To power electrically tractioned railway vehicles, a connection between the vehicle and the source of energy is needed. This is achieved with the combined action of a train device called pantograph catenary, a powered line positioned along the railway. Guaranteeing a constant contact force between the two parts is essential to allow continuous energy feed to the vehicle. The geometry of the catenary along the line generates important force fluctuation. A rigid and periodical geometry is considered for the study of these fluctuations under different pantograph velocities and contact stiffness rigidities. Its periodical nature is studied together with its behaviour in the frequency domain.