Patent Publication Number: US-7903041-B2

Title: Magnetic antenna apparatus and method for generating a magnetic field

Description:
FIELD OF THE INVENTION 
     This invention relates generally to radio communications and more particularly to communications based on magnetic transmission. 
     BACKGROUND OF THE INVENTION 
     Magnetic transmit antennas are typically configured as loops of wire having a modulated current driven through them. The higher the current at the transmitted frequencies, the greater the strength of the magnetic field and, hence, the greater the transmission range of the antenna. Conventional transmit antenna designs often use a power amplifier coupled directly to the antenna, along with a tuning capacitor to cause the antenna loop to be resonant at the transmission frequency. Loop resonance is one way to increase the current and hence the magnetic field strength of the transmit antenna. However, inducing resonance in the loop antenna may undesirably generate high voltages at the resonant frequency. Such high voltages can be in the range of 1,000 to 4,000 volts, for example. These voltages can create electrical arcs that could ignite explosive gasses within the transmitter&#39;s operational environment (e.g. a coal mine) and/or cause other undesirable effects. 
     On the other hand, if the additional tuning circuitry is not used in conjunction with a power amplifier directly coupled to the magnetic transmit antenna so as to cause resonance within the loop antenna (and thereby increase the magnetic field strength) then a much more powerful amplifier must be used in order to provide a substantial drive current to the loop antenna for most practical applications. For example, if a loop antenna presented a load impedance of 2 ohms, and if 100 amperes of current is needed in each loop of antenna wire for a sufficient magnetic field strength for a given application, then the amplifier would be required to provide about 200 volts of drive voltage at 100 amperes (i.e. 20,000 Watts or 20 KW). Such high power amplifiers are extremely costly, heavy and generally impractical to implement in most environments. Moreover, such a high power amplifier would severely drain a portable battery, present both a large and weighty mass element, and further generate significant heat losses. Such undesirable effects tend to preclude implementation of such a structure, particularly in environments requiring portable operations. Alternative mechanisms for increasing transmission range of magnetic loop transmit antennas is desired. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a magnetic transmit antenna apparatus comprising: a toroidal core transformer having a primary winding inductively coupled to a secondary winding supplying a low voltage and high current to a magnetic transmit antenna wherein the magnetic transmit antenna includes a wire loop having multiple turns for generating a magnetic field. The toroidal core transformer includes a primary winding that operates in association with the secondary winding to match the impedance of a signal source to the magnetic transmit antenna. 
     The invention also relates to a process for generating a magnetic field comprising supplying a high voltage, low current to a primary winding of a toroidal core transformer, inductively coupling the primary winding to a secondary winding of the toroidal core transformer for supplying a low voltage and high current to a magnetic transmit antenna, thus generating a magnetic field. 
     Still further, a magnetic transmit antenna apparatus for transmitting communications data comprises: a power amplifier  160  having an input  160   a  for receiving a communications data signal waveform  105   a  for transmission, and an output providing an amplified output signal waveform  105   a ′ corresponding to said received communications data signal waveform; and a non-resonant toroidal core transformer driver  130  coupled between the power amplifier and a magnetic loop transmit antenna  140 , the toroidal core transformer driver having a primary winding inductively coupled to a secondary winding and responsive to the output signal waveform  105   a ′ from the power amplifier to supply an increased current signal waveform  107  to the magnetic loop transmit antenna, wherein the magnetic loop transmit antenna includes a wire loop having multiple turns for generating a magnetic field according to the current signal waveform from the driver to transmit the communications data. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Understanding of the present invention will be facilitated by consideration of the following detailed description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which like numerals refer to like parts and: 
         FIG. 1  illustrates a block diagram of a magnetic transmit antenna system according to an embodiment of the invention; 
         FIG. 2  illustrates a schematic circuit diagram of a magnetic transmit antenna system according to an embodiment of the invention. 
         FIGS. 3 and 4  illustrate graphical representations of selected operational characteristics of a magnetic transmit antenna system according to an embodiment of the invention; and 
         FIG. 5  illustrates a flow chart of a process for generating a magnetic field according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiments is merely by way of example and is in no way intended to limit the invention, its applications, or uses. 
