Selective Intrusion Alarm System

Abstract

A radio object detection, communication, and alarm system for aiding in maneuvering a truck for the delivery of goods at an unloading platform and for protecting the open truck from unauthorized entry during unloading includes portable radio communication means for inhibiting alarm operation when authorized personnel are present in a protected zone.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention pertains to selective area intrusion monitoring systems comprising radio object detection and radio communication and alarm elements and more particularly concerns such systems for monitoring a region of restricted size which may be entered, without alarm triggering, by authorized personnel equipped with miniature transmitters broadcasting a signal of predetermined character.

2. Description of the Prior Art

Generally, prior art intrusion alarms operate when any person or object enters the field in which the alarm sensor is sensitive. Access to the protected region is possible without alarm triggering only by time-consuming repeated disabling and enabling operations as an authorized person repeatedly enters and leaves a protected region. The actual boundary of the protected region varies with ambient conditions and other factors and is generally not well defined. As a consequence, undesired or false alarms are often generated which may embarrass customers in addition to being a general nuisance. Further, repeated false alarms undesirably degrade the credibility of the alarm system and lead to its improper use.

SUMMARY OF THE INVENTION

The present invention relates to radio object detection, communication, and alarm systems for selectively protecting specific regions of space from unauthorized intrusion. Entry by authorized personnel carrying portable transmitters may be made, the alarm operation being inhibited by the particular character of the transmitted signal. The detection system operates in an accurately and selectively defined volume and false alarm events are minimized. The intrusion detection system employs means for the reception, selective gating, and wave form conversion of base-band or sub-nanosecond electro-magnetic signals and includes means for reception and selective utilization of such base-band signals for the generation of control signals according to the presence or to other characteristics of such base-band signals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevation view showing an installation of the novel detection-alarm system for protecting a delivery vehicle.

FIG. 2 is a block diagram showing components of the invention and their electrical interconnections.

FIG. 3 is a more detailed block diagram of the object detector of FIGS. 1 and 2.

FIG. 4 is a circuit diagram of a part of the apparatus of FIG. 3.

FIG. 5 is a perspective view of an antenna used in the preceding figures.

FIG. 6 is a perspective view showing an application of the invention in the protection of an area in a store.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates the novel intrusion protection and alarm apparatus in an application wherein protection of a vehicle 1 from unauthorized intrusion is desired, though authorized personnel may enter through its rear without sounding an alarm. The invention is of particular interest where a protected volume, such as that within the interior of truck 1, must often be entered or left by one or more authorized persons. Entry and exit occurs sufficiently often that repeated locking and unlocking of the rear of the truck would be expensively time consuming. Thus, the protected volume is often left unattended for short periods of time. According to the invention, the problem of protecting goods within the interior of truck 1 is solved by providing an intrusion alarm or alarms which may not be activated when an operating key transmitter carried by authorized personnel is present about the protected zone.

In FIG. 1, the truck 1 may be backed toward an unloading platform 2 by the truck operator after he has adjusted control 3 of the system to register an alarm as soon as the truck is backed sufficiently that front parts 6 of the dock 2 fall within the alarm activating or protected zone 4 between ellipsoidal radiation envelopes 7 and 8. When a front part 6 thus falls within ellipsoid 7, the truck driver is told by the operation of an alarm within the truck cab, such as a buzzer or the lamp 3b of FIG. 2, that the rear 10 of truck 1 is within, say, two feet of frontal surface 6. He then moderates the backing of the truck in the conventional manner so that the dock is not harshly struck.

Once the truck is halted in position for the loading or delivery of goods, the driver operates control 3 so that the protected zone is now moved inward to a new zone 5 bounded, for example, by ellipsoidal surfaces 8 and 9. Reflecting parts 6 of dock 2 do not fall within this second protected zone, and an alarm is accordingly operated only when a person or an object is moved into the new zone 5. Truck 1 is equipped with external antennas 11, 12, and 20a for cooperation in performing the above described functions, and with an externally mounted alarm element 14 such as a bell or siren. Antennas 11, 12, and 20a may be mounted within radomes or other suitable protective dielectric enclosures.

The structure and operation of the radio system cooperating with antennas 11, 12, and 20a and alarm 14 will be considered in connection with FIGS. 1 and 2, and will be seen to consist of a range gated object detection system associated with directive antennas 11 and 12 and with a continuous wave radio system for controlling the operation of the object detector. The transmitting antenna 11 of the object detector is excited by a pulse transmitter 23 driven by a system pulse timing synchronizer 24. Synchronizer 24 also excites zone control 15 whose adjustment 3a determines whether zone 4 or zone 5 is selected by the truck driver.

