Distance boundary of antenna isolation calculation in engineering

When wireless base station antennas share a tower or site, the electromagnetic compatibility should be considered, one of which is to calculate the antenna isolation. For horizontal isolation, calculation is based on antenna pattern and the traditional far-field boundary is d>2D²/λ. In practical scenarios of antenna co-site, however, antenna distance cannot fit these boundary conditions. Regarding this issue, Magnitude Error on Sphere (MES) is defined as a new indicator of antenna far-field and near-field approach degree. The analysis has shown that the distance boundary in horizontal isolation calculation can be set as d>8λ for typical base station antenna structure. While for vertical isolation calculated by I_v(dB)=28+401g(d/λ), the distance boundary can be set as d >4λ.