Microstrip

Microstrip is a type of electrical transmission line used to convey microwave-frequency signals. It consists of a conducting strip on top of a dielectric substrate, with a conducting ground plane on the opposite side of the substrate. The line is planar and compatible with printed circuit board fabrication.

The electromagnetic fields are mostly confined to the dielectric between the strip and the ground plane, with some fringing into air above the strip. This gives a quasi-TEM mode, with a characteristic impedance Z0 that depends on the strip width W, the substrate height h, and the relative permittivity εr. The effective dielectric constant ε_eff lies between 1 (air) and εr and is often estimated with standard empirical formulas. As a result, the width-to-height ratio is chosen to achieve a desired impedance, commonly 50 ohms.

Advantages include ease of fabrication on standard PCBs, low profile, and compatibility with planar devices. Disadvantages include radiation and conductor losses at higher frequencies, sensitivity to substrate material and humidity, and larger cross-sectional area for low-εr substrates compared to other transmission lines.

Variants such as coplanar waveguide (CPW) and CPW with ground offer alternative field configurations and circuitry integration. Materials used for the dielectric substrate include FR-4, Rogers RT/duroid, and PTFE-based laminates.

Applications span RF and microwave circuits: interconnects, filters, couplers, power dividers, patch antennas, and integrated circuits (RFICs/MMICs). Design relies on either closed-form impedance formulas or full-wave electromagnetic simulators to determine W for a target Z0 and to model losses and dispersion.