US3895368A - Surveillance system and method utilizing both electrostatic and electromagnetic fields - Google Patents

Surveillance system and method utilizing both electrostatic and electromagnetic fields Download PDF

Info

Publication number
US3895368A
US3895368A US279097A US27909772A US3895368A US 3895368 A US3895368 A US 3895368A US 279097 A US279097 A US 279097A US 27909772 A US27909772 A US 27909772A US 3895368 A US3895368 A US 3895368A
Authority
US
United States
Prior art keywords
low frequency
signal
space
signals
source
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
Application number
US279097A
Other languages
English (en)
Inventor
Lloyd L Gordon
Robert D Williamson
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Sensormatic Electronics Corp
Original Assignee
Sensormatic Electronics Corp
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Sensormatic Electronics Corp filed Critical Sensormatic Electronics Corp
Priority to US279097A priority Critical patent/US3895368A/en
Priority to DE2316411A priority patent/DE2316411C3/de
Priority to IT49239/73A priority patent/IT982964B/it
Priority to ES413541A priority patent/ES413541A1/es
Priority to NL7305216A priority patent/NL7305216A/xx
Priority to GB2143473A priority patent/GB1406925A/en
Priority to FR7320784A priority patent/FR2195812B1/fr
Priority to BE132513A priority patent/BE801202A/xx
Priority to CA175,580A priority patent/CA1003522A/en
Priority to ZA734480A priority patent/ZA734480B/xx
Priority to JP8055073A priority patent/JPS5724598B2/ja
Priority to BR6051/73A priority patent/BR7306051D0/pt
Application granted granted Critical
Publication of US3895368A publication Critical patent/US3895368A/en
Anticipated expiration legal-status Critical
Expired - Lifetime legal-status Critical Current

Links

Images

Classifications

    • G—PHYSICS
    • G08—SIGNALLING
    • G08B—SIGNALLING OR CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
    • G08B13/00—Burglar, theft or intruder alarms
    • G08B13/22—Electrical actuation
    • G08B13/24—Electrical actuation by interference with electromagnetic field distribution
    • G08B13/2402—Electronic Article Surveillance [EAS], i.e. systems using tags for detecting removal of a tagged item from a secure area, e.g. tags for detecting shoplifting
    • G08B13/2405—Electronic Article Surveillance [EAS], i.e. systems using tags for detecting removal of a tagged item from a secure area, e.g. tags for detecting shoplifting characterised by the tag technology used
    • G08B13/2422—Electronic Article Surveillance [EAS], i.e. systems using tags for detecting removal of a tagged item from a secure area, e.g. tags for detecting shoplifting characterised by the tag technology used using acoustic or microwave tags
    • G—PHYSICS
    • G08—SIGNALLING
    • G08B—SIGNALLING OR CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
    • G08B13/00—Burglar, theft or intruder alarms
    • G08B13/22—Electrical actuation
    • G08B13/24—Electrical actuation by interference with electromagnetic field distribution
    • G08B13/2402—Electronic Article Surveillance [EAS], i.e. systems using tags for detecting removal of a tagged item from a secure area, e.g. tags for detecting shoplifting
    • G08B13/2465—Aspects related to the EAS system, e.g. system components other than tags
    • G08B13/2468—Antenna in system and the related signal processing
    • G08B13/2471—Antenna signal processing by receiver or emitter

