Patent Publication Number: US-2022216614-A1

Title: Low-profile magnetic antenna assemblies

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application relates to and claims priority benefits from U.S. Provisional Application No. 63/133,307, entitled “Low-Profile Magnetic Antenna Assemblies,” filed Jan. 2, 2021, which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF EMBODIMENTS OF THE DISCLOSURE 
     Embodiments of the present disclosure generally relate to antenna assemblies, and more particularly, to low-profile magnetic antenna assemblies. 
     BACKGROUND OF THE DISCLOSURE 
     An antenna typically includes an array of conductors electrically connected to an electronic receiver or a transmitter. An electronic transmitter provides a time-varying voltage to terminals of the antenna, which, in response, radiates electromagnetic radio waves at a frequency corresponding to the time-varying voltage. Alternatively, as radio waves are received by the antenna, a time-varying voltage corresponding to the frequency of the radio wave is generated at the terminals, which, in turn is provided to the electronic receiver. Various types of known passive antennas are configured to transmit and receive radio waves equivalently with such a reciprocal behavior. 
     In some aerospace applications, there is a need for antennas that are capable of being positioned on conformal or non-planar surfaces, such as wings and fuselages of aircraft. Small aircraft, such as unmanned aerial vehicles (UAVs) or drones, in particular, have surfaces with low radii of curvature. Such aircraft typically need light weight antennas with low aerodynamic drag and low visibility. Further, various surfaces of aircraft may be formed from conductive or carbon fiber materials, which are known to change the electrical behavior of antennas, such as monopole and dipole antennas and derivatives (for example, whip, blade, Yagi, and other such antennas). 
     Various known planar antennas that include microstrip feeds and pin feeds exhibit low bandwidth, due to their narrowband impedance matching. However, the bandwidth can be increased by using a proximity-coupled feed line. Still, planar antennas generally have low gain and bandwidth due to their thin nature. Further, antennas operating at low frequencies (for example, less than 500 MHz) are typically large (in both height and area), as antenna size scales inversely with frequency. 
     SUMMARY OF THE DISCLOSURE 
     A need exists for a microstrip-based antenna that exhibits increased or otherwise improved gain and bandwidth, and with reduced size. 
     With that need in mind, certain embodiments of the present disclosure provide an antenna assembly including a first magnetic substrate having a first surface and a second surface opposite from the first surface. One or more antenna elements are disposed on the first surface of the first magnetic substrate. A microstrip feed line is disposed on the second surface of the first magnetic substrate. A second magnetic substrate is secured to the first magnetic substrate. The second magnetic substrate includes one or more cavities aligned with the one or more antenna elements and the microstrip feed line. 
     In at least one embodiment, a ground plane is connected to the second magnetic substrate. The one or more cavities are disposed between the one or more antenna elements and the ground plane. In at least one embodiment, the microstrip feed line is disposed between the one or more antenna elements and the one or more cavities. 
     In at least one embodiment, the antenna assembly further includes a third magnetic substrate. The third magnetic substrate is secured to the second magnetic substrate. 
     In at least one embodiment, the antenna assembly is devoid of electrical vias. 
     The second magnetic substrate may be secured to the first magnetic substrate by a first laminate layer. 
     In at least one embodiment, the second magnetic substrate has a first surface and a second surface opposite from the first surface. The one or more cavities extend through and between the first surface and the second surface of the second magnetic substrate. 
     As an example, the one or more antenna elements are one or more rectangular patch antenna elements having inclusive slots. 
     In at least one embodiment, the one or more cavities extend below the one or more antenna elements and the microstrip feed line. In at least one embodiment, the one more cavities define an axial envelope that contains the one or more antenna elements and the microstrip feed line. 
     The first magnetic substrate and the second magnetic substrate have a magnetic permeability greater than 1. As an example, the first magnetic substrate and the second magnetic substrate have a dielectric permittivity of at least 6 and a magnetic permeability of at least 6. 
