Precipitation radars in Low-Earth-Orbit (LEO) provide vertically resolved profiles of rain and snow on a global scale. Nevertheless, observations available from LEO platforms are sparse in time, and they do not allow the observation of short time scale evolution of most atmospheric processes, nor do they properly sample the statistics of the diurnal cycle within short temporal windows. CubeSats and SmallSats enable cost-effective deployment of multiple copies of the same instrument to achieve these goals. As part of a JPL R&TD initiative called RaInCube (Radar In a CubeSat), we have demonstrated the feasibility of a Ka-band precipitation profiling radar instrument in a 10x10x20cm3 volume, excluding the antenna. For optimum synergy with the Global Precipitation Measurement Mission Dual-Frequency Precipitation Radar, 5km radar footprint is desired, requiring a 0.75m aperture size from 400km LEO orbit. The primary goal of this ACT proposal is to design, fabricate and test a 0.75m aperture reflector antenna with 45dB of gain at 35.7GHz that occupies less than 2.5U volume (10 x 10 x 25 cm3) when stowed for launch, bringing the complete radar instrument to TRL 5, thus paving the way for a flight demonstration of RaInCube in a 6U CubeSat (RIC-6U). In order to achieve a compact design with higher efficiency in the presence of blocking from the sub-reflector, we will use an optimized Cassegrainian antenna geometry with a shaped sub-reflector or a sub-reflectarray to partly compensate for the main reflector distortion. As a secondary goal, we will investigate a range of more capable antennas (i.e., larger, scanning and/or multi-frequency) suitable for a 50kg SmallSat platform (RIC-S50) with antenna stowed volume 20 x 20 x 50 cm3.