How do I calculate the effective dielectric constant of a microstrip line versus a stripline?
Microstrip vs Stripline Dielectric
In microstrip, the signal trace sits on top of the dielectric substrate with air (εr=1) above. The electromagnetic field extends both into the substrate and into the air, experiencing a weighted average dielectric constant. This effective dielectric constant determines the propagation velocity and wavelength of signals on the line.
| 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 |
Frequently Asked Questions
Why does εeff matter for design?
εeff determines the physical length of quarter-wave transformers, stubs, and resonators. A quarter-wave section at 10 GHz on microstrip (εeff=3.3) is 4.1 mm long. On stripline (εeff=4.4), it is 3.6 mm. Using the wrong εeff causes the structure to resonate at the wrong frequency.
How accurate are the closed-form equations?
The Hammerstad-Jensen equations are accurate to about ±0.2% for εeff and ±1% for Z0 when W/h is between 0.1 and 10 and εr is between 1 and 16. Outside these ranges, or at very high frequencies, full-wave electromagnetic simulation is needed.
Does the solder mask affect εeff?
Yes. Solder mask (εr ≈ 3.5-4.5) applied over the microstrip trace replaces some of the air above the trace with a higher-εr material, increasing εeff by 2-5% and lowering the impedance by 1-3 Ω. For impedance-critical designs, either remove the solder mask over RF traces or include it in the impedance model.