Abstract: A microwave transceiver comprising a Gunn diode microwave radiation source, a Schottky-barrier diode mixer, a closed rectangular waveguide cavity, a coaxial-waveguide joint, and a monopole antenna providing a microwave radiation dispersion/collection pattern occupying a spatial region elevated from and extending radially outward and 360.degree. in azimuth with respect to said antenna, said spatial region having a substantially spatulate radial cross-section.

Abstract: A continuous wave (CW) leaky cable sensor for an intrusion detector uses a pair of elongated parallel cables, one for establishing an RF field and the other for receiving the field. A modulator is connected at an intermediate location of the receiving (or transmitting) cable, subdividing a detection zone into detection regions on each side of the modulator. The modulator selectively modifies a signal received by a portion of the receiving cable connected to its input. By processing the signal at the output of the receiving cable, the detection region for either side of the modulator in which an intrusion has occurred can be determined. The resolution of a CW sensor is thus increased at low cost. Multiple modulators can be used at spaced locations increasing the number of detection regions, and thus the resolution.

Abstract: A microwave horn having a narrow radiation beam characteristic is energized by a Gunn oscillator coupled to a driver/detector that turns the Gunn oscillator on for a small fraction of a period to produce a sequence of detected pulses representative of an item being sensed that is amplified by an A.C. amplifier and filtered with gain controlled by a sensitivity potentiometer that energizes a threshold detector to produce threshold pulses that trigger hold-off circuitry to produce an output pulse when the threshold pulse is outside the duration of the output pulse. A hold-off potentiometer controls the duration of each output pulse. An output driver amplifies the output pulse and delivers it to an output terminal.

Abstract: A method of scanning intrusion detection which is capable of monitoring a large volume of either interior or exterior space from a single relatively inexpensive unit. This method of intrusion detection comprises the steps of scanning a beam of infrared radiation about a field of view and receiving the radiation of the beam reflected from the field of view, generating a signal indicative of the distance from the device at which the beam has been reflected for each of a plurality of azimuthal sectors of the field of view, storing a plurality of reference signals which are indicative of the distance of reflection of the beam from each azimuthal sector of the field of view during a reference time period, comparing the signals from a selected time period with the reference signals, and generating an output signal if one of the signals is different from the respective reference signal.

Abstract: An electronic communication system monitor in one embodiment has sensors for sensing electrical and environmental conditions for communication system elements. Control relays connect the elements to a programmable controller. The programmable controller has a keyboard, display, printer and modem for remote communication of information; especially alarms providing notice of operating conditions outside acceptable parameters. A second embodiment provides a logic circuit connected to selected sensors which determines when a combination of conditions exist to cause alarm and automatically activates a remedial device to alleviate a potential problem.

Abstract: A bistatic Doppler radar intrusion detection system which utilizes a high duty-cycle phase-coded pulse compression signal to provide a range-gated detection zone is described. The combined transmit and receive antenna beam patterns define the shape of the detection zone in azimuth and elevation. The distance from the antennas to the detection zone and depth of the detection zone are determined by the pulse compression code sequence selections. The system consists of a transmitter having a code sequence generator for generating one of a selected number of codes and a code sequence. An antenna transmits the modulated signal through the detection zone. A programmable digital delay circuit is used to provide a delayed replica of the code sequence and thereby specify the detection zone range. A receiver is provided having an antenna for receiving signals scattered from the detection zone.

Abstract: A method of intrusion detection which comprises the steps of: projecting a beam of infrared radiation toward a field of view from a detection device and receiving the radiation of the beam reflected from the field of view, determining the distance from the device at which the beam is reflected by the field of view, generating a signal indicative of the distance from the device at which the beam has been reflected by the field of view during a selected time period, storing a reference signal which is indicative of the distance of reflection of the beam from the field of view during a reference time period, comparing a signal from a selected time period with the reference signal, and generating an output signal if the signal differs from the reference signal in a preselected manner.

