Study On Near-field Coupling For EMC Wire Antenna At Low Frequency

Antenna factor is considered as one of main uncertainties of radiated EMI tests. If antennas are not calibrated accurately, they will affect the following processes of tests. To calibrate antennas accurately requires a high-quality standard test site, which should be validated by antennas with known antenna factors. There exists value circulation between the quality of sites and the calibrations of antennas. So far there are some problems about methods of calibrations of antennas, such as near-field coupling and balun effects to tests, etc. Near-field coupling, presented in ANSI C63.5, is especially obvious in the frequency range of 30MHz to 180MHz, which brings large errors. This is because the formulation of antenna calibration is based on some assumptions, which ignores high-order terms and influences of antenna coupling to the ground plane, mutual coupling between antennas on the characteristics of antennas. In addition, balun effects may increase the uncertainty of tests too.For these problems, this paper firstly analyzes baluns effects on different kinds of EMC wire antennas by using numerical methods. Then it, in detail, describes processes of two numerical methods which have considered balun effects. This paper also uses the known balun method to conduct a numerical analysis to quantify inevitable existing near-field coupling for a specified balun, including influences of antenna coupling to the ground plane, mutual coupling between antennas on directivities, input impedances of antennas and site attenuation. At last this paper calculates the coupling factors which are based on a specified balun for four ordinary EMC tests and validates the numerical datum as much as possible.Due to the imperfection of domestic test sites, including size shortage, surrounding objects and interferences outside, this paper starts to study interferences of the test site from time domain and analyzes an actual site for a specified balun by using numerical methods. It makes a simulation model based on size and position of the site, the controlling room and lightning rods. Then this paper calculates the effects of them on the characteristics of antennas and tests. Finally it corrects each factor and all factors which are based on a specified balun for four ordinary EMC tests. This paper also includes a lot of tests to validate the key part of numerical datum and analyze the errors.