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 waveguides and a new cluster of six antennas have been successfully installed in Autumn 2005. The increase of the signal-to-noise ratio using these new transmission lines is found to be on the order of magnitude of 20 dB, thus enabling good quality of reflectometry measurement. With the benefit of this improvement, first X-mode reflectometry measurements of density fluctuations in the core region of the JET plasma are reported in this article.