Microstrip Line Types: Overview and Applications

Microstrip lines are essential components in microwave and RF circuits, offering compact and efficient signal transmission. These planar transmission lines are widely used in antennas, filters, amplifiers, and oscillators due to their versatile design and performance characteristics.

There are several types of microstrip lines, including basic microstrip, stripline, suspended stripline, slotline, coplanar waveguide (CPW), and finline. Each type is tailored to specific applications, providing unique advantages and limitations.

This article explores these microstrip line types, their features, and applications.

Basic Microstrip Line

Microstrip line is used to carry Electro-Magnetic Waves (EM waves) or microwave frequency signals. It consists of 3 layers: a conducting strip, a dielectric, and a ground plane.

It is used to design and fabricate RF and microwave components such as directional couplers, power dividers/combiners, filters, antennas, MMICs, etc. Microstrip lines will have low to high radiation, support 20 to 120 ohm impedance, and a Q factor of about 250. It’s difficult to mount chips in shunt mode but easy in series mode.

The RF/microwave products made using microstrip lines are less expensive and lighter in weight compared to their waveguide counterparts. Usually, FR-4 dielectric substrate is used as PCB for microstrip-based etching due to its low cost.

Fig.1 microstrip line

Different types of discontinuities exist in RF/microwave layout design. These include bends, T-junctions, cross-junctions, steps in width, coupling, and open ends. Parasitic effects introduced by these discontinuities will be reduced by the use of constant impedance tapers; for example, curved bends are used in place of sharp bends. Sharp edges are changed to curved edges in RF layouts. In certain cases, radial stubs of different types (series and shunt) are employed.

Microstrip-based layout design is carried out by various RF & Microwave simulation software from companies such as Agilent, Eagleware, Microwave office, applied computational sciences, CST, Ampsa Multimatch etc.

Advantages:

• Compact and lightweight.
• Simple design and fabrication.
• Suitable for high-frequency applications.

Applications:

• Antennas and microwave filters.
• RF amplifiers and oscillators.

Types of Microstrip Line

Microstrip lines are planar transmission structures used in RF and microwave engineering. They are integral to compact, efficient, and cost-effective circuit designs for communication systems, sensors, and radar applications. Below are the main types of microstrip lines, their characteristics, advantages, and applications:

Stripline

Fig.2 stripline

This type of microstrip line has low radiation, a Q factor of about 400, and will support a 35 to 250 ohm impedance range. This type is poor for chip mounting.

Example:

Calculate the width of the strip (W) provided the following details of the stripline: Impedance (Zo) is 50 Ohm, the dielectric constant is 4.5 (ε), and the PCB is 5mm thick (T).

W = 377 T / Zo sqrt(ε)

Width W is about 17.78 mm

Advantages:

• Excellent shielding from external interference.
• Supports TEM mode for precise signal propagation.

Applications:

• High-speed digital circuits.
• Phase shifters and delay lines.

Suspended Stripline

Fig.3 suspended stripline

This type of microstrip or transmission line will have low radiation, a Q factor of about 500, and impedance from 40 to 150 range. It is fair to mount chips on suspended striplines.

Advantages:

• Lower dielectric losses.
• High Q-factor for resonators.

Applications:

• High-performance filters and resonators.
• Low-loss transmission lines in radar systems.

Slotline

Fig.4 slotline

This type of microstrip line will have medium radiation, support a Q factor of about 100, and impedance from 60 to 200. It is easy to mount chips in shunt mode and difficult to mount in series mode.

Advantages:

• Suitable for planar integration with active devices.
• Compatible with unbalanced signal propagation.

Applications:

• Planar antennas and hybrid microwave circuits.
• High-frequency mixer circuits.

Coplanar Waveguide

Fig.5 coplanar waveguide

This type of microstrip line will have medium radiation, support a Q factor of 150, and impedance from 20 to 250. It is easy to mount chips in both series and shunt mode.

Advantages:

• Supports uniplanar circuit integration.
• Reduced parasitic effects compared to microstrip.

Applications:

• MMICs (Monolithic Microwave Integrated Circuits).
• Broadband components such as couplers and splitters.

Finline

Fig.6 finline

This type of microstrip line has no radiation and a Q factor of 500. It is fair to mount chips using this type of transmission line.

Advantages:

• Excellent for waveguide-based systems.
• Supports high power handling capabilities.

Applications:

• Millimeter-wave systems and sensors.
• Satellite communication systems.

Conclusion

Microstrip lines form the backbone of modern RF and microwave design, offering solutions for various applications through their unique configurations. From the simplicity of basic microstrip to the advanced design of finline, each type addresses specific needs in communication, sensing, and signal processing. Understanding their differences and applications ensures optimal circuit design for engineers and researchers.