The guard trace is effective for suppressing common-mode generation caused by placing a signal trace too close to the return edge on a printed circuit board. In our previous study, a guard trace with a periodic structure was proposed not only to reduce common-mode radiation but also to maintain signal integrity and was found to be effective. In the current work, the shape of the periodic structure and the termination at both edges to improve the features was investigated through full-wave simulation and measurement from the viewpoint of transmission coefficient as well as far-field radiation. Results show that the smaller the widths of the narrow portion, the further the periodic structure helps reduce common-mode radiation and maintain signal integrity. Also shown is that the periodic guard trace with both ends connected to the return plane is an appropriate structure.