Any echo reflected from dock 2 during zone 4 operation or from an intruder during zone 5 operation is reflected into receiver antenna 12 and is coupled to pulse receiver 16 made active for a particular zone by zone control 15. The output of receiver 16 is coupled via terminal 46 to driver circuit 17 which may be a simple amplifier or wave shaping device suitable for driving lamp 3b or other internal or cab mounted alarm devices such as device 3c. Driver circuit 17 may involve conventional latching or reset circuits, as desired.

Driver circuit 17 is also adapted to drive an outer alarm 14 located on the exterior of the truck body as seen in FIG. 1, inhibit circuit 18 being interposed between driver 17 and outer alarm 14. Inhibit circuit 18 may be operated to prevent outer alarm 14 from sounding an alarm, though the inner or cab alarms 3b and 3c always operate if signal reflections are passed through pulse receiver 16 by zone control 15. For example, inhibit circuit 18 may be operated to prevent outer alarm 14 from operating by the generation of outputs from selective tunable receivers 19, 19a, which outputs pass through a conventional sum circuit 21 to the inhibit circuit 18. Receivers 19, 19a may be respectively equipped with suitable generally omnidirectional antennas 20, 20a. Receivers 19, 19a may be similar devices so that either may receive signals from one or more small, portable induction transmitters, such as key transmitters 219, 219a. The communication may be narrow band, continuous wave or coded by a distinctive modulation, if desired, and the carrier or modulation frequency may be changed at suitable times. Receiver 19a may be mounted so that antenna 20a is located, as seen in FIG. 1, externally at the rear of truck 1, while receiver 19 and antenna 20 may be located within the truck cab. Antenna 20a may be a ferrite loaded loop antenna for operation at 10 to 50 kHz.

Transmitter 219a represents such a portable transmitter in use by the truck driver or a helper when he is to be outside of the parked truck handling goods. It is seen that transmitter 219a has contacts 223a, 224b for charging a battery contained in the instrument case. It is further equipped with a generally non-directional antenna 220a of simple conventional type. It may be equipped with a switch 233a in the usual manner permitting the person using it to turn its radiation on or off. A preferred alternative switch is a spring actuated switch 221a of conventional type in which the user has to push the switch in and hold it in order to turn transmitter 219a off. It is seen that the person using transmitter 219a may carry it in a shirt or coat pocket and that it will continuously transmit carrier signals over space path 232a to antenna 20a in such a circumstance.

The power transmitted by key transmitter antenna 220a and the output level of its cooperating receiver 19a are such that inhibit circuit 18 is faithfully operated when the transmitter user passes through zone 4 or 5. The power transmitted by transmitter 219a will generally be adequate to disable alarm 14 at three or four times the range characterizing ellipsoid 8. On the other hand, if the user of transmitter 219a moves out of sight of the rear 10 of truck 1, the gain of the transmitter 219a--receiver 19a system is not sufficient to operate inhibit circuit 18, or transmitter 219a may be deliberately turned off. Thus, if an intruder then moves toward the truck rear and into the protected zone 5, outer alarm 14 will sound at a loud level to frighten the intruder away or to summon help. It will be seen that one or more transmitters 219, 219a may be used in this manner.

During intervals when the protected truck is moving, the transmitter 219a will not generally be carried by truck personnel and may be mounted as illustrated at 219 in a receptacle 222 fastened to an interior wall of the truck cab. The base of receptacle 222 is equipped with contacts 225, 226 corresponding to mating contacts 223, 224 for charging the battery within the casing of transmitter 219 from a source such as the vehicle battery or generator 227. Transmitter 219 may be permitted to transmit carrier signals via space path 232 to antenna 20 and receiver 19 while its battery is charging, or the operator may stop such transmissions by operation of switch 233. In the preferred alternative, a push-to-disable switch 221 such as switch 221a may be used. During charging, switch 221 may be held down within detent 228 in the off position by holder 230 pivoted at 229 in a yoke 231 affixed to the interior wall above receptacle 222.

