What Is Microstrip?

Microstrip is a planar transmission line structure consisting of a conducting strip separated from a ground plane by a dielectric substrate. It is the most widely used interconnect technology at radio frequency (RF) and microwave frequencies, appearing in printed circuit boards, monolithic microwave integrated circuits (MMICs), and antenna feed networks. The flat, open structure makes microstrip easy to fabricate using standard photolithographic techniques and straightforward to integrate with active components such as transistors and diodes.

The configuration was introduced in the 1950s as engineers sought a way to realize microwave circuits in a form compatible with printed circuit manufacturing. Unlike coaxial cable or waveguide, microstrip exposes the signal conductor to the air above the substrate, which means the propagating wave mode is quasi-TEM rather than pure TEM. This quasi-TEM behavior produces frequency-dependent phase velocity and effective permittivity, so designers must account for dispersion when operating at millimeter-wave frequencies.

Transmission Line Theory

The electrical behavior of a microstrip line is determined primarily by three parameters: the strip width, the substrate thickness, and the relative permittivity of the dielectric. Together, these set the characteristic impedance and the effective dielectric constant. As detailed in the IEEE Xplore chapter on microstrip line design in Modern Planar Transmission Lines, the effective permittivity lies between the substrate permittivity and that of air, because the fields are distributed partly in the substrate and partly in the air above the strip.

The characteristic impedance of practical microstrip lines spans roughly 20 to 120 ohms, with 50 ohms being the most common design target for RF systems. Losses arise from three mechanisms: conductor losses due to the finite conductivity of the strip and ground plane, dielectric losses from the substrate material, and radiation losses that become significant as frequency increases. Substrate materials such as Rogers RO4003C, PTFE composites, and alumina are chosen to minimize dielectric loss at microwave frequencies.

Microstrip Design and Fabrication

Designing a microstrip circuit begins with closed-form or numerical synthesis equations that relate the physical dimensions to the target electrical parameters. Commercially available electromagnetic simulation tools solve Maxwell's equations on the physical layout to predict S-parameters before fabrication. The design flow aligns with the guidance in the Wiley-IEEE Press book on Foundations for Microstrip Circuit Design, which covers impedance synthesis, discontinuities at bends and T-junctions, and coupled-line structures used in filter and coupler design.

Fabrication follows standard PCB processes: a copper-clad laminate is patterned by photolithography and chemical etching. At millimeter-wave frequencies, surface roughness of the copper conductor becomes a significant loss contributor, driving the use of electrodeposited or rolled copper with controlled surface finish. Vias through the substrate connect the top conductor to the ground plane and set the boundary condition for resonant structures.

Broadband Antennas and Radiation

Microstrip finds extensive use as the feed structure for broadband antennas, including the broadband microstrip patch antennas described in the IEEE Xplore chapter on planar circuit design in RF and Microwave Circuit Design. Quarter-wave transformers, Wilkinson power dividers, and branchline couplers are standard microstrip circuit blocks that shape the bandwidth and impedance match of antenna feeds. At millimeter-wave frequencies, microstrip transitions into substrate-integrated waveguide (SIW) or coplanar waveguide to reduce radiation loss.

Applications

Microstrip has applications in a range of fields, including:

• Wireless communication systems, as the feed network for base station and handset antennas
• Radar systems, implementing low-profile planar arrays and T/R module circuits
• Satellite communications, providing compact, lightweight circuit implementations for transponders
• Medical imaging and therapy, in microwave hyperthermia applicators and MRI coil networks
• Automotive sensors, feeding 77 GHz radar antennas for adaptive cruise control