The impedance of a loopstick antenna is directly modulated utilizing a mechanically actuated magnetoelastic material preferably placed in the core (or center) of looped wires forming the loopstick antenna. Using one or more mechanical actuators the permeability in the center of the loopstick antenna can be modulated at a rapid rate (such as with data or audio information), allowing the magnetic field outside of the antenna to be modulated at large bandwidths without requiring switches or modulators capable of high voltage thus reducing the overall complexity and cost of the transmitter. The external magnetic field is created by an AC source which is preferably impedance matched to the loopstick antenna by means of a matching network and is FM modulated according to a modulating signal applied to the one or more mechanical actuators.

TECHNICAL FIELD

The disclosed technology related to a loopstick antenna which may be used to transmit or communicate at frequencies below 100 MHz. These frequencies are useful for communication over the horizon or in highly conductive environments such as underground or underwater.

BACKGROUND

Traditional compact antennas below 100 MHz consist of electrically small dipoles, monopoles, and loops. In general the latter is often the most efficient for size constrained applications. Furthermore, loop antennas can be loaded at their cores with high permeability materials and may comprises numerous wire loops (which may be called a coil), dramatically improving its efficiency for a given size. This type of antenna is known as a loopstick antenna. For extremely subwavelength applications (i.e. <λ/100) loopstick antennas have a large inductive reactance associated with their impedance. To match this reactance, capacitors can be utilized. However, if good transmit efficiency is to be achieved with these matching networks the transmit bandwidth associated with the antenna will be exceedingly small.

A technique to circumvent this issue which has been proposed previously is to directly modulate the reactive elements of the matching network instead of modulating the signal incident on the network. This technique, known as direct antenna modulation, allows for large fractional bandwidths to be achieved (near 100%); however, for reasonable power levels, the high quality factors associated with reactive elements in this network create large voltages which increase the cost, complexity, and reliability of switches needed to achieve modulation. See, for example,a. Xu, J. X. and Wang, Y. E., “A direct antenna modulation (DAM) transmitter with a switched electrically small antenna”, 2010International Workshop on Antenna Technology(iWAT), 1-3 Mar. 2010;b. Azad, U. and Wang, Y. E., “Direct Antenna Modulation (DAM) for Enhanced Capacity Performance of Near-Field Communication (NFC) Link”,IEEE Transactions on Circuits and Systems I: Regular Papers, Vol. 61, Is. 3, March 2014;c. Yao, W. and Wang, Y., “Direct antenna modulation—a promise for ultra-wideband (UWB) transmitting”, 2004IEEE MTT-S International Microwave Symposium Digest,6-11 Jun. 2004d. Babakhani, A., Rutledge, D. B.; and Hajimiri, A., “Transmitter Architectures Based on Near-Field Direct Antenna Modulation”,IEEE Journal of Solid-State Circuits,pp. 2674-2692, 12 Dec. 2008;e. Keller, S. D., Palmer, W. D. and Joines, W. T., “Direct antenna modulation: analysis, design, and experiment”, 2006IEEE Antennas and Propagation Society International Symposium,9-14 Jul. 2006;f. Suh, S-Y, et al, “Time-variant antenna module for wireless communication devices”, U.S. Patent Publication 2017/0040674, 2017 Feb. 9;g. Manteghi, M. “Transmitting Wideband Signals Through an Electrically Small Antenna Using Antenna Modulation”, U.S. Patent Publication 2017/0279471, 2017 Sep. 28;h. Manteghi, M. “An electrically small antenna concept design for transmitting a baseband signal”, 2017IEEE International Symposium on Antennas and Propagation&USNC/URSI National Radio Science Meeting,9-14 Jul. 2017;i. Manteghi, M. et al., “A time variant antenna for transmitting wideband signals”, U.S. Patent Publication 2016/0294056, 2016 Oct. 6;

At electromagnetic (EM) frequencies less than 100 MHz, compact and efficient antennas and transmitters have been difficult to produce. At very low frequencies (VLF) 3-30 kHz and ultra low frequencies (ULF) 0.3-3 kHz electromagnetic waves are challenging to produce, because the physical size of the antenna, which is a fraction of a single wavelength, becomes impractically large. Antennas for VLF and ULF transmitters can be as large as many acres and require wires suspended hundreds of feet off the ground for efficient operation. Loopstick antennas, which are essentially solenoids loaded with high permeability materials allow for significant reductions in the size of antennas at VLF and ULF. However, because these antennas are generally matched with a reactance in a resonant circuit to improve the efficiency, the bandwidths are often extremely narrow.