The versatility of the invention is seen to permit its use when the rear of the truck is left open during transit from one delivery location to another. With neither transmitter 219, 219a operating, an alarm within and without truck 1 is sounded if an attempt is made by an intruder to enter the truck from the rear when it is in motion or is stopped in slow traffic or at a traffic light. With the rear doors locked and with the outer alarm 14 inhibited, operation of alarm 3b or 3c may be caused to indicate the dangerously close following of a second vehicle within zone 4. It is understood that FIG. 2 is intended, for example, to illustrate two alternate situations. In one case, two transmitters 219 and 219a are in general use. In the other, only one transmitter 219 is employed and alternate positions and modes of operation are represented at 219 and 219a.

It will be seen qualitatively simply by viewing FIG. 1 that the quite special requirement of very short range operation is imposed upon the object detection system cooperating with directional antennas 11 and 12. It will be seen that an object detection system is required of the type disclosed by G.F. Ross in the U.S. Pat. application Ser. No. 137,355, filed Apr. 26, 1971, assigned to the Sperry Rand Corporation and entitled: "Energy Amplifying Selector Gate for Base Band Signal". The Ross application concerns an object detector transmitter and receiver system for very short range object detection including an electromagnetic pulsed energy system for receiving and selectively gating very short base-band electromagnetic pulses, and for supplying an energy amplified output useful for operating utilization equipment. The Ross system employs a substantially dispersionless wide band transmission line arrangement cooperating directly with a biased semiconductor gating or range selector device located in the transmission line for detecting the total energy of the incoming base-band pulse. A cooperating circuit coupled to the gating device supplies a corresponding output signal suitable for application in utilization circuits and permits the system to recycle, making it ready for the receipt of a succeeding short duration base-band echo pulse. Since the total energy of the base-band echo pulse is instantaneously supplied by the dispersionless transmission line system to the semiconductor gating device, the gating system may operate with base-band pulse signals having spectral components the amplitudes of which are all incapable of detection by conventional relatively narrow band receivers.

The total energy in each base-band pulse can, however, be relatively larger than the level of noise or other interfering signals in the vicinity of the receiver-detector. Thus, by appropriately adjusting the sensitivity or threshold of the receiver-detector, base-band signals not affecting other receivers are readily received, detected, and gated without the detector being affected in substantial degree by other radio energy transmissions. The major processing of the echo signals is accomplished by simple base-band circuits, thus avoiding the need for signal frequency conversion and the problems associated with alignment and operation of conventional radio and intermediate frequency amplifiers.

The essential parts of the above-mentioned Ross apparatus as employed in the present invention are disclosed in FIGS. 3, 4, and 5; in FIGS. 3 and 4, parts corresponding to those shown in FIGS. 1 and 2 have corresponding reference numerals. In FIG. 3, the base band pulse generator 23 is triggered by a pulse such as pulse 25A originating in the system synchronizer 24 at time t.sub.o. A base-band signal of sub-nanosecond duration propagates along dispersionless TEM mode transmission line 22 and is radiated by directive antenna 11 toward a reflecting object. Reflected signals are received by the dispersionless receiver antenna 12, which also operates in the TEM mode, and are coupled by transmission line 43 to the gated receiver detector 44. When receiver detector 44 is in its conductive state, output signals appear on lead 45 for supply to utilization apparatus which may be of known type and function, such as to a target presence or object range display or alarm of generally conventional nature.

Synchronizer 24, base-band pulse generator 23, transmission line 22, and transmitter antenna 11 may, for instance, be elements of the integrated type of transmitter-radiator system taught by G.F. Ross and D. Lamensdorf in the U.S. Pat. application Ser. No. 46,079 for a "Balanced Radiation System", filed June 15, 1970 and assigned to the Sperry Rand Corporation. The latter device employs a constant impedance transmission line system for propagating TEM mode electromagnetic waves. The transmission line system is also employed for the cooperative cyclic storage of energy and for its cyclic release by propagation along the transmission line and radiation at an end of the transmission line formed as a directive antenna. Thus, cooperative use is made of the transmission line system for signal generation by charging the transmission line at a first rate of charging and also for signal radiation into space by discharging the line in a time much shorter than required for charging. Discharge of the transmission line causes a voltage wave to travel toward the radiating aperture of the antenna structure. The process operates to produce, by differentiation, a base-band impulse of sub-nanosecond duration that is radiated into space toward a reflecting object. The antenna system has a wide instantaneous band width, so that it may radiate such very sharp impulse-like signals with low distortion. Further, the antenna has an energy focusing characteristic such that maximum energy is radiated in a predetermined direction, as is desirable in object detection systems.

