The maximum gain in on-frequency repeaters is limited by the isolation level between their two antennas due to close-loop stability conditions. In this letter, a novel pixel-layer isolator for tunable on-frequency repeaters is presented. The scattered field of the reconfigurable pixel layer is used to dynamically minimize the mutual coupling between the antennas. Insight is given on the linkage between coupling and reflection coefficients, and a useful design procedure is proposed. The optimized pixel repeater can tune the operating frequency over a 40% range, with tunable instantaneous bandwidth (1%-3%) and achieving a reliable isolation of 50 dB. The repeater has a radiation efficiency of 93% and a constant radiation pattern for the different configurations. Good agreement is obtained between simulations and measurements.