Inductor core choice at high frequencies For RF and high-frequency applications, which core type is preferred for inductors to minimize core losses and maintain high Q?

Introduction / Context:
At high frequencies, inductors suffer increased core losses from eddy currents and hysteresis if magnetic metals are used. Designers therefore prefer core materials with very high resistivity or non-magnetic supports to maintain a high quality factor Q and stable inductance.

Given Data / Assumptions:

• Frequency in the RF to microwave regime.
• Objective: minimize eddy current and hysteresis losses.
• Core options include ferromagnetic metals versus nonmagnetic ceramics (air-equivalent).

Concept / Approach:

Ferromagnetic metallic cores (cast iron, steel, iron alloys) exhibit significant eddy current and hysteresis losses that grow with frequency. In contrast, ceramic (or air) cores are nonconductive and nonmagnetic, eliminating magnetic core losses and yielding higher Q, albeit with lower inductance per turn. Ferrites (ceramic, magnetic but high resistivity) are also common at HF/VHF, but when the choices include only metallic versus ceramic, the best HF choice is a ceramic (air) core support.

Step-by-Step Solution:

Verification / Alternative check:

RF coil construction commonly uses air/ceramic forms; where increased μr is needed, ferrite ceramics (not metallic steels) are chosen for their high resistivity.

Why Other Options Are Wrong:

Cast iron, sheet steel, and iron alloys have large core losses at HF; mild steel is even worse due to conductivity and hysteresis.

Common Pitfalls:

Confusing ferrite ceramics (magnetic but resistive) with metallic iron cores; overlooking Q degradation due to core loss.

Final Answer:

Ceramic cored