Abstract:
A body coupled antenna system usable in a personal locator unit includes an antenna element for connection to an RF circuit; a coupling plate for conductively coupling to an adjacent body for broadening the tuning frequency and reducing the loading sensitivity; a first capacitance interconnected between the coupling plate and the antenna element; and a second capacitance interconnected between the coupling plate and the RF circuit; the first and second capacitances being set to restore a predetermined matching impedance level while maintaining the broadened tuning frequency.

Description:
FIELD OF THE INVENTION 
     This invention relates to body coupled antenna system and to a personal locator unit using such a body coupled antenna system. 
     BACKGROUND OF THE INVENTION 
     Personal locator devices which enable a missing person to be quickly located are in ever greater demand. The personal locator device should be securely attachable to the person&#39;s body so it cannot be accidentally easily discarded with clothing, for example, but be small enough and light enough for comfort and compatibility. One problem is that an RF transmitter or transmitter/receiver in such personal locator devices requires an antenna that is fairly large and can be extremely sensitive to the proximity and movement of the person&#39;s body or other bodies human or otherwise, e.g. bodies such as cars, buildings . . . . There have been a number of approaches to designing body worn RF transmitters/receivers that will function efficiently when worn on the body, typically on wrists or ankles. One approach is to shield the antenna from the body either with RF opaque materials or through physical separation. Another is to use the body to improve antenna functions. In one attempt the antenna is coupled to the wrist or ankle of the person. This requires an electrical antenna band around the wrist or ankle. This not only makes for an awkward and cumbersome arrangement for the wearer but adds to the manufacturing cost and difficulty. In addition the size of the antenna loop is subject to the size of the wearer&#39;s wrist or ankle and the looseness or tightness of the fit of the band attachment. See U.S. Pat. Nos. 4,873,527; 4,977,614; 6,175,729; 6,927,739. See also U.S. Pat. Nos. 5,532,705; 5,465,098, 5,673,054 referring to electric and magnetic fields and effects of antennas close to a body. 
     SUMMARY OF THE INVENTION 
     In accordance with various aspects of the subject invention in at least one embodiment the invention presents an improved body coupled antenna system and an improved personal locator unit utilizing such a body coupled antenna system which can be worn in close proximity to a human body or other body, with minimal effect on antenna efficiency, is small, easy to incorporate in a body coupled device and does not require electrical continuity around the body. 
     The subject invention results from the realization that, in part, an improved body coupled antenna system and an improved personal locator unit utilizing such a system can be achieved where the antenna is coupled to the ground plane of the body and the body coupling helps to efficiently convert the magnetic and electrical fields and the effective resistance of the body secures to further tune the antenna over a range of frequencies. The antenna system may include an inductance antenna element for connection to an RF circuit, a coupling plate for conductively coupling to an adjacent body for broadening the tuning frequency and reducing the loading sensitivity; a first capacitance interconnection between the coupling plate and the inductance antenna element; a second capacitance interconnection between the coupling plate and the RF circuit; the first and second capacitances being set to restore a predetermined matching impedance level while maintaining the broadened tuning frequency. 
     The subject invention, however, in other embodiments, need not achieve all these objectives and the claims hereof should not be limited to structures or methods capable of achieving these objectives. 
     In one embodiment a body coupled antenna system includes an antenna element for connection to an RF circuit and a coupling plate for conductively coupling to an adjacent body for broadening the tuning frequency and reducing the loading sensitivity. A first capacitance is interconnected between the coupling plate and the antenna element and a second capacitance is interconnected between the coupling plate and the RF circuit, the first and second capacitances are set to restore a predetermined matching impedance level while maintaining the broadened tuning frequency. 
     In preferred embodiments the antenna element may include a helical conductor. The coupling plate may be capacitively coupled to the adjacent body. The first and second capacitances may be adjustable. The body may be a human body. The antenna system may be worn on the body wrist. The antenna system may be contained in a housing adapted to be mounted on the body. The coupling plate may be separated by a gap from the body. The RF circuit may include a transmitter circuit. The RF circuit may include a transmitter and receiver circuit. The RF circuit may be contained in the housing. The housing may include a power supply for the RF circuit. The RF circuit may include a receiver circuit. 
