This paper investigates the spatial-frequency channel characterization of Ultra-wideband (UWB) wireless communication systems. Firstly, a novel frequency dependent UWB channel model is constructed based on the theory of electro-magnetic diffraction mechanism, which causes the field strength to vary with the frequency in each multipath. Secondly, we build a space-frequency model, which includes spatial characteristics such as angular power spectrum, and physical sampling points in space. The space-frequency model has two special cases (i) discrete multipath model, and (ii) cluster model, which can be readily used to generate channel data for any arbitrary set of sensor locations. The reconstruction results from channel measurements show the accurateness of the novel frequency dependent model, with reconstruction error decreasing by 40%, compared to the traditional Turin model.