What is the difference between quasi-TEM mode in microstrip and pure TEM mode in stripline?
TEM vs Quasi-TEM Propagation
A true TEM mode exists only in transmission lines with a homogeneous dielectric filling between the conductors (coaxial cable, stripline). In TEM mode, the electric and magnetic fields are entirely perpendicular to the direction of propagation, and the propagation velocity is exactly c/√εr at all frequencies. This frequency independence simplifies wideband design because the electrical length of a structure is constant with frequency.
| 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 |
Cable Selection Criteria
Microstrip cannot support a true TEM mode because the dielectric boundary between the substrate and air forces a longitudinal (along the propagation direction) field component to exist. This longitudinal component increases with frequency, causing the effective dielectric constant to increase from its static value toward the bulk substrate εr. The propagation velocity decreases with frequency, and different frequency components travel at different speeds.
- 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
Loss and Phase Stability
The practical impact of dispersion is that microstrip structures designed for a specific frequency response (filters, couplers, matching networks) shift in frequency when operated far from their design frequency. For narrowband designs (< 10% bandwidth), dispersion is usually negligible. For wideband designs (> 30% bandwidth) above 10 GHz, dispersion must be accounted for in the design using frequency-dependent εeff models or full-wave electromagnetic simulation.
Frequently Asked Questions
Does dispersion affect loss?
Indirectly. Higher εeff at higher frequencies means slightly different impedance, which can cause additional mismatch loss. But the primary loss mechanisms (conductor and dielectric) are separate from dispersion. The practical impact of dispersion on loss is small compared to its impact on phase and impedance.
Can I compensate for dispersion?
Yes. Dispersion-compensated designs use frequency-dependent models (Kirschning-Jansen, Kobayashi) to adjust the physical dimensions so that the circuit performs correctly across the full bandwidth. Modern EM simulators handle dispersion automatically.
When should I use stripline instead?
Use stripline when wideband phase linearity is critical (wideband filters, multi-octave couplers, phased array feed networks where phase accuracy matters across the band). Use microstrip when component mounting is needed or when the simplicity of a two-layer design outweighs the dispersion penalty.