slab line

A slab line is a planar transmission line consisting of a single central conductor strip positioned between two parallel ground planes, typically made from copper or another conductive metal. The ground planes act as shields and return paths, confining the electromagnetic field largely to the region between them. This geometry makes it simpler to manufacture and integrate than cylindrical coaxial cable, while maintaining controlled impedance over a defined frequency range.

The physical spacing between the central conductor and each ground slab determines the characteristic impedance, which is calculated from the geometry and the dielectric material filling the gap. Common impedances range from 50 ohms to 75 ohms in signal applications. The width of the central conductor and the separation distance must be held to tight tolerances, typically within tens of micrometers, to maintain impedance consistency along the line's length. Air, polytetrafluoroethylene (PTFE), or other low-loss dielectrics fill the gap, depending on frequency requirements and mechanical constraints.

Slab lines appear frequently in microwave and millimeter-wave systems where printed circuit board integration is necessary. They form the backbone of stripline circuits in phased array antennas, filter networks, and high-frequency measurement equipment. The planar construction allows multiple transmission lines and active components to be fabricated on the same substrate, reducing assembly time and parasitic coupling between separate cables.

Loss and Frequency Range

Performance degrades at higher frequencies due to conductor skin effect and dielectric loss in the substrate. At microwave frequencies (above 1 GHz), the conductor surfaces become rough on the scale of skin depth, increasing attenuation. Dielectric absorption in the insulating material also rises with frequency. For frequencies beyond 100 GHz, air-filled slab lines or waveguide geometries often replace conventional dielectric-filled designs.

Field confinement between the slab planes suppresses unwanted radiation and crosstalk between adjacent lines, making slab lines attractive for dense circuit layouts. However, fringing fields at the conductor edges must be accounted for in impedance calculations, and fabrication variations can shift the actual impedance away from design values. Quality control during manufacturing, including thickness measurement and surface finish inspection, directly affects transmission performance and yield.