The X-mode radial correlation reflectometry diagnostic on JET has long suffered from strong signal attenuation in the original transmission lines. To reach higher performances, new low attenuation corrugated circular wave-guides and a new cluster of six antennas have been supplied and successfully installed in autumn 2005. The increase of the signal-to-noise ratio using these new transmission lines is found to be in the order of magnitude of 20dBs, thus enabling good dynamic of reflectometry measurement. First X-mode reflectometry measurements of density fluctuations in the core region of the JET plasma are reported in this paper.