Abstract: The present invention relates to a railing selected among hollow railings, grilles, tubes or the like in which a continuous sensor made of an optical fiber extends, which railing is filled with a suitable filling material, the sensor fiber being connected to outside control and/or alarm units to form a complete circuit. The optical fiber has advantageously a numerical aperture (NA) of 0.08.ltoreq.NA.ltoreq.0.5 preferably the following parameters:Bending loss .alpha.exp(-.beta.r), r-Bending radius .beta..alpha.N.A. (b/a) .gamma. b-cladding radius, a-core radius of the fiber, 0.1.ltoreq..gamma..ltoreq.3.0.The filling material has preferably the form of pellets and is suitably selected among glass, steel, ceramics or sand.The railing preferably comprises a binder selected among soft rubber and plastics, e.g. a polyester. The binder may comprise also a material selected among an irritating material and a dye.The railing may also be a system of railing wherein the sensor extends through all the railings.

Abstract: The circuit operates a main switch SCR to pass an a.c. supply V through a load L when a metallic object is in proximity to coil L1. Coil L1 is driven by an oscillator comprising transistor T2 and associated components. SCR is gated on simultaneous occurrence of an output from level detector T3 and a signal from T5, these being ANDed by T4. The timing of the signal from T5 in the supply cycle is governed by the RC time constant of C4,Z1,R14 and is such that the power supplied to the oscillator and level detector is the minimum necessary, for a wide range of supply voltage and load current.

Abstract: A movable door incorporates a proximity detection system for sensing obstructions in the path of the door. The system comprises an array of transmitter/receiver pairs (21, 22 and 23), each pair comprising a transmitter (1) for transmitting an energy pulse, a receiver (2) located adjacent the transmitter and directed to receive any reflected transmitter pulses, and a shielding (3) for shielding the receiver (2) from receiving energy pulses directly from the transmitter (1). At least two transmitter/receiver pairs (21, 23) are mounted on a leading edge or face of the door (12) to transmit energy pulses in a direction which is not at right angles to the leading surface or edge of the door, to monitor for obstructions in the path of the door (12). Another two transmitter/receiver pairs (26 and 28) are mounted on the leading edge or surface of the door (12) to transmit energy pulses at right angles to the surface or edge of the door (12).

Abstract: A monitoring device comprises a transmitter and a receiver for transmitting and receiving, respectively, a signal in a medium to be monitored. During a cycle of m sampling periods n sampling values are taken of the signal received during each sampling period. After completion of each cycle a series of n means sampling values is determined by determining a mean value from the m sampling values taken at corresponding instants in the sampling periods. This series forms an actual pattern word which is sub-divided into sub-pattern words, each sub-pattern word being combined with a corresponding sub-pattern word from a reference pattern word in order to form a combination word. The combination word is translated into a code number which is selected from a predetermined series of code numbers. The code number assigned characterizes the time shift of the actual sub-pattern word with respect to the reference pattern word.

Abstract: An intrusion detection system of the type using cables arranged along a perimeter to be protected and sensing changes in the electrical field around the cables caused by the presence of an intruder. The cables are divided into sections or blocks and typically only one of the sections is energized at any time. The variations caused by an intruder at the selected sections are transmitted through the intervening section to the receiver portion of a transceiver located at one end of the cables. This indicates in which section intrusion has occurred. The system uses continuous wave excitation whereby the expense and complexity of high speed switching and timing of r.f. signals are avoided.

Abstract: A dual sensing intrusion detection system includes a passive infrared radiation detection sensor that generates a first output signal in response to the detection of an intruder in the volume of space. A second detection sensor is directed to the same volume of space and generates a second output signal in response to detection of the intruder. A switch activates the second detection sensor in response to the detection of the intruder by the infrared radiation detector. Logic circuit receives the first and second output signals and produces an alarm signal in response thereto to indicate the detection of the presence of the intruder in the volume of space.

Abstract: An intrusion detection system using waves guided by the conductive outer sheath of a coaxial cable. An r.f. signal from a transmitter is supplied between the inner conductor and outer conductor or shield in conventional fashion. Coupled wave devices are spaced along the cable; each coupled wave device transferring r.f. energy between a transmission mode within the cable and a guided mode propagated along the exterior of the conductive outer shield. In one embodiment a second coaxial cable similarly provided with coupled wave devices is spaced from the transmitter cable and has one end connected to a receiver. The change in r.f. coupling between the cables caused by an intruder produces variations in the r.f. energy coupled to the second cable which is detected at the receiver. Alternative embodiments include the use of a single cable with coupled wave devices adapted for both transmission and reception.

