US3828335A

US3828335A - Radio-wave detector for discovering the movement of persons or objects in a confined space

Info

Publication number: US3828335A
Application number: US00341744A
Authority: US
Inventors: G Salmet
Original assignee: Individual
Current assignee: Individual
Priority date: 1972-03-21
Filing date: 1973-03-15
Publication date: 1974-08-06
Anticipated expiration: 1991-08-06
Also published as: DE2312484A1; FR2176499A1; GB1378280A; FR2176499B1

Abstract

Movements of persons or articles in a monitored space are detected by a change in the effective capacitance between an antenna and an associated counterpoise defining that space, the antenna being energized by an oscillator whose tank circuit is tuned to a predetermined radio frequency f''''e. The oscillator works into a tuned monitoring circuit resonant at a different frequency fo, a capacitive feedback path extending from a tap on the inductive branch of that circuit to an input of the oscillator for applying thereto a control voltage which shifts its operating frequency from f''''e to a value fe closer to fo. This shift in oscillator voltage is reduced by a lowering of the control voltage through a further detuning of the monitoring circuit by a movement to be detected, with resulting change of the operating frequency to a value f''e between f''''e and fe whereby the change in output voltage due to such detuning is intensified. A load circuit connected to the monitoring circuit includes a normally de-energized relay whose energization in response to the aforementioned voltage change produces a voltage drop across a supply resistor common to the relay and the oscillator whereby this voltage change is further stepped up.

Description

(3F? BQGEIBQBBS United st;

Salmet RADIO-WAVE DETECTOR FOR DISCOVERING THE MOVEMENT OF PERSONS OR OBJECTS IN A CONFINED SPACE [76] Inventor: Gaston Raoul Salmet, 15 avenue des Arlantes, Saint-Maur, Val de Marne, France [22] Filed: Mar. 15, 1973 [21] Appl. No.: 341,744

[30] Foreign Application Priority Data [111 3,828,335 [451 Aug. 6, 1974 ,Zt; i)

jrimary Examiner-David L. Trafton [5 7] ABSTRACT Movements of persons or articles in a monitored space are detected by a change in the effective capacitance between an antenna and an associated counterpoise defining that space, the antenna being energized by an oscillator whose tank circuit is tuned to a predeter mined radio frequency f 'e. The oscillator works into a tuned monitoring circuit resonant at a different frequency f,,, a capacitive feedback path extending from a tap on the inductive branch of that circuit to an input of the oscillator for applying thereto a control voltage which shifts its operating frequency from f"e to a value fe closer to f,, This shift in oscillator voltage is reduced by a lowering of the control voltage through a further detuning of the monitoring circuit by a movement to be detected, with resulting change of the operating frequency to a value fe between 1 'e and fe whereby the change in output voltage due to such detuning is intensified. A load circuit connected to the monitoring circuit includes a normally deenergized relay whose energization in response to the aforementioned voltage change produces a voltage drop across a supply resistor common to the relay and the oscillator whereby this voltage change is further stepped up.

11 Claims, 8 Drawing Figures RADIO-WAVE DETECTOR FOR DISCOVERING THE MOVEMENT OF PERSONS OR OBJECTS IN A CONFINED SPACE This invention relates to a radio-wave detector for discovering the movement of peoples or articles in a confined space.

It has already been proposed that the movement of people or objects in a confined space be detected with the aid of radio waves in response to the alteration produced in the field reaching a receiver from a transmitter when a person or article enters or leaves the space in which the radio transmission between the transmitter and the receiver takes place. A system of this kind is of course fairly expensive, needing as it does two complete radio frequency transducers, namely a transmitter and a receiver.

