Patent Publication Number: US-7898389-B2

Title: Radio frequency identification (RFID) tags and methods of communicating between a radio frequency identification (RFID) tag and an interrogator

Description:
RELATED PATENT DATA 
     This patent resulted from a continuation of and claims priority to U.S. patent application Ser. No. 10/075,791, filed on Feb. 12, 2002, entitled “Communication Systems, Communication Apparatuses, Radio Frequency Communication Methods, Methods of Communicating Using a Radio Frequency Communication System, and Methods of Forming a Radio Frequency Communication Device”, naming Freddie W. Smith as inventor, now U.S. Pat. No. 7,075,901 which is a continuation of U.S. patent application Ser. No. 09/020,595, filed on Feb. 4, 1998, entitled “Communication Systems, Communication Apparatuses, Radio Frequency Communication Methods, Methods of Communicating Using a Radio Frequency Communication System, and Methods of Forming a Radio Frequency Communication Device”, naming Freddie W. Smith as inventor, now U.S. Pat. No. 6,356,535 which issued on Mar. 12, 2002, the disclosure of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to communication devices, communication systems and methods of communicating, and more particularly to radio frequency communication devices. 
     BACKGROUND OF THE INVENTION 
     Electronic identification systems typically comprise two devices which are configured to communicate with one another. Preferred configurations of the electronic identification systems are operable to provide such communications via a wireless medium. 
     One such configuration is described in U.S. patent application Ser. No. 08/705,043, filed Aug. 29, 1996, assigned to the assignee of the present application and incorporated herein by reference. This application discloses the use of a radio frequency (RF) communication system including communication devices. The disclosed communication devices include an interrogator and a transponder, such as a tag or card. 
     Such communication systems can be used in various identification functions and other applications. The interrogator is configured to output a polling signal which may comprise a radio frequency signal including a predefined code. The transponders of such a communication system are operable to transmit an identification signal responsive to receiving an appropriate command or polling signal. More specifically, the appropriate transponders are configured to recognize the predefined code. The transponders receiving the code subsequently output a particular identification signal which is associated with the transmitting transponder. Following transmission of the polling signal, the interrogator is configured to receive the identification signals enabling detection of the presence of corresponding transponders. 
     Such communication systems are useable in identification applications such as inventory or other object monitoring. For example, a remote identification device is attached to an object of interest. Responsive to receiving the appropriate polling signal, the identification device is equipped to output an identification signal. Generating the identification signal identifies the presence or location of the identification device and the article or object attached thereto. 
     It is preferred to maximize communication range between communication devices of the identification system while providing robust communications. Increasing the range also increases the applications of the identification system. Providing robust communications ensures reliability and integrity of the system. 
     However, limitations exist upon the wireless communication components utilized within the communication devices. For example, given the nature of use of such electronic devices (i.e., attachment of the transponder to other devices or objects), it is preferred to minimize the size of the electronic device. Compact electronic devices also have cosmetic and utilitarian advantages over larger conventional communication devices. Size limitations impose limitations upon the wireless communication components themselves. In addition, the Federal Communication Commission also imposes power limits upon the wireless communication components. 
     Therefore, it is desirable to provide an identification device which achieves the benefits of increased range and robust wireless communications in consideration of size and power limitations. 
     SUMMARY OF THE INVENTION 
     According to one aspect, the present invention provides a communication device including a first antenna operable to receive wireless communication signals and a second antenna having plural leads and operable to output wireless communication signals. The communication device further comprises a connection coupled with the second antenna and a switch. The switch is operable to provide selective shorting, and insulation or electrical isolation of leads of the second antenna. The connection provides low load impedance of the second antenna during receiving of wireless communication signals in a preferred embodiment of the invention. 
     According to some embodiments of the invention, the communication devices comprise one of a radio frequency identification device and a remote intelligent communication device. 
     Another communication device of the present invention includes a first antenna operable to receive wireless communication signals and a second antenna operable to output wireless communication signals. The second antenna is selectively configured between high load impedance and low load impedance. The communication device includes a switch selectively operable to electrically short and insulate the leads. Further, a transformer is provided intermediate the switch and the a second antenna and the transformer is configured to provide low load impedance of the second antenna responsive to the switch being open. 