Abstract: This invention relates to a method and a system for selectively filling or emptying a tank by a filling or discharging device for the tank. In accordance with the invention, a passive responder and an aerial element of an electromagnetic detection system are respectively provided in the vicinity of the inlet or outlet of the tank and on the part of the filling or discharging device cooperating with the tank inlet or outlet. The aerial element forms part of a detection system for generating an electromagnetic interrogation field, which detection system generates a signal when detecting a responder. In one embodiment of the invention, the responder is a passive responder generating a uniquely coded signal by which the detection system identifies the responder.

Abstract: The invention provides an apparatus and method to confirm or deny the presence of an object within the scan area of an ultrasonic sensing system by requiring that any object be identified on at least two consecutive scans of the sensing system and using scan energy patterns which occur at irregular repetition times to selectively reject spurious direct or reflected signals present in the area. A generator of predetermined pattern burst of ultrasonic energy is arranged to produce one such burst of ultrasonic energy and a timing signal is coupled thereto so as to apply timing signals to cause production of the bursts. The timing signal source produces timing signals only according to a varying pattern whereby separation between adjacent signals meet certain present parameters. The energy patterns are applied to a transmitter which applies them to a scan area whereby the echo or returned energy from an object in the scan area is received and stored.

Abstract: The invention relates to a type of proximity sensor. In one form, two sinusoidal signals travel along two transmission lines near an object. When the distance between one or both of the lines and the object changes, the speed of travel of one or both of the signals changes. There is a correlation between the speed change and the distance, thus allowing one to infer distance from speed change. One way to measure the speed change is to measure the phase relationship between the two signals.

Abstract: A remote controlled object counter has a data counter which may e.g. be buried beneath a trail for counting the passage of people along the trail and a data collector separate from the data counter. The data counter employs an inductive loop with an oscillator and a processor for detecting the oscillator frequency and transmitting a corresponding data signal through radio transceivers in the data counter and the data collector for processing in the latter. To enable adjustment of the data counter sensitivity without physical contact, an adjustment signal generated in the data collector is sent through the transceivers to the data collector processor.

Abstract: An electric power monitor system for radio communication (trunking and paging repeater base station) systems includes a plurality of input switches, power sensors, a power monitor having a display, keyboard, and microprocessor, a printer port, and communications port. The input switches facilitate alarm inputs, for example, unauthorized entry, flooding, fire, and temperature of the antenna site. The power sensors include bidirectional and unidirectional sensors connected to a communication system having tunable components for producing analog signals indicative of system component performance. The power monitor measures, determines, and reports locally and/or remotely the power output and standing wave ratio of transmitters and antennas, insertion loss of combiners and power to and SWR of antennas and alarms.

Abstract: The present invention utilizes a latching relay having on and off inputs for controlling power to a load. The on and off inputs of the relay are controlled by pulse generators which are in turn controlled by an occupancy sensor circuit. These pulse generators provide pulses when movement is initially detected in a room and when no movement has been detected for a predetermined time, respectively. The outputs of the pulse generators are coupled to an isolation circuit which isolates the power supply for the occupancy sensor from the pulses during each pulse to prevent damage to the power supply.

Abstract: A proximity switch has at least three Hall effect magnetic field sensitive transducers mounted in a transducer housing and has at least two magnets mounted in a magnet housing. The transucers produce logic signals, and a logic circuit responsive to these logic signals activates an electrical switch only when the magnets are in a proper position relative to the transducers. The electrical switch may be used to control a machine, and the machine is not activated until the proximity switch is activated.

Abstract: A novel missed-hit detection and player proximity detection system for an electronic, automatically scored projectile target game apparatus wherein the scoring system thereof is automatically deactivated whenever a player comes within a proscribed distance range from the game and wherein the impact of a projectile which misses the target, yet strikes the front of the game registers as a non-scoring "missed-hit" and is deducted from the number of remaining throws in that round.

Abstract: In this system for the securing of rooms, one or more transmitting elements emit suitably modulated light pulses into given regions or sectors which are produced by specially shaped optical components and/or by a special alignment of the transmitting elements and/or by the application of absorbent layers to an infrared filter which is located in front of the transmitting elements and receiving elements. One or more receiving elements receive the incident radiation. Changes in the radiation field with respect to intensity and/or speed of change of the intensity and/or phase are processed in subsequent evaluation logic circuitry. The room security system can also be combined into a larger unit with common evaluation logic.

