The problem of a correct modeling of the electromagnetic field distribution in the near field region is one of the most challenging and interesting due to its direct implication in the engineering evaluation of the shielding effectiveness of materials and structures in close proximity of radiating sources. This paper deals with a detailed analysis on the applicability of the Schelkunoff theory of transmission lines for shielding assessment in near field. The contribution is divided in two Parts. The present Part I analyzes the characteristics of two principal closed form set of equations for the quick evaluation of the near field from an electric dipole. It discusses their assumptions and compares the values of electric (E) and magnetic (H) fields, and their statistics, with those coming from accurate three-dimensional full waves simulations considered as reference. The computed E and H are the input for Part II where the wave impedance and the reflection, transmission, and shielding effectiveness coefficients are computed and discussed.