Patent GB-1298381-A (GB)
Title: Passive labels for use in electronic surveillance systems
Inventor: COLE P H
IPC: G01V3/10
Filed: 1970-02-20
Abstract: 1298381 Theft alarms UNISEARCH Ltd 20 Feb 1970 [26 Feb 1969] 8326/70 Heading G4N [Also in Divisions B6 and H4] In an electronic surveillance system, signals from a transmitter are received by a passive label, which is arranged to retransmit a reply signal to a receiver for processing. The label is provided with storage means and time delay means, so that the reply signal is delayed and can be readily distinguished at the receiver from the original transmitted signal. Labels may be attached, for example, to items of merchandise in a shop and receivers installed at exits can detect unauthorized removal of said items. In a first embodiment, a transmitter provides a pulsed output at a carrier frequency of 897-5 MHz. A label has a protective outer section 15, Fig. 4, and inner sensitive elements 16. Descriptive matter 17 may be included if required. The elements 16 include an aerial system 18, such as a lump loaded omni-directional loop, and a transmission line 19. Said line is formed upon a quartz or other piezo-electric substrate 20, Fig. 5, and includes array elements for returning a ten bit binary code reply to the receiver. As shown, there is an end array 22, containing twentysix electrodes and seven coding arrays 23, each containing sixteen electrodes disposed according to the code 1101001111. The spacing of the array electrodes is approximately 2 microns, i.e., one half-wavelength of a surface electro-acoustic wave at the operating frequency, and alternate electrodes are connected to opposite conductors of the line 19, as shown in Fig. 6. A full set often coding arrays may be provided initially and those which are not required for a particular code disconnected from the line before use. In operation, a pulse is received via the aerial 18 and the entire array is energized along the transmission line 19. The arrays 22 and 23 launch electro-acoustic waves along the piezo-electric substrate 20 and, after a time delay due to propagation along the blank portion 24, the coded reply signal is re-radiated from the aerial 18 to the receiver (Figs. 8, 9, not shown), where it is processed. If full coding facilities are not required, a simplified structure may be used. In one such structure (Fig. 10, not shown) a single array corresponding to the array 22, Fig. 6, launches and receives an acoustic pulse to and from a surface wave reflector which is spaced at a suitable distance away on a quartz substrate. In another structure (Fig. 11, not shown) the surface wave from a single array is constrained by suitable grooves etched in the surface of a quartz substrate so that it propagates in a circular path. In a second embodiment, the transmitter provided has a pulsed output at a carrier frequency of 10 MHz (Figs. 14 to 19, not shown). A label (Fig. 20, not shown) consists of a 4 x 6 inch epoxy glass card carrying a printed circuit tuned and damped loop aerial, and a 2cm. thick 2.5 x 5cm. quartz substrate with a conductive pattern 75, Fig. 21, providing a delay of approximately 6 Ásec. and a five-bit binary replay code. Because of the lower carrier frequency, the spacing between individual electrodes of the array elements is greater than in the first embodiment i.e., approximately 0.1625 mm. A suitable receiver is described, schematic diagrams being given (Figs. 22 to 25, not shown).