This thesis focuses on implementing a small, low-cost, Frequency-Modulated-Continuous-Wave (FMCW) radar sensor using Software-Defined Radio (SDR) platforms. Extensive investigation and comparisons of existing SDRs are provided. Two of the sensor platforms are tested using both commercial FMCW radar kit and USRP-N210 radio. The main application of the effort will be detecting small unmanned aerial vehicles (UAVs) using low-cost, reconfigurable FMCW radar sensors. Simulation studies of various SDR system configurations are done in MATLAB, and hardware calibration using loop-back testing is performed. Detailed discussions on system performance predictions and enhancements, as well as design considerations, are provided. The implemented sensor systems are then brought to outdoor experiments with targets such as reflective plates, water-towers and small drones.