Abstract: An apparatus for detecting intruders comprising a housing provided with at least one window, a passive infrared detector (for detecting the radiation emitted by an intruder around a wavelength .lambda..sub.1), and an anti-obscuring device (detecting by infrared radiation having a wavelength .lambda..sub.2 the presence of an obscuring of the appparatus for detecting intruders). The apparatus further includes an electronic circuit intended to operate an alarm when the presence of an intruder or an obscuring element has been detected. The apparatus for detecting intruders has a detector for detecting an obscuring element arranged at small and at large distances. It further includes a self-verification circuit. An obscuring element is detected, inter alia, by a mirror arranged at the end of the zone to be supervised, which returns radiation .lambda..sub.2 emitted by an emitter to a detector, both being situated very close to the detector of radiation having a wavelength .lambda..sub.1.

Abstract: Disclosed herein is a dual technology intruder detection system which features a modular optical system by which the field of protection of one subsystem can be changed relative to the field of view of the other by merely altering the orientation of one optical module relative to another. Such modular optical system includes a multifaceted reflector having two sets of planar reflective facets. Depending on the orientation of such multifaceted reflector relative to a separate spherical reflector, two different fields of view are provided, whereby false alarm-producing can be avoided.

Abstract: A low-cost transmission line fault detector visually indicates the nature and location of transmission line failures. A current detection circuit inductively coupled to the transmission line produces an output signal when current is flowing through the line. A voltage detection circuit directly coupled across the transmission line monitors the relative potential across the line and produces an output signal when the relative potential is within predefined limits. By observing the states of the outputs of the current and voltage detection circuits (e.g., through visual indications provided by light emitting diodes), one can tell if the transmission line has shorted or open-circuited, and if it has, the location of the failure (e.g., between the source and the detector coupling point or between the detector coupling point and the load).

Abstract: A safety system for use with apparatus having a conductive member and where the apparatus changes the attitude and/or altitude of the conductive member during its operation includes at least one sensor which senses an electric field and provides a signal representative of the strength of the electric field. A safety device responsive to the signal from the sensor prevents the further changing of the conductive member's attitude and/or altitude when the electric field strength is greater than a predetermined safe value.

Abstract: A simulated target for use in conjunction with detection systems using coupled transmission lines. The simulated target is positioned in the vicinity of the transmission line and the response used both to monitor system operation and to calibrate system response levels. The simulated target can be of a type having its electrical length variable under system control or can be purely passive serving to alter the stationary response profile of the system.

Abstract: To accommodate for a change in the level of a received wave which occurs as a function of the distance between an object, the existence of which is to be detected, and a detector, the detector includes a receiver circuit which has an amplifier circuit having a gain which changes exponentially with an elapsed time since an ultrasonic wave has been transmitted. The amplifier circuit includes ramp signal generating means having an output level which changes in proportion to an elapsed time since a control signal assumes its on level, non-linear signal processing means for non-linearly processing a signal which is output from the ramp signal generating means, and a variable amplification amplifier having an amplification factor which depends on a signal level which is output from the non-linear signal processing means.

Abstract: A fail-safe sensor system to provide a positive indication of the proper operation of a sensor system by accounting for the return signals from an active zone and a monitor zone beyond. Search signals, provided by a transducer operating in the transmit mode, are transmitted within an active zone within which objects to be detected are expected to be located. Unreturned search signals are permitted to travel beyond the active zone into a monitor zone beyond and objects in such monitor zone cause return signals which are also employed. A counter feed from a free running clock source is permitted to run up a count whose predetermined limit represents the estimated maximum time period during which a sensor system should operate without contacting an object. Return signals, whether from objects in the active zone or the monitor zone, reset the counter back to zero.

Abstract: An intrusion detection system that comprises a transmission line for RF energy, a transmitter coupled to one end of the transmission line to supply pulses of RF energy into the line, and a range gated doppler receiver for receiving RF energy from the other end of the transmission line. The receiver includes means for producing a detection signal indicating the presence of doppler components in a range gated portion of the RF energy received from the transmission line.

Abstract: A dual-tech intruder detection system includes a pair of intruder-detecting subsystems, each functioning to detect intrusion by a technology different from the other, and apparatus for activating an alarm in response to both subsystems detecting intrusion within a predetermined time interval. A supervisory circuit serves to detect a malfunction in one of the subsystems. Default apparatus, responsive to the output of the supervisory circuit, causes the alarm activating apparatus to activate an alarm in response to the still functioning subsystem's detection of intrusion.

