A microwave identification system comprising a transmitter/receiver provided with an antenna system, a plurality of responders provided with an antenna system, the communication between transmitter/receiver and responders taking place by means of electromagnetic waves in the microwave range, wherein the antenna system of the responders comprises an array with at least two antenna elements (12, 13) interconnected via a modulation device (14, 15, 16) and, for obtaining a retroreflective effect, the signal generated in an interrogation field by one of the two antenna elements is applied to the modulation device and, after being modulated with a code that is characteristic for the responder, is applied to the other antenna element to be radiated by the other antenna element.
An RF identification transponder comprises a substrate layer, and an array of M*N antenna elements formed on the substrate layer, where M and N are integers. The antenna elements have a common predetermined resonant frequency f.sub.r. The resonant frequency of the transponder's antenna element array allows it to be identified apart from other transponders, each having an antenna element array with a different resonant frequency.
A transponder comprises a retro-reflective antenna array in which the length of the transmission lines is given by I=.+-.A n.lambda. where: A is an arbitrary length; n is zero, or an integer, .lambda. is the wavelength of the retro-reflected signal. The array is fabricated on a board (71) of insulating material, using microstrip techniques. Rectangular patches (not shown) form the individual antennae and these are interconnected by transmission lines (75 to 82), each of which pass through a respective modulation device (43) whereby the retro-reflected signal is modulated. A positioning/tracking system incorporating one or more modulated transponders is also disclosed.
Security systems which include an adjustable structure and a motion detector which detects changes in electromagnetic energy from within a zone of protection are described. The structure has a base for mounting the motion detector and wall members which extend away from the base and to form walls around the detector. The wall members are adjusted to positions between the plane of the base and planes perpendicular to the base to alter the shape of the zone of protection and conform it to the area or object of interest. The motion detector is electrically connected to a control unit and alarm module which includes a shock sensor and a current sensor. The motion detector and the control unit and alarm module are connected to a power supply and operated by remote control.
