June 2011

Overview of EMI Shielding Compounds

Electromagnetic radiation that adversely affects device performance is generally termed EMI (ElectroMagnetic Interference). Interference takes many forms such as distortion on a television, disrupted/lost data on a computer, or “crackling” on a radio broadcast. Many electronic devices not only emit electromagnetic fields which might cause interference in other systems, but they are also susceptible to stray external fields which could affect its performance. As a result, they must be shielded to ensure proper performance.

Utilities operating nuclear power plants have been dealing with electromagnetic interference (EMI) problems for over two decades. Many early problems that affected the operation of instrumentation and control (I&C) equipment in plants stemmed from the use of wireless transmission devices (WTDs) (e.g., radio walkie-talkies, cellular phones, etc) inside the plant in the vicinity of system cabinets and cable trays carrying bundles of cables. A simple and partially effective method of reducing EMI events caused by WTDs has been to mark off exclusion zones around system cabinets and areas where I&C equipment is installed. The use of these zones has presented some problems for existing plants. For example, some plants have had to expand the area of some zones that became ineffective upon the use of new WTDs that evidently presented an increased risk to the operation and EMI protection of I&C equipment. The sizes of some expanded zones are larger than 2,000 square feet. In addition, some zones encroach upon human traffic areas used by plant personnel to move from area to area within a plant.

There may be a better use for PCB planes than to just distribute power, namely to provide shielding.

Antenna characteristics, such as input impedance are considerably deviated from those in free space when another antenna is located in the vicinity of the antenna. This often causes system degradation problems. In this article, effects of a parasitic wire located beneath the slotted ground plane are investigated on the coupling between monopole antennas above the ground plane. The method of moments is applied to the problem and a combined matrix formulation that includes mutual coupling effects between the elements located in both regions is newly introduced.  It is shown that a wire beneath the ground plane considerably affects coupling characteristics between two monopoles above the ground plane when the slot resonates.