Abstract:
A method of transmitting data over lines of power line network comprising: inductively generating opposite polarity currents representing the data on first and second lines of a pair of lines of the network at a first location along the lines that propagate along the lines; and sensing the propagated currents at a second location along the lines.

Description:
RELATED APPLICATIONS 
     The present application is a continuation-in-part (CIP) of PCT Application No. PCT/IL02/00454, filed on Jun. 12, 2002, which claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application No. 60/297,643, filed on Jun. 12, 2001, the disclosure of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to transmission of data and in particular to transmission of data over power lines. 
     BACKGROUND OF THE INVENTION 
     It is known to use power lines and power line networks to transmit data. An industry standard protocol referred to as CEBus has been established for data transmission between appliances sensors and control devices in a household over the power network in the household. Utility companies use power lines to transmit data to control and monitoring base stations from sensors that monitor power line equipment, such as transformers and switches, and/or power line operating conditions at different locations along a power line. 
     Transmitters and receivers (hereinafter denoted collectively as “T/R”s) used to transmit and receive data over a low power line network (up to about 250 volts), such as a typical household network, are often electrically connected to power lines in the network via direct conductive contact with the power lines. For high and medium voltage power line networks, such connections are generally undesirable. Connecting a T/R to such power line by direct conductive contact is usually labor intensive and can often be dangerous. 
     U.S. Pat. No. 5,933,073 to Shuey, the disclosure of which is incorporated herein by reference, describes a communication system that transmits data over power lines in a household power network in which T/Rs are connected via conductive contacts to a power line and a ground line in the network. U.S. Pat. No. 5,485,040 to Sutterlin, the disclosure of which is incorporated herein by reference, describes a communication system in which “power line communication apparatus” is electrically connected via conductive contacts to the “hot” line and neutral line of a power network of a household to transmit data over the network. 
     Russian Patent SU 554623, the disclosure of which is incorporated herein by reference, describes a system for generating signals on power lines of a first power line network responsive to signals received on power lines of a second power line network. The first and second power line networks are connected by a power transformer and the system routes the signals from the second power line network around the power transformer. Signals on the second power network are inductively sensed. The system is coupled to power lines in the first power line network using conductive contacts. 
     PCT Publication WO 98/20658, the disclosure of which is incorporated herein by reference, describes a “Non-Invasive Powerline Communication System” in which T/Rs are coupled capacitively or inductively to a power line in a power line system to transmit and sense signals. Signals are transmitted between a first and a second T/R over a same single power line to which both the transmitting and receiving T/R are coupled. To mount a T/R on a power line, the T/R is stressed mechanically to enlarge an opening in the T/R through which the power line is passed into the T/R. The publication states that it is an object of the invention that the T/R be capable of being installed inexpensively and safely without interrupting service to the customer. However, the publication does not describe how this may be accomplished for high voltage power lines (contact with which is not allowed) and in particular for uninsulated high voltage power lines. 
     Further systems for inductive and capacitive coupling of a T/R to a power line are described, for example, in PCT publication WO 92/16920, the disclosure of which is incorporated herein by reference. 
     U.S. Pat. No. 6,407,987 to Abraham, the disclosure of which is incorporated herein by reference, describes couplers that have capacitive circuits serially connected with an air-core transformer. One of the described couplers is a differential capacitive coupler. 
     Not all coupling methods suitable for low voltage wires (e.g., up to about 250 Volts) may be used for medium voltage wires, i.e., above 2000 volts, generally above 6000 volts. For example, wires carrying higher voltages generally require larger capacitors which add higher distortion levels. In addition, the distance between high voltage wires is generally larger than for low voltage wires and therefore requires a relatively long wire length between the T/R and the high voltage wires. Such long wires generally add high distortion levels (e.g., phase skew) to the transmitted signals. 
     UK patent application GB 2,048,622, the disclosure of which is incorporated herein by reference, describes a device for reducing the amplitude of a signal in a conductor, in order to allow detection of one wire from a group of wires, without signals from the other wires interfering in the detection. The device includes an inductive sensor for sensing signals from the wire and an inductive signal injector to apply a counter signal to the wire. 
     SUMMARY OF THE PRESENT INVENTION 
     An aspect of some embodiments of the present invention relates to providing methods and apparatus for transmitting and receiving data signals over lines of a power line network, in which signals are transmitted and received differentially via a pair of lines in the network using inductive couplers. In accordance with an embodiment of the present invention, inductive coupling comprises inductively generating in the first and second lines first and second opposite polarity electromotive forces (EMFs) respectively. Optionally, the EMFs are substantially equal in magnitude. The opposite polarity EMFs generate opposite polarity currents in the respective lines that propagate along the pair of lines so as to propagate the signal along the pair of power lines. 
     According to an aspect of some embodiments of the present invention, a data signal transmitted over a pair of lines is sensed by sensing the opposite polarity currents in the pair of lines. 
