air core transformer

Crazymechanic

Hello , so basically what are the main parameters describing these types of transformers?
What is the typical efficiency of a good winded one?
Also at what frequency range it becomes better in terms of efficiency and at what worse?

I tried to search the info in internet but no good luck , except for some "FREE ENERGY" bs tesla coil fanatic sites and so on, but my intended use is different so i couldn't find any good info other than crackpot texts and some amateur sites.

Thanks.