The object of my invention is to provide a detector of this nature using just a single such transducer, i.e., a transmitter. In accordance with my present invention, the space to be monitored is defined by an antenna and an associated counterpoise connected across a tuned circuit which is coupled to an oscillator whose tank circuit is tuned to a predetermined radio frequency, designated f 'e hereinafter, differing from the resonance frequency of that tuned circuit. Owing to the connection of the latter circuit to the antenna and its counterpoise, resonance frequency f is codetermined by the effective antenna capacitance which in turn is variable by a movement to be detected in the monitored space, the sense of capacitance variation due to such move-' ment being so chosen as to increase the difference between the two frequencies f 'e and f,,. A feedback path between the tuned monitoring circuit, serving to energize the antenna, and the oscillator delivers to an input of the latter, such as a transistor base, a control voltage establishing an operating frequency fe intermediate and frequencies fl, and f 'e, this control voltage varying with changes in the effective antenna capacitance for shifting the operating frequency fe toward frequency f'e, i.e., to a new value fe more remote from resonance frequency 11,, in response to a movement to be detected; the resulting voltage change in the tuned monitoring circuit actuates a responder in a load circuit, connected to the monitoring circuit, for indicating that change.

Advantageously, pursuant to a further feature of my invention, the load circuit includes a differentiation network of long time constant (e.g. seconds); a diode in this network may serve to bypass voltage changes in the monitoring circuit whose polarity is opposite that caused by a movement to be detected.

In a system of this kind, when the effective capacitance between the antenna and its counterpoise is constant or slowly varying, no signal reaches the responder which preferably includes a relay triggerable by an amplifier; when, however, that capacitance varies at a critical rate, a signal is applied to that amplifier to energize the relay.

Preferably, the amplifier comprises an integrator so that the device does not respond to interference picked up via the antenna. The integrator must have a time constant longer than the average very brief duration of domestic, industrial and atmospheric interference.

In cases in which the device is to detect the presence of living beings, more particularly people, whose approach acts to increase the circuit capacitance and also to damp the tuning circuit, the device is so adjusted that the transmitter operating point is positioned on the descending part of the resonance curve of the tuning circuit i.e., at a frequency a little beyond the exact resonance frequency. The reason for this is that since the capacitance increase in the tuned circuit is the result of the presence of a living being in the transmitter field, so that the resonance frequency of this circuit decreases, and since a living body has a small dielectric constant and therefore increases damping, the difference in transmitted energy due to the displacement of the resonance curve toward the origin and to the flattening thereof is at a maximum. This appreciable reduction in the transmitted field is detected by the differentiation circuit which actuates the responder.

The deactivation of the responder can be delayed, preferably via the differentiation network, by using negative detection and by energizing the complete system via a common resistance such as the internal resistance of the power supply or an auxiliary resistance in series therewith.

When, as a result of detecting a movement, an integrating network in series with the differentiation circuit registers a voltage drop (negative detection) and triggers the responder, the resulting power consumption causes an even greater voltage drop at the output of the integrating network; of the detector, owing to this large voltage drop and the long time constant of the differentiator cascaded with the integrator, upon termination of the triggering event (movement of a person) the differentiating capacitor will take much longer than that time. constant to return to its former state of charge and thus stop the operation of the responder. Once the system has ceased to operate, it can be restored to its normal supervisory or monitoring state with removal of the charge associated with normalization of the voltage.

The system according to the invention is of use for protecting the monitored premises or other spaces from intruders and for transmitting recorded messages for advertising, tourist and museum purposes. In the latter case delayed operation of the responder can be produced and maintained by detection, with a short time constant, of message signals going to a loudspeaker, so that the delay ceases when the speech or sound transmission ceases and the device returns to its supervisory or stand-by state.

My invention will now be described in detail with reference to the accompanying drawing in which:

FIG. 1 is a diagrammatic view of a systemaccording to the invention;

FIGS. 2a and 2b are diagrams explaining the operation of the system of FIG. 1;

FIG. 3 is an equivalent circuit diagram of the transmission circuit of the system;

FIG. 4 is a partial block diagram of the complete system;

FIG. 5 is a diagram of its power supply;

FIG. 6 is a diagram showing how deactivation can be delayed when the system is used with a loudspeaker;

FIG. 7 is a simplified detail of the diagram shown in FIG. 4.