     Certain embodiments of the present disclosure provide a method of forming an antenna assembly. The method includes disposing one or more antenna elements on a first surface of a first magnetic substrate; disposing a microstrip feed line on a second surface of the first magnetic substrate, wherein the second surface is opposite from the first surface; forming one or more cavities in a second magnetic substrate; and securing the second magnetic substrate to the first magnetic substrate. Said securing includes aligning the one or more cavities with the one or more antenna elements and the microstrip feed line. 
     In at least one embodiment, the method also includes connecting a ground plane to the second magnetic substrate. Said connecting includes disposing the one or more cavities between the one or more antenna elements and the ground plane. Said connecting may also include disposing the microstrip feed line between the one or more antenna elements and the one or more cavities. 
     As an example, said securing includes securing the second magnetic substrate to the first magnetic substrate by a first laminate layer. 
     In at least one embodiment, said securing includes containing one or more antenna elements and the microstrip feed line within an axial envelope defined by the one more cavities. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a schematic block diagram of an antenna assembly, according to an embodiment of the present disclosure. 
         FIG. 2  illustrates a perspective top view of the antenna assembly, according to an embodiment of the present disclosure. 
         FIG. 3  illustrates a top view of the antenna assembly of  FIG. 2 . 
         FIG. 4  illustrates a cross-sectional view of antenna elements and a microstrip feed line disposed on a first magnetic substrate through line  7 - 7  of  FIG. 2 . 
         FIG. 5  illustrates a cross-sectional view of a second magnetic substrate through line  7 - 7  of  FIG. 2 . 
         FIG. 6  illustrates a ground plane disposed on a third magnetic substrate through line  7 - 7  of  FIG. 2 . 
         FIG. 7  illustrates a cross-sectional view of the antenna assembly through line  7 - 7  of  FIG. 2 . 
         FIG. 8  illustrates a perspective top view of an antenna element disposed on the first magnetic substrate of the antenna assembly. 
         FIG. 9  illustrates a graph of predicted antenna gain in relation to elevation angle for the antenna assembly. 
         FIG. 10  illustrates a graph of predicted voltage standing wave ratio (VSWR) in relation to frequency for the antenna assembly. 
         FIG. 11  illustrates a flow chart of a method of forming an antenna assembly, according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     The foregoing summary, as well as the following detailed description of certain embodiments, will be better understood when read in conjunction with the appended drawings. As used herein, an element or step recited in the singular and preceded by the word “a” or “an” should be understood as not necessarily excluding the plural of the elements or steps. Further, references to “one embodiment” are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. Moreover, unless explicitly stated to the contrary, embodiments “comprising” or “having” an element or a plurality of elements having a particular property may include additional elements not having that property. 
     Certain embodiments of the present disclosure provide a low-profile magnetic antenna assembly with cavity backing. The antenna assembly has at least one proximity-coupled antenna element on a top surface of a composite magnetic radio frequency (RF) board, an embedded microstrip feed line below or within the RF board, a ground plane on the backside or bottom surface of the RF board, and a cavity between the antenna element and ground plane. The use of a magnetic substrate significantly reduces the size of the antenna assembly by utilizing a magnetic permeability μ r &gt;1, in contrast to a dielectric substrate that has a μ r =1. The ground plane minimizes or otherwise reduces any change in antenna behavior while the antenna assembly is placed on conductive surfaces (for example, wings, fuselage, tail fin, and the like of an aircraft). 
     The antenna assembly according to embodiments of the present disclosure includes one or more antenna elements (such as rectangular patch antenna assemblies having inclusive slots) electrically coupled to an embedded microstrip feed, which minimizes or otherwise reduces power loss and simplifies planar arraying. The antenna assembly also includes a ground plane to minimize or otherwise reduce any change in electrical behavior due to conductive surfaces. The antenna assembly utilizes one or more magnetic substrates or layers (such as one more magnetic RF boards) to reduce antenna size (both height and area) and weight. One or more cavities are formed between the one or more antenna elements and the ground plane, which thereby improves the gain of the antenna assembly. 
