Conformity Magazine - December 2008 - (Page 31)

Mutual Coupling of Antennas Mutual coupling of antennas to the product under test is a significant source of measurement error at three meters but a smaller source of error at 10 meters. The average biconical antenna has a physical lateral size of about 1.5 meters (the broadband dipole assemblies), and a height and depth (length) of about 0.6 meter. The mutual coupling between the physical dimensions of the antenna and the equipment under test (EUT) is significant at 3-meters for both desktop and floor-standing equipment. The standard log-periodic antenna has a physical lateral size of about 1.5 meters, a depth (length) of 1.3 m, and a height of only about 0.06 meters. Again, the mutual coupling to this antenna is similar to the biconical antenna. The mutual coupling effect increases for the so-called hybrid antenna (a new development in antennas in the last ten years) covering the approximate frequency range of 30-1000 MHz. The physical size of the hybrid antenna approaches 1.35 meters in lateral-size (width), a depth (length) of 1.3 meters, and a height of 0.75 meters. This larger combined antenna is a significant near-field physical influence at 3-meters and it has a disturbing effect on the field being measured. The percentage influence on the emission levels at three-meters is that the general antenna size (1.5 meters) is 1/2 (50%) of the measurement distance, while at 10 meters it is only 3/20 (15%) of the measurement distance. Some EMC laboratories are using a “two-antenna” approach to speed up testing on an EUT. This approach typically uses two antenna masts, with each antenna mast approximately 15 degrees offset from the perpendicular line between the antenna center of the ground screen location and the center of the turntable. The near-field coupling at 3 meters is significant, while the coupling from one antenna mast to another at 10-meters is typically 0.3 dB, and primarily due to mutual coupling effects from cables off the antenna masts. We recommend additional research on the topic of near-field mutual coupling of the EUT and the measuring antenna, especially at 3 meters. Size of the EUT Many EMC testing labs (both internal to manufacturing corporations and independent testing labs around the world) make measurements on distributed products and not on “point sources.” The distributed size of the source of the emanations is beginning to be investigated by the standards bodies in EMC, including CISPR. This realization that the size of the product may influence the near-field emissions needs to be investigated, and we encourage further research into this area of concern. Our experience has been that measurements made at three meters are not reliably correlated to the emission levels measured at 10 meters. This was true 30 years ago when peripheral devices (printers, disk-drives, magnetic tape units, etc.) were standalone units connected to a computer mainframe (central processing unit) by input/ output cables up to 10 meters in length. It is also true today, whether it is large units being assimilated with a large server, or displays or other peripheral devices being added to desktop computers. Laboratory Measurements Radio-frequency radiated emission testing of typical products has confirmed the fact that 3-meter test results cannot reliably predict test results at 10-meters. http://www.nec-tokin.com/english http://www.nec-tokin.com/english

Table of Contents Feed for the Digital Edition of Conformity Magazine - December 2008

Conformity Magazine - December 2008 Contents Editor's Note FCC Warns Regulatory Fee Payers About Potential "Phishing" Commission Prowls the Web to Enforce Its Regulations First DTV Rollout Initiated in Wilmington, NC New Standards List Released for the EU's Directive on the Safety of Toys ESD Association Call for Papers ESDA. JEDEC Announce Standards Development Cooperation Commission Releases Data on Local Telephone Competition The Future of ESD Testing ESD Open Forum Emerging Issues in Standards: China RoHS: One Year and Counting Challenges in Testing: Revisiting 10-Meter and 3-Meter Radiated Emission Measurements Design Fundamentals: Meeting the Shielding Challenges of Injection-Molded Plastics Focus On...Microwave Absorbers Buyer's Guide FDA Recalls Automated External Defibrillators FDA Revises Guidance on Medical Device Tracking Guidance Document Safety of Passive Implants Released FDA Offers Guidance on Marketing Diagnostic Ultrasound Systems EU Commission Issues Correction on Machinery Directive Standards Other CPSC Actions in the News Product Reviews IEC Standards Update UL Standards Update Looking Back: Items from Past Issues of Conformity Advertisers

Conformity Magazine - December 2008

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