Modified uncertainty estimation of antenna factor measurement by standard site method Mikulas Bittera ⇑ , Viktor Smiesko, Karol Kovac Slovak University of Technology, Dept. of Measurement, Ilkovicova 3, 81219 Bratislava, Slovakia article info Article history: Available online 18 July 2011 Keywords: Measurement uncertainty Antenna factor Electromagnetic compatibility abstract The standard site method was used to measure antenna factor. Based on calculation of antenna factor uncertainty, ways of more precise estimation of uncertainty, leading to uncertainty reduction, are presented. Using uncertainty standards together with known approaches to radiated emission measurement uncertainty calculation, a model of antenna factor measurement and an equation for calculation of its uncertainty were derived. Possi- ble steps to reduce resultant measurement uncertainty are described, including an analysis of correlation, and acknowledgment of frequency dependence. The uncertainty was calcu- lated in 25 frequency points in the range from 30 MHz to 1000 MHz. Furthermore, various calculations of uncertainty are executed to examine the suggested effects on the uncer- tainty value. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction In electromagnetic compatibility (EMC) area, some mea- surements, such as radiated emission measurement, require the use of antennas. The most important parameter of an an- tenna is the antenna factor which represents the transfer properties of the measuring antenna. Depending on the measurement method, there are two types of the antenna factors commonly used [1]: – a free-space antenna factor – measured under the free space conditions (recommended by CISPR standards), – a standard site method antenna factor – measured at standardized test site, i.e., within the space with refer- ence ground plane (adopted by ANSI C63.5 standard). Both antenna factors are used in the EMC area, with re- spect to their advantages or disadvantages. The antenna factors can be accurately determined in several ways. The best way is to consider instrumentation, facilities, etc., of each specific case [2]. The standard site method provides antenna factor measurement generally in an open-area test site or in an adequate site with referencing the ground plane. Such a measurement is specific, because measure- ment distance and transmitting antenna height are fixed, whereas the receiving antenna changes its height between 1 m and 4 m over the ground plane. An uncertainty is used to express measurement quality. Therefore low values of measurement uncertainty not only mean a high-class measurement but they also ensure repeatability of a measurement. The uncertainty of anten- na factor measurement will be estimated by the procedure suggested in the Guide to the Expression of Uncertainty in Measurement [3]. In the EMC area the same approach is used [4,5], even though decibel units are applied. Such units may be used due to known properties of log-normal distribution [6]. In the experiment described in [7,8] authors made an attempt to estimate uncertainty of the measurement on the basis of theoretical calculation of antenna factor. Such calculation assumes two waves, the direct and the reflected wave from the reference ground plane, and it is suitable only for easy-to-describe antennas, such as dipole antennas. Nowadays, broadband antennas, like biconical, log-periodic or Bilog, are often used in the EMC area and therefore the shown approach is inapplica- ble. Then, an estimation of radiated emission measurement 0263-2241/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.measurement.2011.07.002 ⇑ Corresponding author. Tel.: +421 260291166; fax: +421 265440227. E-mail addresses: mikulas.bittera@stuba.sk (M. Bittera), viktor. smiesko@stuba.sk (V. Smiesko), karol.kovac@stuba.sk (K. Kovac). Measurement 45 (2012) 190–198 Contents lists available at ScienceDirect Measurement journal homepage: www.elsevier.com/locate/measurement