Abstract: A system includes conductive windings extending around a magnetic core and impedance-varying windings extending around the magnetic core. The impedance-varying windings include positive windings and negative windings. The conductive windings and the impedance-varying windings conduct electric current around the magnetic core. The system includes a first impedance tap changer that is electrically coupled with the positive windings of the impedance-varying windings and a second impedance tap changer electrically coupled with the negative windings of the impedance-varying windings. A controller controls the first impedance tap changer and the second impedance tap changer to change an impedance of the system by changing which portion of the positive windings and which portion of the negative windings are conductively coupled with the conductive windings, and which portion of the positive windings and which portion of the negative windings are disconnected from the conductive windings.

Abstract: Provided is a choke coil capable of improving a noise reduction effect by sufficiently attenuating magnetic field coupling between the choke coil and a metal part. A connector connection line includes: a first connection line led out from a connector conductor side of a coil main body of a winding wire along a y-axis direction away from the coil main body; a second connection line led out from the first connection line at a corner portion of a first pier column or a second pier column along a x-axis direction away from a connector conductor; a third connection line led out from the second connection line along a z-axis direction toward a lower yoke; and a fourth connection line led out from the third connection line along the x-axis direction toward the connector conductor.

Abstract: The regulation, power-handling capability, and reliability of a transformer is improved by a connected switching network which changes the effective numbers of turns and effective wire size of the windings synchronously during each cycle of an applied AC input voltage.

Abstract: A transformer comprises first winding portions (101, 102) constituting a first foil winding and second winding portions (103, 104) constituting a second foil winding having a substantially same magnetic axis as the first foil winding. The first and second winding portions are interleaved in directions substantially perpendicular to the magnetic axis so as to reduce the leakage inductances of the first and second foil windings. The first winding portions are electrically interconnected so that at least one end-portion of each first winding portion is split to constitute two strips (105a, 105b) folded to mutually opposite directions substantially parallel with the magnetic axis, and ends of the strips of different first winding portions are interconnected to constitute connection bridges over a particular one of the second winding portions located between these first winding portions. The second winding portions are electrically interconnected in the corresponding way to constitute the second foil winding.

Abstract: Disclosed is an inductor structure. The inductor structure includes a base material, a plurality of bottom spiral conductors disposed on the base material, and at least one top spiral conductor disposed on the at least one bottom spiral conductor, and dielectric material separating the bottom, middle and top spiral conductors. A current path for high frequency operation is disclosed. Also disclosed is a method for determining the number of turns in the at least one top spiral conductor and the at least one bottom spiral conductor.

Abstract: A chip-type coil component capable of reducing the resistance of the coil while minimizing a decrease in the inductance of the coil includes magnetic layers composed of a multilayer body. The chip-type coil component further includes internal electrodes laminated on the magnetic layers. The internal electrodes are connected to each other to form a coil. The chip-type coil component further includes an auxiliary internal electrode laminated on each of the magnetic layers. Each auxiliary internal electrode is connected in parallel to the internal electrode laminated on the magnetic layer that is different from the magnetic layer on which the auxiliary internal electrode is laminated.

Abstract: Disclosed is a flexible ultracapacitor having a flexible cloth including an autotransformer-type connected switch made with nanoporous carbon electrodes impregnated by an electrolyte. Each of the nanoporous carbon electrodes may be configured to have a circuit. The circuit may include a first inductor and a second inductor. Each of the first inductor and the second inductor may have a first terminal and a second terminal. The second terminal of the first inductor may be coupled to the first terminal of the second inductor. A switch may be coupled between the first terminal of the first inductor and the second terminal of the second inductor.

Abstract: An inductive device includes an inductor having an inductance associated therewith, and a tuning ring disposed around the inductor. The tuning ring has an inductance associated therewith, wherein the tuning ring is coupled to the inductor to establish a mutual inductance between the tuning ring and the inductor. The inductance of the inductor, the inductance of the tuning ring, and the mutual inductance between the tuning ring and the inductor contribute to a total inductance of the inductive device. The tuning ring is configurable, and is selectively configured to achieve a certain value for the mutual inductance, and a certain value for the inductance of the tuning ring, without changing a footprint of the tuning ring.

