This paper describes the design and construction of a crossed slot antenna array to be applied to an Incoherent Scatter Radar for measuring parameters of the Ionosphere. The slot antenna was designed and optimized with electromagnetic simulation software. An analysis of the characteristic modes of the elementary antenna of the array is performed, in order to obtain a physical understanding of the radiating behavior. A prototype of a linear crossed-slot antenna array has been constructed and measured, as an intermediate step for the development of a two-dimensional array.