Definitions

  • ABSTRACT of A microwave signal generator projects an electromag- ⁇ 731 Assignee: Sensormatic Electronics netic wave into a space under surveillance to establish Corporation, Hollywood, Fla. a first field.
  • a pulse or frequency modulated low frequency generator is used to apply a voltage to a dis [22] Filed 1972 continuous conductor for establishing a second field, [2 l] App]. No.: 279,097 electrostatic in nature, throughout the space.
  • the front end of a receiver system is tuned to the microwave frequency and feeds a suitable detector circuit [56] References Cited responsive to the low frequency signal.
  • a coincidence UNITED STATES PATENTS circuit energizes an alarm circuit whenever the de- 3.7()7 7ll 12/1972 Cole eta].
  • the present invention relates to a method and appa ratus for pilferage control. More particularly. it is directed to a method and apparatus for detecting the presence of a telltale element in an unauthorized zone.
  • tags For the purpose of controlling pilferage. it has been proposed heretofore to secure specially constructed tags to the merchandise which tags must be deactivated or removed for authorized removal of the merchandise from the controlled area.
  • the tags are provided with electrically resonant circuits embedded therein which serve to dctune the tank circuit of an electronic oscillator when brought in proximity thereto for triggering an alarm.
  • Use has also been made of tags incorporating a non-linear device in conjunction with an antenna element for reradiating the second harmonic ofa microwave signal which has been directed into the controlled space. Detection of said second harmonic signal has been used to trigger an alarm
  • these known methods have various limitations on their reliability and sensitivity. They are often susceptible to false triggering by metallic structures coincidentally manifesting similar properties to the special tags. Proximity of the human body to the electromagnetic field generating equipment or to the tags tends to mask the effect of the equipment and to interfere with reliable operation.
  • a non-linear device will function as a signal mixer producing sum and difference frequencies when excited by two signals of differing frequencies. It has been suggested. heretofore. to establish two low frequency electromagnetic fields of slightly different frequency within a space to be supervised and to detect the presence of signals corresponding to the difference between said two frequencies. In this manner it is asserted to be possible to detect the presence ofa non-linear device within the controlled region.
  • such system has many shortcomings not the least of which is the cost of producing a circuit ofpractical small size that can be incorporated in a tag and which is resonant at the low frequency.
  • the frequency of one of the two electromagnetic fields be chosen in the microwave region. While this avoids some of the prob lems with the tag encountered when both fields are of low frequency. a different disadvantage exists.
  • the microwave energy produces reflections and standing waves in the vicinity of the space being supervised. This coupled with the increased propagating characteristics of such high frequency energy results in considerable overrange and false triggering of the surveillance system by tags outside of the intended controlled space.
  • the present invention has for its object to provide a method for detecting the presence in a controlled space of a miniature passive electric signal receptor-reradiator which is superior to any method heretofore known.
  • a method of detecting within a confined space an electric signal receptor-reradiator which has signal-mixing capability comprising the steps of simultaneously establishing in the controlled space first and second energy fields.
  • the first energy field is chosen to be electromagnetic in nature and is produced by a continuous microwave signal for causing the receptor-reradiator to return a signal therefrom.
  • the second field is chosen to be electrostatic in nature established by applying signal voltage to a discontinuous conductor relative to a point of reference potential and having a sufficiently low frequency to enable it to be confined substantially to the controlled space. Detection in the space of a signal consisting of a carrier and modulation components where the components are due respectively to said first and second fields is indicative of the presence of the receptorreradiator therein.
  • a surveillance system for detecting the presence in a controlled space of receptorreradiator receptor-reradiator of the foregoing type.
  • said system comprising in combination a source of continuous microwave signals.
  • means coupled to the source of microwave signals for propagating through said space an electromagnetic wave corresponding to the microwave signals.
  • a source of low frequency signals a discontinuous conductor coupled to the source of low frequency signals for establishing through the space an electrostatic field corresponding to the low frequency signals, signal detecting means, means for coupling the detecting means with the space for receiving signals therefrom the detecting means being constructed and arranged to detect the low frequency sig nals only when received as modulation on a carrier signal whose frequency bears a predetermined relationship to that of the microwave signals. and means cou pled to the detecting means for providing an alarm responsive to the detection of the low frequency signals.
  • FIG. I is a block diagram of one surveillance system constructed in accordance with the invention.
  • FIG. 2 is a series of curves showing the signal waveforms at various locations identified by the corresponding reference letters in the system of FIG. 1;
  • FIG. 3 is a schematic diagram of a typical diodedipole receptor-reradiator with signal-mixing capability
  • FIG. 4 is a block diagram of another embodiment of the present invention.
  • FIG. 5 is a series of curves showing the waveforms at various locations identified by the corresponding reference letters in the system of FIG. 4.
  • an ultrahigh frequency transmitter 10 operating at 9l5 MHz functions as a source of microwave signals and has its output connected over path 11 to one input of a circulator 12.
  • the circulator I2 passes the signal from the source 10 to a path 13 leading to a splitter-combiner (tce) element 14.
  • the splitter-combiner element I4 divides the signals received from the circulator into two components which it feeds along the paths l5 and lb to the respective microwave antenna elements 17 and 18.
  • the antenna elements l7 and I8 may be located in respective pedestals or enclosures represented symbolically by the phantom line boxes 19 and 20.
  • any signals that might be received by the antenna elements l7 and 18 from the adjacent space are fed back through the path and 16 to the splitter-combiner l4 whereupon they are combined and fed through path 13 back to the circulator 12. Such signals are then passed by the circulator 12 to the path 21 leading to the first detector 22.
  • the nature of the circulator I2 is such that while most of the signal from the source 10 is fed from the path II to the path 13. some leakage docs feed through from path 11 to path 21. This leakage component of the microwave signal from the transmitter 10 is utilized in the first detector 22 for a purpose which will be described more fully hereinafter.
  • the output of the first detector is fed over a path 23 through a bandpass filter 24 to an AM detector 25 via a path 26.
  • the output from the AM detector 25 is fed over a path 27 to one input of an AND circuit 28.
  • the signal on path 27 is fed through a NOT or inverting circuit 29 to one input of a second AND circuit 30.
  • a free-running multivibrator operating at 100 Hz and designated by the reference numeral 31 has its output fed over a path 32 through a buffer amplifier 33 to a path 34 leading to the input to a pulse-controlled low frequency generator 35.
  • the output of the generator 35 is fed over a path 36 to a discontinuous conductor 37 for a purpose to be described.
  • the generator 35 operates at a frequency of 22 KHZ.
  • the signal output from the multivibrator 31 on path 32 is also fed in parallel to the second input of each of the AND circuits 28 and 30.
  • the output from AND circuit 28 is fed over path 38 to the input of a step counter 39 whose output is directed over path 40 to an input of pulse stretcher 41.
  • the output from pulse stretcher 41 is directed over path 42 to an alarm device 43.
  • a reset signal for step counter 39 is derived over path 44 from the output of AND circuit 30.
  • the two antenna elements 17 and 18 would be mounted on either side of the exit doorway so as to produce an electromagnetic field in the space therebetween.
  • the elements 17 and 18 have a radiation pattern generally confined to the space to be controlled.
  • the conductor 37 may be extended across the areaway so as to establish an electrostatic field throughout the same space when energized by the output from pulsecontrolled generator 35 relative to a point of reference potential.
  • a grounded conductor (not shown) may be located in the floor in order to provide a return path for the signals. if necessary. and to establish said point of reference potential.
