Coil winding quality

Hello guys,

Imagine we have two cylindrical coils: - one with a neat winding, tight, where each turn is right after the previous one, etc. Something like this: neat coil - the second one with the same number of turns, same diameter and approx. the same length, but wound like a little mess.

I'm thinking of a coil relatively short compared to its radius so the messy one should not be much shorter than the neat one.

What loss of inductance would one expect with such a poor quality winding?

The inductance might change only a little, but when used in a LC circuit, the quality determines how accurate the LC circuit will oscillate.

The 'quality' factor of a coil is called 'Q factor'. http://en.wikipedia.org/wiki/Inductor#Q_factor It has a large influence. I had once an FM transmitter for indoor use, but when I exchanged the coil with a pcb printed coil it got 3km range. To increase the Q factor, there are many ways to do that: using a gap between the wires; or use flat copper strips; or gold plated wires; or a little sideways winding; and so on.

In the beginning of the radio, they used all kind of coil windings, like this one : http://museumvictoria.com.au/collections/items/369111/coil-awa-high-frequency-inductance-1914-1920

If you make a coil for a solenoid, it might change the inductance perhaps by 20%, not a big deal. Every winding will create the same magnetic flux, and all the flux is added. In theory, you get the same force for a solenoid. The mechanical strength of the coil itself will be less of course. The wires get more stress and might break when wound too tight.

[ADDED] Not gold plated, but silver plated. Thanks russellz.

Peter_n: To increase the Q factor, there are many ways to do that: using a gap between the wires; or use flat copper strips; or gold plated wires; or a little sideways winding; and so on.

Gold is a poorer conductor of electricity than copper. To improve the Q-factor for high frequency coils where skin effect is significant silver plated wire is sometimes used. An old method that seems to have gone out of fashion is to use Litz wire. Wave winding can be used to reduce self capacitance.

A useful general guide to coil winding is available here from my old Alma Mater.

Russell.

Thx for your replies.

@Peter_n: I feared that the inductance loss would be greater, but 20% is acceptable for some applications indeed.

@russellz: I'll have a look at this guide, thx :)

russellz: An old method that seems to have gone out of fashion is to use Litz wire. Wave winding can be used to reduce self capacitance.

No, neither is "out of fashion". This method (Litz, wave winding used together) is appropriate for coils up to a few megahertz, and that limit mostly for air cored coils. The use of ferrite "pot" cores allows the use of far fewer turns on the coil such that skin effect and self-capacitance are no longer so significant as was also always the case at higher frequencies.

Also, IC (and digital) technologies have taken over at lower frequencies, reducing the number of coils.

OP, please note that you can go back and correct the spelling in the subject line of your first post.

Damn I didn’t even notice! >_<

Paul__B: The use of ferrite "pot" cores allows the use of far fewer turns on the coil such that skin effect and self-capacitance are no longer so significant as was also always the case at higher frequencies.

Yes, that's what I was getting at. I guess "higher frequencies" is rather subjective. I used to work in the range 1 GHz to 15 GHz and skin effect is certainly significant there.

Sachsa, this spreadsheet might be of use to you for designing a single layer coil.

Russell

russellz: I guess "higher frequencies" is rather subjective. I used to work in the range 1 GHz to 15 GHz and skin effect is certainly significant there.

And of course that is true - skin effect only increases with frequency.

Why Litz wire and wave winding can not be used at higher frequencies is that the very structure itself introduces spurious inductance, resistance and capacitance as the "skin effect" concentrates the current onto the surface of the whole wire bundle and in turn, coil.