Abstract:
An animal control system and method generates an on-off correction stimulus using a small transformer and rectifies the high voltage output to give an unfiltered unipolar voltage applied to electrodes as a correction stimulus. A small, efficient power isolation system is disclosed. An omnidirectional antenna is disclosed that reduces authentication time.

Description:
FIELD OF THE INVENTION 
     This invention relates to a system for controlling the behavior of an animal. 
     BACKGROUND OF THE INVENTION 
     Animal control systems are useful to control an animal&#39;s behavior. Examples include a) containment or exclusion systems to contain an animal within a region such as a yard or to exclude an animal from a region such as a room, food table, sofa, bed or chair, b) training systems to modify the animal&#39;s behavior, and c) bark inhibitor systems for dogs. The systems are usually attached to a collar for the animal. A behavior modification signal or correction stimulus is typically an audio sound. A stronger correction stimulus may be an electric shock. While such control systems have been miniaturized as technology has improved, they are still too large and too heavy for a small animal such as a lap dog or small cat. There remains a need for control system circuits that minimize the number of parts and that perform functions in a different mode to permit a reduction in weight and volume. 
     Animal control has long been accomplished by application of electrical shock. The means for generation of the electrical shock fall into 4 modes. 
     1 induction coil, 
     2 alternating current, typically with a step-up transformer, 
     3 direct current, typically rectified from a step-up transformer and filtered, 
     4 pulse, typically by capacitor discharge into a pulse transformer 
     All of these approaches are difficult to miniaturize and to effect with limited battery power while maintaining an effective stimulus. The induction coil mode stores energy in the coil&#39;s magnetic field, and the energy is delivered by the collapsing inductive field. As the coil is made smaller, less energy can be stored and the shock becomes insufficient to control the animal. 
     The alternating current mode is limited to low frequencies because the physiological response diminishes with increasing frequency. For a given frequency, as the step-up transformer is made smaller, it is not possible to maintain the necessary primary inductance to keep the transformer core from saturating, which leads to excessive current consumption and diminished output. 
     The direct current mode stores the energy on a high voltage filter capacitor. Typically the high voltage to be stored on the capacitor is generated by circuitry similar to the alternating current mode just described, except the high voltage is rectified either by half wave, full wave, or voltage multiplier rectification. D.C.-to-D.C. step-up circuits are known for photomultiplier and photoflash circuits. In modem photoflash applications a high frequency is used with a small transformer. A rectifier is used to supply charge to a storage capacitor. If the circuit were to be used in an animal control system, the high voltage storage capacitor would be large and is difficult to miniaturize. The charge may also remain on the capacitor and shock the animal at an inappropriate or unexpected time. Typically a high current is taken from an AA size cell, which is comparatively large and heavy for an animal control system. Further such circuits are not designed for microprocessor control. 
     The pulse mode uses a pulse transformer with excellent high frequency capability; however, the primary to secondary turns or voltage ratio is limited to low ratios in order to maintain a high self-resonant frequency. As consequence, to achieve high voltages, a high voltage must be supplied to the primary. The high voltage may be supplied by circuitry similar to the direct current mode just described and with the inadequacies just described. 
     Thus there remains a need for a shock system that can be controlled by a microprocessor, is small and is operable by the limited voltage and current capabilities of a small battery. 