  • a small non-linear device in the form of a semi-conductor rectifier chip or the like is connected to dipole antenna elements of the proper length the device will function as a signal mixing circuit. taking the signals corresponding to both the microwave transmitter and the low frequency pulse generator and modulating the latter signal upon the former for reradiation.
  • a device i.e.. a diode-dipole.
  • FIG. 3 shows schematically in FIG. 3 with the non-linear device or rectifier 45 connected to dipole elements 46 and 47 which are all embedded in a tag 48.
  • Lumped capacitance and inductance elements are not needed.
  • the tip-to-tip length of the elements 45. 46 and 47 should theoretically be of the order of l6.4 cm. In practice slight departure from the theoretical value may be found beneficial By appropriate folding of the ends of the dipole elements back upon themselves, it is possible to incorporate the structure in a smaller overall configuration.
  • the output of free-running multivibrator 31 is a series of square pulses represented by curve A thereof. These pulses have a duration of approximately 2 milliseconds. In the present example. the repetition rate is Hz.
  • the signal returned to splitter-combiner 14 will have a fundamental component at 915 MHZ plus sum and difference frequencies equal to 915.022 and 914.978 MHz. These signals join with the leakage signal at 9 l 5 MHZ fed over path 21 to the first detector 22.
  • the detector 22 may include a rectifier mixer for eliminating the carrier frequency, i.e.. the 9l5 MHz component.
  • the bandpass filter 24 which has a center frequency of 22 KHz and a bandwidth of 2 KHZ there is derived a signal on path 26 having a frequency of 22 KHz and corresponding to the 22 KHz component present in the modulated signal intercepted by antenna elements 17 and 18.
  • the signal fed to the AM detector 25 over path 26 will duplicate the output of generator 35 and have the form shown in curve B of FIG. 2.
  • the AM detector 25 may be any conventional detector. capable of producing a DC. output proportional to the amplitude of the input signal. Where the input signal has the form shown in curve B of FIG. 2. the output of the detector 25 will be as shown in curve C of FIG. 2.
  • a pulse will be developed at the output of the AND circuit 28 fed to the step counter 39 and counted therby. If the preceding condition prevails for a period of time sufficient to enable the step counter to reach its preset capacity. an output pulse will be fed to the pulse stretcher 4] for energizing the alarm 43. At present. it is preferred that the step counter provide an output after 16 input pulses. Any other count may be employed as desired.
  • the AND circuit 30 will not produce an output signal so long as the signals on paths 32 and 27 are similar and coincident. However, as soon as the signals provided by the AM detector 25 disappear. a reset pulse will be furnished by AND circuit 30 to reset the step counter 39. This will occur in any event when the tag 48 is removed from the controlled space.
  • the antenna elements [7 and l8 pick up a signal due to some artifact. it is not likely that such artifact will produce a sequence of 16 properly shaped and timed pulses. If as shown at point 49 on curve C of FIG. 2 there is no signal received. a reset pulse 50 as shown in curve D of FIGv 2 will be applied to the counter 39. If a broken pulse 51 as shown in curve C is provided by the detector 25. then the reset pulse 52 as shown in curve D will be applied to the counter 39. It should therefore be apparent that either through the absence of a return pulse or the reception ofa defective reutrn pulse the counter will be reset to commence another count anew.
  • the step counter is reset a few times before another valid count is received without interrupting the alarm.
  • the time duration of the pulse stretcher 4] and the number of counts required by the counter 39 it is possible to optimize the response of the overall system for distinguishing between valid signals and artifact.
  • the ultrahigh frequency transmitter 55 has its power output fed over path 56 through a 3db isolator pad 57 and a bandpass filter 58 to the splitter 59.
  • the bandpass filter 58 has a center frequency of9l5 MHz.
  • the splitter 59 has two outputs connected over paths 60 and 61 to individual antenna elements 62 and 63, respectively.
  • the antenna elements 62 and 63 should be mounted on opposite sides of the area to be controlled in corresponding enclosures or pedestals such as those represented by the broken line boxes 64 and 65. In this manner, the two antenna elements 62 and 63 establish an electromagnetic field of microwave energy in the controlled spacc therebetween.
  • a second pair of antenna elements 66 and 67 are mounted across the controlled space from the corresponding transmitter antenna elements 62 and 63, respectively.
  • the signals received from the space by antenna elements 66 and 67 are fed over corresponding paths to the two inputs of a combiner element 68 whose common output is fed over path 69 through a bandpass filter 70 to one input of a balanced mixer 71.
  • the second input of the balanced mixer 71 is furnished with a signal at 915 MHz derived from a low power level output of the transmitter 55 over path 72.
  • the bandpass filter 70 has a center frequency of 9i 5 MHz.
  • the output from the balanced mixer 71 is fed over path 73 to the FM detector 74 whose output is fed to the input of a NAND gate 75.
  • the output from NAND gate 75 is fed over one path to one input of NAND gate 76 and over another path to the input of NAND gate 77.
  • the output of NAND gate 77 is fed to one input of NAND gate 78.
  • the output of NAND gate 76 is fed to the input ofa l6-count counter 79 whose output is connected through a pulse stretcher 80 to an alarm circuit 81.
  • the reset terminal of counter 79 is connected to the output of NAND gate 78.
  • a voltage-controlled multivibrator pulse generator 82 operating at selectable rates between 200 and 250 Hz has its output connected over a path 83 to an attenuator 84 whose output is fed to the controlling input of a voltage-controlled multivibrator pulse generator 85.
  • the generator 85 has a center frequency of 50 KHz.
  • the frequency of generator 85 is shifted il KHz between 49 KHz and 51 KHz.
  • the output from generator 85 is fed through a resonant type low pass filter 86 whereby the square wave is converted to a sinusoidal signal of like frequency which is fed over path 87 to a power amplifier 88.
  • the output of the power amplifier is connected over separate paths 89 and 90 to corresponding step-up transformers 9l and 92.
  • the secondary windings (not shown) of the transformers 9i and 92 are connected to apply voltage to the foil elements 93 and 94 associated. respectively. with each of the housings 64 and 65.
  • the foils constitute a special form of discontinuous conductor.
  • the signals fed to the foils 93 and 94 are in parallel and establish an electrostatic field between the respective foils and ground, i.e.. the point of reference potential. Effective results have been obtained across an 8 ft. space with foils or plates measuring no more than about 4 inches X 4 inches and excited by a signal of about 245 V. RMS. Both the energizing voltage and the foil size may be varied depending upon the area to be supervised.
  • a second path 95 conducts the output of the generator 82 through a buffer amplifier 96 to a delay multivibrator 97.
  • the output of the dalay multivibrator 97 is fed over path 98 to the input of a reference pulse multivibrator 99 whose output is fed over path 100 to the second input of each of the NAND gates 76 and 78.
  • a selection switch may be used to select the desired pulse rate for generator 82.
  • the selectable rates may be 200. 225 and 250 Hz.
  • the preferred rate is 200 Hz al though rates between lOO and 500 Hz have been used experimentally.
  • the generator 82 provides a symmetrical square wave output as shown in curve A of FIG. 5. This signal is reduced by the attenuator 84 to the proper level for shifting the frequency of pulse generator 85 between 49 KHz and SI KHz.
  • generator 85 While the output of generator 85 is square wave in nature, it is converted by the resonant type low pass filter 86 to a sinusoidal signal as represented in curve B of FIG. 5. Such signal is then amplified by the power amplifier 88 and employed to drive the foils 93 and 94 for creating the electrostatic field in the controlled space.
  • a modulated signal is developed which will be received by antenna receiving elements 66 and 67 and applied to the balanced mixer 71.
  • the balanced mixer 71 will remove the 9l5 MHz carrier frequency component and supply the 49 KHz and 51 KHz as detected thereby over path 73 to the FM detector 74 for conversion to a square wave pulse having the form shown in curve E in FIG. 5.
  • detector 74 may be a conventional ratio detector or the like.
  • the input to the detector 74 is provided with a high gain amplifier-limiter (not shown) while the output of the detector 74 includes a low pass filter (not shown) to ensure removal of the 49-51 KHz component.
  • Curve C of FIG. shows the pulse output provided by delay multivibrator 97. It will be seen that the leading edge of the pulse produced by multivibrator 97 coincides with the leading edge of the positive going pulse output of generator 82 shown in curve A.
