This work focuses on antennas for wireless IR-UWB sensor network (WSN). A novel small (<5cm²) planar monopole ultra-wideband (UWB) micro-strip fed antenna is presented. WSN are difficult cases for communications, implying the antenna to be adapted to the channel. The first consideration is the path loss compensation, especially for Ultra Wide Band signals. The characterization of the antenna is done in both frequency and time domain and shows a return loss <10d B from 4,2 to 12.4 GHz (including the 6-8,5 GHz), fidelity factors of more than 90% and pulse width stretch ratio lower than 6%. The antenna transfer function is analyzed. It increases with frequency, providing a quasi-constant transmission channel transfer function (antenna&channel). Return loss and radiation pattern are depicted. The time domain analysis, via Fidelity factor and pulse width Stretch Ratio, confirms its non-distortion behavior. The designed antenna has a small size of 20x20x0.8mm 3 , and an omnidirectional radiation pattern over the frequency band considered. Keywords—impulse radio ultrawideband (IR-UWB); wireless sensor network (WSN); UWB printed slot antenna; antenna transfer function; fidelity.