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.