     The antenna assembly is efficiently manufacturable. For example, the antenna assembly does not need any electrical vias. That is, the antenna assembly may be devoid of electrical vias. The antenna assembly can be manufactured using subtractive (for example, laser etch, milling, and/or wet etching) or additive (for example, printing and/or film deposition) methods. 
       FIG. 1  illustrates a schematic block diagram of an antenna assembly  100 , according to an embodiment of the present disclosure. The antenna assembly  100  includes a first magnetic substrate  102 . One or more antenna elements  104  are disposed on a first surface  106  of the first magnetic substrate  102 . A microstrip feed line  108  is disposed on a second surface  110  of the first magnetic substrate  102 . The second surface  110  is opposite from the first surface  106 . For example, the first surface  106  may be a top surface, and the second surface  110  may be a bottom surface. 
     The antenna assembly  100  includes a second magnetic substrate  112  secured to the first magnetic substrate  102 . For example, the second magnetic substrate  112  may be secured to the first magnetic substrate  102  through a laminate layer. The second magnetic substrate  112  includes one or more cavities (or air gaps)  114 . In at least one embodiment, the one or more cavities  114  extend through and between a first surface  116  and a second surface  118  of the second magnetic substrate  112 . The first surface  116  is opposite from the second surface  118 . For example, the first surface  116  may be a top surface, and the second surface  118  may be a bottom surface. Optionally, the one or more cavities  114  may be contained within the second magnetic substrate  112 , such as between the first surface  116  and the second surface  118 . 
     A third magnetic substrate  120  is secured to the second magnetic substrate  112 . For example, the third magnetic substrate  120  may be secured to the second surface  118  of the second magnetic substrate  112  through a laminate layer. 
     A ground plane  122  is secured to the third magnetic substrate  120 . For example, the ground plane  122  is secured below the third magnetic substrate  120 . Alternatively, the antenna assembly  100  may not include the third magnetic substrate  120 . Instead, the ground plane  122  may be secured to the second magnetic substrate  112 . 
     The one or more cavities  114  are disposed between the one or more antenna elements  104  and the ground plane  122 . For example, the one or more cavities  114  are disposed underneath the one or more antenna elements  104  and above the ground plane  122 . Further, the one or more cavities  114  are disposed underneath the microstrip feed line  108 . 
     In at least one embodiment, the one or more cavities  114  are aligned with the one or more antenna elements  104 . In at least one embodiment, the one or more antenna elements  104  and the microstrip feed line  108  are above the one or more cavities  114 . The one or more antenna elements  104  are not within the one or more cavities  114 . Instead, the one or more antenna elements  104  are aligned over, above, or the like from the one or more cavities  114 . 
     The first magnetic substrate  102 , the second magnetic substrate  112 , and the third magnetic substrate  120  provide magnetic layers. For example, one or more of the first magnetic substrate  102 , the second magnetic substrate  112 , and the third magnetic substrate  120  may include magnetic filler sandwiched between metallic (such as copper) plates or sheets. In contrast to a dielectric substrate, which has a magnetic permeability μ r =1, the first magnetic substrate  102 , the second magnetic substrate  112 , and the third magnetic substrate  120  have a magnetic permeability μ r &gt;1. 
     The one or more antenna elements  104  are proximity-coupled (or electrically coupled) to the microstrip feed line  108 . The body of the first magnetic substrate  102  separates the one or more antenna elements  104  from the microstrip feed line  108 . In at least one embodiment, the microstrip feed line  108  may be embedded within the first magnetic substrate  102  between the first surface  106  and the second surface  110 . 
     By using one or more magnetic substrates (such as the first magnetic substrate  102 , the second magnetic substrate  112 , and the third magnetic substrate  120 ) instead of dielectric substrates, the overall size of the antenna assembly  100  is significantly reduced. As such, the antenna assembly  100  has a lower profile, and is both smaller and lighter than if dielectric layers were used. The ground plane  122  minimizes or otherwise reduces any change in antenna behavior while the antenna assembly  100  is placed on conductive surfaces (for example, wings, fuselage, tail fin, and the like of an aircraft). 
