On 04/08/2011 19:58, Electron wrote:
> Is an air core inductor of 1000 winds equivalent to an air core
> inductor of 100 winds each on 10 layers?

usually not.

N squared is an approximation

Short compared to diameter coils are different to coils long compared to=20
diameter.

Multilayer coils behave differently.

PCB Spiral coils are "interesting" for 50MHz to 500MHz, but at higher=20
frequencies  they don't work as imagined and length becomes dominant.

Also Skin Effect varies with frequency. This has a major impact.

Also multilayer has more complicated distributed self capacitance

Here is an interesting air core coil

http://www.techtir.ie/construction/herge-radio-base

It's in 3 parts. Two coils are wound about 25 turns + 25 turns=20
interleaved.. The 3rd coil is 5 turns separate to the side.

If you put two completely separate air core coils (or any inductor) in=20
parallel, then you get 1/2 inductance. Put them in series and you get=20
double. But if the two coils are wound in parallel, then wiring them in=20
parallel is just like having fatter wire. The Q is increased (resistance=20
reduced to 1/2). The inductance remains the same as if there was one=20
coil. Connect them in series, and it's like a flattened two layer coil,=20
the inductance will be about x4

The 3rd coil is used for positive feedback. As long as losses are=20
greater than feedback, it doesn't oscillate. But as feedback is=20
increased more of the coil losses are "cancelled" and the Q rises=20
dramatically, and radio bandwidth is narrower. Just at the point of=20
balance gives dramatic performance as if it was a perfect inductor.





--=20
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