Necessity of charge measurement for radiation evaluation of transmission lines

This paper attempts to arouse people’s attention to charge measurement in electromagnetic compatibility, especially when evaluating the radiation of transmission lines (TLs). Usually the total current (or common-mode current) is supposed to represent the potential radiation of a TL system. However, it is proved that the measurement of charge, which is the dual source quantity of current, is also necessary to evaluate radiation in this paper. Only when the current and charge are both obtained, the radiation field could be determined accurately. First of all, it is pointed out that charge information could not be properly obtained by current measurement. Although charge could be derived from current theoretically, the error transferred from current to charge could be great for measurement. Then, the error transferred from current to near field (which reflects the charge distribution) is studied by simulation of a typical TL case. And it is proved that such error could be reduced effectively if current is modified by charge. In addition, another important reason for charge measurement is given as limited measurement points, since the standing waveform on TLs cannot be determined by current only. Finally, a possible method for charge measurement is proposed.