Future communication scenarios will require massive Multiple-input multiple-output (MIMO) by the use of multi-beam antenna systems. Maximizing the number of beams for a given antenna size is paramount given that the space allocated to the antenna is often limited. This work aims at evaluating the maximum number of beams by analyzing the SIR in different communication scenarios for planar antenna structures. The concept of ‘observable field’ is used to quantify the power received from the desired signal as well as the power associated with the interference. Due to the planarity of the considered antenna structures, the radiating domains introduce scan loss, an effect not previously modelled when considering earlier investigations based on spherical domains. Furthermore, methods of reducing interference in order to improve the SIR were investigated, i.e., the use of a tapered current distributions on the radiating apertures and null placement techniques.