Abstract: An intrusion detector is configured to transmit signal(s) of a very short ration and then process the returned signal(s) reflected from various surfaces and objects within the propagation path in a room to be secured. The return signal(s) is a succession of peaks whose return times represent the location of respective objects. As configured, the intrusion detector apparatus includes, inter alia, a microprocessor that stores processed facsimiles of the return signal(s) for an unoccupied room and continually makes comparisons with subsequent returned signal(s). When the returned signal(s) changes, as a result of an unauthorized entry into the room, for example, a visual and/or audible alarm indication is produced.

Abstract: A coplanar antenna system has an interrogating means adapted to generate time varying magnetic field components within a region proximate thereto and a receiving means adapted to detect magnetic field components within a region proximate to the antenna system. The interrogating means has a plurality of coils configured such that the components thereof have near zero net resultant magnetic field. At least one coil of the receiving means is configured to detect components thereof having a preselected net resultant magnetic field.

Abstract: Apparatus and method for detecting changes in capacitance of an antenna due to intrusion into the antenna field by a body of different dielectric constant, in the presence of induced AC voltages on the antenna from electromagnetic radiation. The apparatus includes four operational amplifiers connected in a circuit for processing the production of voltage pulses. The first amplifier operates to rectify the AC voltages from the antenna to a DC fluctuating voltage, which voltages are averaged to produce a constant voltage above ground potential so long as there is no intrusion of the field. During this period no signal voltage is transmitted to the second amplifier.

Abstract: A sensor for use in a leaky cable intrusion system which is comprised of a pair of parallel buried coaxial cables, each of which has a velocity of propagation factor different from the other. This results in different sensitivity characteristics with length, resulting in a combined sensitivity which has markedly reduced amplitude variation over cables having similar velocities of propagation. It also makes feasible a system having ungraded cables in which the position of intrusion in the field set up by the sensor can be determined.

Abstract: An intrusion detector system with a transmitter having a clock operating at a selected frequency, a pseudorandom code sequence generator for generating one of a selected number of codes, a modulator for spread-spectrum modulating an electromagnetic signal with the generated pseudorandom code sequence signal, and an antenna for transmitting the modulated signal through a zone of protection; and a receiver having an antenna, a demodulator for detecting and demodulating the modulated signal to recover the transmitter pseudorandom code sequence signal, a clock operating at a frequency corresponding to the transmitter clock, a pseudorandom code sequence generator for generating one of a selected number of receiver code signals corresponding to the transmitter code signals, a detector for comparing the received code signal to the receiver code signal and generating a detection signal proportional to the signal amplitude of the received code signal when locked in phase with the receiver code signal, a phase detector

Abstract: A leaky coaxial cable for use as a sensor in a buried or other medium, which has a conductor located parallel to, and in proximity with the shield along the cable, but insulated therefrom. Preferably the two wire transmission line formed by the external conductor and the shield is terminated with an impedance matching the characteristic impedance of the transmission line (for minimum reflections). The external conductor can be oriented to cause the field in a given direction (e.g. above the surface of the burial medium) to be compensated for field variations caused by variations in the burial medium.

Abstract: A control circuit (16) mounted on a faucet (10) operates an ultrasonic transducer (20) that transmits ultrasound into a target region (22) below the outlet (24) of the faucet. The control circuit receives electrical signals from the transducer when the transducer receives echoes from objects within the target region. The control circuit times the echoes to determine the distance of the object causing the echoes, and, if the distance of the object changes between successive measurements, the control circuit operates a valve (12) to permit water to flow. The circuit is powered by a rechargeable battery (18) that is charged by the action of a generator (31) that is driven by a turbine (34) powered by the flow of water through the faucet.

Abstract: A method of producing a leakage graded coaxial cable having a braided shield, in which ends of the shield wire are dropped as braiding progresses. Gaps in the shield having progressively increasing size are produced through which the radio frequency field leakage occurs, which facilitates providing a constant radio frequency field around the cable. The cable is coated with a jell flooding agent prior to extrusion of its protection jacket, which fills the interstices and gaps of the shield, and solidifies to a waxy, semi-resilient consistency. The resulting cable can be coiled and flexed, maintaining its shield reliability, and rejects water and fluids in case pinholes in the jacket occur.

