Shortened dipole calculator

[Dirk_SDR]

On the net there are calculators for shortened dipoles like this one:

Rechner: Verkürzter Dipol | DF7SX.de

www.df7sx.de

I would like to do this calculation in an excel sheet.
Could anyone help me with the formula? How are the extension coils calculated in this case?

 

[merlin]

I think a correct term is a trapped dipole. The ARRL antenna book has a chapter for these antennas.
All the dimentions, formulae, can be found here either for dual band or your 'shortened' dipole.

 

[Dirk_SDR]

I think a correct term is a trapped dipole.

Ok, Google translate was not my friend ...