It's not so smiple any more. You have the choice of standard shape, cone shaped, barrel and probbly a few others. There is the standard winding, bi/tri... filler winding, pullback winding and combing gives you the option for a 3d bifiler design. Now I hear about "flat" wire which is which is more rectangular with rounded sides and in looking that up on the web I find there is foil wire intended for cores. So lets start a thread on the advantages/dissadvantages and effects of these.

I was thinking of trying a 3d bifiller pullback design compairing to a standard coil. I was going wind 2 strands each layer as in a standard bifiller, but insted of bringing it right back between the layers which creates gaps in the layers possibly a reason they do not work as well in some cases, I was going to cut off at the end leaving some extra and start the next layer. After all layers are done bring the ends back over the finished coil. The end of the first wire in the first layer would connect to the start of the first wire in the 3rd (not the second) layer. And that would then connect to the 5th, etc. Once the outer mose odd number layer is reached, take the end back to the first wire in the 2nd, 4th, etc.Then do the same for the second wire and then connet the two interwound coils together in series. The reason for the bifiller design was to maxumize the voltage drop between ajoinging wires. WHen using multi-layers you need to think about the ajoining wire in the ajoing layer as well as in the same layer.

Now what makes a pullback design better? Is it because each layer wire more closly at the same angle as the ajoining ones? Is it because it lets the round wire sit down in the winding grove making the coil more compact and tighter to the core? or a combination of both? If it is soly because of the wire siting down in the grove, then the pulbacks must be ether over the outside or down the core center to get the most of the design. And the use of flat, square or foil wire would eleminate the benifit of the pullback design.

 And which would be better? over the top or down the center? If hollow ferrous cores are only slightly less of not any less effective then a soild ferrous core, and have the advantage of less magnetic drag, then it might be best to run the returns down the center since the magnetic field is over the outside. Less interference from right angle returning wires.

The idea behind bifiler was to make use of the capacitance between ajoining wires. With increased ajoining surface you have increased capacitaince. Flattened wire increases the surface area between ajoining wires on the flat sides, so if you wind it with the flats ar right angles to the core, you should increase the effect of bifiller. Square would add to the effect to a lesser amount to ajoining layers. Lesser because ajoining wires would overlap some.

And then there is this foil wire. copper foil on an adhesive backing that doubles as the insilator. How would you use this stuff? get a foil the same width as the length of the coil? Would eleminate the question of pullback or not and return bifiller to the ariginal design of a single winding. Run to foils at the same time and have the basic design of a capacitor cept the plates would be wirted in series insted of parrallel. How would this effect the need for a cap to tune the coil? wasn't that the idea behind the bifiller? Tesla was trying to reduce or eliminate the need for a cap but building it into the coil. Would make coil shapes a bit tougher. Using a bunch of different widths would be costly and you can't really machine the finished coil as that would likly short the layers.

Add to the wire options High Frequency Litzwire.

Let the theories fly and the bobbins spin up!