In this work the portion of the incident field that can be received by an antenna is investigated: the observable field. This field can be characterized only relying on the volume allocated to the antenna and thus independently from the specific antenna. The observable field is composed by a single spherical wave that first converges in the origin and then diverges to infinity. The power associated to the converging wave is the available power for the considered antenna domain. Previously, an estimation of this spherical wave was obtained by truncating the spectral spherical modal series representation of the incident field. Here we provide a more applicable approximation of the observable field, by truncating a spatial integral representation of the incident field that is based on the use of equivalent ideal currents. Eventually, for the vast majority of antennas, the estimation of the available power that can be obtained by approximating the observable field via the ideal currents is more accurate that the estimation that would be obtained via the spectral modal expansion. The ideas are first set considering the case of single plane wave incidence. Eventually, the extension to multiple plane waves is immediate and rigorous.