Magnetic antennas for ultra low frequency and very low frequency radiation

A communication system and a method of fabricating a communication system are described. The communication system includes a transmit antenna including two or more symmetric coils wound around a closed-loop magnetic transmitter core, the transmit antenna configured to transmit an outgoing signal of very low frequency (VLF) or ultra low frequency (ULF) energy. The communication system also includes a receive antenna including two or more coils formed from two or more wires wound around a closed-loop magnetic receiver core, the receive antenna configured to receive transmitted VLF or ULF energy as an incoming signal. The communication system also includes a processor to process the outgoing signal and the incoming signal.

BACKGROUND

The present disclosure relates to antennas and, more specifically, to magnetic transmission antennas for ultra-low frequency (ULF) or very low frequency (VLF) radiation.

Some applications require the use of very low frequency (3 to 30 kilo Hertz (kHz)) or ultra-low frequency (0.3 to 3 kHz) radiation. For example, mining operations require through the earth (TTE) communication. A mining operation may involve personnel positioned over 1000 feet underground. When an event (e.g., explosion) requiring emergency aid occurs in the subsurface environment, the personnel must be able to convey information about the event to the surface and must also be able to receive instructions from the surface. In such operations, any radiation above the VLF frequency range is absorbed by the earth.

SUMMARY

According to one embodiment, a communication system includes a transmit antenna including two or more symmetric coils wound around a closed-loop magnetic transmitter core, the transmit antenna configured to transmit an outgoing signal of very low frequency (VLF) or ultra low frequency (ULF) energy; a receive antenna including two or more coils formed from two or more wires wound around a closed-loop magnetic receiver core, the receive antenna configured to receive transmitted VLF or ULF energy as an incoming signal; and a processor configured to process the outgoing signal and the incoming signal.

According to another embodiment, a method of fabricating a communication system includes configuring a transmit antenna to include two or more symmetric coils around a closed-loop magnetic transmitter core and to the transmit an outgoing signal of very low frequency (VLF) or ultra low frequency (ULF) energy; configuring a receive antenna to include two or more coils formed from two or more wires wound around a closed-loop magnetic receiver core and to receive an incoming signal of VLF or ULF energy; and configuring a signal processing portion to process the outgoing signal and the incoming signal.

DETAILED DESCRIPTION

As noted above, some operations, such as those involving TTE communication, require transmission and reception of VLF or ULF radiation. Other exemplary applications involve communication through building walls, metal sheathing, and the like. Generally, the transmission antennas for VLF or ULF communication require a large number of coils or a large cross sectional area. One approach to a VLF or ULF transmitter has been the use of a large coil (on the order of up to 100 feet in diameter, for example) with a single loop of the wire coil to obtain a large cross sectional area. Another approach that also requires a long wire involves connecting the wire between two grounding points in the earth (whether at the surface or underground) with the grounding points being separated by distances in excess of 100 feet. In the subsurface environment, a large antenna may be difficult to deploy and may be more easily destroyed by an event such as an explosion. In addition, large coil antennas may have a strong directionality (orientation between the transmitter and receiver must match up to receive sufficient energy) but are difficult to adjust due to their size. Yet another approach that facilitates the use of a smaller coil (e.g., 2 feet in diameter) requires many turns of the coils such that the resulting antenna is heavy and requires a relatively large voltage to drive it. Embodiments of the systems and methods detailed herein relate to transmitter and receiver antennas for TTE communication.

