Large intelligent surface (LIS) is one promising path to leverage 6G performance in sub-10 GHz bands. This two-part paper explores the properties of phase-conjugation focusing for a simplified LIS setup with a two-dimensional (2D) circular antenna array and a user antenna located within the array aperture in the same plane. In Part I of this article, we assume vertical polarization for all antenna elements, whereas Part II assumes horizontal polarization. In Part I, we focus on the effect of array radius on the peak gain, 3 dB focusing width, and sidelobes for two types of circular arrays. The numerical results show that the gain minimum is located at the array center. The peak gain varies by less than 0.5 dB for focal points located within 2 from the array center. Similarly, the focal width and sidelobe level are also stable within this region, irrespective of array radius. From 2 from the center to the array edge, the closer proximity of the focal points to some array elements than other elements results in more drastic changes in these NFF properties. Finally, full-wave simulation using Ansys HFSS is used to partially validate the numerical results.