Abstract: Techniques for providing a switchable inductor network having configurable inductance in response to a control signal. The switchable inductor network may adopt a fully symmetric architecture to reduce the effects of parasitic elements in differential mode operation. The switchable inductor network is particularly suitable for multi-mode communications circuitry applications, e.g., in the design of a voltage-controlled oscillator (VCO) or an amplifier or buffer in such circuitry.

Abstract: An inductor structure including a plurality of solenoids and at least one connecting line is provided. One of the solenoids serves as a core, and the remaining solenoids are sequentially wound around the core solenoid. Axes of the solenoids are substantially directed to the same direction. Each connecting line is correspondingly connected between ends of two adjacent solenoids to serially connect the solenoids.

Abstract: An RF impedance transformer having a parallel low-impedance access Eb and serial high-impedance access Eh and intended for connection onto a printed circuit. The transformer includes a multilayer circuit that includes a long side and at least three layers. A first outer layer is separated from a second outer layer of the same thickness by at least one inner layer having a thickness at least four times greater than the thickness of the outer layers, each outer layer comprising an electrical conductor on each surface for forming a microstrip line, the serial high-impedance access Eh and the parallel low-impedance access Eb being on the long side of the multilayer circuit and near to each other so as to limit the area for connection with the printed circuit.

Abstract: A three phase transformer circuit is provided that includes a supply side, wherein the supply side is selectably configurable, via the use of sets of terminal boards, between a delta connection and a wye connection at a time of use and, a load side, wherein the load side is selectably configurable between a delta connection and a wye connection at the time of use. The three-phase transformer also provides for selecting an input voltage, from multiple input voltages, via the sets of terminal boards, at the time of use, and for selecting an output voltage for the supply side, from multiple output voltage, at the time of use.

Abstract: A transformer has primary coils and secondary coils that are arranged in a laminated manner in a direction of a winding axis and are mutually insulated. The secondary coils are composed of conducting plates in a plurality of layers arranged in the laminated manner in the direction of the winding axis, and the conducting plates are electrically connected in series. Connecting portions between the each conducting plates are arranged in an inner space of the primary coils.

Abstract: There are many varieties of windings. Coils adopting an edgewise winding wire in which winding wires are wound around in the radial direction of the coil have a wide flux linkage area orthogonal to the electric wire, so that stray loss within the wire is increased, winding wire loss is increased and temperature is raised thereby. The present invention provides an arrangement in which a plurality of coil units are prepared and connected via external coupling terminals. At this time, winding is performed so that the contact faces of the coil units have equal potentials, so that there is no need to ensure an insulation distance between coils, and the coils can be downsized. Therefore, the mass of the respective coils can be reduced. Taps disposed on the respective coils are arranged to have equal potentials, according to which the external coupling terminals can double as tap switch terminals, so that there is no need to provide a dedicated tap switch.

Abstract: A chip-type coil component capable of reducing the resistance of the coil while minimizing a decrease in the inductance of the coil includes magnetic layers composed of a multilayer body. The chip-type coil component further includes internal electrodes laminated on the magnetic layers. The internal electrodes are connected to each other to form a coil. The chip-type coil component further includes an auxiliary internal electrode laminated on each of the magnetic layers. Each auxiliary internal electrode is connected in parallel to the internal electrode laminated on the magnetic layer that is different from the magnetic layer on which the auxiliary internal electrode is laminated.

Abstract: A multi-layered structure is disclosed for implementing an inductor. A first spiral inductor is situated on a first substrate layer, and one or more additional spiral inductors are situated on one or more additional substrate layers. The substrate layers are positioned such that they are substantially in parallel with each other and the spiral inductors on the various layers are aligned with each other. The spiral inductors are electrically coupled to each other by coupling structures to enable them to act as a single overall inductor. Such an overall inductor exhibits improved characteristics, such as a higher Q factor. Other components may be incorporated with and coupled to the overall inductor; thus, this multi-layered structure may be used to construct almost any circuit in which an inductor is implemented.

Abstract: Two conducting wires are used in one embodiment of an inductor. Opposite ends of each of the conducting wires are connected to leader lines (terminals) shared by the conducting wires. Each of the conducting wires is wound to make half a round of an annular or ring-like magnetic substance. One of the conducting wires is wound around a lower half area of the magnetic substance to form one winding while the other conducting wire is wound around an upper half area of the magnetic substance to form another winding. In this manner, the distance between the leader lines can be increased to eliminate parasitic capacitance between the leader lines. The magnetic fluxes generated by current flowing in the two windings are in the same direction. Thus, it is possible to provide an inductor whose total parasitic capacitance is reduced. In other embodiments, additional conducting wires are used.

