Patent GB-1365475-A (GB)
Title: Intrusion dtection system
IPC: G01S7/02
Filed: 1973-03-14
Abstract: 1365475 Intrusion detection; aerials; combined transmitter/receiver; mixer circuits OMNI SPECTRA Inc 14 March 1973 [24 March 1972] 12268/73 Headings H4D H4L H4A and H3T An intrusion detection system comprises aerial means for receiving electromagnetic energy including means for removably supporting any one of a plurality of interchangeable aerial elements, each of said interchangeable aerial elements providing a distinctive reception pattern of predetermined configuration, and means associated with the aerial means for detecting a frequency difference between transmitted and reflected received electromagnetic energies for providing an output signal which is indicative of an intrusion. As described, an instrusion detection system 10, Fig. 2, has a base-plate 32 which is provided with a pedestal 36 extending from a universally connected mounting foot 38, and which carries a transmitter section 24, a receier section 26, a mixer section 22, an aerial comprising a parabolic dish 16 and a splash plate or sub-reflector 18, circuitry 28, a connector 34, and a radome 12 with an associated cut-out switch 30. Microwave energy is generated in the transmitter section 24 by means of, for example, an oscillating Impatt diode, and is radiated from a slot 40 into a cavity 42 of the mixer section 22 (Figs. 7, 8). A receiver probe 48 extends into the cavity 42 at a slight angle to the major axis of the slot 40, which is orthogonal to the electric field vector of the linearly polarized waves radiated therefrom. A small proportion of the output from the transmitter section is consequently coupled into the receiver section. Tuning probes 44, 46 are provided to compensate for the effect of the probe 48, and to maintain matching. Waves from the cavity 42 are transmitted through a circular waveguide 50 to the splash plate 18. A dielectric member 20 supports the splash plate 18 and has a cylindrical portion 52 which is within the waveguide 50 and which contains a linear to circular polarizer 54. This polarizer is constituted by three conductive pins 56 which extend partly across the waveguide 50 and which are disposed at 45 degrees to the electric field vector of the waves from the cavity 42. A substanial portion of the circularly polarized waves impinge on the splash plate 18 and is directed towards the reflector 16. The support member 20 has a concave face upon which the splash plate 18 is removably attached, e.g. it may be made of metallic foil having an adhesive backing. The splash plate shape determines the coverage of the system, e.g., a disc shape provides a narrow radiation pattern (Figs. 4a, 4b, not shown), an annular shape provides a broad radiation pattern (Figs. 5a, 5b, not shown), and a semicircular shape provides an asymmetrical radiation pattern (Figs. 6a, 6b, not shown). Waves reflected back from an object such as an intruder are circularly polarized in the opposite sense to those transmitted and the polarizer 54 converts these into linearly polarized waves whose electric field vector is orthogonal to that of the transmitted waves, so that a major part of the received energy is coupled into the probe 48. If the reflecting object is moving, the waves received by the aerial will contain components differing in frequency from that of the transmitted waves. This may arise from the Doppler effect with radial motion of the object, or to amplitude modulation owing to multipath propagation effects as the object traverses the radiation pattern.  The mixed signals in the probe 48 are detected by a diode 58, which may be a Schottky barrier diode, and any difference frequency signals appear at the exposed end 62 of the probe. A dielectric member 60 interposed between the probe 48 and the housing of the receiver section 26 provides a by-pass capacitor for higher frequency signals. The output on the line 62 is received by a linear amplifier with a variable gain control, to provide range adjustment. The amplified output is connected through a threshold circuit, which rejects system noise and low amplitude signals, to an analogue-to-digital converter. Pulses from the converter are received by an integrator, which provides an output signal to actuate an alarm through relays, if a sufficient number of pulses occur during a given time period (Fig. 10, not shown).