In FIG. 1 I have shown a resonant monitoring circuit comprising an inductance L and a capacitor C. One side of the circuit is connected to an antenna A and the other to a counterpoise therefor (here ground). The actual capacitance of the tuned circuit is therefore not C but C Ca, Ca, denoting the antenna-to-counterpoise capacitance. The resonant frequency is therefore:

f =1/(21r V L (C +Ca)) Let us assume now that a person P approaches the antenna; since the human body is a conductor, it will increase the antenna-to-counterpoise capacitance so that the resonant frequency of the tuned circuit be comes:

f= l/(2rr V L (C +Ca +Cx)) Cx denotes the extra capacitance due to the presence of person P.

If the circuit L-C is energized by a constantamplitude but variable-frequency alternating current I, the AC voltages at the circuit terminals in dependence upon frequency are represented by a curve K in FIGS. 2a and 2b. Ifthe capacitance increases, the curve which represents the frequency F is the dotted-line curve K, of FIG. 2a. If, in this case, the transmission frequency f is set at a value fe slightly greater than the resonant frequency f,,, the capacitance increase and therefore the frequency variation will of course produce a variation AV of the AC voltage across the circuit. Owing to the shape of the resonance curve, maximum sensitivity is obtained for a value of the frequency fe such that the normal operating voltage V, developed in circuit L-C is approximately 80 percent of the resonant voltage V Also, for a given change in capacitance the voltage variation AV, is proportional to the Q factor of the circuit and inversely proportional to the tuning capacitance C. A very-high-Q inductance and a low capacitance should therefore be used. To give some idea about system sensitivity, if C 500 Pf and Q 200, a 0.02Pf variation of C gives a relative variation AV,/V,, of approximately 1 percent.

Since the human body has a high ohmic resistance, this choice of position for the frequency fe is advantageous in the case of a human being entering the field.

As the equivalent circuit diagram of FIG. 3 shows, the extra capacitance Cx corresponding to the presence of person P and applied across the circuit is in series with a considerable series resistive component Rx, which helps to damp the circuit and therefore to change the curve K, into the dotted-line curve K i.e., to reduce the voltage across the circuit. This leads to an extra voltage reduction AV so that the total variation due to a person entering the field becomes:

AV AV, AV,

The damping efiect is therefore cumulative with the effect of the extra capacitance if the frequency fe is above the frequency f,,.

In some cases, however, the movement to be detected causes a decrease in capacitance, as for instance in the case of a surreptitious opening of an armored door, metal shutter, grid or closure lattice; in this case, as shown in FIG. 2b, the oscillator frequency fe is advantageously smaller than the resonant frequency f, of the transmitting circuit L-C. When the capacitance across the inductance L decreases, the resonance curve shifts to the right (curve K and as in the previous case there occurs a voltage reduction AV,; in this instance, however, there is no damping variation.

In both cases (FIGS. 20 and 2b) the value of AV, can be increased to AV as will be shown hereinafter. Conversely, to reduce the system sensitivity to prevent accidental operation, the offset between the frequencies fe and f can be increased so that operation is shifted to the skirts of the curve K. The choice of the offset or difference between the frequencies fe and f, determines therefore the sensitivity of the system.

These considerations are used in the design of a detector Det for which a circuit diagram is shown in FIG. 4. A fixed-frequency oscillator 0 works through an adjustable resistance R, into an amplifier A, which has a high internal output impedance so as not to damp the oscillatory output circuit consisting of an autotransformer L, preferably of the ferrite-core kind, and a capacitor C,. The lower terminal end of the oscillatory circuit is connected to the counterpoise i.e., to ground in the present case whereas its upper terminal is tied to the antenna A. The resistance R, serves to control the drive of the amplifier A, so that at resonance of the circuit L-C, the AC voltage between a point a in the amplifier output and ground is near the maximum which the amplifier A, can provide without being saturated.

