GB1030537A - A system for detecting specified objects by radio waves - Google Patents

Abstract

1,030,537. Radar; radio receiving apparatus; valve oscillating circuits. PINKERTON ELECTRO-SECURITY CORPORATION. Nov. 5, 1964, No. 45165/64. Headings H3F, H3Q and H4D. [Also in Division G4] In an electromagnetic object detection system (e.g. an intruder alarm system) where a standing wave pattern of radio waves is set up by reflexion from the object thereby influencing receiving aerial impedance, a pair of aerials are spaced apart by a distance greater than the distance at which they respond substantially identically to changes in the standing wave pattern by change in position of the reflecting object, separate means sense impedance changes at each aerial with the output of one sensing means being inverted with respect to the other and the outputs of the sensing means are differentially combined. Spurious fluctuations in the system are cancelled and Doppler-type response is minimized. In the embodiment, Fig. 1 (not shown), an oscillator (2) feeds 400 Mc/s. energy into spaced aerials (4, 6) inductively via respective capacitors (30, 32). The aerials are connected, Figs. 3, 4 (not shown), via capacitors (72, 72a) to oppositely-poled rectifiers (76, 78) the outputs from which feed a differential circuit (74, Fig. 1) connected to a low-frequency amplifier. Oscillator, Fig. 1 (not shown). The oscillator consists of a self-excited twin-triode valve (16) of sufficient inter-electrode capacitance to afford signal feedback of correct phase and amplitude to sustain oscillation, the valve is in an earthed-cathode configuration with grid bias provided by a grid leak resistor (18), and the anode tank circuit inductance is a U- shaped wire (20) connected between the valve anodes, the tank circuit being tuned by changing the distance between the flat tuning disc (22) of a capacitor and the inductance. Aerial combined with receiving apparatus, Fig. 2 (not shown). The components in a circuit, Fig. 3 or Fig. 4 (not shown), are mounted within the insulated hollow hemi-spherical base of the corresponding aerial (60). Features relating to the reduction of the effects of interfering signals and to the independence of system sensitivity with aerial loading are described. More than one pair of aerials may be utilized. It is stated that changes in transmitted wave frequency caused by changes in aerial impedance leads to increased system sensitivity.