The purpose of this work is to design a frequencyscaled monopulse Radial Line Slot Array (RLSA) antenna intended for a space debris detector radar. Firstly, the slots arrangement is designed and optimized by using a global optimization algorithm where the analysis is based on an inhouse Method of Moment algorithm. A A pattern prototype is manufactured and tested. Secondly, a monopulse feed based on a multimode cavity is introduced for simultaneous E and A operation. The cavity is located below the ground plane of the RLSA and is coupled to the antenna by means of a circular slot and is implemented using gap-waveguide technology. Simulations show good impedance matching and isolation between channels and stable E and A patterns for the monopulse operation.