FIG. 1is a block diagram of TTE communication according to an embodiment of the invention. Although TTE communication is shown and discussed as an exemplary environment in which VLF or ULF radiation would be used, the embodiments detailed herein are not limited to any particular location or environment. Further, the subsurface communication system110in particular is discussed with regard to embodiments of the transmit antenna115and receive antenna125. However, the components of the subsurface communication system110may be located elsewhere in alternate embodiments. The systems shown inFIG. 1are only for explanatory purposes and are not true representations of the size, depth, or shape of the components, which are further detailed below. A subsurface communication system110and a surface communication system120are shown. Both include a transmit antenna115,117, respectively, that transmits VLF or ULF radiation and a receive antenna125,127, respectively, that receives VLF or ULF radiation. The subsurface processor113processes signals for transmission to the surface communication system120or received signals, and the surface processor123processes signals for transmission to the subsurface communication system110or received signals. Both processors113,123may include processors, memory devices that store instructions, an input interface, and an output interface (e.g., audio output). While the transmit antennas115,117may be the same type of antenna, and the receive antennas125,127may be the same type of antenna, the antennas may be different in the two different environments according to alternate embodiments. This is because the constraints that apply in the subsurface environment may not apply on the surface. For example, size is a consideration in the subsurface environment for several reasons. In addition, subsurface equipment may have to be made intrinsically safe (i.e., conform to inherent safety design standards) such that electrical current and voltage are kept relatively low, for example, or be small enough to fit in an explosion proof box such that higher power may be used. The subsurface transmit antenna115and receive antenna125are further detailed below.

FIG. 2details the subsurface communication system110shown inFIG. 1according to embodiments of the invention. The transmit antenna115includes a closed-loop core210and symmetric coils of wire215. The wire215may be a copper wire, for example, or another conductive wire carrying current from a source250. The symmetry of the coils or windings of the wire215is such that there is an additive effect on the magnetic field generated on each side of the core210. According to the exemplary orientation shown inFIG. 2, the coils of wire215on both sides of the core210generate a magnetic field going up. AsFIG. 2indicates, the core210may be oblong. In alternate embodiments, the core210may be a toroid or may be formed of c-shaped or u-shaped cores (c-core, u-core), for example. The core210is magnetic and may be comprised of a high-permeability magnetic material (e.g., permeability >>100). The symmetric windings may have the effect of two parallel high magnetic permeability (e.g., ferrite) rods of infinite length. The wire215may be 0.25 inches in diameter, for example, and may carry 20 to 30 Amperes (amps) of current. These exemplary values are not intended to be limiting but, instead, provide a general range of operation of the transmit antenna115.

The receive antenna125also includes a toroid core220, as shown inFIG. 2, and includes two or more pairs of coils formed by two different wires225a,225b. In alternate embodiments, the core220may be a closed-loop magnetic core of a different shape (e.g., square cross sectional shape). The core220may be comprised of a high-permeability magnetic material (e.g., permeability >>100). As shown inFIG. 2, for example, each pair of the two pairs of coils is arranged orthogonal to the other. The arrangement of the two sets of wires225a,225baddresses the directionality issue noted above. That is, if only one wire225were present, then the orientation of the receive antenna125with respect to the (surface) transmit antenna117would affect the strength of the received signal. When two pairs of coils are present, as inFIG. 2, the received signal out of the pre-amplifier and digitizer230at the filter and conditioner240is given by:
Output Signal=√{square root over (X2+Y2)}  [EQ. 1]

X represents the time-dependent signal intensity or voltage level induced in wire225a, and Y represents the time-dependent signal intensity or voltage level induced in wire225b. Thus, based on the arrangement of the coils of the two wires225a,225b(perpendicular to each other), the received signal (result of EQ. 1) is never zero. Depending on the orientation of the receiver antenna, the voltage generated in one wire225ais maximum when the voltage generated in the other wire225bis minimum and vice versa. As the relative orientation with the transmit antenna117changes from one extreme (where voltage generated in wire225ais maximum) to the other (where voltage generated in wire225bis maximum), the voltage generated in wire225adecreases from the maximum value and the voltage generated in wire225bincreases up to the maximum value. That is, the signal given by EQ. 1 is X (at one extreme) or Y (at the other extreme) or some combination of the two voltages in the two wires225a,225b(between the two extremes) but is never null as a result of the relative orientation of the transmit antenna117. In alternate embodiments, weightings may be applied to one or both of X and Y. That is, alternate embodiments of EQ. 1 include:
√{square root over (A*X2+B*Y2)}  [EQ. 2]
C√{square root over (X2+Y2)}  [EQ. 3]
A, B, and C are variables that may have any value greater than zero. If A, B, or C were zero, then the received signal could be zero based on the relative orientation of the transmit antenna117and the receive antenna125(inFIG. 1, for example). Other variations (e.g., a combination of EQ. 2 and EQ. 3) are also possible. For example, another alternate embodiment of EQ. 1 is given by:
√{square root over (A*X2+B*X*Y+C*Y2)}  [EQ. 4]
In the case of EQ. 4, A and C would be greater than zero (e.g., A and C are both 1) and B would be any real number (including zero).

