In this paper, the portion of the incident field that can be received by an antenna is investigated: the observable field. This field can be estimated relying only on the volume allocated to the antenna and thus independently of the specific antenna geometry. The observable field is composed by a single spherical wave that first converges into the origin and then diverges to infinity. The power associated with the converging wave is the power available to an antenna located within the defined volume. Previously, an estimation of this observable spherical wave was obtained by truncating the spectral spherical modal series representation of the incident field. Here, instead, we provide 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 than the estimation that would be obtained via the spectral modal expansion. Moreover, analytical expressions for the observable field are provided here. The ideas are set here considering the case of single plane wave incidence, but the extension to multiple plane waves is straightforward.