The Quality Factor $Q = \frac{X_L}{R} = \frac{\omega L}{R}$ at a specific frequency. A high Q indicates low loss relative to stored energy. The handbook details all loss components that reduce R and thus Q.

  1. Winding (Copper) Loss ($R_{ac}$): The dominant factor at high frequencies.

    • Skin Effect: Current crowds to the conductor’s surface, increasing resistance.

    • Proximity Effect: Magnetic fields from adjacent turns/ layers induce eddy currents, further increasing resistance. This is often worse than the skin effect in multi-layer windings.

    • Mitigation: Use Litz wire or very thin foil (thickness < skin depth $\delta$). Use sectional winding to reduce layer count and proximity effect.

  2. Core Loss: Magnetic hysteresis and eddy current losses within the core material itself. This loss appears as a resistance in parallel with the inductive reactance.

    • Core Material: Ferrites have much lower loss than powdered iron or laminated steel at high frequencies. Select the correct material grade (e.g., Mn-Zn for 10kHz-1MHz, Ni-Zn for >1MHz).

    • Flux Density ($B$): Core loss increases non-linearly with $B$. Operating at a lower peak AC flux swing increases Q.

    • Frequency: Core loss increases with frequency, typically following a power law.

  3. Radiation Loss: At very high frequencies (VHF/UHF), the inductor can radiate electromagnetic energy like an antenna. This lost energy reduces Q. Shielded cores or can-type enclosures are used.

  4. Dielectric Loss: Insulation between winding turns and between winding and core has a loss tangent. At very high frequencies, the capacitance between turns can draw a displacement current through this lossy dielectric, creating loss. Use low-loss insulation materials (e.g., PTFE, ceramic).

  5. Parasitic Capacitance ($C_p$): While capacitance itself doesn’t cause loss, it forms a resonant circuit with L. The self-resonant frequency (SRF) is where $Q$ drops to zero. Operating near the SRF causes current to circulate through the dielectric and winding resistance via the capacitive path, increasing effective loss. Winding techniques that reduce inter-turn capacitance (e.g., spaced winding, “bank” winding) help maintain high Q over a wider frequency range.

Handbook’s Synthesis: Designing a high-Q inductor is an exercise in minimizing all parasitic resistances ($R_{ac}$ from windings, $R_{core}$ from core loss) and managing the reactive parasitics (Cp) to push the SRF well above the operating frequency. The optimal design is highly frequency-specific.