I have two solenoid coils, one is wounded on ferrite rod and other is wounded in larger diameter in proxmity of ferrite rod, see picture. It is used as wireless comm (simplex) between RX and TX where I use resonance on TX to maximise signal gains After bit of reading and now familar with mutual inductance between two air solenoid, impact of cross section area and so on. (1) How to calculate or estimate mutual inductance as well as coupling coeff under this diagram. (2) If possible tip how to calculate/estimate the reluctance for this circuit. My gut feeling that not all flux path will goes to ferrites as flux concreator and need to estimate out how much fraction of flux goes to ferrite rod.