The amplifier A, therefore behaves like a constantcurrent generator. The tap a is so chosen that the total supply direct current under these conditions does not exceed a given low value, e.g. 1 mA, when the apparatus is on standby, so that the system can run for a long time (several months) if battery-energized.

FIG. 7 is a very simplified circuit diagram of the oscillator O and the amplifier A. The oscillator O is a Hartley circuit comprising a transistor Tr,, a ferrite-core inductance L, and a capacitor C the elements L, and C forming a tuned circuit. Depending upon the number of stages in it, the amplifier A either inverts or does not invert the phase of the signal which it transmits; in this particular case the amplifier A, which comprises a single transistor Tr inverts the phase. A feedback coupling between the tap a and a point g (i.e., the base lead of the transistor Tr, of the oscillator O) is provided by a capacitor C, which therefore feeds back a control voltage that is always in phase quadrature with the normal voltage on lead g.

Ishall first describe the case of FIG. 2a and of an amplifier which inverts the phase of the output voltage relatively to the input voltage. In the absence of feedback coupling the oscillator frequency fe would be the same as the frequency of the tuned circuit L,-C constituting the tank circuit of oscillator 0.

If the circuit L-C, is tuned exactly to the oscillator frequency fe (f fe), the -out-of-phase control voltage fed back via capacitor C to point g lags with reference to the regenerative feedback voltage from tank circuit L,C, present at point g in the absence of such capacitive feedback, and so the oscillator frequency is altered. It can be shown that the operating frequency decreases in this case. Conversely, if the oscillatory detector circuit L-C,, is detuned, the lagging control voltage decreases and the frequency fe increases, tending toward the natural frequency f'e of the tuned circuit L,-C,.

Consequently, the movement of curve K towardK; when capacitance C22 is 66666151 in parallel with capacitances C and Ca (FIG. 3) is enhanced by the effect of frequency fe shifting to a higher value f'e (FIG. 2a), closer to the natural frequency f 'e of tank circuit L C and so the detectable voltage variation becomes AV instead of AV,.

Thanks to the high Q of the circuit LC,, which gives a very sharply peaked curve K, the variation AV and therefore the sensitivity of the system can be increased by a factor of approximately 3 to 5.

In the case of FIG. 2b (detuning of circuit L-C by decrease of capacitance), the circuit arrangement shown in FIG. 7 again increases the frequency fe, but in this case such increase counteracts the effect of curve K shifting toward K To obtain a similar effect i.e., a variation of oscillator frequency fe in the sense opposite to the variation of the detector-circuit frequency the feed-back circuit should be connected to a point g which is symmetrical with reference to the point g relatively to the neutral point h of the oscillator tuned circuit. Of course, the connections to points g and g must be reversed if the amplifier does not invert the phase of the output voltage.

Because of the autotransformer effect of the inductance L of the detector circuit, the voltage between the point a and ground is normally several tens of volts. A proportion of this voltage is taken off at a top b (FIG. 4) and fed via a rectifying connection, constituted by a diode D to an integrating circuit comprising a resistance R in parallel with a capacitor C As will be apparent, the alternating voltage of radio frequency fe (or f'e) developed in the tuned circuit L-C, builds up a negative potential, of a magnitude proportional to the radio-frequency voltage, on the ungrounded terminal of integrating capacitor C Autotransforrner tap b is so chosen that the voltage thus detected is large but its detection does not cause appreciable damping of the circuit LC As an example, the DC voltage across the resistance R can be something like 50V, whereas the AC voltage between the tap a and ground may be only about 5V r.m.s. Of course, and as explained with reference to FIG. 2a, in most cases the inductance L is adjusted to above the resonance frequency (or if such adjustment cannot be provided, the capacitance C is so adjusted) to give a voltage across the circuit L-C, of about 80 percent of the voltage at resonance.