     Electromagnetic noise in a same neighborhood of two relatively closely spaced lines tends to generate same polarity and magnitude currents in each of the lines. However, as noted above, in accordance with an embodiment of the present invention, same polarity changes in current in the lines generate opposite polarity EMFs in the transformer wire. As a result, electromagnetic noise tends not to generate substantial net EMFs in the transformer wire and sensitivity of the T/R to electromagnetic noise is reduced. 
     An aspect of some embodiments of the present invention relates to differentially transmitting data signals over a pair of medium or high voltage power lines, which are both electrified to a voltage level. The transmitted signals carried by each of the lines at any moment are substantially of same amplitude and opposite phase. To achieve as close an amplitude as possible and a highest phase difference as possible, the couplers used to connect the transmitter and/or the receiver to the power lines are optionally substantially identical. It is noted, however, that due to physical constraints, the amplitudes may differ between about 1-3% and/or the phase difference between the wires may be lower than 180°, and an actual phase difference of at least 150° and preferably above 170°, is considered opposite phase in accordance with the present invention. 
     An aspect of some embodiments of the present invention relates to providing a coupling unit for coupling a T/R to lines in a power line network. The coupling unit, in accordance with an embodiment of the present invention, comprises two separate couplers, each coupler connecting to a different line of the pair of lines. Optionally, the couplers are connected in parallel, with opposite polarity, through separate wires, to a single two-terminal port of the T/R. In some embodiments of the invention, the couplers are substantially identical having substantially the same wire length and the same resistance. 
     In some embodiments of the invention, the couplers comprise capacitive couplers. Alternatively, the couplers comprise inductive couplers. Each coupler optionally comprises at least one core formed from a magnetically permeable material that is mounted to the line to which the coupler couples the T/R. A wire, hereinafter a “transformer wire”, connected to the T/R is wound around the at least one core in the coupler. Current generated in the transformer wire by the T/R generates a changing magnetic field in the at least one core and thereby an EMF in the line. A changing current in the line generates an EMF in the transformer wire that is sensed by the T/R. 
     A single transmission system, including a plurality of T/Rs may use only inductive couplers or only capacitive couplers. Alternatively, some of the T/Rs of the system may use inductive coupling while others include capacitive coupling. Further alternatively or additionally, the T/Rs use separate coupling for transmission and for reception. In some embodiments of the invention, the transmission coupling is capacitive while the reception coupling is inductive or vice versa. 
     In an embodiment of the invention a same transformer wire connected to the T/R provides current for the at least one core in each of the inductive couplers. The transformer wire is configured so that current flows around the at least one core in one of the couplers in a direction opposite to that of current flow around the at least one core in the other coupler. Therefore, a current generated in the wire by the T/R generates opposite polarity EMFs and thereby opposite polarity currents in the lines. Similarly, same polarity changes in current in the two lines generate opposite polarity EMFs in the transformer wire. Optionally, the number of windings of the transformer wire around the at least one core in each of the couplers is the same. 
     Alternatively to using a same transformer wire for both the couplers, in some embodiments of the invention, a separate wire is used for each of the couplers. Optionally, the separate wires are connected to the T/R in parallel, such that the signals sensed by the transformer wires are combined at the connection of the transformer wires to the T/R. Alternatively, each of the transformer wires is connected to a separate port of the T/R and the signals are combined within the decoder, for example after digitization and/or decoding of the signals. 
     In accordance with an embodiment of the present invention, each coupler comprises first and second parts and the at least one core in the coupler comprises first and second segments mounted respectively in the first and second parts. The first and second coupler parts may be separated and may be brought together and securely held one to the other using any of various mechanical means and configurations. By way of example, the two parts may be distanced from each other or closed together using a suitable bolt in a manner in which the jaws of a vise are opened and closed using a bolt. When the two coupler parts are brought together, the first and second segments of the at least one core are brought together to form the complete core. 
     To mount the coupler to a line, the two parts of the coupler are separated or distanced from each other. The line is positioned between the parts and the two parts brought together so that after being brought together the line is located and held in the open space at the center of the at least one core. The line may be positioned between the coupler parts and the coupler parts brought together using a suitable elongated insulated tool that enables a person installing the coupler to maintain a safe distance from the line during the installation process. 
     There is therefore provided in accordance with an embodiment of the present invention, a method of transmitting data over lines of power line network comprising: inductively generating opposite polarity currents representing the data on first and second lines of a pair of lines of the network at a first location along the lines that propagate along the lines; and sensing the propagated currents at a second location along the lines. 
     Optionally, the power line network is a low voltage power line network and at least one of the first and second lines is electrified with a voltage in excess of about 100 volts. Alternatively, the power line network is optionally a medium or high voltage power line network and at least one of the first and second lines is electrified with a voltage in excess of about 2 KV. 
     In some embodiments of the present invention, the opposite polarity currents are substantially equal. 
     In some embodiments of the present invention, inductively generating comprises inductively coupling a first magnetic circuit to the first line and a second magnetic circuit to the second line and generating opposite sense changes in the magnetic fields in each of the magnetic circuits, where the senses of the magnetic fields in the first and second magnetic circuits are determined relative to same sense directions along the first and second lines respectively. 
