Measurements on an ultrafast scanning tunneling microscope with simultaneous spatial and temporal resolution are presented. We show images of picosecond pulses propagating on a coplanar waveguide and resolve their mode structures. The influence of transmission line discontinuities on the mode structure is investigated. It is also demonstrated how common and differential modes of electrical pulses are generated. The capacitive coupling between the tip and the transmission line is explained in terms of two contributions: a long range and a local coupling. We also show how these contributions affect the imaging of the propagating pulses. (C) 1997 American Institute of Physics.