     The present invention also provides a communication system including an interrogator and a communication device configured to communicate with the interrogator. The communication device includes a first antenna operable to receive wireless signals from the interrogator and a second antenna operable to output wireless signals to the interrogator. The communication device also includes a connection configured to provide a low load impedance of the second antenna during receiving of wireless signals using the first antenna. 
     One method of communicating according to the present invention includes forming a first antenna, forming a second antenna, receiving wireless interrogation signals using the first antenna and outputting wireless identification signals using the second antenna. The method also provides opening a coupling intermediate plural leads of the second antenna during the receiving, selectively shorting the leads of the second antenna during the outputting, and providing a low load impedance of the second antenna during the receiving. Methods according to additional aspects of the invention also provide beam forming using the first and second antennas. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the invention are described below with reference to the following accompanying drawings. 
         FIG. 1  is a functional block diagram illustrating a wireless communication system. 
         FIG. 2  is a top plan view of one embodiment of a communication device of the communication system of  FIG. 1 . 
         FIG. 3  is a top plan view of the communication device at an intermediate processing step. 
         FIG. 4  is a bottom view of the communication device at an intermediate processing step. 
         FIG. 5  is a diagrammatic representation of antennas of the communication device. 
         FIG. 6  is a diagrammatic representation, similar to  FIG. 5 , of the communication device. 
         FIG. 7  is a schematic diagram of one embodiment of a backscatter switch of the communication device. 
         FIG. 8  is a gain plot of a receive antenna of the communication device. 
         FIG. 9  is a gain plot of the receive antenna of the communication device illustrating enhanced gain. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     This disclosure of the invention is submitted in furtherance of the constitutional purposes of the U.S. Patent Laws “to promote the progress of science and useful arts” (Article 1, Section 8). 
     The present invention provides electronic devices configured to communicate via wireless communication signals, such as radio frequency signals. The present invention additionally provides methods of communicating. 
     Some embodiments of the electronic devices include electronic communication devices such as wireless identification devices. Exemplary electronic communication devices include radio frequency identification is devices (RFID) and remote intelligent communication devices (RIC). A remote intelligent communication device is capable of functions other than the identifying function of a radio frequency identification device. A preferred remote intelligent communication device includes a processor. 
     Some communication devices disclosed herein are implemented upon printed circuit boards (PCBs) according to described aspects of the present invention. The disclosed embodiments are illustrative and other configurations including encapsulated devices which utilize features of the present invention are possible. 
     Referring to  FIG. 1 , a communication system is illustrated. Although the communication system is described with reference to a wireless identification system  12 , the present invention is implemented in other configurations in accordance with other embodiments. The depicted identification system  12  includes a remote communication device  10 , such as a remote intelligent communication device or a radio frequency identification device, and an interrogator unit  14 . Typically, plural communication devices  10  are provided to communicate with interrogator unit  14 . An exemplary wireless identification system  12  is described in U.S. patent application Ser. No. 08/705,043, incorporated by reference above. An exemplary interrogator  14  is described in detail in U.S. patent application Ser. No. 08/806,158, filed Feb. 25, 1997, assigned to the assignee of the present application and incorporated herein by reference. 
     Communication device  10  is configured to communicate via electromagnetic signals with interrogator unit  14 . Preferably, device  10  communicates with interrogator unit  14  via wireless electromagnetic signals, such as radio frequency (RF) signals. Wireless electromagnetic signals or radio frequency signals, such as microwave signals, are utilized for communications in the preferred embodiment of identification system  12 . Interrogator unit  14  further includes an antenna  16  to facilitate wireless communications. 
     In one embodiment of the identification system  12 , interrogator  14  outputs an interrogation signal via antenna  16  during forward link communications. The interrogation signal is received and processed by any communication devices  10  within the transmission range of interrogator  14 . Following processing, appropriate communication devices  10  are configured to return an identification signal during return link communications. The identification signal identifies the individual device  10  transmitting the identification signal in one embodiment of the invention. 
     Referring to  FIG. 2 , one embodiment of communication device  10  is shown. Communication device  10  includes a base substrate  18  which comprises a printed circuit board in the described embodiment. Other substrates, such as a flexible polyester film, are utilized in other embodiments. Substrate  18  includes a first surface  20  shown in  FIG. 2 . Substrate  18  also includes a second surface  22  (shown in  FIG. 4 ) opposite first surface  20 . 