     Before embarking on a detailed discussion, the following should be understood. Near-field magnetic wireless communications utilize non-propagating magnetic induction to create magnetic fields for transmitting (and receiving) as opposed to conventional radio frequency (RF) communications that create time varying electric fields. RF fields are virtually unbounded, tending to decrease in intensity as the square of the distance from the transmitting antenna, whereas magnetic fields decrease as the cube of the distance from the transmitting antenna in certain transmission media (e.g. in air or vacuum). Magnetic wireless communications generally do not suffer from the nulls and fades or interference or that often accompanies RF communications. However, conventional magnetic transmit loop antennas and their power amplifiers and tuning circuitry produce high voltages when operating at resonant frequencies. As previously described, this can cause dangerous power levels in the magnetic antenna loop, creating safety hazards. 
     The strength of the transmitted magnetic field is essentially dependant on the amount of current flowing in the transmit loop, rather than the voltage across the loop. The higher the current at the transmitted frequencies, the greater the strength of the magnetic field. 
     Current flowing in a loop antenna is the primary determinant of magnetic field strength. Magnetic moment (M) is determined as the amount of current in a loop of wire multiplied by the number of loops of wire and the cross sectional area of the loop(s) (i.e. Magnetic moment (M)=(current in a loop of wire)×(number of loops of wire)×(cross sectional area of the loop(s)). Actual total power or voltage applied is not a significant factor in transmission power. 
     In accordance with an aspect of the present invention, employing a transformer driver between a power amplifier and the loop of a transmit antenna provides a means to step up the current in the loop and proportionally step down the voltage, thereby keeping the power essentially constant. This enables operating the system according to an aspect of the present invention such that resonance of the loop transmit antenna is not induced, thereby allowing a broad frequency range for transmission. This is in contrast to prior art configurations that require operation at resonance, which provides only a narrow frequency range at which the transmit antenna device can function. 
     Moreover, the magnetic flux in a toroid is largely confined to the core, preventing its energy from being absorbed by nearby objects, making toroidal cores essentially self-shielding. Therefore, an additional feature of the toroidal transformer driver of the present invention is that it efficiently retains most of the magnetic energy in the transformer itself, thus reducing the amount of electromagnetic interference (EMI) shielding otherwise required in a application where EMI radiation must be kept to a minimum. 
     Referring now to the drawings, there is shown in  FIG. 1  a block diagram of a magnetic transmit antenna system  100  according to an exemplary embodiment of the present invention. The system  100  generates the magnetic component of electromagnetic radiation output from loop transmit antenna  110  that conveys data communications information signals over the air for receipt via an appropriately configured receiver antenna (not shown). 
     As shown in  FIG. 1 , a magnetic transmit antenna apparatus for transmitting communications data comprises a power amplifier  160  having an input  160   a  for receiving a communications data signal waveform  105   a  for transmission, and an output  160   b  providing an amplified output signal waveform  105   a ′ that corresponds to the received communications data signal waveform  105   a . In an exemplary embodiment, input signal  105   a  may be an information carrying signal such as an audio signal such as a 0.5 v, 1 mA audio signal output from a communications source  105  such as a microphone or other such signal source operatively coupled to power amplifier  160 . The communications system or source  105  includes data signals that modulate a carrier and which are conditioned as by way of example, by the application of an 802.11 paradigm (the foregoing not shown). 
     As further shown in  FIG. 1 , a non-resonant toroidal core transformer driver  130  has its primary winding  125  electrically coupled to the output  160   b  of power amplifier  160 , and its secondary winding  120  electrically coupled to loop  140  of magnetic transmit antenna  110 . The primary winding is inductively coupled to the secondary winding of the toroidal core transformer driver  130 . The power amplifier  160  provides an output signal of the same waveform as that of the input  105   a  but with increased power characteristics. For example, for a 0.5 v, 1 mA input signal  105   a , the output from power amplifier  160  to transformer coil driver  130  is a 10 v, 5A signal having increased power relative to the input signal  105   a  but of the same waveform. 
     The toroidal core  130  transformer driver primary and secondary windings are configured such that for a given input voltage and current applied to the primary winding  125 , amplifies the current at the output of the secondary while reducing (e.g. inverting) the voltage output at the secondary. The waveform of the signal is not changed by the non-resonant structure, however, the current input to the loop antenna is magnified while the voltage is reduced. The increased current signal  107  waveform is input to the magnetic loop transmit antenna, wherein the magnetic loop transmit antenna includes a wire loop having multiple turns for generating a magnetic field modulated according to the current signal waveform from the driver to transmit the communications data by modulating the magnetic signal output from the loop antenna. 