FIG. 3illustrates a transmit antenna115core310according to an embodiment of the invention. The particular shape shown inFIG. 3is only an exemplary embodiment of the three-dimensional core310that may be used for the transmit antenna115. The three-dimensional core310is comprised of the closed-loop cores210discussed with reference toFIG. 2. In the exemplary three-dimensional core310shown inFIG. 3, four closed-loop cores210(for example, each formed of two c-cores) are grouped. As detailed below, the three-dimensional core310facilitates an increase in the additive effect in magnetic field noted above. Two specific embodiments are illustrated using two linear parts320a,320bof the core310. While these linear parts320a,320bare referenced for explanatory purposes, it should be clear that coils are not always arranged around parts of the three-dimensional core310that are linear (e.g., when the three-dimensional core310is formed from a grouping of closed-loop cores). According to an embodiment A, each linear part320a,320bis individually wrapped with a coil315a,315b. The coils315a,315bmay be formed from the same wire (though shown differently for explanatory purposes) or may be different wires fed by different sources250. According to another embodiment B, the coil315cmay be wrapped around both linear parts320a,320bof the core310together. According to either embodiment A, B, the wrapping of the coil or coils (generally315) around the linear parts320a,320bas well as the other linear parts of the core310or the direction of current through the coil or coils315is such that there is an additive effect on the magnetic field produced in the coil or coils315a,315b,315c. That is, every coil315wrapped around the core310produces a magnetic field in the same direction as every other coil315wrapped around the core310.

FIG. 4illustrates a transmit antenna115core410according to another embodiment of the invention. The exemplary three-dimensional core410is comprised of two of the closed-loop cores210discussed with reference toFIG. 2. Three sets of coils415are shown inFIG. 4. As noted in the discussion ofFIGS. 2 and 3, the three sets of coils415are arranged (and current is supplied) such that there is an additive effect on the magnetic field produced in each of the three sets of coils415. It bears noting again that the antenna115may be part of the surface communication system120or may be located inside a building or in any environment in which VLF or ULF communication is desired.

FIG. 5is a process flow of a method of fabricating a communication system (e.g., subsurface communication system110) for VLF or ULF communication according to embodiments of the invention. At block510, configuring a transmit antenna115may be according to one of the embodiments detailed with reference toFIG. 2, 3, or4above. The transmit antenna115may include a closed-loop magnetic core210or a three-dimensional core310,410formed from closed-loop magnetic cores (210). The wire215or coils315,415of one or more wires are arranged with respect to a current source250(not specifically shown inFIGS. 3 and 4) such that there is an additive effect on the magnetic field generated at every part of the core210,310,410. At block520, configuring a receive antenna125includes winding two wires225a,225baround a closed-loop symmetric (e.g., toroid, c-core) core220. The arrangement of the two wires225a,225bis such that the received signal is given by EQ. 1 above. At block530, assembling the communication system (subsurface communication system110) with the signal processing portion113includes configuring the signal processing portion113to process incoming and outgoing signals. This configuring of the signal processing portion113refers to including known processing elements and, additionally, a pre-amplifier and digitizer230and filter and conditioner240that output the received signal based on a combination of the voltage generated in each wire225a,225bof the receive antenna125(according to EQ. 1). At block540, disposing the communication system may include disposing the communication system in a subsurface environment and further may include disposing the subsurface communication system110in an explosion proof box. In alternate embodiments, disposing the communication system (540) may include disposing the communication system in a building or other environment.