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

Guided, hybrid propagation in a slotline at microwave and millimeter-wave frequencies is discussed. The analysis is carried out rigorously in the space domain, involving the variational solution of a singular integral equation for an E, field tangential to the slot. Data are obtained for field distributions, dispersion, and characteristic impedance, and are compared with data available in the literature. Results are reported on power lost by the propagation mode in the conductors and in the dielectric substrate. The analysis highlights the onset of leakage into a sufficiently thick substrate due to the excitation of a TM (transverse magnetic) surface wave. This sets a high-frequency limit for lossless operation.<>