     Battery powered apparatus that combines low power microprocessors with high power circuits, such as electronic shockers, require power management because the microprocessor may malfunction if the power supply fluctuates excessively. It is well known that the maximum power is obtained from a source when the load is matched. This applies to batteries as well. In a simple matched system the load resistance would be equal to the source resistance, i.e. the internal battery resistance (more correctly when the source and load are conjugate impedances, but considering only the resistance is sufficient in this application). In such a system the terminal voltage of the battery drops to half the open circuit value. Thus a 3-volt battery will drop to 1.5 volts under a matched load. Microprocessors usually fail when the supply voltage drops by half Also the internal resistance of a battery increases near the end of battery life. Thus, even a load resistance that is higher than the normal battery internal resistance will become significant as the battery nears end of life. To keep the microprocessor from malfunctioning, it is well known to isolate the microprocessor and other circuits from temporary drops in supply voltage by using a diode feeding a capacitor, the latter maintains the voltage supplied to the microprocessor. The typical voltage drop across a silicon diode is 0.7 volts. Even Schottky diodes have 0.3 volts drop or more. This is too much of a voltage drop in a low voltage battery system. For example a fresh 3-volt lithium battery may supply only 2.3-2.7 volts through a diode isolation circuit. This may be insufficient voltage to reliably operate the microprocessor. Batteries also drop in voltage near the end of battery life, and it is desirable to get maximal life from batteries by maintaining operation even at low battery voltages. Voltage losses in load isolation systems thus reduce the amount of useful battery life. While low drop out voltage regulators are available, they do not tolerate an input voltage lower than the load voltage or consume too much power, compromising battery life. The usual protection scheme for the regulator is to use a reverse coupled diode. The strategy is to drag the load voltage down as the supply voltage drops. This protects the regulator but fails to provide the needed isolation. 
     Thus there remains a need for a power management system that provides isolation from transient supply voltage drops, has a minimal voltage drop and is efficient so as to maintain long battery life. 
     Animal control systems that are containment or exclusion systems use an electro-magnetic radiated signal from a boundary wire and have a receiving antenna in the form of an unshielded inductor. Such inductors have a solenoidal reception field. Animals can learn to avoid the boundary signal by orienting themselves and, hence, the receiver to the blind spot of the solenoidal field. Simply adding another inductor physically oriented different to the first and paralleling the electrical circuits results in a new reception field that is the vector sum of the two inductors, i.e. another solenoidal field. One solution is to use two or three orthogonal inductors that are activated or switched on singly or in pairs by a controller or microprocessor, as taught in U.S. Pat. No. 5,425,330 and U.S. Pat. No. 5,435,271 to Touchton et al. The inductor or inductor pair having the strongest signal is then selected for further signal processing. This selection process takes time. This lost time diminishes the deterrent effect for those animals that attempt to run through the boundary. Another solution is to sequentially sample or switch on each antenna for a period of time, as taught in U.S. Pat. Nos. 5,460,124, 5,682,839, and 6,269,776 to Grimsley et al. The switching reduces the time the signal can be received, assuming not all three antennas are receiving sufficiently strong signal. This reduces the ability to authenticate a weak signal because some of the antennas, i.e. part of the time, offer insufficient signal to process. The switching also introduces a 0.1 second latency in detecting the boundary signal as it switches through antennas that are not receiving the boundary signal. 
     Thus there remains a need for an antenna system that is omnidirectional and does not incur the lost time required for selection of the strongest signal or is reduced in ability to authenticate a weak signal or has a detection latency. 
     SUMMARY OF THE INVENTION 
     The animal control method and system of this invention, overcomes many of the deficiencies of the conventional animal control systems of the prior art. A particular object of this invention is achieved by innovative circuitry, which reduces the weight and volume of the control system and uses a small amount of power so that it may be applied to smaller animals such as lap dogs and cats. 
     This invention provides an animal control system comprising a system to receive a control signal such an electromagnetic boundary signal, a audio bark signal, an ultrasonic perimeter signal, a magnetic boundary signal, or a radio signal and to generate a correction stimulus comprising a shock system. The shock system is typically coupled to a microprocessor that controls the shock system. Other controllers may be used. A simple and efficient shock system comprises a correction signal generator coupled to a step-up transformer, the output of which is coupled to a rectifier. The output of the rectifier is coupled to electrodes that contact the animal to provide the correction stimulus. The correction signal generator is typically under a controller or microprocessor control. 
     It was found that alternating current frequencies above about 5 kilohertz were in-sensible and hence ineffective, at least on a human. It is believed that other animals have similar frequency limits. Thus, in order to use a high frequency to permit a small transformer, it is necessary to rectify the high voltage output. But this conventionally requires a high voltage filter capacitor, which is physically large. Surprisingly, it was found that the rectified high frequency voltage without a filter capacitor, i.e. an unfiltered unipolar high voltage, was as effective a correction stimulus as a capacitor filtered voltage. Elimination of a high voltage filter capacitor reduces the weight and physical volume of the shock system. Rectifying means to change a bipolar voltage, a voltage having both positive and negative excursions, into a unipolar voltage, a voltage having only positive or only negative excursions. 
