Microstrip v/s Stripline: Layout Difference and When to Use Them

RF (Radio Frequency) PCB design is where engineering meets art. Among the tools in an RF designer's toolkit, microstrip and stripline transmission lines are the unsung heroes. They ensure signals travel seamlessly across PCBs without succumbing to interference, loss, or impedance mismatches. But what are these lines, and how do you choose between them? Let's dive in, and to know more about PCB, see our detailed blog on PCB manufacturing.

What Are Microstrip and Stripline Transmission Lines?

Microstrip Transmission Line:

A microstrip is a conductive trace on the surface of a PCB, with a ground plane underneath and a dielectric material (PCB substrate) sandwiched in between. The top side of the trace is exposed to air. And its electromagnetic field behaviour is given as:

• Fields propagate partly in the substrate and partly in the air.

• This creates a hybrid dielectric medium, giving microstrip unique properties.

Stripline Transmission Line:

In contrast, a stripline is a conductor buried between two ground planes within a PCB. It is entirely surrounded by dielectric material. And its electromagnetic field behaviour is given as:

• Fields are fully confined to the dielectric material.

• This shielding makes it more immune to external noise.

Why Are These Lines Important in RF Design?

In RF designs, every millimeter counts, quite literally. Transmission lines like microstrip and stripline ensure:

1. Signal Integrity: Prevent distortion over long traces.

2. Impedance Matching: Maintain a consistent impedance (e.g., 50 Ω or 75 Ω) for maximum power transfer.

3. Noise Suppression: Control radiation and susceptibility to external EMI.

4. Power Efficiency: Minimize losses to preserve signal strength.

Where to Use Microstrip vs. Stripline

Choosing between these two is like deciding between sneakers and dress shoes—it depends on the occasion.

Microstrip:

• High-Speed Digital Signals: Simpler design makes microstrip perfect for high-speed applications where cost and efficiency matter.
• Lower Frequencies (up to ~10 GHz): Works well when loss and noise aren’t critical.
• Cost-Sensitive Designs: Surface traces simplify fabrication, making them ideal for budget-friendly projects.

Stripline:

• High-Frequency RF Applications (above ~10 GHz): Better shielding minimizes radiation and interference.
• Noise-Sensitive Circuits: Use in environments with strict electromagnetic compatibility (EMC) requirements.
• High-Density PCB Layouts: Ideal for complex multilayer designs requiring impedance control and minimal crosstalk.

Examples of Usage

Example 1: Microstrip for Wi-Fi Antennas

A microstrip patch antenna is often used in Wi-Fi routers because it’s cost-effective, lightweight, and suitable for the 2.4 GHz band. The exposed surface helps in efficient signal radiation.

Example 2: Stripline in Radar Systems

Radar systems operating at 77 GHz demand precision and noise immunity. Stripline’s shielding properties make it indispensable in such high-frequency, noise-sensitive environments.

Calculating Trace Lengths and Impedance

The impedance of microstrip and stripline traces depends on their dimensions and the PCB’s material properties. Use the following equations for design:

Microstrip Impedance (Z₀)

Where:

• ϵ​: Dielectric constant of the substrate
• h: Height of the dielectric
• w: Width of the trace
• t: Thickness of the trace

Stripline Impedance (Z₀):

Where:

• h: Distance between the two ground planes
• w: Width of the trace

Tips for Modulating Microstrip and Stripline

To tailor these transmission lines for your application:

1. Adjust Trace Width: Wider traces reduce impedance; thinner traces increase it.

2. Use Simulation Tools: Software like HFSS, ADS, or KiCad with integrated RF modules helps fine-tune designs.

3. Control Dielectric Materials: Choose substrates with low loss tangents for better performance at high frequencies.

4. Fine-Tune Layer Heights: In multilayer PCBs, precisely control the thickness between layers.

RF PCB Design Tips Using Microstrip and Stripline

1. Best Possible Routing Practices

• Minimize Bends: Use smooth curves or 45° bends to avoid signal reflection.
• Short is Sweet: Keep traces as short as possible, especially at higher frequencies.

2. Ground Plane Considerations

• Ensure a continuous and unbroken ground plane under microstrip lines.
• For stripline, maintain symmetry between the ground planes to avoid signal skew.

3. Avoid Crosstalk

• Maintain sufficient spacing between adjacent traces.
• Use guard traces or additional ground planes if needed.

4. Test, Test, and Test Again

• Use a vector network analyzer (VNA) to validate impedance and signal integrity after fabrication.

How can interference among RF, IF, and low-frequency circuits be prevented?

Typically, RF circuits in a system are implemented as an independent single board for layout and routing, often enclosed in a dedicated shielding chamber. RF circuits are generally single-layer or double-layer boards with relatively simple circuitry. These measures minimize the influence of distributed parameters on RF circuits and enhance the consistency of RF systems. Compared to standard FR4 materials, RF PCBs tend to use high-Q materials. These materials have a lower dielectric constant, smaller distributed capacitance in transmission lines, higher impedance, and shorter signal transmission delays.

In mixed circuit design, while RF and digital circuits may reside on the same PCB, they are typically separated into RF and digital sections, with distinct layout and routing. Ground vias and shielding boxes are used to separate these sections and prevent interference.

Conclusion

Microstrip and stripline are the backbone of modern RF and high-speed PCB designs. Microstrip offers simplicity and cost-efficiency, while stripline shines in noise-sensitive, high-frequency applications. Knowing when and how to use them can make the difference between a reliable product and a glorified paperweight.

RF design isn't magic it's physics (with a dash of art). Choose wisely, design meticulously, and always keep your soldering iron warm for those inevitable tweaks!

FAQ:

1. Can signal lines between the power and ground planes use the stripline model?

Yes, both the power and ground planes are considered reference planes when calculating characteristic impedance. For instance, in a 4-layer board (top layer - power plane - ground plane - bottom layer), the top layer's trace impedance uses the power plane as a reference in the microstrip model.