     The separation between transmit antenna  110  and an associated receiving antenna (not shown) is about one half (½) the carrier wavelength or less for near field operation. 
     According to an embodiment of the present invention, power amplifier signals (see  FIG. 1 ) may take the form of audio signal for transmission via the transmit antenna  110 . By way of non-limiting example, the power amplifier  160  signal source may have a center frequency between about 90 Hz and 3,000 Hz. At the upper end, signals may carry digitized voice information between transmitters and receivers. At the lower end, the signals may carry data at a rate of around 10 bits per second, which may correspond to about one alphanumeric character a second. Depending upon the distance between transmitter and receiver and the nature of the medium of transmission (e.g., air, solid material such as rock; and/or water) interposed between transmitter and receiver, an appropriate center frequency between about 90 Hz and about 3,000 Hz may be selected. At a lower end of the transmit antenna&#39;s range a nominal carrier frequency would be on the order of 90 Hz to a higher end of 6,000 Hz. 
     Referring again to  FIG. 1 , the toroidal core transformer  130  has a core  135  which operates in conjunction with primary winding  125  and secondary winding  120  to both match impedance of the antenna  110  and power amplifier  160 , and to step down the voltage applied from power amplifier  160 . In one embodiment of the invention the core is fabricated from multiple layers of a ferrite material, such as supplied by Magnetic Metals of Anaheim, Calif. as 1 mil number  48  alloy comprising a magnetic permeable material wound around a form until the core dimensions d, e, f are approximately 0.127 meters×0.0191 meters×0.025 meters, respectively. The toroidal core  135  is then removed from the form. 
     In one embodiment of the invention, the secondary  120  windings are wide strips or ribbons of copper to achieve wide core coverage with least turns for a given turns ratio in primary  125  to secondary  120 . In another embodiment of the invention the primary  125  wire wraps around the entire toroidal core such that primary  125  essentially winds around the entire inside surface of the toroid so as to provide an efficient coupling between the wire and the magnetic field surrounding the wire and the toroid material itself. 
     In yet another embodiment of the invention the secondary  120  utilizes a wire of lower gauge (e.g., AWG 6 gauge) and the primary  125  utilizes a higher gauge (e.g., 22 gauge wire) which is wrapped around the secondary. Alternatively, the thicker secondary wire  120  may be wrapped around the outside of the primary wire  125 . In one version of the embodiment the primary  125  and the secondary  120  are interleaved. In each of the aforementioned embodiments the objective is to achieve an efficient electrical coupling between the primary  125  and the secondary  120  windings. 
     Various combinations of primary wire and secondary wire wound around the transformer core  135  are used to achieve differing goals dependent on transmit power, and voltage and current constraints. By way of example and not limitation, in one embodiment of the invention the transformer  130  comprises a primary of 32 AWG gauge wire having 300 turns. In yet another embodiment the transformer  130  comprises a primary composed of multiple turns of AWG 22 gauge wire wound around a secondary of 4 turns of AWG 6 gauge. 
     Referring to the schematic circuit shown in  FIG. 2 , circuit  200  includes a signal source  210  such as provided by power amplifier  160  ( FIG. 1 ), that supplies a voltage and current to toroidal core transformer  230  having a primary winding  220 , a core  225  and a secondary winding  235 . The secondary winding  235  poles a, b attach to respective ends a′b′ of a magnetic antenna  240 . Antenna  240  comprises at least one loop in the configuration shown in  FIG. 1  as loop  140 . 
     In one embodiment the primary winding  220  and the secondary winding  235  are wound with AWG 22 gauge copper magnetic wire which is lacquered for insulation. The use of AWG 22 gauge wire for the secondary winding  235  limits the current to less than 20 amps due to wire heating and for certain applications is a lower size limit for the wire employed for the toroid core transformer  230  secondary. The size wire also determines the equivalent circuit resistance looking back from the transmit antenna  110  into the secondary winding  235 . The antenna  240  presents to the secondary winding  235  an equivalent circuit  250  comprising a resistor R 1  in series with an inductor L 1 . In one embodiment the input voltage to the primary  220  is 6.48 volts RMS and the ratio of primary windings  220  to secondary windings  235  is 16:1, such that the secondary voltage is less than approximately 0.4 volts passing a current of 54.8 amps through the antenna  240 . 
