HOWS YOUR EMI QUOTIENT?EMI, or ElectroMagnetic Interference, continues to be a relatively unknown nemesis. Not only for product designers, but more importantly for users of electronic devices who can bump into EMI and not realize the problem until symptoms occur - such as CRT screens starting to 'blip'. Current FCC regulations dictate that cell phones, computes and other electronic devices incorporate RFI shielding to block unwanted RF emissions interfering with other equipment found in the home and office. Designers should be aware that densely packed electronic assemblies might have internal components that interfere with each other. While there is no known material that will "block" magnetic fields, electromagnetic shielding relies on the ability to re-direct magnetic fields from unwanted areas. By diverting the magnetic flux to themselves, EMI shielding utilizing dosed shapes is most efficient for the containment of unintended interference. Electromagnetic interference can be classified by its frequency characteristics. The following is an approximate classification of interference frequencies.
While there is no known material for blocking magnetic fields, they can be "redirected" by using highly permeable shielding alloys. Shielding can be designed to be incorporated into something as small as a miniature electronic device or as large as an entire building! OSHA REPORT Continued from page 1In general, an antenna might be one of the conductors in an electronic circuit, a metal object such as your front porch railing, or even nonmetallic objects like a tree limb or an extended arm. The effectiveness of an antenna to transmit or receive EM waves depends on the conductivity of the material used, the antenna's shape, and the physical dimensions of the antenna relative to the wavelength of the EM field. The best broadcast and reception of EM waves is obtained when the dimensions of the antenna properly match the wavelength of the electromagnetic field. That is why the length of TV "rabbit ear" and "whip" antennas need adjustment each time the channel is changed, and why roof mounted TV antennas have so many different sized elements. When measuring worker exposure to non-ionizing radiation (EM fields), it is important to be aware that the probe is also an antenna. The antenna and circuitry of an RF probe are arranged so it can function over a range of operational frequencies. The width of this operational frequency range is called the bandwidth. If measurements are attempted outside the probe's frequency range, the measurements will be inaccurate and could severely damage the probe. Always choose the proper probe based on both power rating and the frequency. This report is excerpted from- FIELD SERVICE MEMO - ELECTROMAGNETIC RADIATION and HOW IT AFFECTS YOUR INSTRUMENTS © Occupational Safety & Health Administraion 200 Constitution Avenue, NW, Washington DC 20210
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