Air Coils
This side will explain how to make coils with accurate inductance.
I have made coils with different number of turns and spacing and measured the inductance with a Wayne Kerr Precision component analyzer 6430A.

Introduction :
How I did the measurement.
I used a drill of 7.1mm for all my test coils.
Look at figure at right to see the shape of the coil and its endwires.

How to understand the table below:
The first measurement is made when the coil is totally compact (no space between the turns).
Then I started to space the end-wires of the coil in 0.1" (2.54mm) step.
Lets look at the measurement for 4 turns as an example.

When I made 4 turns the coil was compact and the length between the end wires was almost 0.1".
The inductance was 199nH.
Then I spaced the coil so the end wires was 0.2" (5.12mm) apart.
The inductance was 155nH.
Then I spaced the coil so the end wires was 0.3" (7.62mm) apart.
The inductance was 128nH...and so on.

 Measurement of air coil 7.1mm diam, Wire = 0.5mm Cu n=turns Pad space Inductance info 1 0.1 " 60nH - 2 - 73nH compact 2 0.1 " 75nH - 2 0.2 " 72nH - 3 ≈ 0.1 " 150nH compact 3 0.2 " 112nH - 3 0.3 " 111nH - 4 ≈ 0.1 " 199nH compact 4 0.2 " 155nH - 4 0.3 " 128nH - 4 0.4 " 127nH - 5 ≈ 0.1 " 275nH compact 5 0.2 " 205nH - 5 0.3 " 197nH - 5 0.4 " 174nH - 6 ≈ 0.15 " 328nH compact 6 2 " 277nH - 6 3 " 223nH - 6 4 " 210nH - 7 ≈ 0.2 " 433nH compact 7 3 " 311nH - 7 4 " 289nH - 7 5 " 242nH - 8 ≈ 0.2 " 508nH compact 8 3 " 399nH - 8 4 " 358nH - 8 5 " 326nH - 9 ≈ 0.2 " 590nH compact 9 3 " 512nH - 9 4 " 442nH - 9 5 " 390nH - 10 ≈ 0.25 " 672nH compact 10 3 " 603nH - 10 4 " 514nH - 10 5 " 461nH - 10 6 " 409nH -

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