Abstract: This invention relates to a transformer (1) for multi-output power supplies such as those commonly found in electronic equipment. The transformer comprises a magnetic core (3) and a plurality of windings (5, 7, 9) at least some of which are fractional windings, arranged about the magnetic core. The transformer comprises a dual transformer structure with a pair of transformers, a main transformer (11) and an auxiliary transformer (13). In a preferred embodiment, the main transformer and the auxiliary transformer are connected together. In this way, readily available magnetic components may be used in the construction of the transformer and the simple construction allows for a large cross-sectional area of transformer to be deployed so that reduced turn counts of windings may be used.

Abstract: A stacked inductor with combined metal layers is represented in this invention. The stacked inductor includes: a top layer metal coil, and at least two lower layer metal coils, the metal coils being aligned with each other; adjacent metal coils being connected at the corresponding ends through a via; wherein, each of the lower layer metal coils is consisted of plural layers of metal lines which are interconnected. With the same chip area, the stacked inductor of the present invention can achieve higher inductance and Q factor because of the mutual inductance generated from the plural layers of metal lines and the reduced parasitic resistance.

Abstract: A reactor is provided in which coil segments (5-1, 5-2) of each of first and second auxiliary winding elements (2-1, 2-2) is of a multilayered and aligned winding structure. The coil segments (5-1, 5-2) of the first auxiliary winding element (2-1) and the coil segments (5-1, 5-2) of the second auxiliary winding element (2-2) are disposed within respective space areas (6-1, 6-2) delimited between the coil segments of the second auxiliary winding element and an outside and between the outside and the coil segments of the first auxiliary winding element. The coil segments of each of those first and second auxiliary winding elements are so combined as to be adjacently alternately positioned in a line to thereby form a main winding body (3). The pair of the auxiliary winding elements are connected parallel to each other.

Abstract: A communication interface employing a differential circuit and method of use is disclosed. In one form, a circuit operable to communicate signals via a communication bus can include a differential signaling circuit operable to be coupled to a communication bus. The differential signaling circuit can include a first current carrying element and a second current carrying element. The circuit can also include a shunt circuit including a first transformer. The first transformer can include a primary transformer element and a secondary transformer element. The first transformer can be operably associated with the communication bus.

Abstract: A DC/DC power converter includes an electrically configurable transformer/inductor. The electrically configurable transformer/inductor receives a power plug. The power plug, depending on its configuration, configures the operation of the transformer/inductor and therefore the DC/DC power converter. The power plug may permit access to power received from the power converter. The power plug may also pass power to the power converter from a remote electrical source.

Abstract: A multilayer printed circuit board (“PCB”) coil that simulates a coil formed from litz wire. The PCB includes a plurality of alternating conductor and insulating layers interconnected to cooperatively form the coil. Each conductor layer includes a trace that follows the desired coil shape and is divided into a plurality of discrete conductor segments. The segments are electrically connected across layers to provide a plurality of current flow paths (or filaments) that undulate between the layers in a regular, repeating pattern. The coil may be configured so that each filament spends a substantially equal amount of time in proximity to the paired coil and therefore contributes substantially equally to the self or mutual inductance of the coil. Each conductor layer may include a plurality of associated traces and intralayer connector that interconnected so that each filament undulates not only upwardly/downwardly, but also inwardly/outwardly in a regular, repeating pattern.

Abstract: An AC neutral bus electromotive power rectification unit includes a first coil unit and a second coil unit. The first coil unit includes a first conductive wire coil having a first end and an opposite second end. The conductive coil is disposed in a first non-conductive tube and is suspended in a ferrous matrix. The second coil unit includes a second conductive wire coil having a first end and an opposite second end. The first end of the second coil unit is electrically coupled to the first end of the first coil unit. The second coil unit is disposed in a second non-conductive tube and is surrounded by a non-conductive material.

Abstract: A meander inductor is disclosed, the inductor is disposed on a substrate or embedded therein. The meander inductor includes a conductive layer composed of a plurality of sinusoidal coils with different amplitudes and in series connection to each other, wherein the sinusoidal coils with different amplitudes are laid out according to a periphery outline. The profile of the meander inductor is designed according to an outer frame range available for accommodating the meander inductor and is formed by coils with different amplitudes. Therefore, under a same area condition, the present invention enables the Q factor and the resonant frequency fr of the novel inductor to be advanced, and further expands the applicable range of the inductor.

