Conformity - November 2008 - (Page 14) Several technical issues were recognized in these seminal articles. Those technical issues included restrictions on the measurement geometry discussed in [1]. Smith identified: 1. 2. Measurement distance as a concern to minimize antennato-antenna coupling and near field effects; The need to keep antenna heights (both transmitting and receiving) high enough to minimize antenna-to-ground plane mutual impedances and to ensure a negligible contribution from the surface-wave component of the ground wave; The preference of horizontal over vertical polarity calibrations because “mutual coupling between the antenna and the orthogonal transmission line is negligible, calculations of the horizontal ground wave are simpler than calculations for the vertical ground wave, the surface-wave component for horizontal ground wave over earth is more tightly coupled to the surface, and the horizontal ground wave is less sensitive to differences in surface conductivity and permittivity than the vertical wave.” During the ensuing years, efforts were made to get the international community to adopt ANSI C63.5 as an international standard for the calibration of antennas within CISPR 16. It turns out that this adoption almost happened well over a decade ago but, for several reasons, it did not “make the cut”. Several concerns were raised, including the fact that the standard allowed for vertical polarizations and did not explicitly restrict such measurements. In addition, the international community considered 3-meter calibration distances insufficient to reduce antenna-to-antenna coupling. This issue was implicitly addressed in ANSI C63.4 with the addition of mutual coupling correction factors for dipole antennas used in normalized site attenuation testing (Table 4 of [4]). The 1998 edition of C63.5 included 10-meter calibration distances and explicitly removed vertical antenna calibrations as CISPR was leaning to this approach. This resulted in unintended consequences for site measurements. The use of horizontal antenna calibrations for vertical polarity site attenuation measurements assumes the issues 1-3 raised by Smith in 1982 were insignificant. This assumption was cast into doubt in light of the ±4 dB test site criteria in [5]. The errors, once thought to be negligible, needed to be redressed, as they represented between 1 and 2.8 dB of the ±4 dB test site criteria. These errors were corrected in the 2004 edition of ANSI C63.5, which introduced numerical corrections for biconical dipole antennas and an alternative method for other hybrid antennas used to measure site attenuation. This alternative method included further restriction on reference sites used to calibrate antennas to be used in site validations. Technical Drawbacks in Earlier Versions To review, the Standard Site Method (SSM) and Normalized Site Attenuation (NSA) were first introduced by Smith et al. in 1982 [1, 2] in a pair of complementary papers noted above. The methods were adopted in ANSI C63.5-1988/C63.4-1992. It was a leap forward for EMC antenna calibration and site validation measurements, as they provided a standard way for calibrating EMC antennas. This had proved to be quite challenging until then because of the wide frequency ranges and relatively low gains (thus wide beamwidths) of these antennas. SSM was quickly adopted worldwide as the most popular EMC antenna calibration method. The basic idea of SSM is quite simple. The method builds upon the far field Friis transmission equation, and adds a ray tracing component from the ground bounce of the wave over the conducting ground plane used for these calibrations. Even though a ground plane is used, the standard site method in recent times now aims to produce free-space antenna factors by removing the ground effect mathematically. The ground plane was introduced at the start to provide a repeatable, consistent, and predictable reference. To avoid signal nulls 3. Due to the reciprocity of antenna calibrations and site attenuation, these concerns also represent sources of error in normalized site attenuation measurements. The standards consensus process, as always, is used to find acceptable compromises to such concerns, and in 1988 ASC C63® drafted the first antenna calibration standard, C63.5 [3]. Site attenuation was also added to ANSI C63.4 [4] soon thereafter. This first draft of ANSI C63.5 recognized the need for an immediate alert, by recommending that antennas used to measure site attenuation not be calibrated on the same site to be evaluated. This first edition of C63.5 simply stated that the standard site method involved three site attenuation measurements. Later editions would refer to taking three “insertion loss measurements.” Notice of Upcoming Workshop A workshop will be held on ANSI C63.5 - Antenna Calibration on Saturday, August 15, 2009, just prior to the 2009 IEEE International Symposium on EMC in Austin, Texas. Held at ETS-Lindgren in nearby Cedar Park, the workshop will consist of lectures by Don Heirman of Don HEIRMAN Consultants, Chair of ANSI ASC C63, Mike Windler of UL, Chair of ANSI ASC C63 Subcommittee 1 on “Techniques and Development” and Dennis Camell of NIST, Chair of the Working Group for revisions to ANSI C63.5. Attendees will have the opportunity to apply what they learn via problem solving and performing an antenna calibration using ETS-Lindgren’s expansive ISO 17205 certified open area test site and A2LA-accredited calibration lab. Registration information will be available on www.c63.org and www.emc2009.org after January 1, 2009. 1 Conformity november 2008 http://www.c63.org http://www.emc2009.org
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