Sevick’s books on inductors are widely regarded as the canonical works on inductors. I’m looking for a more intuitive understanding and not so much math. Here are some rules of thumb for inductors. Not everything here is necessarily 100% accurate. (That is the nature of a rule of thumb.)


  • Z: impedance in ohms
  • F: frequency in Hertz
  • L: Inductance in Henries
  • k is a constant
  • A = area of a turn
  • N = number of turns
  • C = capacitance


  • Formal: The formula for inductance is: L = kAN^2
  • Formal: For a simple choke (not at resonant frequency): Z = 2πFL
  • Quasi-formal: For a choke at its resonant frequency Z, you treat it as parallel L and C. Z = 1/(impedance_of_L - impedance_of_C)


  • Increasing the diameter of turns, adds reactance.
  • Adding a turn, adds reactance.
  • An inductor has reactance (lots), resistance (little) and inter-turn capacitance (little).
  • Because an inductor has an L and a C, it has a resonant frequency.
    • Look up the resonant frequency in the ARRL Antenna Handbook or online. It is hard to calculate.
    • Often, you want the operating frequency to be well below the resonant frequency.
  • The windings of the inductor are usually kept well apart; often several conductor diameters.
  • A “W2DU balun” is just a bunch of ferrite beads (toroids) slipped over the coax feed line. If you need to choke a wide bandwidth, you can use a mix of ferrites to cover the range. Might need lots of beads an low frequencies.