Well-established procedures are consolidated to determine the associated measurement uncertainty for a given antenna and measurements scenario [1–2]. Similar criteria for establishing uncertainties in numerical modelling of the same antenna are still to be established. In this paper, we investigate the achievable agreement between antenna measurement and simulation when external error sources are minimized. The test object, is a reflector fed by a wideband dual ridge horn (SR40-A and SH4000). The highly stable reference antenna has been selected to minimize uncertainty related to finite manufacturing and material parameter accuracy. Two frequencies, 10.7GHz and 18GHz have been selected for detailed investigation. The antenna has been measured in two reference spherical near-field measurement facilities as a preparatory activity for a Facility Comparison Campaign on this antenna in the frame of a EurAAP/WG5 activity. A full CAD model, in step compatible format, has been provided and the antenna has been simulated using different numerical methods from different software vendors [4–7]. Each participant was responsible for generating a suitable mesh and the numerical stability of their solution.