Abstract: An autotransformer for use in low frequency, high power applications that uses a stack of printed wire boards constructed of a top, inner, and bottom layer including electrical trace windings circumventing the transformer core and formed in the inner layer for direct thermal contact with a heat sink interface providing a uniform and consistent heat path down to the heat sink plate. The autotransformer further includes a board to board connection employing solder cups to electrically connect between predetermined printed wire board traces. The printed wire board autotransformer also may use a non-planar interface for thermal interface with a non-planar heat sink plate surface.

Abstract: A flux sharing magnetic circuit has a parallel arrangement of secondary electromagnetic circuits with independent loads. An AC driven primary delivers current to the secondary circuits to maintain charge in their batteries. The batteries deliver DC current to the loads while secondary coils provide battery charging currents to maintain charge in the batteries. When current is not drawn by the battery or the load, flux is delivered to a flux pool in the magnetic circuit so that input AC power drain is reduced.

Abstract: Disclosed is a signal transmitter including a primary winding, a first and at least one second secondary winding which are arranged at a distance from the primary winding in a first direction and are in each case inductively coupled to the primary winding, and each of which has at least two series-connected winding sections, with the at least two winding sections of each of the first and second secondary windings being arranged in at least two different winding levels, and a signal transmission apparatus having a signal transmitter such as this.

Abstract: A coil system for a plasmoid thruster includes a bias coil, a drive coil and field coils. The bias and drive coils are interleaved with one another as they are helically wound about a conical region. A first field coil defines a first passage at one end of the conical region, and is connected in series with the bias coil. A second field coil defines a second passage at an opposing end of the conical region, and is connected in series with the bias coil.

Abstract: Suspension inductor devices are provided. A suspension inductor device includes a dielectric substrate and a suspension induction coil. The suspension induction coil includes an input end disposed on the dielectric substrate. A spiral coil is wound from the dielectric substrate to an interconnection. The interconnection is disposed in the spiral coil and connects the input end and the spiral coil. An output end is disposed on the dielectric substrate and adjacent to the input end.

Abstract: An electrostatic shield and a voltage transformer having a high voltage winding and a low voltage winding. The shield takes the form of a concentric winded coil, made from a round conductive wire having a layer of insulation around it and a radius of at least ten times the radius of the wire used for the high voltage winding. This shield is dimensioned to surround the high voltage winding of the transformer.

Abstract: Provided are an inductor, which is vertically formed, and an electronic device having the inductor, and more particularly, an inductor capable of minimizing loss of a surface area and accomplishing high efficiency impedance by vertically forming the inductor in a plurality of insulating layers, and an electronic device having the same. The inductor includes a plurality of conductive lines disposed in the insulating layers; and vias vertically formed in the insulating layers to electrically connect the plurality of conductive lines. When a board or an electronic device including an inductor proposed by the present invention is manufactured, the inductor can occupy a minimum area in the electronic device or board while providing high inductance. In particular, the surface area of the electronic device or board occupied by the inductor can be remarkably decreased to reduce manufacturing costs.

Abstract: An inductor structure disposed over a substrate and including a coil layer is provided. The coil layer has a plurality of coil turns electrically connected with each other. An innermost coil turn of the coil layer has a portion with a narrower width in a region with a higher magnetic flux density than that in the other region with lower magnetic flux density.

Abstract: An integrated structure of passive elements in an LLC resonance converter realized by flexible circuit boards includes a closed magnetic circuit formed by first and second magnetic cores, a tubular, double-sided, flexible circuit board, tubular magnetic-material layer and tubular, single-sided, flexible circuit board. The single-sided circuit board is coaxially sleeved in turn from inside outwards on a magnetic core column of the closed magnetic circuit. The tubular, double-sided, circuit board is a laminated plate material formed in turn of a first insulating layer, first copper foil, insulating medium layer, second copper foil and second insulating layer. The tubular, single-sided, circuit board is a laminated plate material formed in turn of a third insulating layer, third copper foil and fourth insulating layer. The integration of a resonance capacitor, resonance inductor, shunt inductor and transformer in a LLC resonance converter is realized by using these flexible circuit boards.

Abstract: Magnetic component assemblies including coil coupling arrangements, that are advantageously utilized in providing surface mount magnetic components such as inductors and transformers.