Abstract: An electronic article surveillance system and a marker for use therein, which marker comprises a tuned resonant circuit including inductive and capacitive components formed of a laminate of a dielectric and conductive-multi-turn spirals on opposing surfaces of the dielectric, wherein the capacitive component is formed as a result of distributed capacitance between the opposed spirals, and wherein the circuit thereby resonates at at least two predetermined frequencies which are subsequently received to create an output signal.

Abstract: A ported coaxial cable is positioned atop or wound around a coil of barbed wire and is coextensive therewith. Radio frequency energy applied to the cable is randomly dispersed about the exterior of the coil. Variations in the reflected energy caused by intrusion of the area adjacent the coil, including the areas above or beneath the coil, are processed by detection system signal processing equipment to determine the incidence and location of intrusion events.

Abstract: A glass fiber, strand or a product made of glass fibers is made to pass through a chamber irradiated with high-frequency radiation, so that, if the electrically conductive matters are contained, a high-frequency electric discharge occurs in the chamber accompanied by generation of light. The presence of the electrically conductive matters is made known by detecting this light by a photosensor.

Abstract: A ported coaxial cable is positioned within a coil of barbed wire and is coextensive therewith. Radio frequency energy applied to the cable is confined within the tubular volume of the coil. Variations in the reflected energy caused by physical disturbances of the coil are processed by detection system signal processing equipment to determine the incidence and location of intrusion events.

Abstract: A motion detector system includes an ultrasonic transmitter, receiver and phase detector which produces as output signals of frequency related to the speed of movement of any moving object in a zone being monitored. The output of the phase detector is fed to two separate bandpass filters having different pass-bands characteristic of the frequencies generated by a human being moving in the zone being monitored. A warning is sounded only when signals have been present in both bands for more than a predetermined period.

Abstract: This invention is an intrusion detector which uses codirectionally coupled CW leakly cable sensor techniques. Successive sensors are connected serially through R.F. decouplers, and each is polled and is sent power from a control unit via the serial cables, through the decouplers. The detector thus provides both intrusion detection and a secure data link.

Abstract: Detection apparatus comprising a plurality of elongate transmitting conductors each coupled to a source of electromagnetic energy and arranged for producing an electromagnetic field along the length thereof, at least one receiving conductor arranged alongside the plurality of transmitting conductors for sensing the electromagnetic field produced thereby, and alarm indicating circuitry coupled to the at least one receiving conductor for providing an alarm indication in response to a sensed disturbance of the electromagnetic field, the source of electromagnetic energy for each transmitting conductor comprising an AC voltage source of selected amplitude and phase, the selection of amplitude and phase determining the configuration of the overall electromagnetic field produced by the transmitting conductors, there being a non-zero phase relationship between the voltage sources of at least two of the transmitting conductors.

Abstract: A control system for an illumination lamp installed in a building equipped with a door operation control apparatus. The system comprises a door opening and closing instructor, a control circuit responsive to an instruction signal from the instructor to produce a door control signal and a lamp control signal, a door driver responsive to the door control signal to drive the opening and closing of a door, an illumination lamp for illumination of the interior of the building, a detector for detecting the movement of a human body and producing an electric signal representative of the movement, and a circuit which is responsive to the lamp control signal and the electric signal to turn on the illumination lamp and keeps the illumination lamp turned on for a predetermined time extending from the termination of the lamp control signal and the electric signal.

Abstract: A surveillance apparatus against theft or intrusion includes a box containing a detector reacting to an abnormal presence and operable to selectively activate a camera, a flash and/or an acoustic warning device. The open rear of the box is hingedly attached to a support plate affixed to a wall while flexible wires from the box components include a connector part removably attachable to a mating connector part on the support plate, the latter being supplied with a source of power. A backup source of power may be included in the box as well as cooperating lock components on the box and support plate.

Abstract: A continuous loop transmitting winding assembly is disposed in one panel and a vertically oriented figure eight receiving coil winding assembly is disposed in a second panel, the two panels being mounted on pivots for swinging gate type movement straddling the pathway to be controlled by a magnetic surveillance system. This arrangement confines the surveillance field to the pathway and tends to follow the passage of an object bearing a marker therealong. No marker detecting backfield is generated. Alternatively, one winding assembly is moveable while the other is stationary thereby similarly confining the field.Special winding configurations are used to reduce interference from third harmonic signals produced in adjacent metallic counters. The arrangement also imposes immunity to signals emanating from cash registers, motors and the like.Door and single gate mounted magnetic and microwave surveillance systems are also disclosed.