     Optionally, the first and second magnetic circuits respectively comprise first at least one and second at least one magnetically permeable cores, wherein each at least one core is wound with at least one loop of a conducting wire and inductively coupling the first and second magnetic circuits comprises mounting the first and second at least one magnetic core to the first and second lines respectively. 
     Optionally, generating opposite sense changes in the magnetic field in the first or second magnetic circuits comprises generating opposite sense current changes in wire loops wound about the at least one first and at least one second core, wherein the sense of a current change in the at least one wire loop wound about the first or second core is determined relative to the same direction used to determine the sense of the magnetic field in the first or second core. 
     In some embodiments of the present invention, the at least one loop wound around the first and/or second core comprises a plurality of loops. Optionally, the current change is generated in all the loops of the plurality of loops. Alternatively, the current change is generated in only a portion of the loops of the plurality of loops. In some embodiments of the present invention, the number of loops in which the current change is generated is less than or equal to four. In some embodiments of the present invention, the number of loops in which the current change is generated is less than or equal to three. In some embodiments of the present invention, the number of the plurality of loops is less than or equal to twenty. In some embodiments of the present invention, the number of the plurality of loops is less than or equal to ten. 
     In some embodiments of the present invention, generating opposite sense current changes comprises generating changes in currents that flow in same senses through the at least one loop wound about the first at least one core and second at least one core respectively. 
     In some embodiments of the present invention, generating opposite sense current changes comprises generating changes in currents that flow in opposite senses through the at least one loop wound about the first at least one core and second at least one core respectively. 
     Additionally or alternatively generating opposite sense currents comprises winding first and second wires about the first and second at least one cores respectively and generating a current change in the first wire and an opposite sense current change in the second wire. 
     Optionally, generating the opposite sense current changes comprises connecting the ends of the first wire and the ends of the second wire to a signal circuit that generates the current change. Optionally, each end of the first wire and an end of the second wire are connected to a same terminal connected to the signal circuit. Alternatively, at least one end of each wire is connected to a terminal connected to the signal circuit to which an end of the second wire is not connected. In some embodiments of the present invention, connecting the first and second wires comprises connecting the wires to the circuit via a transformer. 
     In some embodiments of the present invention, generating opposite sense currents comprises winding a same wire in opposite senses about the first at least one core and the second at least one core and generating a change in a current flowing through the wire. Optionally, generating a change in the current comprises connecting the ends of the wire to a signal circuit that generates the current change. Optionally, comprising twisting the wire around itself for a portion of its length between the signal circuit and the first at least one core and/or the second at least one core. Additionally or alternatively, connecting the wire to the circuit comprises connecting the wire to the circuit via a transformer. 
     In some embodiments of the present invention, the transformer is a center tapped transformer. In some embodiments of the present invention, mounting the first or second at least one core to a line comprises: forming first and second matching segments of the at least one core; mounting the first and second segments in first and second matching housings respectively; positioning the housings so that the line is located between the first and second core segments; and closing the first and second housing to each other so that the first and second segments of the at least one core meet and form a magnetic circuit that surrounds the line. 
     Optionally, the method comprises mounting a line guide to the first segment into which the line is placed to properly position the line between the first and second core segments. Alternatively or additionally, the first and second housings are coupled by a bolt that and closing the first and second housing to each other comprises turning the bolt. Optionally, the line is an overhead line and mounting the at least one core comprises hanging the coupled housings on the line by positioning the first housing so that the housing rests on the line and then turning the bolt. Additionally or alternatively, turning the bolt comprises turning the bolt using an insulated tool sufficiently long so that a safe distance from the line may be maintained during mounting. In some embodiments of the present invention, the line is a voltage carrying line that is electrified during mounting. 
     In some embodiments of the present invention, the method comprises sensing opposite polarity currents transmitted along the first and second lines to the first location and wherein sensing a propagated current comprises sensing a change in magnetic flux through a loop of the at least one loop generated by the current. 
     In some embodiments of the present invention, the first and second lines are uninsulated. In some embodiments of the present invention, at least one of the lines is a neutral line. In some embodiments of the present invention, both lines are voltage carrying lines. 
     In some embodiments of the present invention, generating current changes comprises generating current changes characterized by frequencies greater than about 2 MHz. In some embodiments of the present invention, generating current changes comprises generating current changes characterized by frequencies greater than about 4 MHz. In some embodiments of the present invention, generating current changes comprises generating current changes characterized by frequencies less than about 30 MHz. 
     There is further provided, in accordance with an embodiment of the present invention, apparatus for transmitting a signal over first and second lines of a pair of lines of a power line network the apparatus comprising: a first magnetic circuit inductively coupled to the first line; a second magnetic circuit inductively coupled to the second line; and a signal circuit that generates changes in magnetic flux in the first and second magnetic circuits responsive to the signal. 
     Optionally, the power line network is a low voltage power line network and at least one of the first and second lines is electrified with a voltage in excess of about 100 volts. Alternatively, the power line network is a medium or high voltage power line network and at least one of the first and second lines is electrified with a voltage in excess of about 2 KV. 