     Communication device  10  includes plural components mounted upon first surface  20 . Such components include an integrated circuit  24 , and first and second power supplies  26 ,  28  in one embodiment. Other power supply configurations may be utilized. A conductive trace or pattern  37  is provided upon first surface  20  to provide electrical interconnection of the components. Conductive pattern  37  includes conductors  40 ,  42 ,  46 ,  48  to provide electrical interconnection. 
     Further, the illustrated conductive pattern  37  includes a first antenna  30  and second antenna  32  for implementing wireless communications. First antenna  30  is also referred to as a receive or forward link antenna and second antenna  32  is also referred to as a transmit or return link antenna. Conductive pattern  37  comprises copper in one embodiment of the invention. Other materials are utilized in other embodiments to form conductive pattern  37 . 
     One embodiment of integrated circuit  24  includes suitable communication circuitry within communication device  10  for providing wireless communications. For example, in one embodiment, integrated circuit  24  includes a microprocessor  65 , memory  67 , and transponder circuitry  68  in cooperation with one another for providing wireless communications with interrogator unit  14 . An exemplary and preferred integrated circuit  24  is described in U.S. patent application Ser. No. 08/705,043 incorporated by: reference above. The illustrated integrated circuit  24  is packaged in a conventional small outline (SOIC) package. 
     One embodiment of the communication circuitry or transponder circuitry  68  includes a modulator, such as a transmitter, and a receiver operable to respectively communicate (i.e., output) and receive wireless electronic signals. The microprocessor  65  is coupled with transponder circuitry  68  and is configured to process the electronic signals. Responsive to the detection of an appropriate interrogation or polling signal, microprocessor  65  instructs transponder circuitry  68  to output the identification signal. The modulator comprises an active transmitter or a backscatter device according to certain embodiments. Such outputting or communicating of the communication signals via the modulator comprises one of transmitting electromagnetic signals and reflecting received signals. 
     Plural power supplies  26 ,  28  are provided in the described embodiment of communication device  10 . A single power supply is utilized in other embodiments. The illustrated power supplies  26 ,  28  are connected in series to provide operational power to components of communication device  10 . Power supplies  26 ,  28  provide power at approximately 6 volts to components of communication device  10 . The illustrated power supplies  26 ,  28  comprise batteries although other power sources may be utilized. 
     Brackets  36 ,  38  are elevated from first surface  20  and are configured to hold respective power supplies  26 ,  28  upon substrate  18 . Perimetral edges of power supplies  26 ,  28  form positive or power terminals. Upper surfaces (i.e., facing away from surface  20 ) of power supplies  26 ,  28  also form the positive terminals. Brackets  36 ,  38  provide electrical coupling with the positive terminals at edges  27 ,  29  and the upper surfaces of power supplies  26 ,  28 . Brackets  36 ,  38  are formed of stainless steel in the described embodiment of the invention. Alternatively, other conductive materials may be utilized to fabricate brackets  36 ,  38 . 
     The device  10  includes plural vias which extend through respective brackets  36 ,  38  and substrate  18 . A first via receives a conductive post  53 . Post  53  provides electrical coupling of the elevated bracket  38  to first surface  20  of substrate  18 . Post  53  is electrically coupled with conductor  40 . Positive power from power supplies  26 ,  28  is applied to capacitor  34  and integrated circuit  24  via post  53  and conductor  40 . 
     Conductor  42  provides electrical coupling of the negative terminal of power supply  26  with integrated circuit  24 . Conductor  42  is coupled with a pad  54  (shown in  FIG. 3 ) provided below power supply  26 . Conductor  42  is insulated from bracket  36 . 
     The positive terminal of power supply  26  is electrically coupled with the negative terminal of power supply  28 . A via is provided through bracket  36  and substrate  18 . A conductive post  50  is provided within the via and electrically couples bracket  36  with the second surface  22  of substrate  18 . Referring to  FIG. 4 , a conductor  58  upon second surface  22  is coupled with post  50 . Another conductive post  55  provides electrical coupling of conductor  58  at second surface  22  with an electrical pad  56  upon first surface  18  (shown in  FIG. 3 ). Post  55  is provided within a via formed through substrate  18  and is coupled with pad  56  and the negative terminal of power source  28 . 
