How do I account for dispersion in microstrip when designing wideband circuits?
Microstrip Dispersion Effects
Dispersion in microstrip arises because the fraction of electromagnetic energy in the substrate versus air changes with frequency. At low frequencies, the field extends far into the air above the trace, giving a lower εeff. At high frequencies, the field is more tightly confined to the substrate, and εeff approaches the bulk εr. This frequency-dependent εeff changes the phase velocity and wavelength.
| 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
How big is the dispersion effect?
For 50 Ω microstrip on 10 mil substrate (εr=10): εeff changes from 6.9 at DC to 8.5 at 30 GHz, a 23% increase. On FR4 (εr=4.4): εeff changes from 3.3 to 3.8 over the same range, an 15% increase. Lower εr substrates have less dispersion.
Does trace width affect dispersion?
Yes. Wider traces (lower Z0) have less dispersion because the field is already mostly in the substrate at low frequencies. Narrow traces (high Z0) have more dispersion because a larger fraction of the field extends into the air and gradually shifts into the substrate with increasing frequency.
When do I need to worry about it?
When the substrate height exceeds λ0/50 at the operating frequency and the bandwidth exceeds 20%. For example: 10 mil substrate at 20 GHz (h/λ0 = 0.017, just below threshold). At 40 GHz (h/λ0 = 0.034): dispersion correction is essential for accurate design.