In this work, the development of a dual band radio frequency (RF) beamsteering frontend for a frequency modulated continuous wave (FMCW) radar system is presented. The frontend supports both the 2.4 and the 5.8GHz Industrial, Scientific, Medical (ISM) bands. There have been great works on both dual band FMCW radars and digital beamsteering systems. In this work, the antenna signals will be combined in the RF domain while at the same time combining the benefits from spatial and frequency diversity receivers. A signal theory on FMCW radar systems in multipath conditions is formulated. From this theory, requirements for the RF beamsteering frontend are derived. Improved amplitude control stages were developed, modeled and characterized. An improved vector modulating circuit as a building block for a four antenna array has been designed and measured. The dual band RF frontend application specific integrated circuit (ASIC) was designed and fabricated. A system has been developed on a printed circuit board (PCB) combining the designed frontend with an existing FMCW radar system. The system was characterized in different scenarios with heavy multipath effects and even in non line-of-sight (nLOS) conditions. The designed vector modulator features a phase control range of 360° and more than 40dB of amplitude control range. At the same time, the root mean square (RMS) phase and amplitude error are 6° and 0.16dB, respectively. The vector modulator shows a power consumption of 87mW and requires 0.2mm² of chip area. The measurements show that the distance error could be reduced considerably using an RF beamsteering frontend. For the first time, a radio wave distance measurement system was benchmarked in nLOS conditions, showing its benefits compared to single antenna frontends. The mean distance measurement error was reduced from around one meter using only one antenna to around 30cm using the beamsteering frontend, which means that the distance measurement error was reduced to around one third. The standard deviation of the distance measurement error could even be decreased by a factor of four. The realized beamsteering radar system requires 270mA from a 12V supply. It allows for dual band signal processing and thus enables to exploit spatial diversity and frequency diversity at the same time.