Abstract: There is provided a balun transformer, in which first to fourth layer coils are stacked and coupled magnetically; one end of each coil of the first to fourth layer coils is grounded; the second and third layer coils are connected in parallel, an unbalanced signal is input/output to/from a common terminal of the second and third layer coils; a first balanced signal is input/output to/from the other end of the first layer coil; and a second balanced signal is input/output to/from the other end of the fourth layer coil.

Abstract: A multilayer winding inductor. The inductor at least includes multi-level interconnect and single-level interconnect structures. The multi-level interconnect structure includes a plurality of conductive plugs and a plurality of looped conductive traces overlapping and separated from each other. Each looped conductive trace has a gap to define first and second ends and at least two conductive plugs disposed between the neighboring looped conductive traces. The single-level interconnect structure is located over the multi-level interconnect structure, comprising an uppermost looped conductive trace and a second conductive plug. The uppermost looped conductive trace has a gap to define first and second ends, and the second conductive plug is disposed between the second end of the uppermost looped conductive trace and the first end of the looped conductive trace adjacent thereto, thereby electrically connecting the multi-level and single-level interconnect structures.

Abstract: An electrical machine includes a first part-winding, a second part-winding, a core having slots in which the first and the second part-windings are wound, and a switching device for connecting the first part-winding to the second part-winding. In each of the slots, one of the two part-windings forms a bottom layer at the slot base, and the other part-winding is disposed as a top layer parallel above the bottom layer. The two part-windings are capable of being connected in parallel with the switching device during operation of the electrical machine. It is thus possible for a symmetrical field distribution to be achieved in a startup circuit. In addition, no additional components caused by harmonic fields occur in the torque curve which could impede runup.

Abstract: A system for selecting coil voltage in an electrical device comprises a housing and a coil in the housing having a plurality of windings. Each winding has a pair of pins. A slider supports a plurality of moveable contacts and is moveably mounted in the housing proximate the coil so that the plurality of moveable contacts selectively make or break electrical contact between the winding pins to select coil voltage.

Abstract: The invention relates to an inductor comprising a plurality of interconnected conductive segments interwoven with a substrate. The inductance of the inductor is increased through the use of coatings and films of ferromagnetic materials such as magnetic metals, alloys, and oxides. The inductor is compatible with integrated circuit manufacturing techniques and eliminates the need in many systems and circuits for large off chip inductors. A sense and measurement coil, which is fabricated on the same substrate as the inductor, provides the capability to measure the magnetic field or flux produced by the inductor. This on chip measurement capability supplies information that permits circuit engineers to design and fabricate on chip inductors to very tight tolerances.

Abstract: The invention relates to an inductor comprising a plurality of interconnected conductive segments interwoven with a substrate. The inductance of the inductor is increased through the use of coatings and films of ferromagnetic materials such as magnetic metals, alloys, and oxides. The inductor is compatible with integrated circuit manufacturing techniques and eliminates the need in many systems and circuits for large off chip inductors. A sense and measurement coil, which is fabricated on the same substrate as the inductor, provides the capability to measure the magnetic field or flux produced by the inductor. This on chip measurement capability supplies information that permits circuit engineers to design and fabricate on chip inductors to very tight tolerances.

Abstract: The invention relates to an inductor comprising a plurality of interconnected conductive segments interwoven with a substrate. The inductance of the inductor is increased through the use of coatings and films of ferromagnetic materials such as magnetic metals, alloys, and oxides. The inductor is compatible with integrated circuit manufacturing techniques and eliminates the need in many systems and circuits for large off chip inductors. A sense and measurement coil, which is fabricated on the same substrate as the inductor, provides the capability to measure the magnetic field or flux produced by the inductor. This on chip measurement capability supplies information that permits circuit engineers to design and fabricate on chip inductors to very tight tolerances.

Abstract: The invention relates to an inductor comprising a plurality of interconnected conductive segments interwoven with a substrate. The inductance of the inductor is increased through the use of coatings and films of ferromagnetic materials such as magnetic metals, alloys, and oxides. The inductor is compatible with integrated circuit manufacturing techniques and eliminates the need in many systems and circuits for large off chip inductors. A sense and measurement coil, which is fabricated on the same substrate as the inductor, provides the capability to measure the magnetic field or flux produced by the inductor. This on chip measurement capability supplies information that permits circuit engineers to design and fabricate on chip inductors to very tight tolerances.