     The antenna assembly according to embodiments of the present disclosure includes one or more antenna elements  104  (such as rectangular patch antenna assemblies having inclusive slots) electrically coupled to an embedded microstrip feed line  108  (such as between the first magnetic substrate  102  and the second magnetic substrate  112 ), which minimizes or otherwise reduces power loss and simplifies planar arraying. It has been found that the one or more cavities  114  within the second magnetic substrate  112  between the one or more antenna elements  104  and the ground plane  122  improve the gain of the antenna assembly  100 . 
     In at least one embodiment, the antenna assembly  100  is devoid of electrical vias. As such, the process of manufacturing the antenna assembly  100  may be easier due to there being no need to form electrical vias within the antenna assembly  100 . Alternatively, the antenna assembly  100  may include at least one electrical via. 
     The one or more antenna elements  104  may include an inclusive slot. For example, the one more antenna elements  104  may be a rectangular patch antenna element having an inclusive slot. The inclusive slot improves cross polarization of the antenna assembly  100 . Alternatively, the one or more antenna elements  104  may not include inclusive slots. As another example, the one or more antenna elements  104  may be circular antenna elements that may or may not include inclusive slots. 
     As described herein, the antenna assembly  100  includes the first magnetic substrate  102  having the first surface  106  and the second surface  110  opposite from the first surface  106 . The one or more antenna elements  104  are disposed on the first surface  106  of the first magnetic substrate  102 . The microstrip feed line  108  is disposed on the second surface  110  of the first magnetic substrate  102 . The second magnetic substrate  112  is secured to the first magnetic substrate  102 . The second magnetic substrate  112  includes the one or more cavities  114  aligned with the one or more antenna elements  104  and the microstrip feed line  108 . For example, the one or more cavities  114  are below the one or more antenna elements  104  and the microstrip feed line  108 . 
     In at least one embodiment, the ground plane  122  is connected to the second magnetic substrate  112 . The one or more cavities  114  are disposed between the one or more antenna elements  104  and the ground plane  122 . In at least one embodiment, the microstrip feed line  108  is disposed between the one or more antenna elements  104  and the one or more cavities  114 . 
     The antenna assembly  100  may also include the third magnetic substrate  120 . For example, the ground plane  122  is secured to the third magnetic substrate  120 . The third magnetic substrate  120  is further secured to the second magnetic substrate  112 . 
     It is to be understood that the terms first, second, third, fourth, and the like are merely for labeling purposes. A “first,” may be a “second,” “third,” “fourth,” or vice versa. 
       FIG. 2  illustrates a perspective top view of the antenna assembly  100 , according to an embodiment of the present disclosure.  FIG. 3  illustrates a top view of the antenna assembly  100  of  FIG. 2 . Referring to  FIGS. 2 and 3 , for the sake of clarity, portions of the antenna assembly  100  are shown transparent. 
     As shown, the antenna assembly  100  includes four antenna elements  104  disposed above four cavities  114 , respectively. Optionally, the antenna assembly  100  can include more or less antenna elements  104  disposed above more or less cavities  114 . The antenna elements  104  are not within the cavities  114 . Rather, the antenna elements  104  are disposed above the cavities  114 , as described above. 
     The antenna elements  104  can include inclusive slots  124 , which improve the cross polarization of the antenna assembly  100 . Optionally, less than all of the antenna elements  104  include inclusive slots  124 . Alternatively, none of the antenna elements  104  include inclusive slots  124 . 
     The microstrip feed line  108  connects to a power divider  126 , such as in an edge-to-edge fashion. In at least one embodiment, a single contiguous cavity  114  is disposed below the microstrip feed line  108 , the antenna elements  104 , and the power divider  126 . Optionally, separate and distinct cavities  114  may be disposed below the antenna elements  104  and portions of the microstrip feed line  108  and/or the power divider  126 . 
     In at least one embodiment, the one or more cavities  114  extend below the antenna elements  104 , the microstrip feed line  108 , and the power divider  126  outside of the widths, diameters, or axial envelopes of the one or more antenna elements  104 . For example, the one or more cavities  114  may extend below an entirety of the antenna elements  104 , the microstrip feed line  108  and the power divider  126 . 
