There was a post over on EEVblog that got me thinking. What do you do when you are unable to, for various reasons not be able to procure iron core toriods. Well, obviously air core inductors are a thing and highly practical for the low values of inductance used at VHF and UHF. But, what about at HF?
Well it turns out that as long as you do not mind a little bit of size to the inductors, they are still a vary practical option. So what i did was grab out some 1mm enamelled wire and wind 9 turns on a bit if 22mm OD pipe i had laying about to see what sort if inductance such a coil would create. Turns out about 2uH. That is highly usable value in low pass and band pass filters in the lower HF range. And with some thought on the actual construction and mechanics of the coil, very usable inductors could be made this way if you are unable to buy iron core toriods for whatever reason.
Anyway, that is some food for thought. There is nothing magical about iron core toriods, air cores will work just as well and should have a reasonable Q equal to or better than a toriod. So if you are stuck, there is a solution.