One option to avoid this bandwidth issue is to directly modulate the reactive elements of the matching network, however for transmitters of reasonable power levels this can often result in high voltages placing undesirable constraints on the transmitter and its antenna system. Also, losses in the switch used to modulate the reactive element can significantly degrade efficiency.

BRIEF DESCRIPTION OF THE INVENTION

This invention addresses this high voltage issue by directly modulating the impedance of the loopstick antenna with a mechanically actuated magnetoelastic material preferably placed in the core (or center) of looped wires forming a loopstick antenna. Using one or more mechanical actuators the permeability in the center of the loopstick antenna can be modulated at a rapid rate (such as with data or audio information), allowing the magnetic field outside of the antenna to be modulated at large bandwidths without requiring switches or modulators capable of high voltage thus reducing the overall complexity and cost of the transmitter. The external magnetic field is created by an AC current or voltage source which is preferably impedance matched to the loopstick antenna by means of a matching network and is FM and/or AM modulated according to a modulating signal applied to the one or more mechanical actuators.

In accordance with embodiments of the disclosed technology, the permeability of a loopstick antenna is modulated, preferably via mechanical actuation of a magnetoelastic material, allowing the reactance of the loopstick antenna to be modulated instead of the reactance of a matching network. This implementation of direct antenna modulation has the advantage of avoiding the use of high voltage switching elements.

In accordance with embodiments of the disclosed technology, a coil of wire with a magnetoelastic material such as terfenol-D, or galfenol (FexGa1-x) in or adjacent to the coil. Using an alternating voltage source, current is passed through the coil generating a magnetic field external to the coil. By applying a modulated mechanical stress to the magnetoelastic material of this coil its permeability can be modulated, thus modulating the external magnetic field allowing signals and data to be encoded for transmission.

DETAILED DESCRIPTION

The reader's attention is directed to (i) all papers and documents which are filed concurrently with this specification and which are open to public inspection with this specification (the contents of all such papers and documents are incorporated herein by reference) and (ii) all papers and documents which are otherwise incorporated by reference herein (but not physically filed with this specification).

In this description we will first provide an overview of the principle components and function, then provide a technical rational for the operational principle, and finally discuss design variations.

Magnetic antennas typically consist of coiled wire driven by sources of current or voltage. In the case of an electrically small loop antenna consisting of a single or multiple turns of wire the intrinsic impedance seen at the input terminals of the antenna is given by the following equations (see R. C. Hansen & R. E. Collin, “Small Antenna Handbook”, John Wiley and Sons, Inc. 2011):

Za⁢n⁢t=R1+Rr+j⁢Xa⁢n⁢t⁢⁢R1=Rs⁢D2⁢a⁢⁢Rr=5⁢π2⁢N2⁢k4⁢D4⁢μe24X≅60⁢π⁢N2⁢k⁢D⁡(ln⁢9⁢Db-1)
where, Rlis the ohmic loss in windings, Rris the radiation resistance, X is the reactance, Rsis the surface resistivity of the wire windings, D is the loop diameter, a is the tube radius, N is the number of turns, k is the free space wave number, μeis the permeability of the core, and b is the axial length of solenoid.

Since the reactive impedance of the antenna is positive, a matching network consisting of capacitive elements is required for good efficiency. In the case of a small air core loop antenna operating below 100 kHz, the radiation resistance is exceedingly small. Furthermore, the capacitive reactance and quality factor required to efficiently match the antenna are often very large, resulting in either poor efficiency or small bandwidths. One technique for mitigating this issue is load the center of an air core loop with a high permeability material such as a ferrite material. This antenna design is known as a ferrite-loaded loop or loopstick. Assuming that low loss ferrite materials are used the radiation resistance and reactance of the antenna can be rewritten as follows (see R. C. Hansen & R. E. Collin, “Small Antenna Handbook”, John Wiley and Sons, Inc. 2011):

Rr=5⁢π2⁢N2⁢k4⁢L4⁢μe24⁢(L/2⁢A)4⁢⁢X=3⁢0⁢π2⁢N2⁢k⁢L⁢μe(L/2⁢A)2
where L is the length of the core and A is the radius of the windings. In this design, the reactance is linearly proportional to the permeability of the core. Thus for a constant capacitive match, the resonant frequency of the transmitter can be varied by modifying the permeability of the core.