     In another embodiment a personal locator unit includes a body coupled antenna system, an RF circuit, a housing for the body coupled antenna system and the RF circuit. The body coupled antenna system includes an antenna element for connection to the RF circuit, a coupling plate for conductively coupling to an adjacent body for broadening the tuning frequency and reducing the loading sensitivity and a first capacitance interconnected between the coupling plate and the antenna element. A second capacitance is interconnected between the coupling plate and the RF circuit and the first and second capacitances are set to restore a predetermined matching impedance level while maintaining the broadened tuning frequency. 
     In preferred embodiments the RF circuit may include a transmitter circuit. The RF circuit may include a transmitter circuit and receiver circuit. The RF circuit may include a decode circuit responsive to the receiver circuit, for identifying a personal identification code associated with the instant personal locator unit and a switch circuit for enabling the transmitter circuit. The transmitter circuit may transmit at least the personal identifier code associated with the instant personal locator unit. The RF circuit may include a power supply. The housing may include a device for carrying the housing on the body. The device may include a strap. The coupling plate may be capacitively coupled to the adjacent body. The coupling plate may be separated by a gap from the body. The RF circuit may include a receiver circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which: 
         FIG. 1  is a schematic functional block diagram of a personal locator unit (PLU) including a body coupled antenna system according to one embodiment of this invention in the context of the supporting locator system including a station receiver and tracking unit. 
         FIG. 2  is a schematic block diagram of one embodiment of the body coupled antenna system of  FIG. 1 ; 
         FIG. 3  is a schematic block diagram of another embodiment of the body coupled antenna system of  FIG. 1 ; 
         FIG. 4  is a three dimensional diagrammatic view of a personal locator unit including a body coupled antenna system secured by a wrist band on a human body; 
         FIG. 5  is a more detailed schematic diagram of a body coupled antenna system according to one embodiment of the invention; 
         FIG. 6  is a more detailed equivalent circuit representation of a body coupled antenna system according to an embodiment of this invention; and 
         FIG. 7  is an enlarged, exploded view of components of one embodiment of a body coupled antenna system according to this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Aside from the preferred embodiment or embodiments disclosed below, this invention is capable of other embodiments and of being practiced or being carried out in various ways. Thus, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. If only one embodiment is described herein, the claims hereof are not to be limited to that embodiment. Moreover, the claims hereof are not to be read restrictively unless there is clear and convincing evidence manifesting a certain exclusion, restriction, or disclaimer. 
     There is shown in  FIG. 1  a personal locator unit  10  according to this invention including a band  12  to be fastened around a human body, for example, a wrist or ankle using clasp  14 . Band  12  carries housing  16  which contains a body coupled antenna according to this invention as well as an RF circuit and power source. When a person wanders the base station  18  detects that the PLU  10  is missing, and sends an alarm call to authorities. The tracking unit  22  is employed to detect the personal ID code from the PLU  10 , and locate the missing person. The tracking unit  22  has a receiver that seeks the personal ID code from the PLU  10 . The PLU sends out a tracking signal all the time; it is always running. It is a beacon, transmitting a locating chirp once per second. Typically the system operates in a 216 MHz range, but it is not limited to the VHF spectrum. Housing  16  of PLU  10  contains a body coupled antenna system  30 ,  FIG. 2 , according to this invention driven by an RF circuit  32 , such as transmit circuit  34  with a power supply  36  which may be implemented commonly with a battery. Transmit circuit  34  may drive body coupled antenna system  30  continuously so that it always broadcasts its personal ID code or it may operate it to periodically broadcast its ID code to save power. 
     Alternatively, RF circuit  32   a ,  FIG. 3 , may include a transmit circuit  34   a , and a receiver circuit  38 . In operation, an activation signal from a radio frequency network, containing the personal ID code of a missing person&#39;s PLU  10  is received by receiver circuit  38  of each PLU in the area, one of the PLU&#39;s having the correct ID recognizes the incoming personal ID code as its own in decoder  40  and through switching circuit  42  turns on transmit circuit  34   a  which performs as previously to produce a continuous or at least periodic broadcast from body coupled antenna system  30   a  of its personal ID code to be picked by one or more tracking unit  22 . Typically the broadcast from body coupled antenna system  30   a  may be at 216 MHz and its reception may be at 170 MHz. The system may use a band of 60 frequencies in the 216 to 217 MHz range. The system could work on any frequency. When used on a human body PLU  10 ,  FIG. 4 , may be mounted on a wrist  50  of the human body by means of band  12  and clasp  14 . 
