Surveillance system and method utilizing both electrostatic and electromagnetic fields

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.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus 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.

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. In one known arrangement, the tags are provided with electrically resonant circuits embedded therein which serve to detune 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 of a microwave signal which has been directed into the controlled space. Detection of said second harmonic signal has been used to trigger an alarm. However, 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.

For a long time it has been known that 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 of a non-linear device within the controlled region. However, such system has many shortcomings not the least of which is the cost of producing a circuit of practical small size that can be incorporated in a tag and which is resonant at the low frequency.

It has also been suggested that the frequency of one of the two electromagnetic fields be chosen in the microwave region. While this avoids some of the problems 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.

SUMMARY OF THE INVENTION

With the foregoing in mind, 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.

In accordance with one aspect of the present invention there is provided a method of detecting within a confined space an electric signal receptor-reradiator which has signal-mixing capability, the method 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 receptor-reradiator therein.

In accordance with another aspect of the present invention, there is provided a surveillance system for detecting the presence in a controlled space of receptor-reradiator 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 signals only when received as modulation on a carrier signal whose frequency bears a predetermined relationship to that of the microwave signals, and means coupled to the detecting means for providing an alarm responsive to the detection of the low frequency signals.

DESCRIPTION OF THE DRAWINGS

The invention will be better understood after reading the following detailed description of the presently preferred embodiments thereof with reference to the appended drawings in which:

FIG. 1 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 diode-dipole receptor-reradiator with signal-mixing capability;

FIG. 4 is a block diagram of another embodiment of the present invention; and

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.

DETAILED DESCRIPTION

The same reference numerals are used throughout the various figures of the drawings to designate the same or similar parts.

Referring now to FIG. 1, an ultrahigh frequency transmitter 10 operating at 915 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 12 passes the signal from the source 10 to a path 13 leading to a splitter-combiner (tee) element 14. The splitter-combiner element 14 divides the signals received from the circulator into two components which it feeds along the paths 15 and 16 to the respective microwave antenna elements 17 and 18. The antenna elements 17 and 18 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 17 and 18 from the adjacent space are fed back through the path 15 and 16 to the splitter-combiner 14 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 12 is such that while most of the signal from the source 10 is fed from the path 11 to the path 13, some leakage does 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. At the same time, 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. In the present embodiment, 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.

Assuming that the system of FIG. 1 is to be used for controlling the egress from a retail store or the like, 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. Preferably, 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 pulse-controlled 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.

It has been discovered that if 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. Such a device, i.e., a diode-dipole, is shown 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. For efficient operation at 915 MHz, the tip-to-tip length of the elements 45, 46 and 47 should theoretically be of the order of 16.4 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.

Referring to FIG. 2, 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 100 Hz. The pulses from multivibrator 31, after passing through the buffer 33, function to turn on the generator 35 so as to provide frequency or pulse bursts therefrom. This is represented by curve B in FIG. 2. Thus, assuming the presence of a tag 48 in the space between the antenna elements 17 and 18, a signal will be reradiated back to the elements 17 and 18 consisting of a carrier component at 915 MHz modulated by square wave bursts of a 22 KHz modulating signal. That is, 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 915 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 915 MHz component. In cooperation with 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.

Still assuming that the tag 48 is in the space being supervised, 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 D.C. 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.

So long as the signals fed over path 27 from the detector 25 to the AND circuit 28 coincide with the output from the multivibrator 31 represented by curve A 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 41 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.

Because of the inversion caused by the circuit 29, 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.

If, however, the antenna elements 17 and 18 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 FIG. 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 of a defective reutrn pulse the counter will be reset to commence another count anew.

On the other hand, once a full count is received so as to activate the pulse stretcher 41, it is possible for the step counter to be reset a few times before another valid count is received without interrupting the alarm. By properly relating the time duration of the pulse stretcher 41 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.

While the system shown in FIG. 1 is quite effective, increased sensitivity and selectivity is provided by the system now to be described with reference to FIG. 4 to which attention should be directed. As shown therein, 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 of 915 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 space 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 915 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 of a 16-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. In response to the pulse control received through attenuator 84 from generator 82 the frequency of generator 85 is shifted .+-.1 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 91 and 92. The secondary windings (not shown) of the transformers 91 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 .times. 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.

The operation of the circuit of FIG. 4 will now be described with reference to the waveforms shown in the various curves of FIg. 5 wherein the letters appended to the individual curves correspond to the letters appearing on FIG. 4. A selection switch (not shown) may be used to select the desired pulse rate for generator 82. For example, the selectable rates may be 200, 225 and 250 Hz. At present, the preferred rate is 200 Hz although rates between 100 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 51 KHz. 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.

As in the embodiment of FIG. 1, when a receptor-radiator such as shown in FIG. 3 is introduced into 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. In known manner, the balanced mixer 71 will remove the 915 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. It should be understood that detector 74 may be a conventional ratio detector or the like. Preferably, 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. 5 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 of multivibrator 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.

Bearing in mind that the FM detector 74 is arranged to produce a D.C. signal of one level in response to a 51 KHz input signal and a D.C. signal of a second level in response to a 49 KHz signal, it will be appreciated that 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. When 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 100 represented by curve D a pulse count will be introduced into the counter 79. After 16 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 J-K flip-flops connected in known manner to produce an output at the completion of a count of sixteen.

Should the pulse output from detector 74 be interrupted due to removal of the tag from the controlled space or due to some other interference the counter 79 will be reset in a manner whick should be evident from the previous discussion and from the drawings.

By way of further explanation, curve E' shows a possible artifact type signal which might result from extraneous 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 of 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.

It should now be appreciated that with either the circuit of FIG. 1 or FIG. 4, an alarm will be given only when the detected low frequency signal has a wave envelope coinciding with an output of the pulse modulating means. In the system of FIG. 1, the multivibrator 31 represents the pulse modulating means, while in the system of FIG. 4 the pulse modulating means is represented by generator 82. This concept is extended in the system of FIG. 4 wherein an alarm is given only when the detected low frequency signal is frequency modulated with a wave envelope having the same shape as the modulating signal.

In both systems the low frequency signal should be preferably no greater than 100 KHz. It has been found that such low frequencies are best employed for establishing the electrostatic field. On the other hand, the high frequency signal should be in the microwave region so that the physical size of the tags need not be excessive.

Having described the presently preferred embodiments of the invention it should be understood that various changes in construction and arrangement will be apparent to those skilled in the art and are fully contemplated herein without departing from the true spirit of the invention as defined in the appended claims.

Claims

What is claimed is:

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.