The properties of the electron-cyclotron maser radiation produced in the auroral kilometric radiation (AKR) source cavity are investigated by means of two-dimensional particle-in-cell simulations and observations made by the Fast Auroral SnapshoT (FAST) Explorer. The simulations assume a population of primary auroral electrons with a downgoing shell distribution. Driven simulations in a meridional plane with a finite magnetic field gradient demonstrate that the maser radiation builds up very rapidly with decreasing altitude and that bursts of radiation with timescales of the order of 0.5 ms are produced at only 5--6 km below the injection level. Initial value simulations in a longitudinal plane show that the radiation component striking the cavity boundary at normal incidence is damped away via particle absorption and conversion to the Z mode. In contrast, the longitudinally propagating component is able to be amplified along an extended path length. The FAST observations of the electric field in the source region reveal that the polarization in the plane perpendicular to the ambient magnetic field varies between being approximately isotropic to having a substantial (as much as a factor of 100) enhancement of the along-track polarization. No evidence is found that the across-track polarization is ever dominant, and thus the AKR emissions do not form a standing wave structure between the cavity boundaries.