     In some embodiments of the present invention, the first and second magnetic circuits respectively comprise first at least one and second at least one magnetically permeable cores, wherein each at least one core is wound with at least one loop of a conducting wire connected to the circuit and wherein the circuit generates current changes in the at least one loop wound about each at least one core to generate the changes in magnetic flux. 
     Optionally, the at least one loop wound around the first and/or second core comprises a plurality of loops. In some embodiments of the present invention, the signal circuit generates current changes in all the loops of the plurality of loops. Alternatively, the signal circuit is connected to the plurality of loops so as to generate current changes in only a portion of the loops. In some embodiments of the present invention, the number of loops in which the current changes are generated is less than or equal to four. In some embodiments of the present invention, the number of loops in which the current change is generated is less than or equal to three. 
     In some embodiments of the present invention, the number of the plurality of loops is less than or equal to twenty. In some embodiments of the present invention, the number of the plurality of loops is less than or equal to ten. 
     In some embodiments of the present invention, the number of loops wound about the first and/or second at least one core is less than four. In some embodiments of the present invention, the number of loops wound about the first and/or second at least one core is less than three. 
     In some embodiments of the present invention, a same wire is wound about the first and second at least one cores and wherein the sense of the winding of the wire about the first at least one core is opposite to that of the winding of the second at least one core, where the sense of winding about the first and second at least one core is determined relative to same sense directions along the first and second lines respectively. Optionally, the wire is twisted around itself for a portion of its length between the circuit and the first at least one core and/or the second at least one core. In some embodiments of the present invention, the wire is connected to the circuit via a transformer. Optionally, the transformer is a center-tapped transformer. 
     In some embodiments of the present invention, each at least one core comprises first and second matching segments which are brought together to form a magnetic circuit so that the magnetic circuit surrounds the line. Optionally, the apparatus comprises first and second matching housings in which the first and second segments are mounted. Optionally, the apparatus comprises a bolt coupled to the first and second housings that can be turned to separate the housings from each other or close the housings to each other so that the first and second segments of the at least one core meet and form the magnetic circuit. Optionally, the signal circuit senses opposite polarity currents representing a signal that are transmitted along the first and second lines to the first location by sensing changes in flux through a loop of the at least one loop wound respectively around the first and second cores generated by the currents. 
     In some embodiments of the present invention, the signal circuit generates flux changes characterized by frequencies greater than about 2 MHz. In some embodiments of the present invention, the signal circuit generates flux changes characterized by frequencies greater than about 4 MHz. In some embodiments of the present invention, the signal circuit generates flux changes characterized by frequencies less than about 30 MHz. 
     There is further provided, in accordance with an embodiment of the present invention, a coupler for inductively coupling a circuit to an electrified line, the coupler comprising: a first part having a first segment of at least one core formed from a magnetically permeable material; a second part having a second segment of the at least one core that matches the first segment; a wire, electrically connected to the circuit, that loops around at least one segment of the at least one core at least once; and a bolt coupled to the first and second parts turnable to separate the first and second parts from each other or close the first and second parts to each other so that the first and second segments of the at least one core meet and form a magnetic circuit that surrounds the line. 
     There is further provided, in accordance with an embodiment of the present invention, apparatus for transmitting a signal over a power line network, comprising a first coupling circuit adapted to couple to a first line of the network, a second coupling circuit adapted to couple to a second line of the network, a signal circuit, including at least one two-terminal port, adapted to transmit or receive electrical signals representative of data transmitted on the first and second lines, a first wire connected to a two-terminal port of the signal circuit and to the first coupling circuit, so as to transfer signals between the signal circuit and the first coupling circuit and a second wire connected to a two-terminal port of the signal circuit and to the second coupling circuit, so as to transfer signals between the signal circuit and the second coupling circuit. 
     Optionally, the first and second wires are connected to the same two terminal port of the signal circuit. Optionally, the first and second wires are connected to the same two terminal port of the signal circuit, with opposite orientation. Optionally, the first coupling circuit comprises an inductive coupling circuit. Optionally, the first coupling circuit includes a magnetic core and wherein the first wire is wound around the core one or more loops. Optionally, the first wire comprises a resistor parallel to the one or more loops. Optionally, the first coupling circuit comprises a capacitive coupling circuit. Optionally, the first wire comprises a resistor adjacent the first coupling circuit. Optionally, the resistance of the resistor is substantially equal to the typical resistance of the first wire. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
       Non-limiting examples of embodiments of the present invention are described below with reference to figures attached hereto and listed below. In the figures, identical structures, elements or parts that appear in more than one figure are generally labeled with a same numeral in all the figures in which they appear. Dimensions of components and features shown in the figures are chosen for convenience and clarity of presentation and are not necessarily shown to scale. 
