Hi Peter

Hello lf-group,
is there any formula to calculate a loading-coil like  the Decca-type?
Abt 3 mH might be required.

By "Decca-type" I presume you mean "basket weave"
Alan Melia has pictures and some design info for a home made basket weave
coil of 2.3mH and another of 9mH at
http://www.alan.melia.btinternet.co.uk/lfaecoil.htm#LF%20Aerials%20and%20Loa
ding%20coils

I find coil equations rarely give an accurate value of mH, I prefer to wind
some turns and measure the resonant frequency with a capacitor of known
value. Then add or subtract turns until the desired value is obtained.

Hugh M0WYE