  • the trailing edge of the pulse produced by multivibrator 97 may be adjustable by appropriate means not shown.
  • the delay produced by multivibrator 97 is thereby adjusted to be equal to the normal delay encountered by the signals in passing through the equipment both into the electrostatic field and back on the modulated carrier through the balanced mixing and detecting circuitry.
  • the trailing edge of the pulse from multivibrator 97 is employed to trigger the reference pulse multivibrator 99 whose output is shown by curve D.
  • the leading edge of the pulse produced by multivibrator 99 coincides with the trailing edge of the pulse output ofmultivibrator 97.
  • the width of the pulse produced by multivibrator 99 may be adjustable by means not shown so that such pulse width coincides with the pulse width received from an actual tag in the controlled space.
  • the FM detector 74 is arranged to produce a DC. signal of one level in response to a 51 KHZ input signal and a DC. signal ofa second level in response to a 49 KHz signal
  • the output derived from the NAND gate 75 will have the form shown in curve E when a tag 48 is in the controlled space.
  • the subsequent operation of the system is quite similar to that previously described with reference to FIG. 1.
  • the signal at the output of the NAND gate 75 has a waveform or envelope which coincides with the signal at the output of multivibrator 99 on path I00 represented by curve D a pulse count will be introduced into the counter 79. After [6 such counts, an output will be fed to the pulse stretcher 80 to energize the alarm 81.
  • Any suitable counter may be employed for this purpose. For example, use may be made of a tandem arrangement of four Jl( flip-flops connected in known manner to produce an output at the completion of a count of sixteen.
  • curve E shows a possible artifact type signal which might result from extra neous factors. It should be mentioned that the counter 79 is actuated both as to its input and reset terminals by negative going pulses. Input pulses ol'the type shown in curve E will cause count pulses of the type shown in curve F interspersed with reset pulses as shown in curve G'. Consequently, the counter will be repeatedly reset and will not reach its output count.
  • the low frequency signal should be preferably no greater than [00 KHz. It has been found that such low frequencies are best employed for establishing the electrostatic field.
  • the high frequency signal should be in the microwave region so that the physical size of the tags need not be excessive.
  • the method of maintaining surveillance within a confined space to detect the presence in said space of an electric signal receptor-reradiator with signal mixing capability comprising the steps of simulataneously establishing in said space first and second energy fields, said first field being electromagnetic in nature and produced by a continuous microwave signal for causing said receptor-reradiator to return a signal therefrom, said second field being electrostatic in nature established by applying a signal voltage to a discontinuous conductor relative to a point of reference potential and having a sufficiently low frequency to enable it to be confined substantially to said space. and detecting the presence in said space of a signal consist ing of a carrier and modulation components where said components are due respectively to said first and second fields.
  • a surveillance system for detecting the presence in a controlled space of a minature passive electromagnetic wave receptor-reradiator with signal mixing capability, said system comprising in combination a source of continuous microwave signals, means coupled to said source of microwave signals for propagating through said space an electromagnetic wave corresponding to said microwave signals, a source of low frequency signals, a discontinuous conductor coupled to said establishing of low frequency signals for extablishing through said space an electrostatic field corresponding to said low frequency signals, said low fre' quency signals having a sufficiently low frequency to enable said electrostatic field to be confined substantially to said space, signal detecting means, means for coupling said detecting means with said space for receiving signals therefrom, said detecting means being constructed and arranged to detect said low frequency signals only when received as modulation on a carrier signal whose frequency bears a predetermined relationship to that of said microwave signals, and means coupled to said detecting means for providing an alarm responsive to detection of said low frequency signals.
  • a surveillance system wherein said discontinuous conductor comprises a plate-like member.
  • a surveillance system for detecting the presence in a controlled space of a miniature passive diode-dipole electromagnetic wave receptor-reradiator with signal mixing capability, said system comprising in combination a source of microwave signals, means coupled to said source of microwave signals for propagating through said space an electromagnetic wave corresponding to said microwave signals. a source of low frequency signals. a discontinuous conductor coupled to said source of low frequency signals for establishing through said space an electrostatic field corresponding to said low frequency signals. said low frequency signals having a sufficiently low frequency to enable said electrostatic field to be confined substantially to said space. signal detecting means. means for coupling said detecting means with said space for receiving signals therefrom. said detecting means being constructed and arranged to detect said low frequency signals only when received as modulation on a carrier signal having the same frequency as said microwave signals. and means coupled to said detecting means for providing an alarm responsive to detection of said low frequency signals.
  • a surveillance system wherein means are coupled to said source of low frequency signals for pulse modulating the latter. and wherein said means for providing an alarm are coupled to said pulse modulating means for providing said alarm only when the detected low frequency signal has a wave envelope coinciding with an output of said pulse modulating means.
  • a surveillance system for detecting the presence in a controlled space of a miniature passive electromagnetic wave rcceptor-reradiator with signal mixing capability.
  • said system comprising in combination a source of microwave signals. means coupled to said source of microwave signals for propagating through said space an electromagnetic wave corresponding to said microwave signals. a source of low frequency signals, means coupled to said source of low frequency signals for fre quency modulating the latter with a modulating signal. further means coupled to said source of low frequency signals for establishing through said space an electrostatic ficld corresponding to said low frequency signals, said low frequency signals having a sufficiently low frequency to enable said electrostatic field to be confined substantially to said space. signal detecting means. means for coupling said detecting means with said space for receiving signals therefrom.
  • said detecting means being constructed and arranged to detect said low frequency signals only when received as modulation on a carrier signal whose frequency bears a predetermined relationship to that of said microwave signals. and means coupled to said detecting means for providing an alarm responsive to detection of said low frequency signals.
  • a surveillance system wherein said means for providing an alarm are coupled to said frequency moduating means for providing said alarm only when the detected low frequency signals are frequency modulated with a wave envelope having the same shape as said modulating signal.
  • a surveillance system according to claim 8. wherein said modulating signal has the form ofa square wave.
  • a surveillance system according to claim 7. wherein said modulating signal has the form ofa square wave.
  • a surveillance system wherein said frequency modulation of said source of low frequency signals is characterized by a frequancy deviation of the order of lKHz.
  • a surveillance system for detecting the presence in a controlled space ofa miniature passive electromagnetic wave receptor-reradiator with signal mixing capability; said system comprising in combination a source of microwave signals; means coupled to said source of microwave signals for propagating through said space an electromagnetic wave corresponding to said microwave signals: a source of low frequency signals; means coupled to said source of low frequency signals for establishing through said space an electrostatic field corresponding to said low frequency signals; said low frequency signals having a sufficiently low frequency to enable said electrostatic field to be confined substantially to said space; said source of low frequency signals comprising a voltage-controlled multivibrator pulse generator.
  • said detecting means being constructed and arranged to detect said low frequency signals only when received as modulation on a carrier signal whose frequency bears a predetermined relationship to that of said microwave signals; and means coupled to said detecting means for providing an alarm responsive to detection of said low frequency signals.

