aircore

Air-core refers to inductors and transformers that use air as the magnetic core, with no ferromagnetic material such as iron or ferrite. In such devices the magnetic flux travels through air (or another non-magnetic medium) rather than through a solid magnetic core. The result is that the inductance or mutual coupling is determined by coil geometry and the permeability of free space, μ0.

In practice, an air-core coil stores energy in its magnetic field and has no core losses or saturation effects associated with ferromagnetic materials. The inductance of a long solenoid is approximately L ≈ μ0 N^2 A / l, where N is the number of turns, A is the cross-sectional area, and l is the length. For a given target inductance, a larger coil or more turns are typically required compared with ferrite-core designs. Air-core devices can exhibit high quality factors (Q) at radio frequencies because they avoid core hysteresis and saturation losses, though parasitic capacitance and resistance of the windings become critical at higher frequencies.

Advantages of air-core designs include broad linearity, wide temperature tolerance, and absence of magnetic saturation and core losses. They are especially favored in high-frequency RF circuits, tuners, and certain impedance-matching networks where stable inductance and low losses are important. Disadvantages include larger size for the same inductance, weaker magnetic coupling over distance, and greater influence of parasitics, which can limit their self-resonant frequency and practical usefulness at lower frequencies.

Applications typically encompass RF inductors and transformers in tuners, filters, and impedance networks, where predictable, frequency-dependent behavior without magnetic materials is desired. Design considerations focus on winding geometry, conductor losses, stray capacitance, and achieving the desired self-resonant frequency.