Air Coil Inductance

[codan]

I have noticed there are different Inductance formula's for a various types of air coils & there design, single layer spiral, flat wound spiral, multilayer etc.

My question is for the design of Jumble wound air coils:

What is the Inductance formula for a Jumble wound---Random wound air coil & the necessary dimension points needed to be taken for the caculation.
Some say you use the single layer spiral air coil formula, others say use the multilayer formula & some say neither apply?

Is there a site that i can go to to look this up or can anybody help me out?

Thank You

 

[Bob Scott]

My father's old engineering handbooks state that the formula for a multilayer coil is only approximate, being most accurate where the cross sectional area is close to being a square (10:9) and the proportional diameter is close to the illustrated drawing. I suppose the formula for a single layer coil is also a close approximation.

{EDIT} Multilayer coil formula used is L=(0.8a^2N^2)/(6a+9b+10c) where L is microhenries, (a) is mean radius, (b) is length and (c) is winding thickness in inches. I entered the dimensions into a spreadsheet years ago and found the optimum dimensions for creating the most inductance from a given length of wire using this formula, but I can't post the results with 100% confidence that I remember them correctly. I suspect the ratio was also 6:10:9.{/EDIT}

The REAL formulas I saw in the book are infinitely long and contain undecipherable math.

I would think that if the coil is flat, use the single layer formula. If the cross sectional area is close to square, use the multilayer formula. If it is neither, use the technique used by design engineers in the 1930s: experiment with prototypes until it works.

 

[Hero999]

What inductance do you require?

If it's under 1:mu:H or so a single layer coil is probably the best way to go.

Another question is, why not buy it? This is probably the most sensible option for inductances over a few :mu:H.

 

[chemelec]

What is the Inductance formula for a Jumble wound---Random wound air coil & the necessary dimension points needed to be taken for the caculation.
Some say you use the single layer spiral air coil formula, others say use the multilayer formula & some say neither apply?

Is there a site that i can go to to look this up or can anybody help me out?

Thank You

I have some calculators here that I have created:

**broken link removed**

But the Problem with Scramble wound is How Loose or How Tight is it Wound.
Sorry, You can't get an Accurate Formula because of this.

 

[stevez]

I recall seeing a chapter in an amateur radio reference - ARRL Handbook for Radio Amateurs - that included the various theoretical formulas as well as some suggested adjustment factors that might help to account for the difference between theory and actual result. Those were for the simpler coils but might be of some help. As already suggested, scramble wound has so many variables that it might be impossible to accurately predict the end result.


If I had this to do, I would do some experiments to compare a multilayer prediction to random wind - to see what direction the inductance takes. I would think that the relative density of turns would be less for random wind.

 

[codan]

Many thanks for the replies, it has been helpful.

Thank You Again.