The
approximate inductance of a single layer close wound aircoil may be
calculated from the below formula.
Where
L
= Inductance in µH
r
= Former radius in mm
=
Length of winding in mm
N
= Number of coil turns
Example:
Coil
has 20 turns close wound on to a 25mm diameter (12.5mm radius) former and the wire is 1.2mm
diameter which will make the overall length of the coils 24mm.
A
slightly more practical formula for determining the number of coil
turns for a given value of inductance.
Example:
Required
Inductance of 7µH on close wound single layer air inductor with a
25mm former and the wire is 1.2mm diameter which will make the
overall length of the coils 24mm.
The
below chart shows inductance in uH of inductors with 7 different
former sizes against the number of coil turns.
Fig 1 The
chart shows inductance in uH of inductors with 7 different former
sizes against the number of coil turns.
Reference http://hamwaves.com/antennas/inductance.html