The frequency chosen is approximately 30 KHz, which is low enough for the transmitted voltage and its harmonics not to interfere with radio broadcasting, yet high enough to be able to use high-Q inductances of reduced size.

The negative voltage detected at the ungrounded terminal c of network R C is transmitted to a DC amplitier A via a differentiation network (I -R The function of this network, whose time constant is on the order of seconds, is to transmit at a point d only relatively rapid variations of the voltage at point c signaling the approach of a person, and not to transmit very slow variations, due for example to variations of the ambient temperature or of the supply voltage (upon exhaustion of the cells).

Since the voltage at c is negative and the approach of a person reduces this voltage, such approach produces a positive voltage at d.

The amplifier A energizing a relay R is so designed that for zero or negative voltage at d the voltage across the relay R1 is zero, whereas for even a very small positive voltage (e.g. on the order of 0.1V) at the point d the amplifier A energizes the relay Rll sufficiently for the same to become operative.

Preferably, the input impedance of the amplifier A is very high several megohms so that a very high detected voltage can be produced at the point c with low power consumption. The magnitude of the resistance R should therefore itself also be very high.

Advantageously, the amplifier A is a conventional NPN transistor connected as a cathode follower, so as to have a high input impedance, followed by another NPN transistor arranged as a voltage amplifier, in turn followed by a voltage-amplifying PNP transistor driving the relay R One of the stages of amplifier A includes an integrating network Int, having a time constant of 0.2 to 0.5 sec, for general interference suppression.

Relay R1 has two contacts r r the former actuating a responder, e.g. triggering an alarm AL, whereas the latter preserves the response by the application of an appropriate voltage +v to one of the transistors of amplifier A This obviates the need for a direct holding contact on the relay; the holding effect of voltage +v can be controlled by any parameter, e.g. as described below with reference to FIG. 6.

Since the circuit arrangement responds to the appearance of a positive voltage at the point d, a diode D can provide very rapid absorption of negative potential variations at that point so that when such variations occur, for instance, at switch-on, the apparatus is immediately ready for operation i.e., there is zero voltage at the point d.

Detection of increasing rather than decreasing voltage swings at the input terminal b is possible if the amplifier A is sensitive to negative voltages (PNP stage), in which case the shunt diode D would be inverted.

If the detector is self-restoring to the standby or monitoring state, it is preferable for many uses of the invention to have a signal of limited duration rather than a steady signal. Various auxiliary means are known for providing a delay giving a signal lasting for a few tens of seconds; in the present case, however, there is a very simple way of achieving this result with virtually no addition of extra items.

The delay procedure can be clearly understood from the following example.

If the system consumes, say, 1 mA on standby, its consumption is e.g. 30 mA when relay R1 operates, because of the energy used up by this relay. If a resistance Rs (FIG. 5) is connected in series with the associated power supply S and is of such magnitude that the supply voltage energizing the detector part Det of the system drops by e.g. 10 percent when the relay is thus energized, all the AC and DC voltages will decrease in substantially the same proportion. More particularly, the voltage integrated at the point c, which was 50 V, becomes -45 V, and the initial voltage change which triggered the alarm and which was just a few tenths of a volt is converted into a much greater swing as a result of the voltage drop developed across resistor Rs. A positive voltage above 5 V therefore appears at the point d. To dissipate this voltage by way of the differentiation network, capacitor C must first discharge through resistor R sufficiently for the voltage at d to decrease to e.g. 0.1 V or less. When this voltage has been reached,

the relay R1 returns to normal and stops the responder.

Consequently, if the value of capacitance C is chosen appropriately, a signal lasting for approximately 1 minute can be produced without any other delay means being used.

Conversely, however, when the relay R1 releases, a very large negative voltage variation occurs at the point d; as previously described, this variation is, however, absorbed very rapidly by the diode D and the detector is ready almost immediately for further operation.