     It is well known that smaller transformers may be used, for the same power transferred, by raising the frequency of operation. Furthermore, a large step-up ratio is required in order to raise a low battery voltage, such as 3 volts, to a voltage suitable for a correction stimulus, such as 330 volts. This requires a transformer turns ratio of 1 to 110. For example if the primary winding requires 12 turns to achieve a sufficient inductance to keep the transformer from saturating at the operating frequency, the secondary winding must be 1320 turns. Such a large number of turns will exhibit a large inductance, proportionate to the square of the number of turns. The otherwise small interwinding capacitance resonates with the large inductance to give a relatively low self-resonant frequency. Attempts to operate the transformer above the self-resonant frequency results in high losses because the secondary of the transformer acts as a low-pass filter. However, operation at the resonant frequency provides good low loss performance. For a small “E” core ferrite transformer, less than a centimeter on a side, with 1320 turns for the secondary, the resonant frequency measured 19 kilohertz. For a small “pot” core ferrite transformer, approximately 7 millimeters in diameter, with 440 turns for the secondary, the resonant frequency measured 80 kilohertz. For the purposes of this invention, a high frequency is any frequency greater than that where the physiological effect of the alternating current begins to decline, i.e. above about 5 kilohertz. 
     In order to effectively use the limited battery power available, it is useful to maximize the physiological effect. This can be accomplished by allowing the animal&#39;s nerve endings to repolarize. Once the nerve has depolarized in response to the correction stimulus, further application of the stimulus is a waste of power. Thus it is more effective to apply an on-off control stimulus. For example, to repetitively apply a voltage of sufficient intensity and duration to substantially depolarize the nerves, then to turn the voltage off and allow the nerves to repolarize and then to reapply the voltage, etc. It is desired to apply the voltage 5 to 200 times per second. Fewer than 5 fails to impress an urgency of response while greater than 200 looses the insistence of sensibly discrete actions. Thirty repetitions per seconds are preferred. In other words, the controlled on time of the stimulus signal is one sixtieth of a second and the off time of the stimulus signal is one sixtieth of a second, where upon the stimulus signal is again reapplied for a sixtieth of a second and so forth. 
     A less strong or graded correction stimulus can be achieved by reducing the duration and/or intensity of the voltage so as to not extensively depolarize the nerves. For example the on time of the stimulus signal may be reduced. To keep the perceived stimulus signal constant the off time of the stimulus signal may be increased by the amount of time the on time is reduced, so as to keep the sum of the on time and off time constant. 
     Furthermore, a more effective correction stimulus may be created by continuing the correction stimulus voltage for periods of time, including the on time and off time control of the correction stimulus, then discontinuing the correction stimulus voltage for periods of time. The animal receives the aversive correction stimulus and is allowed to react to it without being so affected as to be unable to respond to the correction stimulus. The stimulus may be continued or applied for 0.5 to 20 seconds then discontinued or stopped for 0.5 to 20 seconds. The correction stimulus may then be continued to further control the animal. Of course, the control signal should still present to continue the correction stimulus. 
     Also it may be desirable to stop the control signal all together after a time because the animal may be trapped or unable to react to the control stimulus. For example, after 20 to 120 seconds, the correction stimulus may be stopped for an extended time, 4 minutes or more, or until the control signal is not detected, which resets the sequence of the correction stimulus. 
     A method of utilizing a small, efficient, and effective shock system for animal control comprises a) mounting on the animal a controller to receive a control signal, b) generating a correction signal, c) coupling the correction signal to a step-up transformer to produce a stepped-up correction signal, c) rectifying the stepped-up correction signal, and d) applying the rectified stepped-up correction signal to the animal as a correction stimulus to control the animal. Mounting means to put into proper position for use. 
