Conformity - November 2008 - (Page 20) Measurement Uncertainty for C63.5 by Bob DeLisi, Underwriters Laboratories The subject of ANSI C63.5 is calibration of antennas used to measure radiated electric fields. This standard also recognizes the need to define and provide procedures for labs (both calibration labs as well as testing labs that wish to perform their own antenna calibration) performing antenna calibrations to determine the associated measurement uncertainty required in calibration reports. The uncertainty is related by both systematic and random error over the frequency range of calibration. Depending on the method of calibration (Standard Site Method (SSM), Reference Antenna Method (RAM) or the Equivalent Capacitance Substitution Method (ECSM)) a different major influence factor will affect the overall uncertainty for each method. The question then is, what factor is a major contributor for antenna calibration measurement uncertainty? For the SSM, it is the quality of the test site that is used during measurement. The accuracy of the calibration of the reference antenna weighs heavily in the RAM method, and the accuracy of the simulated capacitance plays the major role for the monopole antenna method. ANSI C63.5 provides both the elements of uncertainty and the definitions and equations on how a lab calculates its particular calibration measurement uncertainty. In each of the above calibration methods, there are provisions on how to reduce the overall contributors of error terms during measurement. For example, by instructing the lab to properly impedance match the connection of the antenna to cable and cable to receiver, the calibration uncertainty can be reduced. Another example is the effect on cable (used in the calibration for the source field and the antenna under calibration as a receiving antenna) insertion loss related to temperature for those calibration sites which are exposed to the weather, such as an open area test site. This effect is an important error term that some fail to recognize in an uncertainty budget. The temperature influence factor is described in ANSI C63.5. If testing is not done at a time of year that provides a small varying temperature range over the antenna calibration interval, the heating or cooling effects of the cable can introduce errors that are not corrected for and thus need to taken into consideration in the uncertainty budget. Not only does ANSI C63.5 identify this contributor and provide information on how to calculate the contribution value, it also recommends ways to reduce the contributor by monitoring the site’s ground plane temperature over the duration of the measurement as temperatures change. In future editions of ANSI C63.5, measurement uncertainty will be removed and included in a new standard which has been given the number ANSI C63.23 and entitled: “Guide for American National Guide for Electromagnetic Compatibility— Calculations (Computations) and Treatment of Measurement Uncertainty.” This standards is currently being drafted by a working group in ASC C63 Subcommittee 1. The goal of C63.23 will be to provide guidance on how to calculate the uncertainty of measurement for emissions testing, i.e., ANSI C63.4 and antenna calibrations (ANSI C63.5) in the first edition. In future editions, it will also cover immunity type tests and other emissions methods, as the goal is to provide measurement uncertainty considerations for all ASC C63 measurement standards. In addition to providing some basic understating of uncertainty contributors, the document will provide guidance on where contributors can be located, for example, within instrumentation specification sheets. ANSI C63.23 will also introduce techniques to use nested Type A analysis in determining overall uncertainty. The Type A process involves repeat measurements using the same testers, equipment and test site. This facilitates a statistical approach to determining an overall measurement uncertainty. ANSI C63.23 will also include calculations for antenna calibrations beyond the standard site method that is currently described in ANSI C63.5. It is expected that the first edition of ANSI C63.23 will be published by mid 2009 or possibly earlier. Bob DeLisi is a senior staff engineer with Underwriters Laboratories, and can be reached at bob.delisi@us.ul.com. 20 Conformity november 2008
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