In embodiments of the present invention, the traditional ferrite material core used to load the center of a prior art loopstick antenna10, for example, is replaced with a ferromagnetic core12having magnetoelastic (“magnetoelastic” is also called “magnetostrictive” in the art) properties. As such, at least a portion of core12should comprise a magnetoelastic material12MS(seeFIG. 2). The magnetoelastic (or magnetostrictive) material12MSpreferably comprises galfenol (an alloy of iron and gallium: FexGa1-x) or terfenol-D (an alloy of iron, terbium, and dysprosium for example, TbxDy1-xFe2and related alloys) or some other magnetoelastic (or magnetostrictive) material such as, for example, Ni, FeGaB, iron cobalt and/or Alfenol (FexAl1-x).

Magnetoelasticity (sometimes also called magnetostrictivity) is the coupling of an internal magnetization of a ferromagnetic material to its mechanical strain state. The orientation of the ferromagnetic moment is primarily determined by the interaction of the magnetization with applied field (Zeeman energy), the magnetocrystalline anisotropy (MCA), and the applied stress, or magnetoelastic coupling. The orientation of the magnetization within the material will be always found in a direction that minimizes the energies associated with these forces. When stress is introduced into the magnetoelastic material12MSthe effective permeability material changes in response to the applied stress. Depending on the type of magnetoelastic material, different changes may occur upon the application of tensile or compressive stress. For some materials, like galfenol, tension tends to increase the relative permeability of the material (qualitatively making the material more like iron) while compression reduces the effective permeability of the material (qualitatively making the material more like air).

There are other magnetostrictive materials than those identified above which may be utilized or which may be developed in the future for magnetostrictive material12MS. It is to be understood that the discovery of new materials which are highly magnetostrictive (also called magnetoelastic in the art) is ongoing and moreover the discovery of new materials which exhibit very little magnetostrictiveness is also on going. So current magnetic materials can exhibit little or relatively large amounts of magnetostrictiveness. In this disclosure the term magnetostrictive (or magnetoelastic) material is intended to refer to magnetic materials whose magnetostrictiveness λ is at least 20 microstrains (50 microstrains is approximately the value of Ni). Generally speaking, the higher the number of microstrains the better for material12MS, but there can be an engineering tradeoff since materials with greater magnetostrictiveness may be more expensive to utilize and may have greater temperature sensitivities (for some magnetostrictive alloys such as FeGaB, the Curie temperature can be as low as 350° C.). For the purposes of this disclosure, non-magnetostrictive (or non-magnetoelastic) material has a λ that is less than 20 microstrains.

The antenna10will likely work with any magneto-elastic material, it will work best with materials that have stress induced permeability changes that range from high values to very low values. Galfenol is much better in this regard than Terfenol-D, and thus galfenol is the presently preferred material for magnetoelastic material12MSin ferromagnetic core12.

As is depicted byFIGS. 1 and 2, wire11is wound around a ferromagnetic core12which includes a magnetoelastic (or magnetostrictive) material12MSthereby forming the coil or loop11of the magnetoelastic loopstick antenna10. As can be seen fromFIG. 2, the core12may include a relatively smaller amount of a magnetoelastic (or magnetostrictive) material12MSsuch as galfenol and a relatively larger amount of a ferromagnetic, but non-magnetoelastic material12MGsuch as MetGlas®. In some embodiments, the ferromagnetic core12may comprise a unitary body of a magnetoelastic (or magnetostrictive) material or have a greater portion of a magnetoelastic (or magnetostrictive) material (compared to a non-magnetoelastic (or non-magnetostrictive) material), especially if the magnetoelastic (or magnetostrictive) material selected does not enjoy the same degree of magnetoelasticity as does galfenol.

