Patent Publication Number: US-2022239001-A1

Title: High gain tightly coupled dipole antenna array

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. Section 119(e) of co-pending and commonly-assigned U.S. Provisional Patent Application No. 63/140,412, filed Jan. 22, 2021, by Grant E. Davis and Matthew G. Rivett, entitled “HIGH GAIN TIGHTLY COUPLED DIPOLE ANTENNA ARRAY,” Docket No. (19-2893-US-PSP), which application is incorporated by reference herein. 
    
    
     BACKGROUND 
     1. Field 
     The present disclosure relates to antenna systems and methods of making the same. 
     2. Description of the Related Art 
     Tightly Coupled Dipole Antenna Arrays (TCDAs) comprise an array of dipoles that provide broadband and wide angle performance for transmitter and receiver applications. For some applications, however, it is desirable to have gain with increased directivity over a narrower angle. The present disclosure satisfies this need. 
     SUMMARY 
     Antenna systems having increased directivity are disclosed herein. Illustrative, non-exclusive examples of inventive subject matter according to the present disclosure are described in the following enumerated paragraphs: 
     A1. An antenna system, comprising:
         an array of conductors coupled to a feed line, wherein the array is configured to:
           emit electromagnetic radiation in response to an input signal being input to the array through the feed line; or   output an output signal to the feed line in response to electromagnetic radiation being received on the array; and   
           a director disposed in front of the array, wherein the director has a first reactive load having a first complex impedance that is tailored to increase a directivity of the antenna system by reactively loading the conductors.       

     A2. The antenna system of paragraph A1, further comprising a reflector disposed behind the array, wherein the reflector is configured to cause a reflection of a portion of the electromagnetic radiation, received on the reflector and comprising received electromagnetic radiation, toward the director. 
     A3. The antenna system of paragraph A2, wherein:
         the reflector comprises a second reactive load; and   the second reactive load has a second complex impedance that tailors the reflection of the received electromagnetic radiation toward the director.       

     A4 The antenna system of paragraph A3, wherein:
         the reflector comprises a printed circuit board;   the printed circuit board comprises a conductive track; and   the conductive track comprises at least one of a thickness or meander varying as a function of position along a length of the reflector so as to tailor the second complex impedance.       

     A5. The antenna system of any of the paragraphs A1-A4, wherein:
         the director comprises a printed circuit board;   the printed circuit board comprises circuitry; and   the circuitry has one or more reactive impedances that form the first reactive load.       

     A6. The antenna system of paragraph A5, wherein:
         the circuitry comprises circuit elements configured to control a phase of the electromagnetic radiation at different positions along a length of the array so as to increase the directivity by tailoring at least one of a destructive interference or constructive interference of the electromagnetic radiation at the different positions.       

     A7. The antenna system of any of paragraphs A5-A6, wherein the one or more reactive impedances comprises a capacitive reactance and an inductive reactance. 
     A8. The antenna system of any of the paragraphs A1-A7, wherein the first reactive load comprises an array of circuit elements, and wherein each of the circuit elements comprises:
         a first capacitor; and   a second capacitor in parallel with an inductor;   wherein the first capacitor is in series with the combination of the second capacitor and the inductor.       

     A9. The antenna system of any of the paragraphs A1-A8, wherein:
         the conductors are periodically positioned along the array with a period P; and   the first reactive load comprises the array of circuit elements positioned along a length of the director with the period P.       

     A10. The antenna system of any of the paragraphs A1-A9, further comprising:
         a first microstrip comprising the array, wherein the first microstrip further includes:
           the conductors;   a conductive backplane;   a first dielectric disposed between the conductors and the conductive backplane; and   a plurality of loads, wherein each of the loads connects one of the conductors to an adjacent one of the conductors; and   
           a second microstrip comprising the director, wherein:
           the second microstrip further comprises the first reactive load;   the first reactive load comprises a plurality of conductive components separated by one or more dielectric layers; and   the plurality of conductive components comprise at least one of a capacitive pad or a wire having an inductance.   
               

     A11. The antenna system of paragraph A10, further comprising:
         a third microstrip comprising the reflector positioned behind the array, wherein the third microstrip comprises a second reactive load including a wire having at least one of a varying thickness or a meander varying an inductance of the wire along a length of the third microstrip.       