         FIG. 1  is a schematic drawing of a system for transmission of data on a pair of lines in a power line network, in accordance with an embodiment of the present invention; 
         FIGS. 2A ,  2 B,  2 C and  2 D are schematic drawings of systems for transmission of data on a pair of lines in a power line network, in accordance with alternative embodiments of the present invention; 
         FIGS. 3A and 3B  are schematic drawings of systems for connecting a T/R to a line in a power line network, in accordance with an embodiment of the present invention; and 
         FIGS. 4A-4C  show a coupler suitable for use in coupling a T/R to a line in a power line network, in accordance with an embodiment of the invention. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIG. 1  schematically shows a system  20  for transmitting data signals over a pair of lines  22  and  24  in a power line network, in accordance with an embodiment of the present invention. System  20  is suitable for transmitting signals over lines in low voltage as well as medium and high voltage power line networks. However, the system is expected to be particularly advantageous for use in transmitting signals over medium and high voltage power line networks, such as for example power line networks that carry voltages in voltage ranges above about 2 Kilovolts. By way of example, lines  22  and  24  are assumed to be above ground lines in a medium or high voltage power line network. Both lines  22  and  24  may be high voltage lines or one of lines  22  and  24  may be a neutral line in the power line network. Lines  22  and  24  generally run parallel to each other distanced by a safety distance required due to the voltage levels they carry. Although the present invention may be employed with lines  22  and  24  at any distance from each other, the present invention is especially advantageous when lines  22  and  24  are separated by more than half a meter or even a meter. As mentioned above, prior art methods are not suitable for lines separated by such distances. 
     System  20  optionally comprises a transmitter/receiver (T/R)  26  coupled to lines  22  and  24  by a coupling unit  28 . In some embodiments of the invention, as is now described, the same coupling apparatus is used for both transmission by T/R  26  onto lines  22  and  24  and reception of signals from lines  22  and  24  by T/R  26 . In an exemplary embodiment of the invention, coupling unit  28  comprises two inductive couplers  30  and  32 . Inductive couplers  30  and  32  respectively optionally comprise at least one magnetically permeable core represented by a single core  34  and  36 . Cores  34  and  36  are mounted on lines  22  and  24  respectively. Only features and elements of conductive couplers  30  and  32  germane to the discussion of system  20  are shown in FIG.  1 . 
     In accordance with an embodiment of the present invention, a single wire  40 , i.e. a transformer wire  40 , is wound about both cores  34  and  36 . Thus, a single transmission by T/R  26  onto wire  40  is sufficient to induce signals in both lines  22  and  24 . Wire  40  is optionally wound about core  34  in an opposite sense to the sense in which wire  40  is wound about core  36 . The winding of the wire in opposite directions causes inductive effects between wires  22  and  24  to intensify the transmitted signals rather than reduce or even cancel the transmitted signals. Optionally, the number of turns of transformer wire  40  about each of cores  34  and  36  is the same. Alternatively, the number of turns about each of cores  34  and  36  is different, for example when the noise conditions are expected to be different. 
     It is assumed that if lines  22  and  24  are uninsulated, transformer wire  40  is insulated with a suitable insulation known in the art that protects the wire from voltage on the lines. In addition assuming that lines  22  and  24  are outside lines and that transformer wire  40  is exposed to the weather, the insulation is also weather proof insulation that does not readily deteriorate as a result of exposure to sunlight and rain. 
     Ends  42  of transformer wire  40  are optionally connected to a two-terminal port of T/R  26  by a suitable connector, connectors or cable. In  FIG. 1  ends  42  are shown, by way of example, connected to T/R  26  by a coaxial cable  44  which in turn is connected to T/R  26 . It is assumed that transformer wire  40  is insulated with a suitable insulator that protects the wire from voltage on line  22  or  24 . Optionally, the length of wire  40  from ends  42  to cores  34  and  36  is substantially the same, so that the amplitude of the transmitted signals on lines  22  and  24  is substantially the same and the phases of the signals are generally opposite, for example at least above 170° or even 175°. 
     T/R  26  transmits a signal over lines  22  and  24  to another T/R (not shown) coupled to the lines by transmitting a signal via coaxial cable  44  to transformer wire  40  that generates a corresponding current in the transformer wire. Since wire  40  is wound around cores  34  and  36  in opposite senses, the current in transformer wire  40  generates opposite polarity EMFs in lines  22  and  24 . The opposite polarity EMFs generate opposite polarity currents in lines  22  and  24  and an associated potential difference between the lines, which propagate along the lines to the other T/R. The other T/R senses the transmitted signal by sensing the transmitted currents and/or associated voltage difference. 
     T/R  26  senses a signal transmitted over lines  22  and  24  by sensing opposite polarity changes that represent the signal in current in the lines. Opposite polarity changes in currents in lines  22  and  24  generate EMFs in transformer wire  40  that add and provide a voltage difference between ends  42  of the wire that is a sum of the generated EMFs. The voltage difference is transmitted to T/R via coaxial cable  44 . 
     As mentioned above, coupling unit  28  in the embodiment shown in  FIG. 1  is adapted to handle both transmission and reception of signals. In this embodiment, the number of turns of transformer wire  40  about each core  34  and  36  is optionally determined as a compromise between the optimum for transmission and the optimum for reception. Optionally, the number of turns about each core is relatively small, between 1 and 5, for example between 2-4. The relatively small number of turns of transformer wire  40  about cores  34  and  36  enables generating currents in lines  22  and  24  at relatively high frequencies in a range of between about 2-30 MHz and higher. As a result, data can be transmitted by system  20  over lines  22  and  24 , in accordance with an embodiment of the present invention, at high data rates. 
