Patent ID: 6909369

Claim:
Device for monitoring an area, which device comprises a transmitter unit for transmitting a signal and a receiver unit for receiving a reflected signal via an antenna, a first slotted cable being arranged as transmitting antenna for the transmission of a high-frequency signal and a second slotted cable being arranged as receiving antenna for the reception of the reflected signal, the slotted cables being arranged in association with each other and essentially parallel to each other, and the transmitter unit being connected to one end of the first slotted cable, a delaying device connected between the first slotted cable and the receiver unit, and the receiver unit being connected to the end of the second slotted cable furthest away from that end of the first slotted cable, characterized in that the relative speed between the monitoring device and a reflecting object is determined by utilizing the Doppler principle, a mixer being arranged with one input connected to the transmitting antenna and the other input connected to the receiving antenna, which mixer generates at its output the difference frequency between the transmitted signal and received signal.