air then means more windings tho because research tells me air permeability is 1
Hi,
As a rule the most important thing about a magnetic circuit is distance which in a magnetic circuit means length.
The next most important thing is the magnetic circuit itself, in that the flux is said to circulate similar to the way an electrical current circulates in an electrical circuit.
This means to understand your setup you have to figure out what the magnetic circuit is and what the circulation path lengths are.
For a magnetic circuit that is made from all core material and no air, the permeability is just about the permeability of the core because the flux is always through a magnetically active material with high permeability relative to air. For a mag circuit that goes partly though air though, the decrease in permeability is abrupt and very significant. Even a small gap 0.1 inches can have a profound effect on the permeability, and the permeability is what increases the self inductance of an air coil when inserted in the space where the air was originally.
The problem with a 'rod' is that the air gap is as long as the core itself. That big of an air gap will make the construction look only a little better than air. The other problem is that it is hard to complete the magnetic circuit which must start at one end directed outward and end at the other end directed inward. Whatever you use as a receiver has to be able to be part of that circuit, and because one end of the rod will be very far from the receiver it will make a very poor magnetic path.
The idea is to keep the entire magnetic path as short as possible, with emphasis on the shortness of the air part of the path. So if you look at different geometries you see different results. A round flatter coil works better because the turns at the back of the coil are not that much farther from the receiver than the turns at the front of the coil.
I am not sure if you should get too hung up on the value of the self inductance. There will be an optimum value yes but it may be hard to get.
What you could try if you feel you need to use a rod is use two rods, one for the transmitter and one for the receiver. Keep the two rods parallel and as close together as possible.
Take a look at ron simpson's post and see how that setup words, then compare it to the two rods. Note the difference in the air gaps, which makes the most difference in power transfer. The other idea is to get the mutual inductance as high as possible, and the smallest air gap(s) achieves that.