>why would I do this? Cant you just buy this shit? You can buy many standard value inductors off the shelf, but those standard values may not adequately fit your needs, especially if you need a high inductance value. In many RF projects, custom coils are pretty much an essential because the project parameters are often either very broad or very narrow which means inductors with specific characteristics are a must. Some characteristics also require different core material such as air, ferrite or even fiberglass to meet the needs of the circuit. Ferrite can have too much magneto-reluctance and impede high frequency signals or change their waveform shape. Air core inductors air often used to reduce or eliminate that high frequency saturation and attenuation. It's complex and heavy on the math to deal with inductive tuned circuits. Oh, and just FYI, if you find an air core inductor in a device and your OCD kicks in because a few of the coils are lopsided and there's a gap between them and the rest of the coil, DON"T FIX IT! That little bit of separation and angle change is often deliberate as it changes and tunes the coil to a more optimal set of characteristics for the tuned circuit. You could throw the whole circuit's behavior off by tidying up the "sloppy" coils. This is another reason why the standard value off the shelf inductors aren't a fix-all for all inductive needs.