Difficulty: Easy
Correct Answer: air core
Explanation:
Introduction / Context:
At very-high frequencies (VHF), magnetic core materials suffer significant eddy-current and hysteresis losses, degrading the Q factor and detuning circuits. The choice of inductor construction is crucial for RF performance.
Given Data / Assumptions:
Concept / Approach:
Air-core inductors eliminate core losses because there is no ferromagnetic material. Although inductance per turn is lower, the reduction in losses and improved linearity make air cores the standard choice at VHF. Ferrites may be used at lower RF, but metallic cores such as iron or steel are highly lossy at VHF.
Step-by-Step Solution:
Evaluate losses: metallic cores → large eddy currents at VHF.Air core → no magnetic core losses; Q limited by conductor skin effect and proximity effect.Thin-film processes relate to fabrication method, not core material; they still typically realize air-core microinductors at high RF.Select air core as the conventional VHF solution.
Verification / Alternative check:
RF design handbooks specify air-core coils or low-loss ferrites only up to certain MHz; at VHF and above, air cores dominate.
Why Other Options Are Wrong:
Cast iron/sheet steel have unacceptable losses and low Q at VHF; “thin film technique” is a method, not a core type, and does not guarantee low loss without air core.
Common Pitfalls:
Confusing ferrite usage at HF with VHF practice; overlooking conductor layout to mitigate skin/proximity effects.
Final Answer:
air core
Discussion & Comments