Landscapes

  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Signal Processing (AREA)
  • Automation & Control Theory (AREA)
  • Computer Security & Cryptography (AREA)
  • Electromagnetism (AREA)
  • General Physics & Mathematics (AREA)
  • Acoustics & Sound (AREA)
  • Burglar Alarm Systems (AREA)
  • Geophysics And Detection Of Objects (AREA)
  • Radar Systems Or Details Thereof (AREA)
US279097A 1972-08-09 1972-08-09 Surveillance system and method utilizing both electrostatic and electromagnetic fields Expired - Lifetime US3895368A (en)

Priority Applications (12)

Application Number Priority Date Filing Date Title
US279097A US3895368A (en) 1972-08-09 1972-08-09 Surveillance system and method utilizing both electrostatic and electromagnetic fields
DE2316411A DE2316411C3 (de) 1972-08-09 1973-04-02 Mit Mikrowellen- und elektrostatischem Feld arbeitendes Überwachungssystem zum Feststellen der Anwesenheit eines Empfänger-Rückstrahlers mit Signalmischeigenschaft
IT49239/73A IT982964B (it) 1972-08-09 1973-04-04 Sistema per rivelare la presenza di intrusi in zone sorvegliate
ES413541A ES413541A1 (es) 1972-08-09 1973-04-10 Un sistema de vigilancia para detectar la presencia en un espacio controlado de un receptor-retransmisor de senales electricas.
NL7305216A NL7305216A (es) 1972-08-09 1973-04-13
GB2143473A GB1406925A (en) 1972-08-09 1973-05-04 Surveillance system and method utilizing both electrostatic and electromagnetic fields
FR7320784A FR2195812B1 (es) 1972-08-09 1973-06-07
BE132513A BE801202A (fr) 1972-08-09 1973-06-20 Procede et dispositif de surveillance utilisant a la fois des champs flectrostatique et electromagnetique
CA175,580A CA1003522A (en) 1972-08-09 1973-07-04 Surveillance system and method utilizing both electrostatic and electromagnetic fields
ZA734480A ZA734480B (en) 1972-08-09 1973-07-09 Surveillance system and method utilizing both electrostatic and electromagnetic fields
JP8055073A JPS5724598B2 (es) 1972-08-09 1973-07-19
BR6051/73A BR7306051D0 (pt) 1972-08-09 1973-08-08 Processo para manter vigilancia dentro de um espaco confinado e sistema de vigilancia

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
US279097A US3895368A (en) 1972-08-09 1972-08-09 Surveillance system and method utilizing both electrostatic and electromagnetic fields

Publications (1)

Publication Number Publication Date
US3895368A true US3895368A (en) 1975-07-15

Family

ID=23067622

Family Applications (1)

Application Number Title Priority Date Filing Date
US279097A Expired - Lifetime US3895368A (en) 1972-08-09 1972-08-09 Surveillance system and method utilizing both electrostatic and electromagnetic fields

Country Status (12)

Country Link
US (1) US3895368A (es)
JP (1) JPS5724598B2 (es)
BE (1) BE801202A (es)
BR (1) BR7306051D0 (es)
CA (1) CA1003522A (es)
DE (1) DE2316411C3 (es)
ES (1) ES413541A1 (es)
FR (1) FR2195812B1 (es)
GB (1) GB1406925A (es)
IT (1) IT982964B (es)
NL (1) NL7305216A (es)
ZA (1) ZA734480B (es)

