Patent GB-1374948-A (GB)
Title: Intrusion detection system
IPC: G08B13/24
Filed: 1973-03-14
Abstract: 1374948 Doppler radar; aerials OMNI SPECTRA Inc 14 March 1973 [24 March 1972 10 April 1972] 12273/73 Headings H4A and H4D [Also in Division G4] An intrusion detection system includes a transmitter for generating electro-magnetic energy plane polarized in a first direction, a duplexer including polarizing means converting said plane polarized energy into circularly polarized energy and communicating it to antenna means for radiating and receiving circularly polarized energy, said duplexer further converting received circularly polarized energy into energy plane polarized in a second direction substantially orthogonal to said first direction and passing it to a receiver for energy polarized in said second direction, and means for detecting a low frequency component in the received energy. The antenna unit (Fig. 2) comprises a base-plate 32 covered by a radome 12 and incorporates a switch 30 which signals tampering with the radome. The microwave generator, e.g. an Impatt diode in unit 24 provides energy which is coupled through a tuning cavity to a propagation path and thence to splash plate 18 which reflects the energy out on to parabolic reflector 16 which directs the energy beam out into the area to be protected. By varying the shape of the splash plate 18, the shape of the radiated beam can be varied. The plate may be for example a circle, semicircle or a ring. Reflected energy is coupled back via the reflector 16, plate 18 and tuning cavity to receiver section 26 including a self-biased diode detector. The duplexing device is shown in Fig. 4. Microwave energy is produced in generator 24 and coupled via an elongate aperture (40, Fig. 7, not shown) into cavity 42. The aperture ensures that the generated wave has its electric field orthogonal to the aperture. The energy passes along path 54 to be reflected by splash plate 18. In path 54 are sited three rods 56 inclined at 45 degrees to the plane of the electric field of the transmitted wave. This converts the energy into circularly polarized waves. Reflected waves are 180 degrees out-of-phase with the transmitted waves and thus after passing back along path 54 past the polarizers 56, they become plane polarized in a direction orthogonal to the generated waves. In the cavity 42 is situated a receiver probe 48. This is aligned substantially with the electric field of the reflected waves but is arranged to be slightly skewed (Fig. 7, not shown). This ensures that it receives a small portion of the generated wave as well as the major portion of the received wave. The two tuning screws 44, 46 are provided to counterbalance the effects of the probe 48 in the cavity 42. The two waves mix in the probe and the low-frequency resultant is fed to diode detector 58 and thence via blocking capacitor 60 to the output. Input and output signals to the alarm are provided via a sealed connector 34, Fig. 2. The detected low frequency signals which may be due to Doppler shift or amplitude variations in the received signals pass to a variable gain amplifier, threshold, detector, pulse former, integrator and relay driver to an alarm relay (Fig. 8, not shown). The amplifier gain is adjusted on installation to set up the required range of operation. The gain control may be calibrated in range. Specification 1,365,475 is referred to.