Other types of transmitter-radiator systems may be employed. For example, there is known in the art a variety of transmitter systems for producing single positive or negative going pulses or trains of such pulses, each pulse having very short duration, and for radiating such pulses from a suitable antenna 11. Spark gap transmitters, for instance, readily produce short electromagnetic pulses. Delay line pulse generators are well understood in the art to be capable of adjustment such that very short electromagnetic pulses may be radiated. One device for producing such short-base-band pulses is disclosed by G.F. Ross in the U.S. Pat. No. 3,402,370 for a "Pulse Generator", issued Nov. 30, 1965, and assigned to the Sperry Rand Corporation.

For controlling operation of the novel wave amplifier zone selecting gate, the synchronizer pulse 25A is coupled by line 25 to a variable delay trigger circuit 26 for generating on output line 31 a corresponding pulse 31B. Pulse 31B may be generally similar to pulse 25A, though delayed by an arbitrary time interval. Variable delay trigger circuit 26 may be any of several well known adjustable pulse delay circuits, including those, for instance, whose delay characteristic may be varied according to the setting of a tap 28 adjustable along potentiometer 27 relative to lead 29, an appropriate potential being supplied to the opposite end of potentiometer 27 from a voltage source (not shown) connected to terminal 30 and which may also be grounded at its opposite end. It is seen that the position of tap 28 is controlled by the knob 3a of the zone control 15 of FIG. 2 so that the starting point of the selected range zone 4 or 5 is determined. Knob 3a may be moved to one position for initiation of protected range zone 4 at ellipsoid 8 of FIG. 1, or to a second position for initiation of the protected range zone 5 at ellipsoid 9.

Thus, variable delay trigger circuit 26 determines the initiation of the wave selector gate, while range gate generator 32, whose input is supplied via line 31, determines the duration of the zone selector gate. This duration is determined, as will be further explained in connection with FIG. 4, according to the length L of transmission line 33, whose center conductor is adapted to supply necessary operating voltages via resistor 34 from terminal 35 to active circuit elements of range gate generator 32. The range gate thus formed is the wave 36C.

Wave 36C is supplied by line 36 to a low pass filter 37, whose function is to provide a moderate integration to wave 36C, removing any transients or over-shoots from the edges of wave 36C and thus preventing false operation of succeeding circuits. Wave 38D is the modified output of filter 37 and is passed through inverter 39 to produce on line 40 the inverted or negative going wave 40E. Wave 40E is generally similar to wave 48D, but is inverted in polarity.

The inverted wave 40E operates gated receiver-detector 44 and current source circuit 41 which forms, as will be explained, the actual gating potential used to control flow of signals through the gated receiver-detector 44 from receiver antenna 12 to output lead 45 (wave 45F). Gated detector 44 is normally desensitized when a gating signal is present at the output of inverter 39, the gated detector 44 is made sensitive to the presence of millivolt signals collected by dispersionless antenna 12 and propagated into gated detector 44 along transmission line 43. Such sensitivity produces an amplified selected output wave 45F on lead 45 of the order of 3 volts. Such a signal is adequate to operate display apparatus, such as the alarm or presence indicators 3b, 3c, or 14 of FIG. 2. The signal on lead 45 may be used directly to operate alarms 3b, 3c, or 14; on the other hand, to reduce false alarms, successive signals 45F may be integrated in a conventional integrator circuit 42 before application via terminal 46 to driver 17. As previously noted, driver 17 may include a suitable amplifier and may employ a latching or other relay operating the alarms 3b and 3c or 14 when the signal at terminal 46 reaches an appropriate value.

In FIG. 4, circuit details of the device of FIG. 3 are further illustrated, with elements which appear also in FIG. 3 bearing the same reference numerals as used in FIG. 3, including range gate generator 32, low pass filter 37, inverter 39, current source circuit 41, gated detector 44, and receiver antenna 12. The output line 31 of variable delay trigger circuit 26 supplies wave 31B via coupling capacitor 50 and junction 52 to the base 54a of transistor 54, which transistor may be of the 2N 5130 type. Junction 52, and therefore base 54a, is coupled to ground through resistor 51. The collector 54b of transistor 54 is coupled via the inner conductor of coaxial transmission line 33 of length L through resistor 34 to a source (not shown) of positive potential connected between terminal 35 and ground. The length L of open-circuited delay line 33 is adjusted according to the desired duration of the sampling or gate wave 40E. The emitter 54c of transistor 54 provides an output connection via lead 36 to low pass filter 37. In a representative circuit, resistor 34 has the value of 47 K ohms, while the voltage on terminal 35 may be from +200 to +300 volts. Various avalanche transistor delay-line pulse generators of known type may be employed as the gate generator 32.