Cited By (44)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4087802A (en) * 1976-05-24 1978-05-02 Williamson Robert D Method and apparatus for electronic surveillance of precisely defined control zone
US4139844A (en) * 1977-10-07 1979-02-13 Sensormatic Electronics Corporation Surveillance method and system with electromagnetic carrier and plural range limiting signals
FR2398357A1 (fr) * 1977-07-19 1979-02-16 Nedap Nv Dispositif de detection electromagnetique
FR2416514A1 (fr) * 1978-02-03 1979-08-31 Williamson Robert Procede et dispositif de protection contre le vol
US4206453A (en) * 1976-05-24 1980-06-03 Williamson Robert D Method and apparatus for electronic surveillance
US4212002A (en) * 1976-05-24 1980-07-08 Williamson Robert D Method and apparatus for selective electronic surveillance
DE3002195A1 (de) * 1979-02-05 1980-08-07 Sensormatic Electronics Corp Umhuellung zur befestigung eines blattfoermigen elements an einer flachen oberflaeche eines gegen unbefugte befoerderung durch eine ueberwachungszone zu ueberwachenden gegenstands
EP0020062A1 (en) * 1979-05-18 1980-12-10 Parmeko Limited Method and surveillance system for detecting the presence of an e.m. wave receptor reradiator
US4281321A (en) * 1980-06-09 1981-07-28 Sensormatic Electronics Corporation Surveillance system employing a floor mat radiator
WO1982001437A1 (en) * 1980-10-09 1982-04-29 Deterrent Tech Corp Dual frequency anti-theft system
FR2503379A1 (fr) * 1981-04-07 1982-10-08 Sensormatic Electronics Corp Ensemble de surveillance pour detecter la presence d'un recepteur-reemetteur d'ondes electromagnetiques
FR2512558A1 (fr) * 1981-09-10 1983-03-11 Sensormatic Electronics Corp Appareil de surveillance electrique muni d'elements d'antenne mobiles
DE3231595A1 (de) * 1981-09-04 1983-03-17 Sensormatic Electronics Corp., 33441 Deerfield Beach, Fla. Auf hf- und magnetenergie ansprechende diebstahlueberwachungsplakette und system
EP0084400A2 (en) * 1982-01-14 1983-07-27 N.V. Nederlandsche Apparatenfabriek NEDAP Detection system
US4471344A (en) * 1980-10-09 1984-09-11 Ici Americas Inc. Dual frequency anti-theft system
US4472641A (en) * 1983-01-28 1984-09-18 Westinghouse Electric Corp. Power supply apparatus
FR2544871A1 (fr) * 1983-04-25 1984-10-26 Sensormatic Electronics Corp Element de reception et de retransmission de signaux pour une etiquette dans une installation de surveillance
EP0133317A2 (en) * 1983-08-02 1985-02-20 Ici Americas Inc. Electronic article surveillance system
EP0135049A1 (en) * 1983-08-01 1985-03-27 Ici Americas Inc. Electronic article surveillance system
US4679035A (en) * 1985-07-30 1987-07-07 Sensormatic Electronics Corporation Tri-signal electromagnetic article surveillance system
FR2593927A1 (fr) * 1986-01-31 1987-08-07 Sensormatic Electronics Corp Dispositif d'etiquetage pour la surveillance electronique d'articles
USRE32627E (en) * 1981-09-10 1988-03-22 Sensormatic Electronics Corporation Electrical surveillance apparatus with moveable antenna elements
FR2645309A1 (fr) * 1989-04-04 1990-10-05 Usil Flandres Artois Dispositif de detection du passage d'au moins un mobile en au moins un point determine de son deplacement
US5099227A (en) * 1989-07-18 1992-03-24 Indala Corporation Proximity detecting apparatus
US5109217A (en) * 1990-08-09 1992-04-28 Sensormatic Electronics Corporation Method and apparatus for enhancing detection of electronic article surveillance tags in close proximity to electrically conductive objects
AU625452B2 (en) * 1988-01-14 1992-07-09 Sony Corporation A portable data transmitter device and a system using the same
US5206626A (en) * 1991-12-24 1993-04-27 Knogo Corporation Stabilized article surveillance responder
US5241923A (en) * 1992-07-23 1993-09-07 Pole/Zero Corporation Transponder control of animal whereabouts
EP0592781A1 (en) * 1992-10-13 1994-04-20 Sensormatic Electronics Corporation Article surveillance system with frequency hopping
GB2306080A (en) * 1995-10-10 1997-04-23 Roke Manor Research Detection systems
EP0922924A1 (de) 1997-12-10 1999-06-16 Wegmann & Co. GmbH Dicht- und Führungseinrichtung für hochdynamisch beschleunigte, abstandswirksame Schutzelemente
US5990791A (en) * 1997-10-22 1999-11-23 William B. Spargur Anti-theft detection system
US6064308A (en) * 1996-10-25 2000-05-16 Pole/Zero Corporation RF signaling system and system for controlling the whereabouts of animals using same
US6166643A (en) * 1997-10-23 2000-12-26 Janning; Joseph J. Method and apparatus for controlling the whereabouts of an animal
US6446049B1 (en) 1996-10-25 2002-09-03 Pole/Zero Corporation Method and apparatus for transmitting a digital information signal and vending system incorporating same
US20030022645A1 (en) * 2001-07-26 2003-01-30 Runzo Joseph Donald System and method for signal validation and leakage detection
US6518884B1 (en) * 1998-03-27 2003-02-11 Matsushita Electric Industrial Co., Ltd. Electric resonance element, detection apparatus and moving vehicle control system
US6752837B2 (en) 2002-06-28 2004-06-22 Hewlett-Packard Development Company, L.P. Security tags with a reversible optical indicator
US20050242954A1 (en) * 2004-04-29 2005-11-03 Franklin Todd B Error-avoiding anti-theft surveillance system
US20070296609A1 (en) * 2006-06-21 2007-12-27 Dave Thomas Method and apparatus for object recognition and warning system of a primary vehicle for nearby vehicles
US7397373B2 (en) 2002-07-19 2008-07-08 Gatekeeper Systems (Hk) Limited System and method for security and electronic article surveillance
US20090066538A1 (en) * 2006-06-21 2009-03-12 Dave Thomas Method and apparatus for object recognition and warning system of a primary vehicle for nearby vehicles
US20090072995A1 (en) * 2006-06-21 2009-03-19 Dave Thomas Method and apparatus for transmitting information between a primary vehicle and a secondary vehicle
US20090267669A1 (en) * 2005-04-04 2009-10-29 Shigeru Kasai Microwave Generating Apparatus and Microwave Generating Method

Families Citing this family (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4069472A (en) * 1975-12-25 1978-01-17 Tokyo Shibaura Electric Co., Ltd. Foreground subject-identifying apparatus
DE2833509A1 (de) * 1978-07-31 1980-02-14 Siemens Ag Einrichtung zur identifizierung und/oder 0rtung von gegenstaenden und personen
JPS5768070U (es) * 1980-10-09 1982-04-23
RU2099789C1 (ru) * 1996-07-30 1997-12-20 Кулагин Николай Павлович Способ обнаружения вторжения в охраняемое пространство и устройство для его осуществления
ATE271244T1 (de) 1999-10-08 2004-07-15 Activerf Ltd Verbesserungen in beziehung zur sicherheiteinrichtungen
GB9923682D0 (en) * 1999-10-08 1999-12-08 Active Rf Limited Improvements relating to security-pushout prevention

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3707711A (en) * 1970-04-02 1972-12-26 Peter Harold Cole Electronic surveillance system
US3711848A (en) * 1971-02-10 1973-01-16 I D Eng Inc Method of and apparatus for the detection of stolen articles
US3740742A (en) * 1971-05-11 1973-06-19 T Thompson Method and apparatus for actuating an electric circuit
US3806905A (en) * 1971-09-08 1974-04-23 Sperry Rand Corp Transducer and condition monitor

