Stripline

Stripline is a planar transmission line used at microwave frequencies in which a flat conducting strip is sandwiched between two parallel ground planes within a homogeneous dielectric substrate. The strip is usually centered between the planes, and the surrounding dielectric confines the majority of the electromagnetic field, producing a quasi-TEM propagating mode with low radiation loss.

Key characteristics include stable characteristic impedance, reduced crosstalk, and good shielding against environmental variations. The characteristic impedance Z0 depends on the strip width W, the substrate thickness (the distance between the ground planes, often denoted 2h), the conductor thickness t, and the dielectric constant εr of the substrate. Common target values are 50 Ω or 75 Ω. Exact expressions are intricate, so engineers use dedicated calculators or design tables.

Construction methods vary. In printed circuit forms, a copper strip is embedded between two dielectric layers or within a sandwich substrate and metallized to provide ground planes. Stripline can also be formed in embedded modules or ceramic packages. Materials are typically low-loss dielectrics such as PTFE-based laminates or ceramic/glass composites.

Advantages include good shielding from external interference, reduced radiation loss, and tight impedance control over a broad frequency range. Disadvantages include the need for precise manufacturing tolerances for the dielectric thickness and alignment, more complex interconnections to components, and potentially higher cost compared with microstrip.

Applications include microwave integrated circuits, filters, couplers, phase shifters, and RF front-end modules in communications and radar systems.