RFStripline vs. Microstrip: A Detailed Comparison
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Microstrip and stripline are two fundamental types of transmission line structures used in RF and microwave circuits. Both play a crucial role in guiding electromagnetic waves, but they differ significantly in terms of construction, electrical characteristics, and performance. Microstrip, typically implemented on the top surface of a printed circuit board (PCB), offers simplicity and ease of manufacturing. In contrast, stripline is sandwiched between two ground planes, providing superior shielding and lower loss.
Let’s understand the differences between microstrip and stripline, highlighting their unique properties, pros and cons, and applications in various high-frequency design circuits.
Microstrip Line
A microstrip line is a type of transmission line that consists of a conducting strip placed on top of a dielectric substrate with a ground plane beneath it. This configuration is widely used in RF and microwave circuits due to its simple structure and ease of fabrication. The microstrip line’s electric field is concentrated mostly between the conductor and the ground plane but also extends into the air above the dielectric. This results in a quasi-TEM (Transverse Electromagnetic) mode of propagation, where the effective dielectric constant is a combination of the substrate and air.
Fig.1 Microstrip line
- Microstrip line is a simple structure consisting of a single ground plane, conductive trace, and dielectric layer separating the single conductor and the ground plane.
- As mentioned, it uses quasi-TEM mode of propagation in which dispersion occurs as functions of frequency and transmission line length.
- Microstrip lines are commonly used in various applications, including antennas, filters, couplers, and impedance-matching networks.
- Advantages: It offers easy access for mounting surface-mount components and relatively low cost.
- Disadvantages: These lines are more susceptible to signal radiation and crosstalk due to their unshielded nature.
Stripline
A stripline is a transmission line structure that is embedded within a dielectric substrate and sandwiched between two ground planes. The central conductor is located at an equal distance from both ground planes, creating a fully shielded environment. This configuration results in a pure TEM (Transverse Electromagnetic) mode of propagation, where the electric and magnetic fields are confined entirely within the substrate, ensuring excellent signal integrity and minimal radiation.
Fig.2 Stripline
- Stripline is a simple structure consisting of a center conductor surrounded by dielectric material with top and bottom metal ground planes.
- It uses fully transverse Electro-magnetic (TEM) propagation which is dispersion-free and frequency independent.
- Stripline is commonly used in high-frequency circuits requiring high isolation and precise impedance control, such as microwave amplifiers, phase shifters, and delay lines.
- Advantages: The conductors in stripline are well-isolated by dielectric around them and hence they do not radiate. Hence, they are regarded as non-dispersive in nature.
- Disadvantages: They are difficult to manufacture due to additional layers compared to microstrip.
Difference Between Microstrip and Stripline
The following table mentions the difference between stripline and microstrip line with respect to various parameters.
| Features | Microstrip | Stripline |
|---|---|---|
| Structure | Conducting layer, dielectric layer, and ground layer | Top and bottom ground plane, dielectric layer, embedded conducting layer within dielectric |
| Types | • Normal microstrip line • inverted microstrip line • suspended microstrip line • shielded microstrip line | • Normal stripline • Double conductor stripline • Offset stripline • Suspended stripline |
| Characteristic Impedance | 20 to 120 Ohm | 35 to 250 Ohm |
| Width of line at same impedance | Wider | Narrower |
| Manufacturing tolerances | Tighter | Lighter |
| Radiation Loss | High for low ε and Low for high ε | Low |
| Dielectric Loss | Less | More |
| Dispersion | Low | None |
| Component mounting | Easy for series and difficult for shunt | Very difficult because of totally covered structure |
| Quality factor | 250 | 400 |
| Advantages | • Smaller size • Easy to fabricate • Easy to troubleshoot | • Good EM shielding can be achieved with this structure • Low attenuation loss • Wider bandwidth • Better isolation |
| Disadvantages | • Unwanted radiation in uncovered microstrip-based configuration • Higher loss • Poor isolation among adjacent lines | • Complex and expensive in fabricating it. • Stripline Trace width is smaller compared to microstrip line of same impedance and height. • Tuning or troubleshooting is complex. |
| Applications | It is preferred to use in miniature microstrip patch antennas due to their radiating structures. | It is preferred to use in multilayer circuits via PTHs (Plated Through Holes) since it is easy to route signals between layers. |
Conclusion
Comparing stripline vs. microstrip line helps in selecting the right transmission line structure for a given RF or microwave design. Microstrip line is preferred for its simpler construction and ease of access for components, making it ideal for lower-cost applications. On the other hand, stripline’s balanced and shielded configuration makes it a better choice for high-frequency circuits requiring minimal signal interference. By evaluating factors like impedance control, loss, and space constraints, designers can make informed decisions to optimize performance and efficiency in their designs.
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