     The antenna assembly  100  shown in  FIGS. 2 and 3  is merely exemplary and includes the antenna elements  104  arranged in a 2×2 array. The antenna elements  104  are disposed on the first (for example, top) surface  106  of the first magnetic substrate  102 . The microstrip feed line  108  is embedded within the antenna assembly  100 , such as between the first magnetic substrate  102  and the second magnetic substrate  112  (shown in  FIG. 1 ). The dimensions of the antenna assembly  100  (for example, length, width, slot length, slot width, and the like) including the one or more cavities  114  (for example, length, width, height, and the like) are numerically determined to maximize or otherwise increase signal propagation at a desired operating frequency. 
     The antenna assembly  100  may include more or less antenna elements  104  than shown. For example, the antenna assembly  100  may include a single antenna element  104 . As another example, the antenna assembly  100  may include two antenna elements  104 . As another example, the antenna assembly  100  may include eight or more antenna elements  104 . 
       FIG. 4  illustrates a cross-sectional view of the antenna elements  104  and the microstrip feed line  108  disposed on the first magnetic substrate  102  through line  7 - 7  of  FIG. 2 . The antenna elements  104  are disposed on the first surface  106  of the first magnetic substrate  102 . The microstrip feed line  108  is disposed on the second surface  110  of the first magnetic substrate  102 . As shown, the microstrip feed line  108  is disposed underneath the antenna elements  104 . The body  111  of the first magnetic substrate  102  separates the antenna elements  104  from the microstrip feed line  108 . The antenna elements  104  and the microstrip feed line  108  may be formed of an electrically conductive material, such as silver or copper, and may be additively formed on the first magnetic substrate  102 , such as through printing or film deposition. Additionally, the electrically conductive material may be subtractively formed, such as through laser etching, milling, wet etching, or the like. 
       FIG. 5  illustrates a cross-sectional view of the second magnetic substrate  112  through line  7 - 7  of  FIG. 2 . As shown, the cavities  114  may extended between and through the first surface  116  and the second surface  118  of the second magnetic substrate  112 . The cavities  114  may be subtractively formed, such as through laser etching, milling, wet etching, or the like. 
       FIG. 6  illustrates the ground plane  122  disposed on the third magnetic substrate  120  through line  7 - 7  of  FIG. 2 . The ground plane  122  may be an electrically conductive material, such as silver or copper. The ground plane  122  may be additively or subtractively formed on the third magnetic substrate  120 . 
       FIG. 7  illustrates a cross-sectional view of the antenna assembly  100  through line  7 - 7  of  FIG. 2 . As shown, a first laminate layer  130  may secure the first magnetic substrate  102  to the second magnetic substrate  112 . Further, a second laminate layer  132  may secured the second magnetic substrate  112  to the third magnetic substrate  120 . 
     The cavities  114  are formed underneath the antenna elements  104  and the microstrip feed line  108 . The cavities  114  have a width or diameter  140  that is greater that a width or diameter  142  of the antenna elements  104  and a width  144  of the microstrip feed line  108 . As such, the cavities  114  define an axial envelope  150  that contains the antenna elements  104  and the microstrip feed line  108 . 
       FIG. 8  illustrates a perspective top view of an antenna element  104  disposed on the first magnetic substrate  102  of the antenna assembly  100 . The first mode (resonant frequency) of a rectangular antenna element  104  is proportional to the length, L, of the antenna element  104 . As noted, a dielectric substrate (in contrast to a magnetic substrate) has a permeability (μ r )=1, which results in: 
       λ= c/f √{square root over (ε r )}
 
     Where λ, is the wavelength of a frequency within an a dielectric substrate, and ε r  is the dielectric permittivity of the substrate. In contrast, a magnetic substrate, such as including the first magnetic substrate  102 , has a magnetic permeability greater than 1 (that is, μ r &gt;1), which results in 
       λ= c/f √{square root over (ε r μ r )}
 
     Where μ r  is the magnetic permeability of the magnetic substrate. The permittivity for a magnetic substrate with a magnetic permittivity greater than 1 is effectively scaled up by a factor of μ r . For example, a magnetic substrate with a magnetic permittivity of 6 and permeability of 6 has a permittivity effectively six times greater than a dielectric material with a permittivity of 6. 