     This invention provides a power isolation system for the low battery voltages commonly used, such as 3 volts, to maintain operational voltage for the microprocessor and other signal processing circuits by utilizing a pass transistor. A FET would be ideal in terms of low voltage drop but it is symmetric so that it will conduct from the higher to the lower voltage whether it is the power source or the load. Additional circuitry and power consumption would be required to overcome this deficiency. A bipolar transistor has a small voltage drop when saturated, about 50 to 300 millivolts. The bipolar transistor is symmetric in that the emitter and collector can be interchanged. However, most bipolar transistors are optimized for other performance characteristics and are not symmetrical in performance. It was found that high forward gain transistors (forward current gain greater than 50) tend to have a low reverse gain, (reverse current gain less than 1), i.e. when the collector and emitter are interchanged. Thus a bipolar transistor with a large forward gain and a small reverse gain can be used to supply a relatively large current from the power source to the load but will supply only a small reverse current from the load to the “source” when the source is a lower voltage than the load voltage. With the collector connected to the load, the transistor base current is supplied by a resistor to ground of a preferred value just sufficient in resistance to saturate the transistor in normal operation so that the voltage drop is minimized. Other sources of base current may be used. The base current may be fixed such that additional circuitry is not required, for example to turn the base current on and off, to achieve the power isolation. In this configuration, when the “source” voltage drops below the load voltage the transistor is substantially non-conducting and only a small reverse or “leakage” current will flow from the load. Thus good isolation is provided with a minimal voltage drop and with a simple compact circuit. 
     An animal containment or exclusion system is a particular kind of animal control system that uses a boundary signal to restrict the animal to approved regions. A boundary signal is created by applying an “RF” signal (“radio frequency” although the frequency may be as low as 7 kilohertz) to a perimeter wire to radiate an electromagnetic signal that a receiver mounted on the animal can receive to activate a correction stimulus. The boundary signal is received by an inductor and it is necessary to use two or more inductors to overcome the blind spots in the characteristic solenoidal receiving field of a single inductor. Connecting the inductors together will result in an equivalent solenoidal receiving field that is equivalent to a single inductor. Thus the blind spots will not have been removed. 
     It is desired to combine the signals from a plurality of inductors, each physically oriented in a different direction without getting an equivalent solenodial field. To do this, the axes of the inductors are physically oriented in different directions, substantially orthogonally, and the individual received signals modified so that combining or adding the signals does not result in an equivalent solenoidal receiving field or a field with nulls. 
     This invention shifts the electrical phase differently of the received signals before combining them, particularly signals at the carrier frequency. The combined signals do not become an equivalent solenoidal field because signals of different electrical phase cannot cancel each other to give a blind spot. 
     In a preferred embodiment two inductors receive the boundary signal, the received signals are shifted 90 electrical degrees relative to each other so that they can be considered as a sine and cosine wave. It is well known mathematically that the sine and cosine are orthogonal (not in the physically orthogonal sense above) and cannot cancel each other. However, any electrical phase shift greater than about 45 electrical degrees will substantially eliminate the signal cancellation that results in nulls or an equivalent solenoidal field of response, provided that the inductors are oriented differently. Shifting the relative electrical phase 135 degrees gives performance similar to that of 45 degrees but as if one of the inductors were physically inverted. 
     In a preferred embodiment two inductors receive the boundary signal, the received signals from each inductor are amplified and the amplifier is constructed to shift the electrical phase of the signal. The two amplifiers are constructed to shift the phase differently. It is preferred that the phase is different by at least 45 electrical degrees and 90 degrees is more preferred. 
     It is common to use the inductor as part of a tuned circuit that is resonated by a capacitor to a frequency matched to the boundary signal frequency, also called the carrier frequency. This is advantageous because interfering signals can be rejected. A high “Q” (“quality factor” affecting the width of the frequency response) inductor-capacitor circuit may be used to further reject interfering signals. In a preferred embodiment of this invention, at least one of tuned circuits is tuned slightly away from the boundary signal frequency so that the electrical phase of the received signal is shifted. 
