Apparatus and associated systems for transmission of signals within a wide bandwidth (e.g., from DC to 40 GHz and above) include a conduction path and ground structures in an arrangement to provide a smooth transition between propagation in a coplanar waveguide mode and propagation in a microstrip waveguide mode. Some embodiments may be provided without vias, for example, by providing low impedance connections between ground structures on different layers, where the connections are made external to a medium between the layers. Some embodiments may feature a monotonically decreasing gap between a signal conduction path and a coplanar ground structure. Such embodiments may be used, for example, to provide a low loss, wide bandwidth interface between a coaxial transmission line and a microstrip transmission line. As another example, one or more such structures may be used in an electro-optic modulator to control an optical signal.