       FIG. 3  shows a graph of the transmitted power as a function of frequency for the circuit parameters depicted in  FIG. 2 . As the frequency of the signal source  210  increases the output circuit reactance increases, which decreases current flow and in turn decreases transmit power. Under the circuit conditions illustrated in  FIG. 2 , a frequency of transmission of approximately 90 Hz produces a current of 200 amps in the secondary winding  235  and a voltage across the antenna of 0.404 Vrms, which combined deliver approximately 80 watts of output power. As the frequency of the source  210  is increased the power drops off as the current through the secondary winding  235  decreases. At 5,000 HZ the power has dropped to 4 watts as a result of a current of 10 amps and a voltage across the antenna of 0.404 Vrms. 
     With reference to the circuit shown in  FIG. 2 ,  FIG. 4  illustrates a total impedance Z  410  of the transmit antenna  110  comprised of the additive inductor L 1  impedance and R 1  resistance as a function of frequency  405 . Note that the reactance X 1  of the transformer  230  having a core  225  tracks or matches the output impedance Z of the transmit antenna  110 . Rac  430  represents the increase of effective R 1  resistance as a function of frequency  405 . 
     With reference now to  FIG. 1  in conjunction with  FIG. 2 , the larger the cross section of the transmit antenna  110  loop  140 , the greater the range. Although the invention herein describes antenna  110  having x and y dimensions in the range of substantially between 0.0125 and 0.0375 meters, there is no practical limit on the dimensions, which will depend on the application. Thus the x and y dimensions might in some applications be several meters in each direction. 
     Still referring to  FIG. 1 , the more turns of wire on loop  140  of the transmit antenna  110  the greater the transmission range. The greater the current in the loop  140  (as opposed to power) the greater the transmission range. The magnetic antenna  110  wire loop  140  may have multiple turns in the configuration of one of a square, rectangle, circle, ellipse, or triangle configuration. 
     One non-limiting embodiment of the antenna  110  comprises a loop  140  of 60 turns 32 gauge wire in the form of a rectangle essentially having x and y dimensions substantially between 0.0125 and 0.0375 meters in each respective dimension. The rectangular opening may have an area between 0.00016 and 0.00014 meters square. In another non limiting embodiment of the invention the loop  140  has dimensions of about 2.5 cm to 3.75 cm wide×5.0 cm high. 
     In an exemplary embodiment, and with reference to  FIG. 2 , the toroidal transformer  230  having a 200 to 1 turns ratio (primary  220  to secondary  235 ), could be driven by source  210  supplying 10 volts at 1 ampere (10 watts). The secondary  235  operates at 200 amps and 50 milli-volt levels, which would still be at substantially the 10 watt level. 
     In yet another non-limiting example, allowing for efficiency losses, loop  140  current of 90 amperes produced by 0.10 volt RMS in the secondary winding  235  requires a 10 watt source  210  as may be provided by power amplifier  160  ( FIG. 1 ). Essentially the toroidal transformer  230  coupling provides high current to the antenna  240  at very low voltages, thereby contributing to safer operation. 
     Referring still to  FIG. 1 , according to another embodiment of the present invention, transmit antenna  110  also may have a circular configuration having a space bounded by the wire loop  140  comprising an internal round area of about 0.071 meters square. Antenna  110  may be about 0.0125 meter thick, and have approximately 3 or 4 turns, each separated by about 0.018 meter. In one embodiment, transmit antenna  110  may be composed of AWG 0000 copper wire. The antenna  110  is typically wound around an air coil. The greater the number of turns of wire on antenna  110  the greater the range between the antenna  110  and a complementary antenna such as by way of example a magnetic receiving antenna (not shown). As indicated above, other cross sectional configurations of the wire loop may be used such as a square, rectangle, circle, ellipse, or triangle. 
       FIG. 5  depicts an exemplary flow diagram of a process  500  for generating a magnetic field according to an aspect of the invention. The process comprises supplying  510  a high voltage low current to a primary winding of a toroidal core transformer, inductively coupling  520  the primary winding to a secondary winding of the toroidal core transformer for supplying  530  a low voltage and high current to a magnetic loop antenna, thus generating  540  a magnetic field. 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.