The emitter 54c is coupled to junction 55 to provide an input to low pass filter 37, which filter is of generally conventional nature and whose components include in series relation junction 55, resistor 57, junction 58, resistor 60, junction 61, resistor 63, resistor 64, junction 65, resistor 66, and a ground connection. Junction 55 is coupled to ground via resistor 56 and the respective junctions 58 and 61 are coupled to ground through filter capacitors 59 and 62. Junction 65 serves as an output terminal for the filter.

Junction 65 is coupled through coupling capacitor 67 to junction 68 of the inverter circuit 39 and thence to the base 69a of transistor 69, which may be of the 2N 4258 kind. The emitter 69b of transistor 69 is coupled through a series circuit including junctions 79 and 74, resistor 70, and junction 73, to a source (not shown) of positive potential applied at terminal 71 and connected to ground at its opposite end. Junctions 73 and 79 are respectively coupled to ground via capacitors 72 and 80, while junction 74 is connected through potentiometer 75 and resistor 78 to ground. Capacitors 72 and 80 serve as radio frequency by-pass and decoupling components in the conventional manner. The tap 76 of potentiometer 75 is connected through resistor 77 to junction 68. The collector 69c of transistor 69 is connected as an output of the inverter 39 through diode 85. The resistance network associated with potentiometer 75 serves to adjust the potential across resistor 87 which determines the steady state hold off bias on the detector 44.

Diode 85 is connected by line 40 to junction 86 through resistor 87 to ground via line 88 to the emitter 44c of gated detector transistor 44, which may be of the 2N5130 type. The collector 44b of transistor 44 is connected through junction 91 to the gate electrode 93a of field effect transistor 93, which latter may be of the 2N4274 type. The drain electrode 93b of transistor 93 is connected to a source (not shown) of positive potential applied at terminal 98 which may be of the order of +75 to +100 volts with respect to its grounded terminal. The source electrode 93c of transistor 93 is coupled via resistor 92 to junction 91 and via coupling condenser 95 to output leads 45 across output load resistor 96; lead 45 is normally connected to integrator 42 of FIG. 3.

Base-band or sub-nanosecond signals to be gated are applied by line 43 to the base 44a of detector transistor 44. Such base band signals may be found across a matching load resistor 90 attached across the nondispersive TEM mode transmission line forming a continuous two-wire line comprising the constant impedance or uniformly spaced parallel conductors 100, 100a of receiver antenna 12.

The receiver antenna 12 and its associated transmission line system may take the form shown in FIG. 5, where antenna 12 comprises a structure having mirror image symmetry about a median plane at right angles to the direction of the vector of the electric field propagating into the antenna. The general structure may also be used in transmitter antenna 11. Symmetry may also preside in the cooperating transmission line 43 which comprises parallel wire transmission line conductors 100 and 100a; conductors 100 and 100a are spaced wire conductors of a material capable of conducting high frequency currents with substantially no ohmic loss. Furthermore, conductors 100 and 100a are so arranged as to support TEM mode propagation of high frequency energy, with the major portion of the electric field lying between conductors 100 and 100a.

The TEM receiver antenna 12 further consists of a pair of flared, flat, electrically conducting planar members 110 and 110a. Members 110 and 110a are, for example, generally triangular in shape, member 110 being bounded by flared edges 112 and 113 and a frontal aperture edge 114. Similarly, member 110a is bounded by flaring edges 112a and 113a and a frontal aperture edge 114a. Each of triangular members 110 and 110a is slightly truncated at its apex, the truncations 119 and 119a being so arranged that conductor 100 is smoothly joined without overlap at truncation 119 to antenna member 110. Likewise, conductor 100a is smoothly joined without overlap at truncation 119a to antenna member 110a. It is to be understood that the respective junctions at truncations 119 and 119a are formed using available techniques for minimizing impedance discontinuities.

