Question E5D08:
What is the primary cause of loss in film capacitors at RF?  

Choices:  
A: Inductance  
B: Dielectric loss  
C: Self-discharge  
D: Skin effect    

Correct Answer is  D: Skin effect  

Note: The official Extra Class question pool gives the correct answer as B: Dielectric loss, not skin effect.

Why B (Dielectric loss) is correct:
- In film capacitors, RF loss is dominated by the dielectric’s dissipation factor (loss tangent), represented by $\,\tan\delta\,$.
- That loss is modeled as an equivalent series resistance (ESR) that converts RF energy to heat.
- Key relations:
  - $$\text{DF} = \tan\delta \approx \frac{1}{Q}$$
  - $$R_{\text{ESR}} \approx \frac{\text{DF}}{\omega C} = \frac{\tan\delta}{\omega C}$$
  - $$P_{\text{loss}} = I_{\text{rms}}^{2}\,R_{\text{ESR}} = \frac{V_{\text{rms}}^{2}}{R_{\text{ESR}}}$$
- For common film dielectrics (e.g., polypropylene), DF is small but increases with frequency, making dielectric loss the primary RF loss mechanism.

Why the other choices are incorrect:
- A: Inductance
  - The capacitor’s equivalent series inductance (ESL) limits high-frequency performance and sets the self-resonant frequency, but inductance is reactive, not dissipative. It does not cause power loss by itself.
- C: Self-discharge
  - Leakage (modeled as a large parallel resistance) is mainly a DC/low-frequency phenomenon. In quality film capacitors, leakage is very low and not the dominant RF loss.
- D: Skin effect
  - Skin effect raises the resistance of leads/foils at high frequency, contributing to ESR, but in film capacitors the dominant RF loss is typically the dielectric’s loss tangent. Skin-effect-related conductor loss is secondary for good RF-grade film caps.

Rules of thumb:
- Lower DF (tan δ) → higher $Q$ and lower ESR → lower RF heating.
- For a quick estimate: $$R_{\text{ESR}} \approx \frac{\tan\delta}{2\pi f C}$$
- Check datasheets: DF rises with frequency; polypropylene usually beats polyester for RF use.