Magneto hydrodynamic (MHD) modes have a strong influence on the plasma behaviour. The measurement of their characteristics, i.e. amplitude, poloidal (m) and toroidal (n) numbers is essential for the early detection of potentially dangerous modes, and for the better characterization of plasma dynamics. Even though the phase characteristic of the coils can be measured before installation, an in-situ characterization is needed to include the effect of the various structural elements surrounding them and the long cables connecting them to the ADC. Six non uniformly toroidally distributed coils are used on JET to determine the n mode number. Ideally, in presence of a single mode, the phase difference between the coils should be proportional to einfk where fk are the coil toroidal positions, while their amplitude should be the same. In practice, a systematic error is still present. A correction factor can be derived measuring the difference in phase and amplitude among the coil signals after having taken into account the factor einfk. This correction factor mainly depends, as expected, on the mode frequency.