     Referring to  FIGS. 1-8 , the first magnetic substrate  102 , the second magnetic substrate  112 , and the third magnetic substrate  120  have a magnetic permeability of μ r &gt;1. In at least one embodiment, the first magnetic substrate  102 , the second magnetic substrate  112 , and the third magnetic substrate have a magnetic permittivity of at least 6 and a magnetic permeability of at least 6, thereby providing an effective permittivity of at least 36. 
       FIG. 9  illustrates a graph of predicted antenna gain in relation to elevation angle for the antenna assembly  100  shown and described with respect to  FIGS. 1-8 .  FIG. 10  illustrates a graph of predicted voltage standing wave ratio (VSWR) in relation to frequency for the antenna assembly  100 . Referring to  FIGS. 9 and 10 , a numerical model of a low-profile magnetic cavity backed antenna in a 2×2 array designed to operate near 350 MHz was developed using a finite element method (FEM) solver to predict the performance. It has been found that such an antenna assembly  100  has an antenna gain of 0.36 dBi with a 3 dB beamwidth of 97 deg. The 2:1 VSWR bandwidth of the antenna is ˜145 MHz or 41%. 
       FIG. 11  illustrates a flow chart of a method of forming an antenna assembly, according to an embodiment of the present disclosure. Referring to  FIGS. 1-11 , at  200 , an antenna element  104  is disposed on a first surface  106  (such as a top surface) of the first magnetic substrate  102 . At  202 , a conductor forming the microstrip feed line  108  is disposed on the second surface  110  (such as a bottom surface) of the first magnetic substrate  102 . At  204 , material is etched or otherwise removed from the second magnetic substrate  112  to form a cavity  114 . At  206 , a conductive material forming the ground plane  122  is disposed on a bottom surface of the third magnetic substrate  120 . At  208 , the first magnetic substrate, the second magnetic substrate, and the third magnetic substrate  120  are laminated together to form the antenna assembly  100 . 
     As described herein, embodiments of the present disclosure provide microstrip-based antenna assemblies having improved characteristics, such as increased or otherwise improved gain and bandwidth, and having reduced antenna size. 
     While various spatial and directional terms, such as top, bottom, lower, mid, lateral, horizontal, vertical, front and the like may be used to describe embodiments of the present disclosure, it is understood that such terms are merely used with respect to the orientations shown in the drawings. The orientations may be inverted, rotated, or otherwise changed, such that an upper portion is a lower portion, and vice versa, horizontal becomes vertical, and the like. 
     As used herein, a structure, limitation, or element that is “configured to” perform a task or operation is particularly structurally formed, constructed, or adapted in a manner corresponding to the task or operation. For purposes of clarity and the avoidance of doubt, an object that is merely capable of being modified to perform the task or operation is not “configured to” perform the task or operation as used herein. 
     It is to be understood that the above description is intended to be illustrative, and not restrictive. For example, the above-described embodiments (and/or aspects thereof) may be used in combination with each other. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the various embodiments of the disclosure without departing from their scope. While the dimensions and types of materials described herein are intended to define the parameters of the various embodiments of the disclosure, the embodiments are by no means limiting and are exemplary embodiments. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the various embodiments of the disclosure should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Moreover, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. Further, the limitations of the following claims are not written in means-plus-function format and are not intended to be interpreted based on 35 U.S.C. § 112(f), unless and until such claim limitations expressly use the phrase “means for” followed by a statement of function void of further structure. 
     This written description uses examples to disclose the various embodiments of the disclosure, including the best mode, and also to enable any person skilled in the art to practice the various embodiments of the disclosure, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the various embodiments of the disclosure is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if the examples have structural elements that do not differ from the literal language of the claims, or if the examples include equivalent structural elements with insubstantial differences from the literal language of the claims.