A monostatic multibeam radar sensor device for a motor vehicle, including a directional characteristic of an antenna unit having at least one transceiving channel and at least one receiving channel, and including a mixer system, which has an at least approximately isolating mixer for at least one of the receiving channels. The at least approximately isolating mixer includes a Gilbert cell mixer, which, due to a non-ideal isolation between an input of the local oscillator signal and the corresponding receiving channel, emits a transmission power via this receiving channel, using an overcoupling signal, the transmission power influencing the directional characteristic of the antenna unit and the directional characteristic being switchable by controlling the phase position of the overcoupling signal.