How do I minimize radiation loss from microstrip discontinuities at millimeter wave frequencies?
Reducing mmWave Radiation Loss
Every discontinuity in a microstrip line creates a localized change in the electromagnetic field distribution that launches radiation into free space and surface waves into the substrate. At low frequencies, this radiation is negligible. At millimeter wave frequencies, where the substrate thickness becomes a significant fraction of a wavelength, radiation from discontinuities can dominate the total loss budget.
| 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
Is GCPW better than microstrip for mmWave?
Yes. GCPW confines the fields between the signal trace and adjacent ground pads, reducing radiation from both straight sections and discontinuities. The via-fenced ground pads suppress surface wave propagation. GCPW is the preferred transmission line for circuits above 30 GHz.
How thin should the substrate be?
Thin enough that h/λ0 < 0.01 for negligible radiation. At 60 GHz (λ0 = 5mm), this means h < 50 μm (2 mil), which is challenging to fabricate. Practical designs accept some radiation with h = 4-6 mil and use shielding and via fencing to contain it.
Does packaging help?
Yes. A metal lid or cavity over the microstrip circuit contains the radiation and prevents coupling to other circuits. The cavity dimensions must be chosen to avoid cavity resonances within the operating frequency band. Absorber material can be placed on the lid to dampen cavity modes.