     Alternatively to using the same number of turns for both transmission and reception, coupling unit  28  includes a switch (not shown) which changes the number of turns connected to T/R  26 . Optionally, when T/R transmits signals, a command is sent to the switch to connect only some of the turns to wire  40 . When T/R  26  is receiving signals, the switch is set to connect all the turns to wire  40 . In some embodiments of the present invention, whereas a relatively small number of turns of wire  40  about a core are used to transmit signals over lines  22  and  24 , a larger number of turns are used to sense signals transmitted over the lines. The larger number of turns used to sense signals improves sensitivity of T/R for sensing signals. In some embodiments of the present invention, a total number of turns about a core is between about 10-20, all of which are optionally utilized to receive signals, but only about 1-3 turns are used to transmit signals. 
     Further alternatively or additionally, separate couplers are used for transmitting and receiving signals, such that the number of turns, as well as other parameters of the coupling apparatus, may be optimized separately for transmission and reception, without requiring the use of a switch. Optionally, the coupling apparatus for the transmission and for the reception include the same cores  34  and  36 , with different wires connecting the cores to T/R  26 . Alternatively, different cores are used for transmission and reception. In some embodiments of the invention, the different cores are spaced from each other to reduce interference. Various methods known in the art may be used to couple a different number of loops of wire  40  to T/R  26  for transmitting signals than are coupled to the T/R for sensing signals. For example, for transmitting a signal, a tap may be connected to a loop intermediate a first and last loop of the loops wound about a core  34  and or core  36 . Current changes used to transmit signals may then be generated in loops between the tapped loop and the first or last loop of the loops. Alternatively, first and second wires may optionally be wound around a core different number of times. One of the wires may be used for transmitting signals and the other wire used for sensing signals. For simplicity of presentation, it is assumed that a same number of loops is used to transmit and sense signals and that the number is sufficiently small to enable efficient generation of currents in lines  22  and  24  characterized by frequencies between 2 MHz and 30 MHz. 
     Whereas opposite polarity current changes in lines  22  and  24  provide EMFs that add, same polarity current changes in lines  22  and  24  generate EMFs in transformer wire  40  that subtract. A voltage difference between ends  42  of transformer wire  40  caused by same polarity current changes is substantially equal to a difference between the generated EMFs. As a result, sensitivity of T/R  26  to electromagnetic noise, which tends to generate same polarity current changes in lines  22  and  24 , is substantially reduced. 
     In some embodiments of the invention, one or more high voltage protection devices  46  are connected between the ends  42  of wire  40  to a ground, which may be a ground line in the power line network. Since wire  40  feeds directly into low voltage circuitry, these protection devices protect both the circuitry and the locations to which they are connected from damaging and possibly lethal voltages. In some embodiments of the invention, high voltage protection devices  46  may be, for example, discharge tubes, or other devices for discharging over-voltages, as known in the art. 
       FIG. 2A  schematically shows a system  50  for transmitting data signals over lines  22  and  24 , in accordance with an embodiment of the present invention, that is a variation of system  20  shown in FIG.  1 . In system  50 , transformer wire  40  is twisted back on itself for a distance from each core  34  and  36  from the core almost to coaxial cable  44 . Twisting transformer wire  40  tends to reduce noise transmitted by the wire to coaxial cable  44 . 
       FIG. 2B  schematically shows a system  80  for transmitting data signals over lines  22  and  24 , in accordance with another embodiment of the present invention. In system  80 , separate wires  82  and  84  connect cores  34  and  36  to coaxial cable  44 . Using separate wires  82  and  84  generally reduces the noise added to transmitted signals from the inductive coupling, especially for high frequency signals and/or when the distance between coaxial cable  44  and lines  22  and  24  is relatively large. 
     Optionally, as shown in  FIG. 2B , wires  82  and  84  are connected in parallel to coaxial cable  44 . Alternatively, T/R  26  includes separate ports for each of wires  82  and  84 . the signals from wires  82  and  84  are optionally combined after digitation and/or decoding. It is noted that wires  82  and  84  may be totally separate or may include a single wire connected in two middle points to coaxial cable  44 . 
       FIG. 2C  schematically shows a system  90  for transmitting data signals over lines  22  and  24 , in accordance with still another embodiment of the present invention. System  90  is similar to system  80 , but is different in two unrelated details, each of which may be implemented separately without relation to the other. In system  90 , instead of using a coaxial cable  44  to connect wires  40  to T/R  26 , a twisted wire  49  is used. The use of twisted wire  49  generally reduces the noise added by the wire. The embodiment of  FIG. 2C  is especially useful when the length of twisted wire  49  is relatively large, e.g., above about half a meter. For short lengths of a few centimeters the use of coaxial cable  44  is generally suitable. 
