In this paper, the problematic of co-design between LNA and antenna is addressed. The targeted application is ultra wide band where this part of front end is a main interest in RF design. A comparison is drawn out between a 50 Ohm and a co-designed version, with same LNA architecture and power consumption. Simulation results show a 7 dB power gain enhancement in the latter version. Noise factor is improved by more than 1.5 dB over the whole bandwidth