In overhead power line analysis, the influence of ground wires is often considered assuming an ideally conducting connection to the model earth. Additionally, for calculating the inductive coupling, conventionally infinitely long conductor-earth loops are examined, assuming a homogenous field along the conductor direction. Deviations from this approach occur, for example, at the beginning and end of the line as well as near transposition towers where a homogenous field can not be expected. Moreover, ground wires and optical ground wires can also not be assumed to be infinitely long as equalizing 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. Utilizing 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 computational complexity, the integral expression is approximated using image theory, facilitating computations 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, 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.