Surveillance system and method utilizing both electrostatic and electromagnetic fields

• US 3,895,368 A
• Filed: 08/09/1972
• Issued: 07/15/1975
• Est. Priority Date: 08/09/1972
• Status: Expired due to Term

• Current Assignee: Lloyd L. Gordon, Robert D. Williamson
• Original Assignee: Lloyd L. Gordon, Robert D. Williamson
• Inventors: Gordon, Lloyd L., Williamson, Robert D.
• Primary Examiner(s): Swann, III, Glen R.
• Application Number:
  US05/279,097
• Time in Patent Office: 1,070 Days
• Field of Search: 340/280,258C,408 343/6.8LC,6.8R
• US Class Current: 340/572.4
• CPC Class Codes: G08B 13/2422 using acoustic or microwave tags
• CPC Class Codes: G08B 13/2471 Antenna signal processing by receiver or emitter

Abstract

A microwave signal generator projects an electromagnetic wave into a space under surveillance to establish a first field. A pulse or frequency modulated low frequency generator is used to apply a voltage to a discontinuous conductor for establishing a second field, electrostatic in nature, throughout the space. Presence in the space of a miniature passive electromagnetic wave receptor-reradiator in the form of a semiconductive diode connected to a dipole antenna causes the reradiation of the low frequency component modulated on the microwave component as a carrier. The front end of a receiver system is tuned to the microwave frequency and feeds a suitable detector circuit responsive to the low frequency signal. A coincidence circuit energizes an alarm circuit whenever the detected signal coincides with the original modulation envelope being applied to the low frequency generator.

Claims

1. 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, said method 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 consisting of a carrier and modulation components where said components are due respectively to said first and second fields.

2. The method according to claim 1, wherein said second field is produced with a frequency modulated signal.

3. 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 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.

4. A surveillance system according to claim 3, wherein said discontinuous conductor comprises a plate-like member.

5. 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.

6. A surveillance system according to claim 5, 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.

7. A surveillance system for detecting the presence in a controlled space of a 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 frequency modulating the latter with a modulating signal, further 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, 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.

8. A surveillance system according to claim 7, 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.

9. A surveillance system according to claim 8, wherein said modulating signal has the form of a square wave.

10. A surveillance system according to claim 7, wherein said modulating signal has the form of a square wave.

11. A surveillance system according to claim 7, wherein said frequency modulation of said source of low frequency signals is characterized by a frequancy deviation of the order of 1KHz.

12. A surveillance system for detecting the presence in a controlled space of a 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, means coupled to an output of said pulse generator for converting a square wave signal to a sinusoidal signal for use in establishing said electrostatic field, and means coupled to said pulse generator for frequency modulating the latter with a modulating signal;

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.

13. A surveillance system according to claim 12, wherein said modulating signal has the form of a square wave.