Abstract: A segmented inductor for use with a tuning circuit, a method of increasing a self-resonant frequency of a tuning circuit and an oscillator employing the circuit and method. In one embodiment, the tuning circuit includes a first inductive element having a first intrinsic shunt capacitance. The tuning circuit also includes a second inductive element, coupled in series with the first inductive element, having a second intrinsic shunt capacitance. The first and second inductive elements cooperate to reduce a total shunt capacitance associated therewith thereby increasing a self-resonant frequency of the tuning circuit.

Abstract: A low leakage inductance, versatile RF transformer with multiple input/output voltage ratios. The transformer has many applications. For example, and not by way of limitation, the transformer can be used with switching power supplies, RF induction power supplies, and RF plasma power supplies. The efficiency of the transformer is on the order of 99% to 99.7%. These efficiencies have been measured at power levels of 3 kilowatts (KW) to 10 KW.

Abstract: The invention relates to a magnetic component having at least two windings electrically connected in series (3, 4, 13, 14, 24-27, 31-33) and a core (1, 2, 10, 11, 20-23, 30) on which the windings (3, 4, 13, 14, 24-27, 31-33) are arranged so that in the event of a current flow through the windings (3, 4, 13, 14, 24-27, 31-33) the generated magnetic stray fields outside the component at least partly compensate each other, the core having at least one inside limb portion and at least two outside limb portions and in that the windings (3, 4, 13, 14, 24-27, 31-33) are arranged on the inside limb portion and/or the outside limb portions.

Abstract: A transformer of a Tesla type, and energy supply and particle accelerator devices that include such transformers. The electrical transformer includes a primary winding, a secondary winding which is electromagnetically coupled to the primary winding, and is characterized in that the primary winding consists of one single turn. The transformer operates without any soft magnetic core. The single turn is formed by at least two sector segments of a rotationally symmetric body, over which segments a voltage is applied. Preferably, the segments are equal in size, the voltages are equal in magnitude and one end of each segment is kept at ground potential. In an energy supply and an accelerator according to the present invention, a switch controlling the application of the voltage over the primary winding has controlled turn-on and turn-off preferably is an IGBT switch.

Abstract: A voltage compensation system for an A.C. power line supplying one or more air conditioners, a telephone system or other telecommunication system, a battery charger, or some other utilization system that should have an input voltage within a given normal range. The voltage compensation system includes a transformer unit containing a transformer having at least two windings coupled electromagnetically by a transformer core; one of those windings preferably is in two segments that are connectable in series or in parallel. One winding is connected to an input terminal and to an output terminal. The other winding adds to the A.C. line voltage when the line voltage drops below a low voltage threshold and/or subtracts from the line voltage when the A.C. line voltage exceeds a high voltage threshold. If the A.C. line voltage is in the range required by the utilization system, the other transformer winding is effectively shorted out or otherwise disconnected.

Abstract: A radio frequency choke for use in a cable television transmission and distribution circuit is disclosed. The radio frequency choke comprises a core of zinc-ferrite material as provided by MMG/NEOSID, Ltd., part number 31P0250012, and a wire coil wound around the core. The coil includes a first lead, a first clockwise winding of N1 turns around the core, the first lead being connected to an input of the first winding, a second counterclockwise winding of N2 turns around the core, a third counterclockwise winding of N3 turns around the core, and a second lead connected to an output of the fourth winding. In a preferred embodiment, there are six turns in the first clockwise winding, five turns in the second clockwise winding, and five turns in the third counterclockwise winding. Moreover, in the preferred embodiment, resonance damping resistors of approximately 2 K.OMEGA. are connected between the windings of the radio frequency choke.

Abstract: Two vertical, parallel, horizontally spaced signal coils are connected in series but in opposite phase across an a.c. source. Thus a buried conductor located centrally beneath the coils experiences an alternating magnetic field whose strength is due to the addition of the fields induced by the two coils. The field strength may be further enhanced by one or more of: providing a ferromagnetic flux diverter between the upper ends of the coils; placing a conductive nonmagnetic screen between them; connecting a tuning capacitor in parallel with the coils; or providing a tuning circuit having series-connected secondary coils which are flux-linked to respective signal coils and are in parallel with a capacitor. There may be a switch arrangement for switching the signal coils to in-phase connection, for use when they are oriented horizontally.