Also, it should be noted that MetGlas® may made from any a number of different metallic glass alloys, some of which are magnetoelastic and others which are not magnetoelastic (or less so). So MetGlas® alloys may be used for both the magnetoelastic (or magnetostrictive) material12MSand for the ferromagnetic, but non-magnetoelastic, material12MG, provided that a magnetoelastic (or magnetostrictive) MegGlas® alloy is selected for material12MSand that a ferromagnetic, but non-magnetoelastic, MegGlas® alloy is selected for material12MG. In some embodiments, a magnetoelastic (or magnetostrictive) MegGlas® alloy (or even a non MetGlas® alloy) may be designed (or selected) which forms the entirety or majority of core12.

The term alloy above is used in its broadest possible sense and is not intended to exclude mixtures or composite materials. For just one example, gaflenol is typically referred to as an alloy of FeGa in the art, but the Ga tends to clump together in the Fe matrix much like “raisins within a cake” according to one researcher.

All ferromagnetic materials tend to exhibit some magnetoelastic (or magnetostrictive) properties. The reference herein to magnetoelastic (or magnetostrictive) materials or properties is intended to refer to those materials which exhibit the greatest change in permeability for a range of applied stresses, such as gaflenol, while the reference to non-magnetoelastic (or non-magnetostrictive) materials or properties is intended to refer to those materials such as common ferrites which are minimally magnetostrictive.

Typically, the coil or loop11is formed by one or more wires disposed, wound or wrapped around core12so that core is preferably located at or near the center of coil or loop11. Other arrangements, such as having the coil or loop11disposed adjacent or inside core12may be utilized, but the then the magnetic coupling between the core12and the coil or loop11likely will be reduced. When wound around core12the length of the wire forming the coil or loop11is preferably less than one 10th of the free space wavelength of antenna10at its resonant frequency. In some embodiments, multiple coils or loop11may be utilized for use with different frequencies of interest. The coil or loop11is coupled to a matching network16.

Mechanical stress (which is modulated with information to be transmitted) is applied to magnetoelastic material12MSin core12via a mechanical actuator14, such as a piezoelectric (PZT) driver. When in compression the permeability of the magnetoelastic material12MSdecreases, reducing the reactance of the antenna10, and as this compressive stress is reduced and/or converted into tension, the permeability of the material12MSis increased thus increasing the reactance of antenna10. By connecting this mechanically tuned loopstick antenna10to (i) a capacitor (or a capacitive matching network16shown inFIG. 1, which network16may be embodied by a single capacitor if desired) and (ii) a variable frequency (current or voltage) source18, an efficient compact frequency modulated RF source of external magnetic energy is created.

InFIG. 1, the output of the modulator20is preferably applied to both the mechanical actuator14and the source18. When a strain is applied to the magnetoelastic core of the loopstick antenna (by the mechanical actuator14), the permeability of the core12changes which in turn changes the resonant frequency of the antenna10. As such, the driving frequency of the source18is also preferably changed to match the resonant frequency of the antenna10utilizing the connection depicted between modulator20and source18. The modulator20modulates the mechanical actuator14using audio or data from a signal source22and also changes the frequency of the RF generated by source18(but without modulating the source18) as mentioned above to account for a high Q of the antenna10.

A preferred embodiment of the present invention comprises a 15 cm long cylinder or core of galfenol with a diameter of 8.2 mm. Copper wire is wrapped around the cylinder in a spiral configuration (forming coil11) and connected to a matching network16and voltage source18(which may comprise an oscillator and a RF amplifier). Multiphysics simulations of this device demonstrate that using between 20 MPa of tension and 60 MPa of compression a greater than 50% change in the resonant frequency of the loopstick antenna can be achieved (seeFIG. 3). To minimize losses, the core12material may comprise material12MSwhich is preferably embodied as galfenol and which preferably constitutes some fraction, less than 100%, of the total material in core12with the remaining material12MGconsisting of some highly permeable but low loss material such as a ferrite material, such as a non-magnetostrictive material as depicted byFIG. 2and described above with reference thereto.

Modifications, additions, or omissions may be made to the systems, apparatuses, and methods described herein without departing from the scope of the invention. The components of the systems and apparatuses may be integrated or separated. Moreover, the operations of the systems and apparatuses may be performed by more, fewer, or other components. The methods may include more, fewer, or other steps. Additionally, steps may be performed in any suitable order. As used in this document, “each” refers to each member of a set or each member of a subset of a set.