Abstract: A unit for detecting a moving person or object by means of doppler frequency shift. The unit comprises a microwave circuit board (1) accommodating an oscillator and a mixer, and an antenna board (2) carrying transmit (8) and receive (9) patch arrays. In the assembled unit the two boards (1,2) are disposed adjacent one another. Coupling between the microwave circuit and the antenna arrays is achieved by two slots (6,11) resonant at the oscillator fundamental frequency and formed in a ground plane (5) on the microwave board (1). Feed striplines (7,10,12) on the two boards (1,2) lie orthogonal to the slots (6,11) and are terminated in T-sections (15,14). The stripline/slot arrangement suppresses emission at the oscillator second harmonic frequency. The antenna board (2) has no electrical connection to the microwave circuit, enabling the radiated beam shape to be changed by simply replacing the antenna board with one having a different patch array pattern.