Hybrid modes, losses and substrate leakage of slotline at millimetre wave frequencies

Slotline has proved its usefulness as a transmission media in the lower microwave range. Its use at higher frequencies requires, however, proper characterization not only of the fundamental guided mode, which is now hybrid, but also of its complete spectrum, as required to describe leakage in the dielectric substrate as well as radiation. The onset of the former sets a high frequency/substrate thickness limit to lossless operation, whereas radiation modes are excited by discontinuities such as diode mounts and steps, the waveguide being open. In this manuscript we approach the problem rigorously in the space domain. Numerical results for the guided mode are compared where possible with those available in the literature. Dielectric and conduction losses in the guide are also considered. Finally, we derive the continuous spectrum of slotline, which is a novel theoretical development for an open structure of this kind.