     In a preferred embodiment two inductors are substantially physically orthogonal and the electrical phase of the received boundary signals outputted by the inductors are shifted by tuning a first inductor and a first capacitor and second inductor and second capacitor to frequencies above and below the carrier frequency, respectively, so that the electrical phase difference between them is greater than 45 degrees, preferably 90 degrees. Tuned above means the resonant or most sensitive frequency of the inductor and capacitor is greater than the carrier frequency, and analogously for tuned below. The Q may be modified by shunting the inductors with resistors. 
     With only two inductors and with phase-shifted signals, a null in the received signal still remains in one plane. Use of three inductors oriented in three different directions will eliminate the null altogether. In a preferred embodiment three inductors are substantially physically orthogonal and receive the boundary signal the received signals are shifted 60 electrical degrees relative to each other. However, any electrical phase shift greater than about 30 electrical degrees but less than about 90 degrees will substantially eliminate the signal cancellation that results in blind spots in the field of response. 
     In a preferred embodiment three inductors are substantially physically orthogonal and receive the boundary signal, the received signals from each inductor are amplified and the amplifier is constructed to shift the electrical phase of the signal. The three amplifiers are constructed to shift the phase differently. It is preferred that the phases are different from each other by 60 electrical degrees. The inductors may also be resonated with capacitors and the Q adjusted with resistors. 
     In a preferred embodiment the electrical phase of the received boundary signals are shifted by tuning a first inductor and a first capacitor and a third inductor and a third capacitor to frequencies above and below the carrier frequency, respectively, so that the electrical phase difference is 60 degrees and tuning a second inductor and second capacitor to the carrier frequency. The sensitivity of second inductor and second capacitor may be matched to that of the others at the carrier frequency. 
     A method of animal control utilizing an antenna system for animal containment which does not need switching, does not give up authentification of weak signal, has no latency, and has fewer or no nulls in the response field comprises the steps: a) mounting on an animal a receiver to receive a boundary signal, b) receiving the boundary signal with a plurality of receiver subsystems, each having substantially orthogonal physical axes, c) shifting the electrical phase of the received boundary signals, forming shifted received boundary signals, d) combining the shifted received boundary signals, e) detecting the combined shifted received boundary signals and f) generating a correction stimulus to control the animal. 
     The phase shifting also may be accomplished by tuning the inductor and capacitor and phase shifting by the amplifier. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following description of this invention will be better understood when read in conjunction with the accompanying drawings in which: 
     FIG. 1 is a partial schematic drawing of a shock system; 
     FIG. 2 is a partial schematic drawing of a power isolation system; 
     FIG. 3 is a partial schematic drawing of a boundary signal receiver; 
     FIG. 4 is an illustration of signals with differing phase, 
     FIG. 4 a  is an illustration of the combining of signals with differing phase; 
     FIG. 4 b  is an illustration of the combining of signals with differing phase; 
     FIG. 5 is a partial schematic drawing of an alternate boundary signal receiver; and 
     FIG. 6 is a graph of phase and amplitude responses of a three tuned circuit antenna. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The shock system of this invention is better understood by reference to FIG. 1 in which a partial circuit is drawn. The controller  101 , possibly a microcomputer, receives a control signal, not shown, and commands a correction stimulus to be generated by dropping the voltage on wire  102  from a value near the supply voltage indicated as ++, to zero volts. Wire  102  is connected from the controller  101  to the emitter of NPN transistor  105 . Lowering the voltage on the emitter causes current to flow in the transistor&#39;s base from the supply voltage through resistor  103 . The base current is increased by the gain of transistor  105  and flows from the collector through resistor  104  to the base of PNP power transistor  106 . Transistor  106  amplifies the base current to flow from its collector through the primary winding of step-up transformer  107 . The current in the primary of transformer  107  creates a primary voltage and is coupled to its secondary to give a much larger voltage. The secondary of transformer  107  is coupled to fast recovery rectifier  108 , the other side of which is connected to one of the stimulus electrodes  111  for applying the correction stimulus to the animal. The other stimulus electrode  111  is connected to ground or common to complete the circuit. The others side of the secondary is connected to the base of transistor  105 . Also connected to the base of transistor  105  is capacitor  109  the other side of which is connected to ground. Capacitor  109  is selected during design to adjust the phase of current through transistor  106  relative to the voltage across transformer  107 , to improve the efficiency of power conversion. 
