Prediction of electric far-field strength from printed circuit boards by measuring the common-mode current

A far-field prediction from the common-mode current on the traces of printed circuit boards (PCBs) was investigated, which had been partly examined by Paul (1989) in the frequency range up to 200 MHz for electrically short traces on the PCB with a narrow bandwidth signal source. Three sample PCBs were prepared, which have two different lengths of balanced parallel traces that were not always electrically short, and a PCB with a ground plane on the reverse. A battery powered signal generator excited each PCB so that we could expand the frequency range up to 1 GHz with sinusoidal waves. A method for calculating the electric far-field from the PCB was given in conjunction with the common-mode current on the traces. A surface current probe consisting of thin wires wounded on the middle of a half-cut ferrite core was used to measure the trace currents. Calculated electric far-field strength based on the common-mode current was compared with the measured far-field strength at a distance of 3 m for three different sample PCBs in the frequency range from 30 MHz to 1 GHz. Good agreement between the common-mode current predictions and the measurements was obtained for the line-type trace PCBs, while a little poor correlation was observed for the PCB with a ground plane