It is also to be understood that the flared members 110 and 110a of antenna 12 are constructed of material highly conductive for high frequency currents. It is further apparent that the interior volume of antenna 12 may be filled with an air foamed dielectric material exhibiting low dielectric loss in the presence of high frequency fields, such material acting to support conductor 110 in fixed relation to conductor 110a. Alternatively, the conductive elements of antenna 12 may be fixed in spaced relation by dielectric spacers (not shown) which cooperate in forming enclosing walls for the configuration, thereby protecting the interior conducting surfaces of antenna 12 from the effects of precipitation and corrosion.

The planar collector elements 110 and 110a of receiver antenna 12 are coupled in impedance matched relation to the two wire transmission line 43. Transmission line 43 has the same impedance as the transmission line comprising antenna elements elements 110 and 110a. Transmission line 43 may have its parallel wire conductors 100 and 100a molded into a dielectric enclosing element 121 for accurately determining the separation of conductors 100 and 100a so that transmission line 43 has a constant impedance along its length. Dielectric element 121 may be surrounded by a braided or other conductive shield 122 which may be grounded at any convenient location. Shield 122 may, in turn, be surrounded by a protective plastic cover element 124 of the well known type. Transmission line 43 is readily coupled to the base of field effect transistor 93, as seen in FIG. 4. Generally, the length of transmission line 43 between antenna 12 and active element 93 will be short. For example, if the rise time of the propagating signal is .tau. seconds, then the length D in question should be in the order of 10D/c, where c is the propagation velocity.

A cooperating antenna 12 and transmission line 43 system of the form shown in FIGS. 4 and 5 is a preferred antenna system, in part because desired TEM mode propagation therein is readily established. The TEM propagation mode is preferred since it is the substantially nondispersive propagation mode and its use therefore minimizes distortion of the propagating subnanosecond pulse signal to be received by antenna 12. By maintaining a continuously constant characteristic impedance and TEM propagation along the structure including antenna 12 and line 43, frequency sensitive reflections are prevented therein and frequency dispersion is eliminated. A received sub-nanosecond impulse therefore flows through antenna 12 into transmission line 43 without substantial reflection and without substantial degradation of its shape or amplitude. Since the full energy of a low-level sub-nanosecond base-band pulse is thus delivered to the gated receiver detector 44 by the antenna-transmission line system, it is seen that the receiver detector 44 can be sensitive to extremely short duration low-level base-band pulses having an extremely wide spectral content, any component of which would be incapable of detection using conventional wide pulse reception techniques.

With reference again to FIG. 4, operation of the novel wave amplifying zone selector circuit will be understood from the foregoing. It is seen that range gate generator 32 relies for its operation upon characteristics inherent in the 2N5130 avalanche transistor 54 and in the open circuited delay line 33 of length L. In response to the positive triggering signal 31B, transistor 54 breaks into conduction and a voltage step wave is propagated into delay line 33. When this step wave reaches the open end of line 33, it is inverted there upon reflection and returns to collector 54b, whereupon the current flow in transistor 54 is brought to zero and the transistor reverts to its nonconducting condition. Thus, the voltage wave 36C across filter resistor 56 is a sharply rising and terminating positive pulse of duration 2L/c seconds, a duration dictated by delay line 33 (c is the velocity of propagation of the step wave in delay line 33).

In the quiescent state of the circuit of FIG. 4, transistor 69 in inverter circuit 39 is normally fully conducting, causing a current of about 30 milliamperes to flow through the emitter resistor 87 associated with detector transistor 44 (resistor 87 may have a resistance value of about 100 ohms). The voltage consequently appearing across resistor 87 will be about +3 volts and assures that detector transistor 44 is in its nonconducting state. The field effect transistor 93 acts as a constant current source, assuring that a constant current is fed via the collector 44b and emitter 44c of detector transistor 44 in its quiescent state so that its bias state is precisely controlled. Resistor 92 in the collector circuit of detector transistor 44 has a positive thermal coefficient and serves to afford temperature compensation for the thermal characteristic of the conduction threshold of detector transistor 44.

When wave 31B triggers range gate generator 32, the positive output wave 36C produced by range gate generator 32 is, as previously explained, fed through low pass filter 37 to inverter 39. In traversing filter 37, wave 36C is acted upon so that the positive wave 38D results, having rounded rise and fall portions. Accordingly, any high level transients near the start or the end of wave 36C are removed, a desirable result since they might otherwise undesirably trigger detector transistor 44 into conduction.

