Abstract: An improved sensing unit for installation in a room to be monitored by a microwave intrusion detection system. The sensing unit comprises an omnidirectional microwave antenna made of a rectangular waveguide having a shaped aperture defined by cutting the free end of the waveguide at an angle from each short side wall. A scattering blade is positioned centrally between the pointed ends of the wide side walls thus formed.The antenna is coupled to a transceiver whose output signal is applied to a sensitivity control serially connected to a filtering amplifier. The amplified output signal is fed to a level detector whose output is applied to a triggering signal generator through a delay circuit which inhibits signals of insufficient duration.