Family Cites Families (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3631442A (en) * 1968-03-22 1971-12-28 Robert E Fearon Anti-shoplifting system

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3707711A (en) * 1970-04-02 1972-12-26 Peter Harold Cole Electronic surveillance system
US3711848A (en) * 1971-02-10 1973-01-16 I D Eng Inc Method of and apparatus for the detection of stolen articles
US3740742A (en) * 1971-05-11 1973-06-19 T Thompson Method and apparatus for actuating an electric circuit
US3806905A (en) * 1971-09-08 1974-04-23 Sperry Rand Corp Transducer and condition monitor

Cited By (63)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4206453A (en) * 1976-05-24 1980-06-03 Williamson Robert D Method and apparatus for electronic surveillance
US4212002A (en) * 1976-05-24 1980-07-08 Williamson Robert D Method and apparatus for selective electronic surveillance
US4087802A (en) * 1976-05-24 1978-05-02 Williamson Robert D Method and apparatus for electronic surveillance of precisely defined control zone
US4308530A (en) * 1977-07-19 1981-12-29 N.V. Nederlandsche Apparatenfabriek Nedap Detection system forming wide gates with superior spatial selectivity
FR2398357A1 (fr) * 1977-07-19 1979-02-16 Nedap Nv Dispositif de detection electromagnetique
US4139844A (en) * 1977-10-07 1979-02-13 Sensormatic Electronics Corporation Surveillance method and system with electromagnetic carrier and plural range limiting signals
FR2405528A1 (fr) * 1977-10-07 1979-05-04 Sensormatic Electronics Corp Procede et dispositif de surveillance d'un espace limite
FR2416514A1 (fr) * 1978-02-03 1979-08-31 Williamson Robert Procede et dispositif de protection contre le vol
DE3002195A1 (de) * 1979-02-05 1980-08-07 Sensormatic Electronics Corp Umhuellung zur befestigung eines blattfoermigen elements an einer flachen oberflaeche eines gegen unbefugte befoerderung durch eine ueberwachungszone zu ueberwachenden gegenstands
EP0020062A1 (en) * 1979-05-18 1980-12-10 Parmeko Limited Method and surveillance system for detecting the presence of an e.m. wave receptor reradiator
US4281321A (en) * 1980-06-09 1981-07-28 Sensormatic Electronics Corporation Surveillance system employing a floor mat radiator
WO1982001437A1 (en) * 1980-10-09 1982-04-29 Deterrent Tech Corp Dual frequency anti-theft system
US4471344A (en) * 1980-10-09 1984-09-11 Ici Americas Inc. Dual frequency anti-theft system
FR2503379A1 (fr) * 1981-04-07 1982-10-08 Sensormatic Electronics Corp Ensemble de surveillance pour detecter la presence d'un recepteur-reemetteur d'ondes electromagnetiques
DE3231595C2 (es) * 1981-09-04 1992-02-06 Sensormatic Electronics Corp., Deerfield Beach, Fla., Us
DE3231595A1 (de) * 1981-09-04 1983-03-17 Sensormatic Electronics Corp., 33441 Deerfield Beach, Fla. Auf hf- und magnetenergie ansprechende diebstahlueberwachungsplakette und system
US4413254A (en) * 1981-09-04 1983-11-01 Sensormatic Electronics Corporation Combined radio and magnetic energy responsive surveillance marker and system
US4394645A (en) * 1981-09-10 1983-07-19 Sensormatic Electronics Corporation Electrical surveillance apparatus with moveable antenna elements
USRE32627E (en) * 1981-09-10 1988-03-22 Sensormatic Electronics Corporation Electrical surveillance apparatus with moveable antenna elements
FR2512558A1 (fr) * 1981-09-10 1983-03-11 Sensormatic Electronics Corp Appareil de surveillance electrique muni d'elements d'antenne mobiles
DE3233028A1 (de) * 1981-09-10 1983-03-24 Sensormatic Electronics Corp., 33441 Deerfield Beach, Fla. Elektrische ueberwachungseinrichtung mit beweglichen antennenelementen
US4551712A (en) * 1982-01-14 1985-11-05 N.V. Nederlandsche Apparatenfabriek Nedap Electronic detection system for detecting a responder including a frequency divider
EP0084400A2 (en) * 1982-01-14 1983-07-27 N.V. Nederlandsche Apparatenfabriek NEDAP Detection system
EP0084400A3 (en) * 1982-01-14 1983-08-03 N.V. Nederlandsche Apparatenfabriek Nedap Detection system
US4472641A (en) * 1983-01-28 1984-09-18 Westinghouse Electric Corp. Power supply apparatus
US4642640A (en) * 1983-04-25 1987-02-10 Sensormatic Electronics Corporation Signal receptor-reradiator and surveillance tag using the same
DE3413839A1 (de) * 1983-04-25 1984-11-08 Sensormatic Electronics Corp., Deerfield Beach, Fla. Signal-empfaenger-ruecksender und ueberwachungsanhaenger dafuer
FR2544871A1 (fr) * 1983-04-25 1984-10-26 Sensormatic Electronics Corp Element de reception et de retransmission de signaux pour une etiquette dans une installation de surveillance
EP0135049A1 (en) * 1983-08-01 1985-03-27 Ici Americas Inc. Electronic article surveillance system
AU573749B2 (en) * 1983-08-01 1988-06-23 Ici Americas Inc. Microwave anti-theft system
EP0133317A3 (en) * 1983-08-02 1985-03-27 Ici Americas Inc Electronic article surveillance system
EP0133317A2 (en) * 1983-08-02 1985-02-20 Ici Americas Inc. Electronic article surveillance system
AU573602B2 (en) * 1983-08-02 1988-06-16 Ici Americas Inc. Anti-theft system using microstrip antennas
US4679035A (en) * 1985-07-30 1987-07-07 Sensormatic Electronics Corporation Tri-signal electromagnetic article surveillance system
FR2593927A1 (fr) * 1986-01-31 1987-08-07 Sensormatic Electronics Corp Dispositif d'etiquetage pour la surveillance electronique d'articles
AU625452B2 (en) * 1988-01-14 1992-07-09 Sony Corporation A portable data transmitter device and a system using the same
EP0391775A1 (fr) * 1989-04-04 1990-10-10 Universite Des Sciences Et Techniques De Lille Flandres-Artois (U.S.T.L.) Dispositif de détection du passage d'au moins un mobile en au moins un point déterminé de son déplacement
US5068655A (en) * 1989-04-04 1991-11-26 Universite Des Sciences Et Techniques De Lille (U.S.T.L.) Flandres Artois Device for detecting the passage of at least one mobile body in at least one location fixed by its travel
FR2645309A1 (fr) * 1989-04-04 1990-10-05 Usil Flandres Artois Dispositif de detection du passage d'au moins un mobile en au moins un point determine de son deplacement
US5099227A (en) * 1989-07-18 1992-03-24 Indala Corporation Proximity detecting apparatus
US5109217A (en) * 1990-08-09 1992-04-28 Sensormatic Electronics Corporation Method and apparatus for enhancing detection of electronic article surveillance tags in close proximity to electrically conductive objects
US5206626A (en) * 1991-12-24 1993-04-27 Knogo Corporation Stabilized article surveillance responder
US5241923A (en) * 1992-07-23 1993-09-07 Pole/Zero Corporation Transponder control of animal whereabouts
EP0592781A1 (en) * 1992-10-13 1994-04-20 Sensormatic Electronics Corporation Article surveillance system with frequency hopping
GB2306080B (en) * 1995-10-10 2000-02-16 Roke Manor Research Improvements in or relating to detection systems
GB2306080A (en) * 1995-10-10 1997-04-23 Roke Manor Research Detection systems
US6064308A (en) * 1996-10-25 2000-05-16 Pole/Zero Corporation RF signaling system and system for controlling the whereabouts of animals using same
US6446049B1 (en) 1996-10-25 2002-09-03 Pole/Zero Corporation Method and apparatus for transmitting a digital information signal and vending system incorporating same
US5990791A (en) * 1997-10-22 1999-11-23 William B. Spargur Anti-theft detection system
US6166643A (en) * 1997-10-23 2000-12-26 Janning; Joseph J. Method and apparatus for controlling the whereabouts of an animal
EP0922924A1 (de) 1997-12-10 1999-06-16 Wegmann & Co. GmbH Dicht- und Führungseinrichtung für hochdynamisch beschleunigte, abstandswirksame Schutzelemente
US6518884B1 (en) * 1998-03-27 2003-02-11 Matsushita Electric Industrial Co., Ltd. Electric resonance element, detection apparatus and moving vehicle control system
US7395548B2 (en) * 2001-07-26 2008-07-01 Comsonics, Inc. System and method for signal validation and leakage detection
US20030022645A1 (en) * 2001-07-26 2003-01-30 Runzo Joseph Donald System and method for signal validation and leakage detection
US6752837B2 (en) 2002-06-28 2004-06-22 Hewlett-Packard Development Company, L.P. Security tags with a reversible optical indicator
US7397373B2 (en) 2002-07-19 2008-07-08 Gatekeeper Systems (Hk) Limited System and method for security and electronic article surveillance
US7034687B2 (en) 2004-04-29 2006-04-25 Comm-Engines Error-avoiding anti-theft surveillance system
US20050242954A1 (en) * 2004-04-29 2005-11-03 Franklin Todd B Error-avoiding anti-theft surveillance system
US20090267669A1 (en) * 2005-04-04 2009-10-29 Shigeru Kasai Microwave Generating Apparatus and Microwave Generating Method
US20070296609A1 (en) * 2006-06-21 2007-12-27 Dave Thomas Method and apparatus for object recognition and warning system of a primary vehicle for nearby vehicles
US20090066538A1 (en) * 2006-06-21 2009-03-12 Dave Thomas Method and apparatus for object recognition and warning system of a primary vehicle for nearby vehicles
US20090072995A1 (en) * 2006-06-21 2009-03-19 Dave Thomas Method and apparatus for transmitting information between a primary vehicle and a secondary vehicle
US8350720B2 (en) 2006-06-21 2013-01-08 Dave Thomas Method and apparatus for object recognition and warning system of a primary vehicle for nearby vehicles