The positive wave 38D, when coupled by capacitor 67 to inverter circuit 39 and thus to the base 69a of transistor 69, causes current conduction through transistor 69 to stop, forcing the voltage across resistor 87 rapidly to fall to zero. This event places detector transistor 44 in its fully sensitive state with respect to any signal to be sampled that is collected by antenna 12, for example, and thereby arriving on line 43 at the base 44a of transistor 44. Any signal sampled by detector transistor 44 appears as a negative amplified and time extended wave 45F on the collector 44b of detector transistor 44 and is supplied by coupling condenser 95 across load resistor 96, for example. It may then be supplied to the aforementioned utilization apparatus via terminal 45 in the customary manner, since wave 45F is amplified and time extended with respect to the received short pulse or echo signal. Upon termination of the gating pulse 38D, the circuit returns to its above described quiescent state, awaiting receipt of the next succeeding triggering wave 31B.

It will be noted that transmission of short-duration pulses from their source generator 23 is also through a transmission line system 22 or other medium that preferably operates substantially solely in the TEM mode, and that propagation modes that permit dispersion of pulses such as subnanosecond or base band pulses are not used. Thus, the full energy of received echo or other base band pulses originally generated by transmitter 23 is effectively directed to processing within the amplifying zone-gated receiver-detector 44.

The novel alarm and protection system, while useful in protecting vehicles and their contents, may advantageously be employed in a wide variety of other applications, including that of FIG. 6. FIG. 6 shows transmitter and receiver antennas 211 and 212 respectively corresponding to the base band object detection antennas 11 and 12 of FIGS. 2, 3, and 4 located on a wall of region 225 in a pharmacy in which drugs or other objects prone to theft are stored in storage locations 221 and 222. While a pharmacist will often be present in the area 225 while compounding prescriptions, he must often serve customers in front parts of the store. An intruder may then find ready access to area 225 by aisle 223 or by exterior door 224. With the base band object detection system of FIGS. 2, 3, and 4 in operation to define a protected zone extending generally upward from the dot-dash line 230, any such intrusion will be proclaimed by alarm 214. On the other hand, the pharmacist and his employees may pass freely through aisle 223 or door 224 when wearing an operating key signal transmitter, such as transmitter 219a of FIG. 2 when its energy is being received by an antenna 220 corresponding to antenna 20 or 20a of FIG. 2. In this manner, particularly sensitive regions in a store or a bank may be constantly monitored, while customers are permitted to move freely about in other parts of the building without fear of setting off alarms.

While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than of limitation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspect.

Claims

I claims:

1. A selective intrusion alarm system comprising:

radiation receiver means for detecting signals reflected by intruding objects,

first alarm means,

receiver gating means for passing said detected signals to operate said first alarm means only for signals from objects within a selectable zone proximate said receiver means, and

radiation transmitter means for inhibiting operation of said first alarm means,

said radiation transmitter means being transportable by authorized personnel for permitting said personnel to enter said selectable zone without operating said first alarm means.

2. Apparatus as described in claim 1 additionally comprising second alarm means always responsive to said detected signals passed by said receiver gating means.

3. Apparatus as described in claim 1 wherein said receiver gating means comprises means for selectively passing said detected signals from at least a first or a second of said selectable zones.

4. Apparatus as described in claim 1 wherein said intrusion alarm system additionally comprises:

pulse synchronizer means, and

base-band pulse transmitter means responsive to said synchronizer means,

said receiver gating means being responsive to said synchronizer means for gating the output of said radiation receiver.

5. Apparatus as described in claim 4 comprising in series connection:

alarm driver circuit means responsive to said radiation receiver,

inhibit circuit means, and

said first alarm means.

6. Apparatus as described in claim 5 wherein:

said inhibit circuit means is responsive to low frequency receiver means, and

said low frequency receiver means is responsive to said radiation transmitter means for inhibiting said first alarm means.

7. Apparatus as described in claim 6 wherein said radiation transmitter means comprises low frequency transmitter means cooperative with said low frequency receiver means.

8. Apparatus as described in claim 3 wherein:

said radiation receiver means is mounted at one end of a vehicle,

said intrusion alarm system for a first setting of said receiver gating means is adapted to detect signals from objects toward which said end of said vehicle may be moved in a first zone, and

said intrusion alarm system for a second setting of said receiver gating means is adapted to detect only objects in a second zone closer to said end of said vehicle than said first objects.