Dielectric Loss
Understanding Dielectric Loss
Dielectric loss, along with conductor loss, determines the total insertion loss of microstrip lines, stripline, PCB traces, and waveguide fills. At higher frequencies, dielectric loss becomes increasingly dominant, making material selection critical for mmWave designs.
Loss Tangent Values
| Material | tan delta | Use |
|---|---|---|
| Air | 0 | Waveguide, coaxial |
| PTFE (Teflon) | 0.0002 | Low-loss substrates, cables |
| Rogers RO4003 | 0.0027 | Microwave PCB |
| FR-4 | 0.02 | Standard PCB (< 3 GHz) |
| Alumina | 0.0001 | MMIC, thick film |
| Silicon | 0.015 | CMOS substrate |
alpha_d = (pi f er_eff tan_delta) / (c x sqrt(er_eff))
Dielectric loss in dB/length:
Loss = 8.686 x alpha_d x length
At 10 GHz, 10 cm microstrip on FR-4:
~2 dB dielectric loss alone
On Rogers RO4003: ~0.3 dB
Frequently Asked Questions
What is dielectric loss?
Dielectric loss is energy dissipated as heat in a dielectric material by the alternating electric field. It is quantified by loss tangent (tan delta). Lower tan delta = less loss. Critical for PCB substrate selection and microwave circuit performance.
Why is FR-4 bad for microwave?
FR-4 has a high loss tangent (~0.02), causing significant signal attenuation above 1-2 GHz. At 10 GHz, a 10 cm FR-4 microstrip line loses ~2 dB from dielectric loss alone. Microwave designs use Rogers, PTFE, or alumina substrates with tan delta < 0.003.
Does dielectric loss increase with frequency?
Yes. Dielectric loss (alpha_d) is directly proportional to frequency. Doubling the frequency doubles the dB/length dielectric loss. This is why low-loss substrates become increasingly important at mmWave frequencies.