Radio Frequency Interference (RFI) is probably one of the most important limiting factors in the accuracy of the brightness temperatures measured by microwave radiometers. Due to the proximity of the 'protected' L-band (1400-1427 MHz) to numerous services (communication, localization, broadcast...), this band is seriously affected by RFI, preventing ESA's SMOS mission or NASA/CONAE Aquarius mission from retrieving soil moisture and ocean salinity over large areas of the world. In this work, a technique is presented as an attempt to mitigate the RFI at its origin, when the synthetic aperture image is being formed. The technique is based on the well-known beam- and null-steering techniques used in today's advanced communication systems, but generalized to the synthetic aperture case, by means of the so-called equivalent array factor, which allows to interpret the 'synthetic aperture array' as a 'real aperture array' with an antenna in the position of each baseline. © 2014 IEEE.