     Referring again to  FIG. 2 , first antenna  30  and second antenna  32  are formed upon first surface  20  of substrate  18 . First antenna  30  is also referred to as a forward link or receive antenna operable to receive wireless communication signals. First antenna  30  comprises a loop antenna in the illustrated embodiment. Other antenna configurations are possible for first antenna  30 . 
     Conductors  46 ,  48  operate to couple first antenna  30  with plural IC connections  63 ,  69  of integrated circuit  24 . IC connections  63 ,  69  provide an RX input to transponder  68 . The RX input has an impedance of about 50 Ohms (real) and is invariant in the described embodiment. Thus, receive antenna  30  sees a constant load of about 50 Ohms. Receive antenna  30  and RX input are “matched” in a preferred embodiment to provide maximum RF voltage to the RX input. 
     Second antenna  32  is formed as a dipole antenna including portions or halves  33 ,  35  upon first surface  20  of substrate  18 . Second antenna  32  may be also referred to as a return link antenna or transmit antenna and is operable to output wireless signals. Halves  33 ,  35  of antenna  32  have corresponding lengths appropriate for the desired transmission frequency. In the illustrated embodiment, halves  33 ,  35  of the dipole antenna  32  have respective sizes appropriate for 2.45 GHz communications. Second antenna  32  is formed in other configurations in other embodiments. 
     Second antenna  32  includes plural leads  44 ,  45  for connection to integrated circuit  24 . In one embodiment, a connection  60  (shown in  FIG. 4 ) is utilized to couple leads  44 ,  45  of second antenna  32  with plural IC connections  64 ,  66  of integrated circuit  24  as described in detail below. Connection  60  is also referred to herein as a coupler or transformer. 
     Conductors  46 ,  48  of trace  37  are formed upon first surface  20  of substrate  18  to connect leads  44 ,  45  of first antenna  30  with integrated circuit  24 . In one embodiment of the invention, conductors  46 ,  48  individually have a predefined length to separate first antenna  30  and second antenna  32  by a distance d 1 . In one embodiment of the invention, distance d 1  is approximately equal to the wavelength of the wireless communication signals. Communication device  10  and interrogator unit  14  of system  12  are operable to communicate via wireless signals having a frequency of 2.45 GHz in the described embodiment. The lengths of conductors  46 ,  48  may be adjusted for utilization of other communication frequencies. 
     Separating first antenna  30  and second antenna  32  by a distance approximately equal to the wavelength of the wireless communication signals beam forms receive antenna  30  to a desired direction. Such interacting and beam forming (phase tuning) of antennas  30 ,  32  enhances the gain of both antennas  30 ,  32 . The amount of interaction between antennas  30 ,  32  depends upon the spacing of the is antennas  30 ,  32  (i.e., distance d 1 ) and the impedance load of antennas  30 ,  32 . Adjusting spacing d 1  adjusts the phase tuning of antennas  30 ,  32 . 
     Referring to  FIG. 3 , first surface  20  of substrate  18  is shown with the components removed. Positioning of brackets  36 ,  38  for coupling with power electrodes of power sources  26 ,  28  is shown in phantom. Further, positioning of integrated  24  and capacitor  34  are also shown in phantom on first surface  20 . Capacitor  34  is provided in the illustrated embodiment to reduce noise in the wireless communications. 
     Conductive pattern  37  includes pads  54 ,  56  for coupling with respective power supplies  26 ,  28 . In the described embodiment, the negative terminals of power supplies  26 ,  28  are electrically coupled with pads  54 ,  56 , respectively. In particular, battery brackets  36 ,  38  utilize spring tension to couple power supplies  26 ,  28  with pads  54 ,  56 . The negative terminals are soldered or attached to pads  54 ,  56  by conductive epoxy in alternative embodiments. Other attachment methods may also be utilized. 
     As shown in  FIG. 3 , plural vias are provided within leads  44 ,  45  of second antenna  32  and through substrate  18 . Conductive posts  47 ,  49  are inserted through the vias and electrically coupled with respective leads  44 ,  45  of second antenna  32 . Posts  47 ,  49  are provided to electrically couple first surface  20  with second surface  22  of substrate  18 . 
     Plural vias are also provided through substrate  18  for electrical connection with IC connections  64 ,  66  of integrated circuit  24 . Conductive posts  23 ,  25  are provided within vias adjacent integrated circuit  24  to provide electrical connection intermediate first surface  20  and second surface  22  of substrate  18 . A connection  60  (shown in  FIG. 4 ) is utilized adjacent second surface  22  to couple conductive posts  47 ,  49  with respective conductive posts  23 ,  25 . 
