This paper presents the design, optimization, and experimental characterization of a planar antenna subarray for space applications. The proposed component is intended to be used as a building block for a large Ka-band antenna systems. It is a one-dimensional linear array fed using a standard rectangular waveguide. It includes a transition between the waveguide and the beam forming network (BFN), the BFN itself and six radiators. A planar BFN and planar radiating aperture-fed double stacked circular patches are used to guarantee the low-profile of the proposed element. To ensure low losses and to reduce the mutual coupling inside the BFN, the suspended stripline (SSL) is selected as the transmission line. Due to the tight requirements on RF performance and the limited space available, novel components, such as waveguide to SSL transition and SSL power dividers, are proposed. The design is optimized using a step-by-step procedure. A complete subarray prototype has been fabricated requiring no-less than six different manufacturing technologies. The experimental results have validated the numerical simulations and show a good overall performance and low losses with efficiency above 70% in the frequency band 25.5–27 GHz (5.7%).