Hello Peter,

I got 2 formulae from 'Grundlagen der Hochfrequenztechnik' :

1. For 'slim' coils (H>>D) : L = 2*Pi*N^2*D(3.47/(1+2.2*H/D)-K*C/H)

where

L = inductance (nH)
N = windings
D = average coil diameter (cm)
H = coil length (cm)
C = thickness of the wirelayers (all together, in cm)
K = correction factor depending on H/C ratio :
        L/C     K
        2       0.10
        3       0.15
        5       0.23
        10      0.27
        15      0.30
        20      0.31
        30      0.32

2. For 'fat' coils (H<=D) : L = 78*L^2*D/(3*D+9*H+10*C)

If the coil is not close wound then a correction factor dL has to be taken
into account : dL = 2*Pi*D*(ln(b/d)+0.155)
where
b = wire spacing (centre wire to centre wire)
d = wire diameter
ln = natural logaritm

dL has to be added to L (but since always b > d , dL will be negative).



I hope the above gives at least an idea where to start with,


73, Rik  ON7YD

I am dismantling a Decca loading coil with the objective of rebuilding it to
load my 16m high antenna. It will be converted so that it is a multi-layer
coil. Does anyone know of a formula for multilayer coils where the spacing
between the layers is several times the wire diameter?

Regards,
Peter, G3LDO

e-mail <[email protected]>

Web <http://web.ukonline.co.uk/g3ldo>