     One embodiment of the body coupled antenna system  30   b ,  FIG. 5 , includes an inductance antenna element in the form of a helical coil  60  connected at one end to the center conductor  62  of coaxial cable  64  from RF circuit  32   b . The other end of antenna element  60  is connected to capacitance C 2   66  which may be adjustable and whose other end is connected to a body coupling plate  68  via conductor  70 . The body coupling plate  68  is also connected to capacitance C 3   72  through conductor  74 . Capacitor  72  C 3  completes the circuit to the outer shield  76  of coaxial cable  64 . Coaxial cable  64  has a typical impedance of 50 ohms and may be connected to a reference point such as circuit or floating ground  78 . A reactive coupling is effected between body coupling plate  68  and the neighboring body  80  which may typically be a human body, but may be other kinds of bodies, such as car bodies, or other bodies which present body ground  82 . The reactive coupling may be resistive, with the body coupling plate  68  in direct contact with the body, or it may be capacitive, with the coupling provided over a gap  84  using for example, standoffs  86  and  88  or an encapsulating or partially encapsulating medium  90  all shown in phantom. In the range of 216 MHz conductance L 1   60  can be in a range of 0.1 μH and capacitances C 2   66  and C 3   72  may be set to somewhere between two and ten ρF. These capacitors may have established values or they may be made adjustable as shown in  FIG. 5 . Body coupling plate  68  would typically have a very small area of approximately one square inch and can be in direct contact with body  80  or can be separated as shown with a dielectric such as air. 
     A more detailed equivalent circuit of the body coupled antenna of this invention is shown in  FIG. 6  where the power supply is shown as a battery  36   c . Capacitances C 2   66   c  and C 3   72   c  are shown as fixed as opposed to variable capacitances and conductors  70   c  and  74   c  are represented by inductances L 2  and L 3 , respectively. The reactive coupling between body coupling plate  68   c  and the body is shown as a reactive capacitance C 1   92  and the body impedance is shown simply as a resistance R 1   94 . L 1 , the inductance antenna element  60   c , is shown as a long helical coil of wire spanning the length of the antenna. Inductances  70   c  L 2  and  74   c  L 3  may be straight pieces of wire which together also span the length of the antenna. Capacitor C 2   66   c  is at the right of the antenna and capacitor C 3   72   c  is at the left of the antenna. The top plate of capacitor C 1   92  is the body coupling plate  68   c  at the junction of inductances L 2   70   c  and L 3   74   c . The bottom plate of capacitor C 1   92  is the wrist of the person or some portion of the person&#39;s body or another body on which the system is mounted. Body impedance R 1   94  is the surface resistance of the person&#39;s wrists and forearm. The body impedance  94  is shown here as a resistance but this is for simplification of explanation and is not a limitation. Body coupled antenna system  30   c  is driven by a 50 ohm coaxial cable  64   c  which has ferrites over it to inhibit currents flowing on the outside of the shield and forcing differential drive of the antenna. The antenna is actually driven from the RF circuit  32   c  which is floating. 