Also Published As

Publication number Publication date
FR2195812B1 (es) 1978-03-10
ZA734480B (en) 1974-06-26
ES413541A1 (es) 1976-02-01
JPS5724598B2 (es) 1982-05-25
BE801202A (fr) 1973-10-15
FR2195812A1 (es) 1974-03-08
GB1406925A (en) 1975-09-17
DE2316411B2 (de) 1979-08-09
BR7306051D0 (pt) 1974-07-11
IT982964B (it) 1974-10-21
JPS4945593A (es) 1974-05-01
NL7305216A (es) 1974-02-12
CA1003522A (en) 1977-01-11
DE2316411C3 (de) 1980-04-17
DE2316411A1 (de) 1974-02-28

Similar Documents

Publication Publication Date Title
US3895368A (en) Surveillance system and method utilizing both electrostatic and electromagnetic fields
US3707711A (en) Electronic surveillance system
US3810147A (en) Electronic security system
US3740742A (en) Method and apparatus for actuating an electric circuit
US3863244A (en) Electronic security system having improved noise discrimination
CA1107835A (en) Surveillance method and system with electromagnetic carrier and plural range limiting signals
US3967161A (en) A multi-frequency resonant tag circuit for use with an electronic security system having improved noise discrimination
CA1169525A (en) Surveillance system employing a floor mat radiator
US4352098A (en) Surveillance systems
US3798642A (en) Recognition system
CA1110726A (en) Detection system
US3624631A (en) Pilferage control system
US4471344A (en) Dual frequency anti-theft system
US4551712A (en) Electronic detection system for detecting a responder including a frequency divider
GB1531485A (en) Theft detection system
US4027303A (en) Apparatus for detecting the presence of objects by sensing reflected energy
US3314066A (en) Method and apparatus for detecting the entrance of an object into a region being monitored
EP0100128A1 (en) Absorption detection system
US3270339A (en) Intruder alarm system
US4212002A (en) Method and apparatus for selective electronic surveillance
JPH0255833B2 (es)
EP0091581B1 (en) Crossed beam high frequency anti-theft system
US3257659A (en) Counter-detection system
US4087802A (en) Method and apparatus for electronic surveillance of precisely defined control zone
CA2075991C (en) Security system for surveilling the passage of commodities through defined zones