The development of efficient ultrawideband (UWB) systems for high-rate wireless links requires an accurate knowledge of the radio propagation mechanisms. Anticipated usage scenarios in office or residential environments predict a significant impact of mobile persons on the UWB radio link. However, the issue of temporal variations in the UWB propagation channel has been scarcely investigated so far. This paper presents a study of the time variance of a UWB channel induced by the motion of people, with a specific focus on the main path of a line-of-sight radio link. A real-time measurement campaign involving moving scatterers was performed in the 4-5 GHz band, in a hallway of a typical office building. The analysis of measured data, regarding the shadowing effect and fast signal fluctuations, provides a detailed description of the phenomena involved in a time varying UWB channel. As a result, a model is described for the temporal variations observed on the main propagation path in a fixed UWB link, accounting for both the nonstationary attenuation due to obstructing people and the small-scale signal variations due to scattered propagation paths.