Since the detector operates basically on the principle of varying the state of tuning of a tuned transmitting circuit, the main variation being capacitive and the secondary variation being in the damping, there is no need to use a vertical antenna in association with a counterpoise forming a horizontal mass plane. The antenna its counterpoise can be e.g. two metal strips or even wires connected to the two ends of the oscillatory circuit and extending parallel to each other. The strips can be placed on the ground, if the same is not conductive (floor), or positioned vertically on either side of an entrance which it is required to protect. The sensing element formed by the antenna and its counterpoise can be devised differently to suit individual cases. Inter alia, in a room or the like the antenna can be in the ceiling and the counterpoise can be below it on the floor.

Thanks to its high sensitivity, the system is highly versatile. For instance, with a vertical antenna 1.50 meters long and a ground plane which is either inherently conductive or made so, e.g.- by latticework, a person can be detected at up to about 8 meters from the antenna i.e., assuming that the antenna is accessible from all directions, the operative area of the system is on the order of 200 m.

An obvious use of the system is for protection against unwanted intrusions. It can also be used to detect movement, e.g. for automatic door opening, lighting of passageways (timers), or counting people.

Also, its delay feature makes it very suitable for advertising purposes as, for instance, to trigger a tape recorder which broadcasts an advertising announcement or a commentary on an article on show in a museum.

FIG. 6 shows one such adaptation of the invention. An endless magnetic tape m contains a text which may be repeated a number of times, the spacing between repetitions being such that the end of the text and the start of its repetition are separated by an interval of a few seconds. For instance, in its commercial application a tape recorder MAG containing the tape m is under the control of the movement detector hereinbefore described. The two systems are located near the place where possible clients may pass by. When a client approaches, the relay of the detector Det starts the tape recorder MAG. For a brief period the tape recorder transmits no signal to loudspeaker H, but the relay R1 remains held by the delay means hereinbefore described; in this case the delay is fairly short, e.g. seconds. At a predetermined time the tape recorder MAG starts to read out the text through a loudspeaker H. After some time the internal delay of the detector Det terminates, but the detector relay R1 locks as a result of detection of the transmitted modulation; the voltage across the loudspeaker H is detected by a network DAL-C, and applied through holding contact r, of relay R1 of FIG. 4 to hold the relay while the lowfrequency modulation i.e., the transmitted message continues. Upon cessation of the message the holding voltage disappears, the relay R1 releases and the tape recorder MAG stops. The procedure can restart when someone else passes nearby.

Clearly, the time constant of the detector network R4 C4 must be long enough for the hold not to be likely to disappear between individual words of the message, and short enough for terminating the hold at the end of the message, e.g. after 2 seconds of silence, so that the system is restored to standby for someone else to pass by. 7

Of course, recording using a continuous tape can be replaced by any other kind of sound recording, such as one using a disk with automatic return of the pickup arm.

I claim:

1. A system for detecting movements in a monitored space, comprising:

an antenna and a counterpoise therefor jointly defining the space to be monitored;

an oscillator provided with a tank circuit tuned to a predetermined radio frequency;

a tuned circuit with a resonance frequency differing from said predetermined radio frequency coupled to said oscillator for energization thereby, said tuned circuit being connected between said antenna and said counterpoise whereby said resonance frequency is codetermined by the effective capacitance between said antenna and said counterpoise, said effective capa ems g filllllbyjl ovemenLtQl fijl lfiilltt in a sense increasing the difference between said resonance frequency and said predetermined frequency;

a feedback path between said tuned circuit and said oscillator for delivering to an input of said oscillator a control voltage establishing an operating frequency intermediate said predetermined radio frequency and said resonance frequency to be radiated by said antenna, said control voltage varying with changes in said effective capacitance for shifting said operating frequency toward said predetermined radio frequency in response to a movement to be detected; and

a load circuit connected to said tuned circuit, said load circuit including responder means for indicating a voltage change in said tuned circuit due to a movement to be detected.

2. A system as defined in claim 1 wherein said load circuit includes a differentiation network of long time constant in series with said responder means.

