Conformity Magazine - July 2008 - (Page 18)

FSNSA is a volumetric test that requires the transmit antenna to be placed at 3 different heights (planes) and 5 different positions (Figures 3 and 4). FSNSA is measured at a 3m or 5m separation, with the separation remaining constant between the transmit and receive antennas. In addition both antennas are aligned towards each other at each position during the test, thus making the calibration much simpler by allowing the use of one individual antenna rather than the multiple geometric factors required for NSA tests. From experience however, it does not simplify the chamber validation. The FSNSA pass criteria from 30-1000MHz are the same as for the NSA measurement, and set at +/-4dB. Although the number of FAR sites is still low compared to SACs, it is generally accepted now that a compliant 3m FAR can be similar in size to a compliant 3m SAC, thus allowing a 3m SAC to be easily converted when lined with the appropriate floor absorber. Larger FARs are rare, with potential 10m type measurements preferring a 5m FAR, mostly due to cost and currently a lack of agreement on limits from the standards committees. Practical issues, such as the handling of large floor standing EUTs typical of 10m sites, have tended to limit the application of FARs to table top equipment. But, as CISPR 16-1-4 now allows both SAC and FAR to co-exist, there should be a method for everyone. Considerations for the Frequency Range Above 1GHz Above 1 GHz, CISPR 16-1-4 specifies a method known as the site voltage standing wave ratio (SVSWR) test for the validation of EMC anechoic chambers. Since this method was only very recently introduced, it had been common historically for another method, known as the free space transmission loss (FSTL) test, to be applied in the absence of any standard. The FSTL method is very similar to the FSNSA method described previously, and typically transmit and receive antennas were both double ridged horns. For many years, critics of the FSTL method saw the use of highly directive antennas as making it too easy to pass, as their high gain meant that the chamber was not really being tested but, instead, the quality of the antenna calibration, and that that alone played the main role in the chamber validation result. In addition, as an unintentional radiator, the EUT would normally have a more complex and less directive radiation pattern and so it was felt that the horn antenna did not give a true representation of this pattern at these frequencies. As a consequence, one of the main differences in the new SVSWR method described by CISPR 16-1-4 is the use of a more isotropic antenna type as the transmitting antenna. The receive antenna is specified to be of the same type that is used subsequently for the product testing, and is typically a broadband horn antenna. A specification of the radiation pattern for the transmit antenna is included in the standard and shown in the Figures 3 and 4. Unfortunately, no calibration method is given for determining the pattern of the transmit antenna. This is something that should be developed in future versions of the standard, although many calibrations laboratories can already offer such a service. Currently, the only available data is supplied by the manufacturer of the antenna, of which there are unfortunately too few, and the lack of calibration method leaves the door open to disparity in the uncertainty of the subsequent measurements. It is hoped that CISPR 16-1-5 will address this in future editions. Figure 3: E-plane requirement of the TX antenna Figure 4: H-plane requirement of the TX antenna 18 Conformity JULy 2008

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Conformity Magazine - July 2008 Contents Editor's Note FCC Adopts 6 Million Dollars in DTV Enforcement Orders Commission Enforcement Bureau Adopts V-Chip Consent Decree FDA Seeks 2 Million Dollars from Medical Device Manufacturer FDA Answers Questions About CFLs and UV Radiation EPA Issues Energy Star Specs for TV, Video Boxes Canada Unveils Sweeping Product Safety Legislation How the Evolution of CISPR Standards Continues to Shape the Requirements for Anechoic Chambers ESD Open Forum Reach Compliance for Non-EU Article Manufacturers The iNARTE Informer Preamplifier Considerations for Emission Measurement Systems Effective Shielding in Medical Electronics Applications Product News Focus On...Power Components Buyer's Guide Updated Standards List for the EU's ATEX Directive EOS/ESD Symposium Set for September CPSC Actions in the News Researchers Work to Develop Non-Flammable Batteries Safety Link Launches Redesigned, Enhanced Web Site IEC Standards Update UL Standards Update Product Reviews Telcordia Standards Update From Our "You Can't Make This Stuff Up" Department Looking Back: Items from Past Issues of Conformity Advertisers

Conformity Magazine - July 2008

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