T. Ma

Papers

Electromagnetic compatibility and interference metrology