 ```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 `````` ```