     Surprisingly it was found that good physiologic response was generated without using an output storage or filter capacitor across stimulus electrodes  111 . However, a small capacitor might be placed across the stimulus electrodes  111  to protect the circuit from static electric discharges. But it is preferred to keep the capacitance across the electrode to a minimum, less than about 10 nanofarads, to allow rapid build up and decay of the correction stimulus voltage. The correction stimulus voltage should decay more than 50% during the off time of the on time/off time sequence. 
     In a preferred embodiment, the transformer  107  was a 8.3 mm wide by 8 mm high by 3.6 mm wide ferrite “E” core with 12 primary turns and 1320 secondary turns, for a voltage step-up of  110 . With a supply voltage of 3 volts, from a lithium battery not shown, nearly 300 volts is output from the transformer  107  depending on the degree to which the voltage of the battery is diminished by the heavy load of the shocker. Component were  103 , 10,000 ohms;  104 , 100 ohms;  105 , 2N3904;  106 , FMMT591A;  108  FR16; and  109 , 1000 picofarads. In a preferred embodiment, an even smaller transformer  107  was a 7 mm diameter by 4 mm high ferrite pot core with 4 primary turns and 440 secondary turns. 
     The power isolation system of this invention is better understood by reference to FIG. 2 in which a partial schematic circuit is drawn. The battery, not shown, supplies a voltage (indicated as ++) to input wire  201 . The wire is connected to filter capacitor  202  and to the emitter of bipolar PNP transistor  205 . The base of transistor  205  is connected to ground through resistor  203 . The collector of transistor  205  is connected to storage capacitor  204  and output wire  207 . NPN transistor  206  is diode connected with its collector and base connected to input wire  201  and its emitter connected to output wire  207 . In operation diode connected transistor  206  supplies current to the output from the input when the battery is first connected to give a rapid voltage rise as required for some microcomputers. But since the voltage drop across transistor  206  is about 0.7 volts, transistor  205  supplies additional current to charge the output capacitor  204  to with in about 80 millivolts of the input voltage. The current flowing at the base of transistor  205  through resistor  203  is small, fixed or nearly constant, about 1 microamp. Because the gain of transistor is chosen to be high, the collector is saturated giving a low emitter to collector voltage. The diode-connected transistor could be eliminated if there is no need to rapidly charge capacitor  204 . Considering the currents of typical animal control systems, a diode such as a 1N4448 could be used in place of the diode-connected transistor. 
     The component values were;  202 , 440 microfarad;  203 , 2 megohm;  204 , 220 microfarad;  205 , 2N5087; and  206 , 2N3904. 
     With 3 volts on wire  201 , the initial current supplied to  207  at ground potential by transistor  205  without  206  in the circuit was 400 microamps. With 3 volts on wire  207  and zero volts on  201 , the reverse current through transistor  205  was 4 microamps, i.e. substantially nonconducting. With the voltages used in this application, the forward to reverse gain of the transistor is 100:1. In normal operation, with 15 microamps drawn from  207 , the voltage drop across transistor  205  was 80 millivolts after charging  204 . 
     A preferred receiver of this invention is better understood by reference to FIG. 3 in which a partial circuit is drawn. The first antenna system  10  comprises the inductor  11 , capacitor  12 , and resistor  14  connected in parallel to selectively receive the boundary signal carrier frequency. The output of circuit  10  is coupled through capacitor  15  to the base of transistor  334 . The second antenna system  20  comprises the inductor  21 , capacitor  22 , capacitor  23 , and resistor  24  connected in parallel, to also selectively receive the boundary signal carrier frequency. The output of circuit  20  is coupled through capacitor  25  to the base of transistor  334 . The third antenna system  30  comprises the inductor  31 , capacitor  32 , capacitor  33 , and resistor  34  connected in parallel, to also selectively receive the boundary signal carrier frequency. The output of circuit  30  is coupled through capacitor  25  to the base of transistor  334 . The inductors are drawn in different orientations to represent the different physical alignments of the physical devices that are preferably physically orthogonal. The collector of transistor  334  is connected to a supply voltage, not shown, through resistor  331 . The operating base current is supplied by resistor  332 , connected between the base and collector of transistor  334 . The cathode of diode  333  is connected to the base of transistor  334  and the anode to ground. Diode  333  protects transistor  334  from excessive reverse voltage when the inductors are very near a boundary wire and the signal is very large. The signal amplified by transistor  334  is coupled to additional conventional and known in the art circuitry, not shown, by capacitor  335  connected to the collector of transistor  334 . 
