<p>Based on the analysis of data from two coherent scatter radars of the Institute of Solar–Terrestrial Physics of the Siberian Branch of the Russian Academy of Sciences (ISTP SB RAS), we show that these radars record a significant (up to 35%) number of signals coming from the back lobe of the antenna pattern, which had not been previously taken into account in automatic data processing. The complexity of such analysis is due to the uncertainty in calculating the elevation angle of the received signals (the spacing between two linear antenna arrays exceeds the wavelength of the radio wave). In this paper, we discuss a fast and simple method for detecting such back lobe signals that is based on threshold processing of the elevation angle of the received radio waves. The optimal threshold elevation angle that minimizes the errors in separating signals into mainlobe and back lobe ones is calculated on the basis of modeling and the Bayesian approach. This elevation angle is 28° for the EKB radar and 38° for the MAGW radar. The presence of the effect and its significance at the MAGW radar have been proven to the first time by direct measurementsof the elevation angle using a specially deployed antenna array perpendicular to two other antenna arrays of the radar. The optimality of the threshold elevation angle has been proven for the first time by a ray tracing analysis of all the signals observed by the radars in 2021, rather than only ground backscatter signals, which are commonly used. Based on these experimental results it has been proven for the first time that the found threshold elevation angle is the optimal angle for separating signals into interpretable and uninterpretable from the point of view of radio wave propagation in the main beam. Statistics of observations of signals outside the main lobe of the antenna pattern of the MAGW and EKB ISTP SB RAS radars are presented for the first time.</p>