Abstract: In addition to a first opening (11) forming a primary output from a waveguide cavity (10) of a microwave oscillator, a coupling aperture (12) in a reflective wall (32) provides a secondary output which does not degrade the performance or tuning characteristics of the source. Less than -20 dB of the source power may be coupled out via the aperture (12) and is used in a feed-back loop (12,21,23,24,36) with an harmonic mixer (21) and frequency discriminator (24) to stabilize the source. The mixer comprises a further waveguide section (15) separated from the source cavity (10) by the reflective wall (32). An advantageous mixer assembly comprises a circuit substrate which is mounted across the further waveguide cavity (15) and which carries an inductive probe by which the signal from the coupling aperture (12) is fed to an antiparallel pair of mixer diodes on the substrate. The whole assembly may be bolted together through holes (34).

Abstract: Information is communicated by microwave radio between transponders (9,10) carried by vehicles (7,8) travelling on a road (1) and a station (22) adjacent the road. The communications from the transponders are effected by the transponders suitably modulating their reflections of beams of microwave energy transmitted by the station from aerials (B) mounted on a gantry (11) above the road, these aerials irradiating respective communication areas (28,29,30). In order to prevent communications from different transponders overlapping and hence interfering with each other the transponders are enabled for their communications by microwave energy from further aerials (A), which energy has a higher frequency to enable it to be beamed at relatively small respective activation areas (23-27) the sizes of which are such that they can each only contain one vehicle and hence one transponder at any given time.

Abstract: A device for indicating whether a target is approaching or receding from a Doppler radar or the like comprising a length of wave-guide having a reflective termination at the end at which microwave energy is introduced into the guide, and two mixer diodes spaced along the guide. The diodes are spaced apart along the guide axis by an odd number of quarter-wavelengths and spaced from the termination by an odd number of eighth-wavelengths. Suitably, the diodes are positioned so as to be loosely-coupled to signals within the guide, thereby rendering the selection of suitable diodes much less critical.

Abstract: A Doppler radar system for controlling portable traffic signals in response to on-coming traffic. Each of two channel amplifiers of the system is fixed at high gain and passes both noise signals and Doppler signals to a phase detector. A threshold element provides a control signal when the average level of the phase detector output between high and low levels changes sufficiently, due to the presence of Doppler signals, from a mean level which is due to noise alone.