RADIO FREQUENCY INTRUSION DETECTION SYSTEM

• US 3,794,992 A
• Filed: 02/07/1972
• Issued: 02/26/1974
• Est. Priority Date: 02/07/1972
• Status: Expired due to Term

• Current Assignee: John B. Gehman
• Original Assignee: John B. Gehman
• Inventors: Gehman, John B.
• Primary Examiner(s): Hubler, Malcolm F.
• Application Number:
  US05/224,130
• Time in Patent Office: 750 Days
• Field of Search: 340/258B,258C 343/5PD
• US Class Current: 342/28
• CPC Class Codes: G01S 1/02 using radio waves G01S19/00 takes precedence
• CPC Class Codes: G08B 13/2497 using transmission lines, e.g. cable

Abstract

In order to detect personnel and other intruding objects in open terrain, one or more long wire antennas are buried in the ground and located orthogonal to a vertically polarized souce of VHF radiation, such as a continuous wave transmitter which feeds an elevated vertical dipole. The intruder couples the radiation to the buried antenna which feeds a receiver. The receiver produces an output responsive to the characteristics of the radiation coupled to the antenna by the intruder to produce an alarm indicative of the presence of the intruder.

Claims

1. A radio frequency intrusion detection system for detecting the presence of an intruding object in a radio frequency radiation field adapted to be produced by a radio frequency transmitting source, which comprises a receiving antenna disposed remotely from the transmitting source of said radiation field at such a distance that the capacitative and inductive coupling from the source is insignificant at the radiation field frequency, said antenna being disposed below the surface of the terrain upon which said radiation field is incident and to which said field is coupled when said object is in proximity to said antenna, and receiver means connected to said antenna and responsive to radio frequency signals generated in said antenna by said radiation field when coupled to said antenna by said object for providing an output indicative of the presence of said object.

2. The invention as set forth in claim 1 wherein said terrain is the ground, and said antenna is buried below the surface of said ground a distance less than the penetration depth for the frequency of said radiation field.

3. The invention as set forth in claim 2 wherein said radiation field is of a frequency in the VHF band and said distance is less than 1 foot.

4. The invention as set forth in claim 1 wherein said antenna is a long wire.

5. The invention as set forth in claim 4 wherein said receiver is connected to one end of said wire.

6. The invention as set forth in claim 4 wherein said receiver is connected to the center of said wire.

7. The invention as set forth in claim 4 wherein said wire is at least 50 feet in length.

8. The invention as set forth in claim 5 wherein said wire is from 50 to 400 feet in length from the point where it is connected to said receiver to an end thereof.

9. The invention as set forth in claim 4 wherein said wire is disposed orthogonally to the direction of propagation of said radio frequency radiation field.

10. The invention as set forth in claim 1 wherein said system is adapted to secure an area and wherein a plurality of said antennas are disposed in end to end relationship about the perimeter of said area.

11. The invention as set forth in claim 10 including a plurality of receivers separately connected to different ones of said antennas.

12. The invention as set forth in claim 11 including common monitoring means connected to all of said receivers for detecting the outputs of said receivers indicating the presence of an object in the prOximity of each of said antennas.

13. The invention as set forth in claim 1 wherein said antenna is a long straight wire, and including a transmitting antenna for radiating waves polarized vertically with respect to said wire to establish said radiation field.

14. The invention as set forth in claim 13 wherein said transmitting antenna is a whip antenna disposed vertically upon said surface and located remotely from said wire antenna.

15. The invention as set forth in claim 1 including a second antenna disposed in side-by-side relationship with said first named antenna means for providing said second antenna with a length approximately one-quarter wavelength different in length from said first antenna, and means for connecting said first and second antenna to said receiver.

16. The invention as set forth in claim 15 wherein said connecting means comprises switch means for connecting said first and second antennas in rapid succession to said receiver.

17. The invention as set forth in claim 15 wherein said receiver comprises first and second receivers connected respectively to said first and second antennas, each of said receivers providing a separate output and means responsive to the sum of said separate outputs for providing said indication of the presence of said object.

18. The invention as set forth in claim 1 wherein said receiver includes a phase lock loop having a narrow band pass.

19. The invention as set forth in claim 18 wherein said phase lock loop comprises a phase discriminator to which a signal corresponding to the radiation detected by said antenna is applied and a variable frequency oscillator and a narrow band pass filter for applying the output of said discriminator to said oscillator for controlling the frequency thereof, a synchronous detector, responsive to said signal, and means for shifting the phase of the oscillations from said oscillator and applying said shifted oscillation to said synchronous detector, and means responsive to the output of said synchronous detector for providing said output indicative of the presence of said output.

20. The invention as set forth in claim 18 wherein said receiver further includes a frequency translator for heterodyning the signal detected by said antenna to an intermediate frequency signal and means for applying said intermediate frequency signal to said phase lock loop.

21. The invention as set forth in claim 20 wherein said frequency translator includes a mixer, first and second local oscillators connected to said mixer, a direct current power supply line for providing operating voltage to said receiver and means for selectively reversing the polarity of direct current applied to said line for selectively enabling said first and second local oscillators.

22. The invention as set forth in claim 18 wherein said receiver further includes means responsive to said receiver output for translating said output into an identification burst of predetermined frequency and duration which corresponds to the reception of said output.

23. The invention as set forth in claim 22 wherein said system comprises monitoring means responsive to said tone bursts which have different frequencies over a band of frequencies, said monitoring means including a wide band amplifier for amplifying said band of frequencies, a mixer coupled to the output of said amplifier, a variable frequency oscillator also coupled to said mixer, detector means coupled to said mixer for providing signals corresponding to said tone bursts which are converted by said mixer to a given frequency, a recycling counter providing a plurality of outputs corresponding to different counts, means for applying clock pulses to said counter causing it to count, a digital to analog converter connected to said counter for converting the counts stored in said counter into an analog signal and applying said analog signals to said oscillator for varying the frequency thereof correspondingly with said counter, and means operated by said counter foR separately displaying said detector means output signals which occur simultaneously with different ones of said counts.

24. The invention as set forth in claim 6 wherein said wire is from 50 to 400 feet in length from the point where it is connected to said receiver to an end thereof.