     System  90  further illustrates resistors  97  in parallel to the wire turns around cores  34  and  36 . Resistors  97  optionally have a resistance which is the typical resistance of wire  40 , in order to maximize the energy in the vicinity of cores  34  and  36 . 
       FIG. 2D  schematically shows a system  70  for transmitting data signals over lines  22  and  24 , in accordance with still another embodiment of the present invention. In system  70 , T/R  26  is coupled to lines  22  and  24  using capacitive couplers  74 . Each of couplers  74  optionally includes a capacitor  72  which is connected on one end to the power line  22  or  24  and on the other end to a wire  40 . A resistor  97 , having substantially the typical resistance of wire  40 , is positioned along wire  40  near the connection point to capacitor  72 . The wires  40  are connected to wire  44  with opposite directions, so that the signals are transmitted on wires  22  and  24  with opposite polarity. As in the above described inductive embodiments, the length of wires  40  and the parameters of couplers  74  are substantially the same, so that the signals have substantially same amplitudes and opposite phases. Wires  40  may be of substantially any length but are generally relatively long, e.g., between 4-7 meters. 
       FIGS. 3A and 3B  show an alternative methodology for connecting transformer wire  40  to T/R  26 . In the embodiments shown in  FIGS. 3A and 3B , ends  42  of transformer wire  40  are connected to a matching transformer  52 . Matching transformer  52  is optionally connected to T/R  26  via a twisted pair  54 . The use of a matching transformer may facilitate matching the impedance of lines  22  and  24  to that of T/R  26 . 
     The connection shown in  FIG. 3A  corresponds to FIG.  1  and the connection shown in  FIG. 3B  corresponds to  FIGS. 2A-2D . Optionally, the twisted pair is shielded, by a shield  56  as shown in FIG.  3 B. Twisted pair  54  can also be a coaxial line  44  (as shown for example in FIG.  1 ). Optionally, as shown in  FIG. 3B  matching transformer  52  is a center-tapped transformer. Use of a center-tapped transformer provides a balanced line for which the opposite polarity EMFs generated in lines  22  and  24  by current in wire  40  have substantially equal magnitude. 
     In any of the embodiments shown, filters for the power line frequency and optionally its lower harmonics, may be inserted in lines along which the signals to or from the transmitter/receiver. Such filters may be as simple as blocking capacitors  58  (shown in  FIGS. 3A and 3B ) for the relatively low line frequency and harmonics or may include Pi or T filter sections, known in the art. 
     It is noted that whereas a single circuit is used to “drive” cores  34  and  36  and that the loops of wire about the respective cores are connected in series, other configurations of driving circuits and wiring configurations may be used to generate and sense opposite polarity currents in lines  22  and  24 , in accordance with an embodiment of the present invention. For example, each core  34  and  36  may be wound with a different wire and each wire connected to a different circuit to generate and sense currents in lines  22  and  24 . Other configurations for generating and sensing currents in lines  22  and  24  will occur to a person of the art. 
       FIGS. 4A-4C  schematically show an inductive coupler  100  suitable for use in a coupling unit for coupling a T/R (not shown) to a line in a power line network, in accordance with an embodiment of the invention. In  FIGS. 4A-4C  coupler  100  is shown being mounted to a line  102  in a power line network, which is by way of example assumed to be an overhead high voltage line. 
     Coupler  100  comprises top and bottom parts  104  and  106  that may be distanced from each other or brought together by a bolt  108  in a manner in which jaws of a vise are separated and brought together. Bolt  108  may conveniently be turned by a suitable optionally insulated wrench that couples to a hole  107  formed in the bolt or a cross-pin  109  formed in or inserted into the bolt. Top part  104  comprises a line guide  110  for positioning and securing line  102 . Line guide  110  is optionally formed from an insulating material sufficient to electrically insulate coupler  100  from voltage carried on line  102 . 
     Coupler  100  comprises, by way of example two cores  112  and  116  formed from a magnetically permeable material, such as a ferromagnetic material. Each of cores  112  and  116  comprises two matching segments, a top segment and a bottom segment. Top and bottom segments of each core  112  and  116  are comprised in top and bottom coupler parts  104  and  106  respectively. In  FIG. 4A  portions of top segments  113  and  117  of cores  112  and  116  are shown.  FIG. 4B , in which top part  104  is partially cut away, shows both top and bottom segments of both cores  112  and  116 . It is noted that a coupler, in accordance with an embodiment of the present invention, similar to coupler  100  may comprise more or less than one magnetically permeable core. The use of a plurality of cores rather than, for example, a single core may be used to reduce undesirable eddy currents. 
     A transformer wire  40  enters bottom part  106  optionally through an insulating collar  121  fit into a suitable aperture (not shown) in the bottom part and loops around each of bottom segments  114  and  118  of cores  112  and  116  as schematically shown in FIG.  4 B. Transformer wire  40  optionally exits bottom part  106  through the same aperture through which it enters the bottom art. In  FIG. 4B  by way of example, transformer wire  40  loops around bottom segments  114  and  118  of cores  112  and  116  twice. The number of times transformer wire  40  loops around bottom segments  114  and  118  may be different from that shown in FIG.  4 B. Transformer wire  40  is used to inductively couple a suitable T/R to cores  112  and  116  and thereby to line  102 , for example, using a method shown in  FIGS. 1-3B . 
