Power supply noise generated by integrated circuits is one of the major sources of electromagnetic radiation from printed circuit boards (PCB). The reduction of power supply noise can be realized by means of devices that bypass the current among power supply planes, such as bypass capacitors and ground vias. In the present work, the effect of current bypass devices on the far field radiation from multilayer PCBs is represented in terms of the ratio between the far field after and before their introduction, and it is estimated by means of the power transported by the ‘radiation effective forward wave’ in infinite power supply planes. This approach is computationally very efficient and yelds improved EMC designs for power supply planes in realistic PCBs, for example by selecting the position of stitching ground vias. The results are confirmed by a comparison with commercial tools. Forward wave analysis can be used also to study the vertical distribution of the power supply noise in multilayer PCBs. This allows to understand some important noise propagation mechanisms that are related to power and signal integrity as well, and to take low-cost countermeasures at early stage of PCB design.