Conformity - November 2008 - (Page 21) 3. The NSA shall be evaluated over a volume (e.g., as an alternate test site). be calibrated only in the horizontal polarity at a 2-meter transmitting height to establish a near free-space condition. The industry consensus has been that the use of all antennas in other geometries, such as vertical polarization at receiving height of 1-meter, introduces only minor variations in the antenna performance. This was true when the antennas were dipoles and dipole like designs (log periodic and biconical dipoles). This is no longer true for larger hybrid antennas with very large low-frequency elements that are orthogonal to the ground plane in the vertical polarity. The influence of the ground plane on the antenna must be limited to ensure the results correlate to dipole-like antennas. Summary ANSI C63.5-2006 is the latest revision of the U.S. standards document available to provide methods of calibration for antennas used in EMC measurements. While this version has addressed issues of concern and incorporated technical advances, several new items have arisen that will need to be considered in this standard. For the next revision of this document, additional text is being added to improve user’s understanding of these concepts and the explanation of their usage. Discussion of standard gain horns and their requirements for calibration is ongoing and If the antenna is to be used for test site validation, then geometry-specific correction factors are also needed. In Annex G, there are tables containing GSCFs for biconical antennas. These GSCF terms can be measured for all types of EMC antennas, and the method is provided in Annex H. These GSCF terms combine with the FSAF to provide a site-to-site comparison of the antenna calibration site (SACS) with the user’s test site. The calibration site has additional requirements that help specify its design and control its environment that are also specified in Annex H. There are three requirements for validation of the antenna calibration site to use for GSCF: 1. 2. 3. NSA shall be measured using biconical dipoles or tuned dipoles. The test site shall be constructed in accordance with C63.7 and Annex H of C63.5. NSA shall comply with the statistical criteria described in this Annex. New advances being considered in the next revision of this standard are an option for time-domain gating to improve antenna calibrations, guidance on a complex-fit NSA for logperiodic antennas, and a limit on the type of antenna to use in this testing based on maximum variations of vertical to horizontal NSA. The use of time-domain gating to determine free-space AFs is discussed in [13]. This method will provide FSAF for any type of antenna, provided that its time-domain pulse length is short compared to the period needed for the reflected signals to be detected. This means that the antenna type will dictate the measurement geometry. Time domain gating can also be used for site validation, as described in [14]. Much of this work was derived from earlier efforts documented in [15]. Antennas used for test site validation will need to have limitations imposed, due to the variations between horizontal and vertical polarization NSA. The goal here is to address the hybrid antennas, which have strong ground plane coupling in one polarity--usually vertical--and much less coupling in horizontal orientation. The standard requires the antenna november 2008 Conformity 21 http://www.spira-emi.com http://www.spira-emi.com
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