Loss Tangent
Understanding Loss Tangent
Loss tangent is a material property that determines how much signal energy is converted to heat as electromagnetic waves propagate through the dielectric. At microwave frequencies, dielectric loss can be the dominant loss mechanism, making loss tangent one of the most important substrate selection criteria.
Loss Tangent Impact
Dielectric loss per unit length is proportional to frequency and loss tangent: alpha_d = (pi f sqrt(er) tan d) / c. Doubling the frequency doubles the dielectric loss. At 77 GHz, a substrate with tan d = 0.02 (FR-4) would have 20x higher dielectric loss than at 3.85 GHz.
Material Comparison
| Material | tan d | Use |
|---|---|---|
| Air | 0 | Waveguide |
| PTFE | 0.0002 | Cable dielectric |
| Rogers 5880 | 0.0009 | Low-loss PCB |
| Rogers 4350B | 0.004 | General RF PCB |
| FR-4 | 0.020 | Low-freq / digital |
| Water | 0.15 (1 GHz) | N/A (absorbing) |
Frequently Asked Questions
What is loss tangent?
Loss tangent measures how much electromagnetic energy a dielectric material absorbs per cycle. Lower values mean less loss. It is the most important material property for RF substrate selection, especially above 10 GHz where dielectric loss becomes dominant.
When does loss tangent matter?
Loss tangent matters most at higher frequencies where dielectric loss increases linearly. Below 3 GHz, FR-4 (tan d = 0.02) is acceptably low loss. Above 10 GHz, low-loss substrates (tan d < 0.005) are needed. Above 30 GHz, ultra-low-loss materials (tan d < 0.002) are essential.
How is loss tangent measured?
Loss tangent is measured using cavity resonator methods (split-post, Hakki-Coleman), waveguide transmission, or free-space techniques. The measurement frequency should match the application frequency because loss tangent can vary with frequency.