     Components were;  11 , 32 millihenry;  12 , 6.8 nanofarads;  14 , 68 kiloohms;  15 , 100 picofarads;  21 , 32 millihenry;  22 , 6.8 nanofarads;  23 , 330 picofarads,  24 , 33 kiloohms;  25  100 picofarads;  31 , 32 millihenry;  32 , 6.8 nanofarads;  33 , 680 picofarads,  34 , 68 kiloohms;  35  100 picofarads;  331 , 470 kiloohms;  332 , 2 megohms;  333 , 1N4448;  334 , 2N5089; and  335 , 220 picofarads. 
     The phase of the received boundary signal is affected by the resonant characteristics of receiving antennas  10 ,  20 , and  30 . In a preferred embodiment, antenna  10  is tuned above the boundary signal carrier frequency, 10.6 kilohertz. Antenna  20  is tuned to the boundary signal carrier frequency by the addition of capacitor  23 . Antenna  30  is tuned below the boundary signal carrier frequency by the addition of capacitor  33 . The resistors  14 ,  24 , and  34  control “Q” of the resonant circuits. Resistor  24  also affects the sensitivity of antenna  20  and can be used to match it to antennas  10  and  30  for reasons illustrated in FIG.  6 . The values of resistance are chosen to allow normal manufacturing tolerance components to be used and still operate according to this invention. Other means may be used to affect the difference in the tuning. For example the value of the inductors might, be made different. Instead of paralleling capacitor  23  across capacitor  22 , a different value of capacitor  22  might be chosen eliminating the need for capacitor  23 , and similarly for capacitor  33 . 
     FIG. 4 illustrates three received signals with differing phase. If signal  402  is taken as zero degrees phase (electrical), then  401  is −60 degrees and  403  is +60 degrees. While  401  and  403  are 120 degrees different as drawn, it is recognized that for example  401  might be negative in amplitude in which case  401  and  403  would equivalently be 60 degrees different rather than 120 degrees. The amplitudes of the signals vectors are drawn as equal in magnitude. As can be seen, for three antennas, the 60 degrees phase difference distributes the phase differences optimally. Of course, the amplitudes will in general be different in use depending on the orientation of the physical antenna to the boundary signal. Also the amplitude and phase depends on the exact value of the components. 
     FIG. 4 a  illustrates the vector resultant of adding equal amplitude signals from  402  and  403  of FIG. 4 as occurs when  402  and  403  are each 45 physical degrees to the boundary signal. The resultant  405  is greater in amplitude than either of the individual signals. 
     FIG. 4 b  illustrates the vector resultant of adding equal amplitude signals from  402  and  403 , except the latter&#39;s amplitude is negative. A negative amplitude is equivalent to a 180-degree shift and naturally occurs if the winding sense of the inductor is reversed or if the receiver is turned 180 degrees about an axis associated with the antenna producing signal  402 . The resultant  406  is approximately equal in amplitude to the individual signals but somewhat less than that of  405 . It is important to note that there is no orientation, which will produce a null, unlike a single antenna. While the resultant amplitude may vary some depending on the orientation, the distance at which the boundary signal is detected is not significantly different. 
     Best performance is achieved with the phases conform to the above, however the actual phase may be more or less and achieve the elimination of nulls in the response. In the case of 2 inductors the phase difference may be from about 45 to 135 degrees. In the case of three inductors the minimum phase difference may be from 30 to 90 degrees. 
     Best performance is achieved with the inductors physically orthogonal (90 degrees) to each other. Substantially physically orthogonal means the orientation of any 2 inductors may be from 45 to 135 physical degrees and still permit the effective elimination of nulls from the receiving field. 