3. A system as defined in claim 2, further comprising diode means in said differentiation network for bypassing voltage changes in said tuned circuit of a polarity opposite that due to a movement to be detected.

4. A system as defined in claim 3 wherein said differentiation network has a resistive branch and a capacitive branch, said diode means being connected across said resistive branch.

5. A system as defined in claim 2 wherein said responder means includes a normally de-energized relay and trigger means for energizing said relay in response to a significant voltage reduction in said tuned circuit due to a movement to be detected, said relay and said oscillator being provided with a common direct-current supply, further comprising resistance means in series with said common supply for generating a voltage drop 9 upon energization of said relay to intensify said significant voltage reduction.

6. A system as defined in claim wherein said differentiation network includes a capacitor and a resistor, said load circuit further comprising a rectifying connection and an integrating network inserted between said tuned circuit and said capacitor for charging the latter upon occurrence of said significant voltage reduction, said resistor enabling delayed discharging of said capacitor upon restoration of said effective capacitance to normal whereby said relay remains energized beyond said restoration.

7. A system as defined in claim 6, further comprisin a source of temporary holding voltage for said trigger means and activating means controlled by said relay for making said source operative over a limited period.

8. A system as defined in claim 7 wherein said source comprises a recording medium carrying a message to be announced and circuitry for deriving said holding voltage from message signals in the output of said recording means.

9. A system as defined in claim 1 wherein said feedback path is connected to said tuned circuit at a point whose alternating voltage decreases upon variation of said effective capacitance by a movement to be detected, thereby diminishing said control voltage in response to such movement.

10. A system as defined in claim 9 wherein said oscillator comprises a transistor with a base lead connected to said tank circuit for receiving a regenerative feedback voltage therefrom, said feedback path including a reactance connected to said base lead for superimposing said control voltage upon said regenerative feedback voltage.

11. A system as defined in claim 7 wherein said relay is provided with contact means for delivering to said trigger means a holding voltage maintaining said relay energized.

Claims

1. A system for detecting movements in a monitored space, comprising: an antenna and a counterpoise therefor jointly defining the space to be monitored; an oscillator provided with a tank circuit tuned to a predetermined radio frequency; a tuned circuit with a resonance frequency differing from said predetermined radio frequency coupled to said oscillator for energization thereby, said tuned circuit being connected between said antenna and said counterpoise whereby said resonance frequency is codetermined by the effective capacitance between said antenna and said counterpoise, said effective capacitance being variable by a movement to be detected in a sense increasing the difference between said resonance frequency and said predetermined frequency; a feedback path between said tuned circuit and said oscillator for delivering to an input of said oscillator a control voltage establishing an operating frequency intermediate said predetermined radio frequency and said resonance frequency to be radiated by said antenna, said control voltage varying with changes in said effective capacitance for shifting said operating frequency toward said predetermined radio frequency in response to a movement to be detected; and a load circuit connected to said tuned circuit, said load circuit including responder means for indicating a voltage change in said tuned circuit due to a movement to be detected.

5. A system as defined in claim 2 wherein said responder means includes a normally de-energized relay and trigger means for energizing said relay in response to a significant voltage reduction in said tuned circuit due to a movement to be detected, said relay and said oscillator being provided with a common direct-current supply, further comprising resistance means in series with said common supply for generating a voltage drop upon energization of said relay to intensify said significant voltage reduction.

6. A system as defined in claim 5 wherein said differentiation network includes a capacitor and a resistor, said load circuit further comprising a rectifying connection and an integrating network inserted between said tuned circuit and said capacitor for charging the latter upon occurrence of said significant voltage reduction, said resistor enabling delayed discharging of said capacitor upon restoration of said effective capacitance to normal whereby said relay remains energized beyond said restoration.

7. A system as defined in claim 6, further comprising a source of temporary holding voltage for said trigger means and activating means controlled by said relay for making said source operative over a limited period.