It is well documented that multipath interference at low grazing angles is an issue for detection and tracking. As the conventional systems, a propagation factor is derived which quantifies system loss due to amplitude fading and phase distortion caused by the interference of 'free space' signals with multipath. For an ultrawide bandwidth (UWB) monocycle waveform operating at UHF, the earth's surface generally appears smooth and supports dominant coherent (specular) reflection. For interference patterns dominated by specular multipath, a closed form of the gain/loss (G) expected at the output of a matched filter (used as the UWB radar's receiver model) was derived and evaluated for a frequency dispersive and non- dispersive terrain model. The frequency dependence of the gain/loss function and the relative weighting of the multipath fields for the scenario analyzed was found to be dominated by the line spectrum of the UWB waveform S(f). In addition, for multipath to have any significant effect in a single monocycle's period, the propagation delay or time- difference-on-arrival (TDOA) of the single bounce and double bounce multipath interference terms must satisfy time coincidence criteria. The propagation factor (PFuwb) derived in this paper averages the range and frequency variations of the derived gain/loss function G(f,R) into a single value which can be used as input to a UWB system loss factor for short range air defense (SRAD) scenarios.