In overhead lines, the impact of ground wires is often considered assuming an ideally conducting connection to the model ground in the line model. Additionally, for calculating the inductive coupling, conventionally infinitely long conductor-earth loops are examined, assuming a homogenous field along the line direction. Deviations from this approach occur, for example, at the beginning and end of the line, as well as near transposition towers, where a homogeneous field is not expected. Moreover, ground and optical ground wires are grounded at each mast, which means that they are not infinitely long as assumed, as compensation currents occur in each span due to the span-by-span earthing, so that the approach of a homogeneous field is not fulfilled here either. Using the Hertzian dipole approach, a complex integral expression for calculating the impedance of conductors of arbitrary length is derived. Since this expression cannot be represented in closed form for finite conductor lengths, numerical methods are employed. To reduce the numerical computational effort, the integral expression is approximated using image theory, enabling calculations for practical uses. Subsequently, a MATLAB-based model is developed and validated with both a fictitious and a real tranmission line, enabling the calculation of line parameters, tower currents and currents in ground and optical ground wires, particularly the zero-sequence impedance, while considering various influencing factors. The developed model is utilized for parameter studies to identify significant factors affecting the zero-sequence impedance.