     Referring to  FIG. 4 , second surface  22  of substrate  18  is shown. A second conductive trace or pattern  57  is formed upon second surface  22 . Conductive trace  57  includes a conductor  58 , connection plane  59  and connection  60 . Second conductive trace  57  is formed of copper in one embodiment. Other conductive materials are utilized in other embodiments. Plane  59  is spaced relative to conductors  46 ,  48  provided upon first surface  20  of substrate  18  and is configured to float at a voltage of approximately 3.2 volts. 
     Conductor  58  electrically couples conductive post  50  with post  55 . Conductor  58  provides electrical coupling of the positive terminal of power source  26  with the negative terminal of power source  28  through bracket  36 , posts  50 ,  55  and pad  56 . 
     Connection  60  comprises plural conductive lines  61 ,  62  in the illustrated embodiment. Lines  61 ,  62  include respective first ends and second ends. First ends of lines  61 ,  62  are coupled with leads  44 ,  45  of second antenna  32 , respectively. Second ends of lines  61 ,  62  are coupled with IC connections  64 ,  66 , respectively. 
     First line  61  is configured to electrically couple conductive posts  23 ,  47  and second line  62  is configured to electrically couple conductive posts  25 ,  49 . Posts  47 ,  49  are electrically coupled with leads  44 ,  45  of second antenna  32 . Posts  23 ,  25  are electrically coupled with IC connections  64 ,  66  (shown in  FIG. 2 ) of integrated circuit  24 . 
     In one embodiment of the invention, lines  61 ,  62  are parallel and configured as transmission lines. Lines  61 ,  62  have a predefined distance d 2  between second antenna  32  and transponder  68 . The distance d 2  is equal to approximately one quarter the wavelength of the wireless communication signals in the described embodiment. 
     Provision of connection  60  as a quarter wave transmission line coupled with second antenna  32  forms a parasitic antenna element that interacts favorably with receive antenna  30  to enhance the receive antenna gain. As described in detail below, quarter wavelength connection  60  operates as a transformer to transform high load impedance for second antenna  32  to low load impedance. Providing low load impedance during receive operations within device  10  provides maximum interaction of transmit antenna  32  with receive antenna  30  (beam forming) and provides the desired enhancement. 
     Referring to  FIG. 5 , the modulator of transponder  68  within integrated circuit  24  includes plural antenna ports BS 1  and BS 2  which are electrically coupled with connections  64 ,  66  of integrated circuit  24 . Antenna ports BS 1  and BS 2  provide backscatter connections with transponder  68 . Connection  60  electrically couples antenna  32  with antenna ports BS 1  and BS 2 . 
     Antenna ports BS 1  and BS 2  (IC connections  64 ,  66 ) form an impedance gap in the described embodiment. Antenna ports BS 1  and BS 2  are additionally coupled with a switch  70  provided within integrated circuit  24 . Switch  70  is referred to as a backscatter switch in some embodiments. Switch  70  is operable to selectively short IC connections  64 ,  66  or insulate (e.g., electrically isolate) IC connections  64 ,  66  by opening a coupling between IC connections  64 ,  66 . Microprocessor  65  is configured to operate switch  70  in one embodiment of the invention. 
     Switch  70  is illustrated as closed in  FIG. 5  thereby shorting IC connections  64 ,  66  across the impedance gap. Switch  70  is referred to as closed or “on” when IC connections  64 ,  66  are shorted. The load impedance of second antenna  32  is low (approximately 30 Ohms in the described embodiment) when switch  70  is on. 
     Referring to  FIG. 6 , switch  70  is open providing a high impedance gap intermediate antenna ports BS 1 , BS 2  (IC connections  64 ,  66 ). Switch  70  is referred to as open or “off” when IC connections  64 ,  66  are not electrically coupled via switch  70 . The load impedance of second antenna  32  is high (approximately 150 Ohms in the described embodiment) responsive to switch  70  being off. 
     Referring to  FIG. 7 , one embodiment of a suitable switch  70  of integrated circuit  24  is shown. Switch  70  is coupled with IC connections  64 ,  66  (ports BS 1 , BS 2 ). Switch  70  includes an n-channel transistor  72  and two n-channel pull-up transistors  74 ,  76 . Transistors  74 ,  76  are respectively connected between a drain voltage Vdd and transistor  72 . 