     The antenna may be more readily explained by deriving the elements one at a time. Thus, if all the elements other than L 1   60   c  and C 3   72   c  are removed that is C 2   66   c , L 2   70   c  and L 3   74   c  are replaced with short circuits and resistance R 1   94  and capacitance C 1   92  are opens, the antenna is then a series resonant tank circuit. In that case, the input impedance would be very low at resonance and the resonance would have a very high Q with sharp falloffs and a consequently high loading sensitivity as is the case for all electrically small antennas. It thus would be difficult to match to and very easy to detune because of its high load sensitivity. Either of these would dramatically reduce the efficiency of the antenna. At this point there are no currents on the outside shield  76  of the coaxial cable  64   c  because there is no parasitic path to ground from inductance L 1   60   c  or capacitance C 3   72   c . If capacitance C 1   92  and impedance R 1   94  are added, there now is a parasitic path to ground through the body of the person represented by the ground symbol  82   c  at the bottom of impedance R 1   94 . The shield  76   c  of the coaxial cable  64   c  would also have a large capacitance to ground, so the coaxial cable would have to be heavily covered with ferrite to discourage the common mode current flowing to ground on the outside of the shield. With capacitance C 2   66   c , L 2   70   c  and L 3   74   c  still shorted, the series circuit of capacitance C 1   92  through impedance R 1   94  sits at the top of the series resonant tank circuit. It would thus de-Q or lower the Q of the circuit considerably. This would solve the easy to detune problem because now the resonance would have a lower Q and a consequent low loading sensitivity. However, this aggravates the matching problem because now the input impedance instead of being very low is very high. It would be difficult to match to such a high impedance without huge losses in the matching circuit. The losses of a matching circuit vary inversely with the physical size of the inductors in the matching circuit, making the matching circuit a problem not just because of the additional cost but the additional size as well. In accordance with this invention the notion is to use the antenna series resonant tank circuit itself to do the matching. Inductor L 1   60   c  being quite large would have low loss. This is exactly what is done by introducing capacitance C 2   66   c . It moves the point of application of the de-Qing resistance impedance R 1   94  to a lower impedance point on the resonant tank circuit. By properly choosing the ratio of C 2   66   c  and C 3   72   c  the input impedance which the coax drive sees can be set to 50 ohms for a perfect match. The setting can be done by trial and error and using fixed value capacitances as  66   c  and  72   c  or adjustable or variable capacitances could be permanently placed there. With such a setting, now, the Q of the antenna may still be too high. If it is, then capacitance C 2   66   c  would have to be made larger and capacitance C 3   72   c  made smaller. This is done by moving the application point of impedance R 1   94  to a higher impedance point on the series resonant tank circuit. The RF circuit  32   c  would need to be replaced with a source with a higher source impedance or another matching circuit would be needed between the 50 ohm coax drive and the antenna. Another advantage of this antenna is that the de-Qing resistance R 1   94  is spread over the forearm and thus the current through it would contribute to radiation. 
     The components of the body coupled antenna system  30   d  according to one embodiment is shown in  FIG. 7  where the helical shape of the inductance antenna element, inductance L 1   60   d  and the a circular form for the body coupling plate  68   d  is apparent. Capacitances C 2   66   d  and C 3   72   d  are shown here as variable or adjustable capacitances and the inductances L 2   70   d  and L 4   74   d  appear as simply conductors between capacitors  66   d ,  72   d  and body coupling plate  68   d.    
     With respect to the body coupled antenna system of this invention it is understood that the body coupling element serves to de-tune the sharp Q factor of a typical tuned loop antenna. Furthermore, this de-tuning of the antenna is achieved without sacrificing the impedance matching of the antenna network to the transmitter: a standard 50 ohm impedance is maintained. The body coupling mechanism also serves to increase the effective ground plane for the antenna. It is believed that the helical antenna and perhaps the gap between the body coupling plate and the body act to increase the effect of the magnetic component of the radio waves. It is understood that very high frequency, e.g. VHF radio waves interact with the human body in a manner that reduces the electric field component and increases the magnetic component. An efficient antenna for VHF radio waves next to the human body then effectively converts the magnetic field to current flow in the antenna for a receiver or conversely when is acting as a transmitter. Although specific features of the invention are shown in some drawings and not in others, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention. The words “including”, “comprising”, “having”, and “with” as used herein are to be interpreted broadly and comprehensively and are not limited to any physical interconnection. Moreover, any embodiments disclosed in the subject application are not to be taken as the only possible embodiments. 
     In addition, any amendment presented during the prosecution of the patent application for this patent is not a disclaimer of any claim element presented in the application as filed: those skilled in the art cannot reasonably be expected to draft a claim that would literally encompass all possible equivalents, many equivalents will be unforeseeable at the time of the amendment and are beyond a fair interpretation of what is to be surrendered (if anything), the rationale underlying the amendment may bear no more than a tangential relation to many equivalents, and/or there are many other reasons the applicant can not be expected to describe certain insubstantial substitutes for any claim element amended. 
     Other embodiments will occur to those skilled in the art and are within the following claims.