Radiation losses due to outward propagating waves are one of the most important loss mechanisms with regards to voice generation. In this work, their effects on vowel production are analyzed by means of time domain finite element simulations with free-field radiation. The radiation impedance and the vocal tract transfer functions of vowels are examined. Vocal tract transfer functions are compared for two different situations: numerical simulations with computational domains ended at the mouth aperture, and an open end boundary condition imposed on it, and numerical simulations with free-field radiation, using a perfectly matched layer approach. Differences in formant location and bandwidth are reported and related to the effects of the real (resistance) and imaginary (reactance) components of the radiation impedance. The relation between these component values and their effects on vowel formants presents a clear non-linear behaviour, due to formant cavity affiliation. Consequently, small changes in their modelling can lead to large variations in vocal tract transfer functions. This highlights the importance of appropriate modelling radiation loss mechanisms, ideally by means of directly simulating outward propagating waves or, otherwise, by resorting to very accurate radiation load models.