     A preferred embodiment is shown in FIG. 5; the antennas  510 ,  520 , and  530  are coupled to individual amplifiers. The antenna  510 , comprising the parallel components inductor  511 , capacitor  512  and resistor  514 , is coupled by capacitor  515  to the base of transistor  554 . The collector of transistor  554  is connected to a power source through resistor  551 . Resistor  552  is connected between the base and collector of transistor  554  to provide operational base current. Diode  553  is to protect transistor  554  from excessive reverse voltage. Capacitor  555  couples the amplified signal to wire  580  that conveys the signal to other common and known in the art circuit circuitry, not shown. The antenna  520 , comprising the parallel components inductor  521 , capacitor  522  and resistor  524 , is coupled by capacitor  525  to the base of transistor  564 . The collector of transistor  564  is connected to a power source through resistor  561 . Resistor  562  is connected between the base and collector of transistor  564  to provide operational base current. Diode  563  is to protect transistor  564  from excessive reverse voltage. Capacitor  565  couples the amplified signal to wire  580  that conveys the signal to other common and known in the art circuit circuitry. Capacitor  566  causes a lag in the phase of the signal. Making the value of capacitor  555  smaller than  565  causes a relative lead in phase of the signal from  510 . The antenna  530 , comprising the parallel components inductor  531 , capacitor  532  and resistor  534 , is coupled by capacitor  535  to the base of transistor  564 . The collector of transistor  574  is connected to a power source through resistor  571 . Resistor  572  is connected between the base and collector of transistor  574  to provide operational base current. Diode  573  is to protect transistor  574  from excessive reverse voltage. Capacitor  575  couples the amplified signal to wire  580  that conveys the signal to other common and known in the art circuit circuitry. The sensitivity or gain of  530  and its amplifier ( 574 ) needs to be lower that that of  510  and  520  and their amplifiers (for reasons illustrated in FIG. 6) this may be accomplished with, for example, selected values of resistor  534  or resistors  571  or  572 . This circuit configuration can utilize a narrower antenna resonance, i.e. higher Q, for a given component tolerance than the circuit of FIG.  3  and eliminate potential interfering signals. The antenna and the associated amplifier is a receiver subsystem. 
     Components were;  511 , 521 , 531 , 33 millihenry;  512 , 522 , 532 , 6.8 nanofarads;  514 , 524 , 534 , 100 kldoohms;  515 , 525 , 535 , 100 picofarads;  551 , 470 kiloohms;  552 , 2 megohms;  553 , 1N4448;  554 , 2N5089;  555 , 220 picofarads;  561 , 470 kiloohms;  562 , 2 megohms;  563 , 1N4448;  564 , 2N5089;  565 , 1000 picofarads; and  566 , 150 picofarads;  571 , 470 kiloohms;  572 , 1 megohm;  573 , 1N4448;  574 , 2N5089;  575 , 1000 picofarads. 
     FIG. 6 illustrates the phase and amplitude response with antenna component values of the preferred embodiment of FIG.  3 . The amplitude response  610  to frequencies received by antenna  10  shows a peak response defined as 100 near 10.9 kilohertz and 55 at the carrier frequency  601 . Antenna  10  shows a phase response  611  of approximately +60 degrees at the carrier frequency  601 . The amplitude response  620  to frequencies received by antenna  20  shows a peak response of approximately 55 at the carrier frequency  601 , 10.62 kilohertz. Antenna  20  shows a phase response  621  of approximately zero degrees at the carrier frequency. The amplitude response  630  to frequencies received by antenna  30  shows a peak response of 100 near 10.4 kilohertz and 55 at the carrier frequency  601 . Antenna  20  shows a phase response  631  of approximately −60 degrees at the carrier frequency  601 . The circle  602  is meant to draw attention to the operational region of the drawing where the three antennas are of approximately equal sensitivity, within the variances of normal production components. 
     With the teachings of this invention, it will be apparent to those skilled in the art as to how to change the components for different applications, such as carrier frequency or greater sensitivity with higher inductance antennas.