I will be winding a couple of toroids with 2-3 turns. It would be a bit artificial to spread these over (the recommended?) 85%.
Should I spread them or place them close together?
Having searched www I find sites which disagree about whether spreading increases or reduces inductance. Does anyone know a definitive answer to this?

 

Hi,

In an air core inductor, when you spread the turns the inductance goes down. The coupling between turns is not as good then.
In an inductor with a core, the core permeability dominates so it probably doesnt matter that much unless the permeability is very low. If the permeability is low then the construction will act closer to an air core inductor so it will matter. Most cores have permeability of at least 100 though.

In theory the inductance of the air core inductor gets multiplied due to the core permeability, so if you start with a lower inductance then you end with a lower inductance, but this could be so little difference because the core couples the turns together much better than air. It has to be lower, but the difference may be hard to measure.

In any of these things though a measurement is always a good idea anyway. A long time ago a toroid transformer i checked was found to have more phase shift when the secondary turns were all wound close together than spread around the circumference. That measurement was done using a scope.

 

I will be winding a couple of toroids with 2-3 turns. It would be a bit artificial to spread these over (the recommended?) 85%.
Should I spread them or place them close together?
Having searched www I find sites which disagree about whether spreading increases or reduces inductance. Does anyone know a definitive answer to this?

Micrometals makes more toroidal cores than anyone. From Micrometals tech sheet -

To obtain the best coupling between the winding and the core, evenly space or position the turns around the core. Be careful not to “bunch up” the windings since the apparent inductance will increase by both the self-capacitance and leakage inductance. As the operating frequency increases above 1 kHz, the winding distribution about the core becomes more important. With lower permeability Iron Powder materials, poor magnetic coupling can result in a 2 to 1 difference in inductance between the calculated and measured value. ​

Full paper...
http://www.micrometals.com/appnotes/appnotedownloads/pct4cipc.pdf