At ASTRON a demonstrator is developed for a future radio telescope. Within the demonstrator an I/Q receiver is used to mix signals to base band. An I/Q mismatch of the analog part results in image signals and this effect is a function of frequency. Since future receivers for radio astronomy will become even more wide band, a compensation for I/Q mismatch over frequency is necessary. In the past a number of narrow band methods are developed to estimate and correct for these errors. Recently a theoretical wide band correction method was proposed and demonstrated by simulations. In this paper the performance of the wide band correction as a function of the number of calibration frequencies is discussed. Furthermore the wide band correction method is applied on data obtained with the I/Q receiver of the demonstrator. It is shown that the average rejection is 70 dB when the correction is applied.