     To mount coupler  100  to line  102  the coupler is “hung” from the line with the line positioned in guide  110  as shown in FIG.  4 A. Bolt  108  is then turned to bring top and bottom parts  104  and  106  tightly together trapping line  102  inside guide  110  between the guide and a top surface  120  ( FIG. 4B ) of bottom part  106 . Bottom part  106  is optionally formed from an insulating material sufficient to electrically insulate coupler  100  and elements thereof from voltage carried on line  102 . Optionally, top surface  120  is covered with a suitable insulating material to insulate bottom part  106  and elements of coupler  100  from voltage on line  102 . 
     Transformer wire  40  is insulated with a suitable, preferably all weather insulation that insulates wire  40  from voltage in line  102 . Any of various insulation materials and configurations known in the art and rated for voltages required to insulate wire  40  from voltage on line  102  may be used in the practice of the present invention. In addition, top segments  114  and  118  are optionally formed and positioned in top and bottom parts  104  and  106  so that they do not come in contact with and are distanced from line  102  when the top and bottom parts are brought together. It is noted that line guide  110  also optionally insulates top segments  113  and  117  from line  102 . 
     It is noted that coupler  100  may be positioned and top and bottom parts  104  and  106  closed together using a properly insulated tool that is sufficiently long to enable an installer to position and close the coupler while maintaining a safe distance from line  102 . For example, assuming that line  102  is an overhead line, any of various tools known in the art may be used to lift coupler  100  from the ground to the line and hang the coupler on the line. A suitable elongated insulated wrench that couples to cross-pin  109  and/or hole  107  may be used by an installer standing on the ground or on a suitable elevated installation platform to turn bolt  108  and close coupler  100 . The wrench, which may be a part of the lifting tool or separate from the lifting tool, may for example comprise a pole having at an end thereof a socket that fits over bolt  108  and locks onto cross-pin  109 . Alternatively, the wrench may for example, have at the end of the pole a pin that fits into hole  107 . 
     When top and bottom parts  104  and  106  are clamped together by bolt  108 , top segments  113  and  117  of cores  112  and  116  meet and are pressed together to bottom segments  114  and  118  respectively to complete magnetic circuits that surround line  102 . To assure proper completion of the magnetic circuits when closed, top and bottom parts  104  and  106  are advantageously accurately registered one to the other to assure that top and bottom segments of cores  112  and  116  are accurately aligned. By way of example, in coupler  100  a surface  122  of top part  104  and a surface  124  of bottom part  106  are used to register the top and bottom parts. Surfaces  122  and  124  are positioned relative to bolt  108  so that coupler  100  cannot be closed unless the surfaces are accurately parallel. Other methods of accurately registering top and bottom parts  104  and  106  of coupler  100  to each other will readily occur to a person of the art. For example, top and bottom parts  104  and  106  may be fitted with guide rods and matching guide rod holes. 
     It is noted that generally, when differential signals are transmitted on two electrical lines, the coupling induces the signals, although to a lesser extent, onto other electrical lines parallel the lines on which the signals are transmitted. In some embodiments of the invention, this coupling between parallel wires is utilized, in communicating between transceivers coupled to different pairs of wires. For example, in neighborhoods in which a first phase line leads to a first group of houses and a second phase line leads to a second group of houses, a single transceiver connects to one of the first and second wires at a point at which they are parallel to each other, such that the signals are applied to both the first and second wires, due to the parallel inductance. Alternatively or additionally, a repeater is used to transfer signals between the first and second lines, when necessary. The repeater optionally includes two pairs of cores. A first pair of cores connects, for example, to the ground line and the first phase line and the second pair of cores connects to the ground line and the second phase line. Optionally, the turns around the cores of the first pair and the respective cores of the second pair are wound in the same direction for the repetition of the signals. 
     Although the above description relates to differential transmission using inductive coupling, the principles of the invention may be used for differential transmission using other coupling methods, e.g., capacitive coupling. Using capacitive coupling is generally simpler as it does not require paying attention to the directions of the turns of the inductive coupling and does not involve the cross talk levels that generally appear in inductive coupling. 
     Although the above methods are particularly useful for transmission of signals on medium and high voltage power lines, some or all of the methods of the present invention may be used for transmission on low voltage power lines, having a voltage beneath 300 volts (e.g., 110 or 220). 
     In the description and claims of the present application, each of the verbs, “comprise” “include” and “have”, and conjugates thereof, are used to indicate that the object or objects of the verb are not necessarily a complete listing of members, components, elements or parts of the subject or subjects of the verb. 
     The present invention has been described using detailed descriptions of embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. The described embodiments comprise different features, not all of which are required in all embodiments of the invention. Some embodiments of the present invention utilize only some of the features or possible combinations of the features. Variations of embodiments of the present invention that are described and embodiments of the present invention comprising different combinations of features noted in the described embodiments will occur to persons of the art. The scope of the invention is limited only by the following claims.