Coupling between repeater's antennas, called Radio Frequency Echo (RFE) deteriorates signal quality and compromises system stability. If the channel insulation is insufficient, the power in the closed-loop raises with time according to the gain margin value. Multiple methods of echo cancellation exists. Generally, these techniques are based on correlation calculation and adaptive filtering. However, their efficiency and algorithm's convergence speed are calling into question under signal's modulation and real-time constraints, specially with narrow band signals having frequency hopping as GSM or TETRA. This paper describes a real time method of RFE detection and estimation independently of the nature of the incoming signal. It is based on frequency sweeping and analysis of power ripples, and it can be implemented on FPGA for real-time purpose. It gives an accurate estimation of corresponding gain margin and loop delay. Index Terms—Radio frequency echo, gain margin, narrow band, frequency hopping.