     When the gate of transistor  72  is high (switch  70  being on), then the two halves  33 ,  35  of antenna  32  are shorted together with a fairly low impedance via IC connections  64 ,  66  and connection  60 . Second antenna  32  becomes substantially similar to a single half-wavelength antenna responsive to switch  70  being on or closed. In a backscatter mode of operation, when halves  33 ,  35  of antenna  32  are shorted together, second antenna  32  reflects a portion of the power being transmitted by interrogator  14 . 
     When the gate of transistor  72  is low (switch  70  being off), then transistor  72  is off, and transistors  74 ,  76  are on. Turning transistors  74 ,  76  on lifts antenna ports BS 1  and BS 2  both up to approximately the drain voltage of Vdd. The two antenna ports BS 1 , BS 2  and halves  33 ,  35  of second antenna  32  are isolated from one another by an open circuit. Second antenna  32  becomes substantially similar to two quarter wavelength antennas when switch  70  is off. In a backscatter mode of operation and the two halves  33 ,  35  of second antenna  32  are isolated, antenna  32  reflects very little of the power transmitted by interrogator  14 . 
     Integrated circuit  24  includes control circuitry  78  in one embodiment for controlling switch  70  between an on state and off state. Control circuitry  78  includes cross-coupled circuitry in one embodiment of the invention. Such cross-coupled circuitry is provided to make sure that both the pull up transistors  74 ,  76  and the shorting device (transistor  72 ) are not on at the same time. 
     The modulated backscatter transmitter further includes another antenna port (not shown) that is intended to be used when integrated circuit  24  is packaged in the standard SOIC package. The additional antenna port provides another option for configuring a backscatter antenna. The additional antenna port is configured to supply a one milliamp current that can drive an external PIN diode that would be situated between the two halves  33 ,  35  of the dipole antenna  32  or any other suitable antenna. The other side of that external PIN diode can be returned to either existing antenna port BS 1  or BS 2 . 
     During return link (i.e., reply mode) operations, switch  70  is turned off and on at a specified rate to form a digital return signal to interrogator  14 . Turning switch  70  on and off changes the load impedance of second antenna  32 . Switch  70  is off during forward link (i.e., receive mode) operations. 
     Maximum interaction (beam forming) of first antenna  30  and second antenna  32  occurs when second antenna  32  has a low load impedance value. However, switch  70  is off (open) during receive mode. Connection  60  operates as a transformer to transform high load impedance of second antenna  32  (responsive to switch  70  being off) into low load impedance during receive mode. Utilization of connection  60  effectively reverses the on/off states of switch  70  to off/on states. Provision of connection  60  intermediate second antenna  32  and antenna ports BS 1  and BS 2  provides low load impedance for second antenna  32  when switch  70  is off. Providing low load impedance yields maximum interaction of second antenna  32  with first antenna  30  in receive mode, enhancement of the receive gain of first antenna  30  and enhancement of communication range of communication device  10  in general. 
     In the described embodiment, first antenna  30  and second antenna  32  are beam formed in forward and backward directions normal to first surface  20  of substrate  18  (the surface containing both antennas  30 ,  32 ). The forward direction faces away from first surface  20  of substrate  18  and the backward direction faces away from second surface  22  of substrate  22 . Antennas  30 ,  32  are arranged in other configurations (e.g., other spacings d 1  are utilized) to beam form antennas  30 ,  32  in other directions in alternative embodiments. 
     Referring to  FIG. 8 , a gain plot for the receive or first antenna  30  is shown. This figure illustrates the normal gain of a loop receive antenna  30  by itself. The directivity equals approximately 3.2 dB. 
     Referring to  FIG. 9 , a gain plot for the receive antenna  30  beam formed with the transmit antenna  32  according to the present invention is shown and demonstrates the enhanced gain. The directivity is approximately 5.3 dB. The gain plot of  FIG. 9  illustrates an enhancement of receive antenna gain by 2-3 dB. The beam of receive antenna  30  is more narrowly focused with the use of transmission line connection  60  as shown in  FIG. 9 , compared with the beam of the receive antenna  30  only shown in  FIG. 8 . 
     In compliance with the statute, the invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features shown and described, since the means herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents.