How does the ground plane spacing affect the impedance and radiation of a microstrip line?
Substrate Thickness Tradeoffs
The dielectric height (h) between the microstrip trace and the ground plane is a critical design parameter that affects impedance, loss, radiation, and surface wave behavior. No single thickness is optimal for all metrics; the designer must balance competing requirements.
| Parameter | Semi-Rigid | Conformable | Flexible |
|---|---|---|---|
| Loss (dB/m at 10 GHz) | 0.8-2.5 | 1.0-3.0 | 1.5-5.0 |
| Phase Stability | Excellent | Good | Fair |
| Bend Radius | Fixed after forming | Hand-formable | Continuous flex OK |
| Shielding (dB) | >120 | >90 | >60-90 |
| Cost (relative) | 2-5x | 1.5-3x | 1x |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
- Margin allocation: include sufficient design margin to account for manufacturing tolerances and aging effects
Frequently Asked Questions
What height is typical for mmWave?
4-6 mil (0.1-0.15 mm) for 60-77 GHz designs. 8-10 mil (0.2-0.25 mm) for 24-28 GHz. 10-20 mil (0.25-0.5 mm) for 10-20 GHz. These heights balance trace width, loss, and radiation for each frequency range.
Can I use multiple dielectric heights?
Not easily on the same layer. All microstrip traces on a given layer share the same dielectric height. To use different heights for different circuits, place them on different PCB layers or use a localized area of reduced ground plane (cavity-backed microstrip), which adds fabrication complexity.
Does the ground plane need to be continuous?
Yes, under all RF traces. Any gap or slot in the ground plane disrupts the return current path, creating radiation, cross-talk, and impedance discontinuities. Ground plane slots under microstrip traces are a common design error that causes unpredictable RF performance.