However, the ferrite- and powdered-iron slug-tuned core coils are adjustable. The air-core coils are not adjustable unless either an expensive roller-contact mechanism or clumsy taps on the winding of the coil are provided. Don't forget to select proper units for all the fields! Tuning inductors can be either air-, ferrite-, or powdered-iron-core coils. If you enter the value of frequency f, the calculator will estimate the expected Q-factor for the coil.