     A12. The antenna system of paragraph A11, wherein the first microstrip, the second microstrip, and the third microstrip are parallel, coplanar, and have the same length. 
     A13. The antenna system of any of the paragraphs A1-A12, wherein:
         a distance between the array and the director is within 10% of λ/4;   a distance between the array and the reflector is within 10% of λ/8; and   λ is the longest wavelength of the radiation.       

     A14. The antenna system of any of the paragraphs A3-A13, wherein the first reactive load and the second reactive load are tailored as a function of:
         a frequency of the electromagnetic radiation in range between 10 MHz and 10 GHz; and   the directivity of the antenna.       

     A15. The antenna system of any of the paragraphs A1-A14, wherein the directivity comprises the electromagnetic radiation converging to or from a sidewall of the array facing the director. 
     A16. The antenna system of any of the paragraphs A1-A15, wherein the director is configured so that the directivity comprises the electromagnetic radiation focused in an elevation direction from or to a horizon. 
     A17. The antenna system of any of the paragraphs A1-A16, wherein the array comprises a tightly coupled dipole array (TCDA) or a multi-tap antenna. 
     A18. The antenna system of paragraph A17, wherein:
         the conductors each have a length within 10% of λ/10;   the conductors are separated by a distance within 10% of λ/100; and   λ is the longest wavelength of the electromagnetic radiation.       

     A19. The antenna of paragraph A17 or A18, wherein:
         the conductors are capacitively coupled or coupled by a near field interaction of an electric field, so that the electric field generated by the electromagnetic radiation at one of the conductors and experienced at a next adjacent one of the conductors has:
           a near-field amplitude proportional to 1/d 2 ; and   a reactive near field amplitude proportional to 1/d 3 , where d is a distance separating the one of the conductors from the next adjacent one of the conductors.   
               

     A20. The antenna system of any of the paragraphs A1-A19, further comprising an aircraft structure, wherein:
         the aircraft structure comprises or is attached to a reflector disposed behind the array;   the reflector is configured to cause a reflection of a portion of the electromagnetic radiation, received on the reflector and comprising received electromagnetic radiation, toward the director; and   the aircraft structure further comprises a skin, a wing spar, a bulkhead, or a leading edge of a wing.       

     A21. An aircraft comprising the antenna system of any of the paragraphs A1-A20. 
     A22. A method of making an antenna system, the method comprising:
         obtaining a multi-tap antenna comprising an array of conductors and a plurality of loads connecting the array of conductors;   coupling a feed line to the array of conductors so that the multi-tap antenna is configured to:
           emit electromagnetic radiation in response to an input signal being input to the multi-tap antenna through the feed line; or   output an output signal to the feed line in response to electromagnetic radiation being received on the multi-tap antenna;   
           positioning a director in front of the multi-tap antenna, wherein the director has a director reactance that increases a directivity of the antenna system; and   positioning a reflector behind the multi-tap antenna, wherein the reflector has a reflector reactance that causes reflection of the radiation toward the director.       

     A23. The method of paragraph A22, further comprising:
         varying the reflector reactance as a function of position along a length of the reflector; and   varying the director reactance along a length of the director, thereby controlling a phase of the electromagnetic radiation at different positions along the length of the director so that at least one of a destructive interference or constructive interference of the electromagnetic radiation is tailored at the different positions.       

     A24. A method of using an antenna system, the method comprising:
         receiving or transmitting radiation using a tightly coupled dipole antenna array (TCDA); and   increasing a directivity of the antenna system using a director positioned in front of the TCDA and a reflector positioned behind the TCDA.       

     A25. The method of paragraph A24, wherein the directivity is toward a horizon or waterline. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic of an example antenna system including a TCDA coupled to a director and a reflector. 
         FIG. 1B  is a schematic of an example antenna system including a TCDA coupled to a director and a reflector, wherein the TCDA, the director, and the reflector comprise microstrips. 
         FIG. 1C  is a graph comparing the directivity of the antenna system of  FIG. 1A  with the directivity of an antenna system without the reflector and the director. 
         FIG. 2  illustrates an example TCDA comprising a multi-tap antenna. 
         FIG. 3A  is a flowchart illustrating an example method of designing a director or reflector. 
         FIG. 3B  is a graph plotting example design parameters, surface impedance, Im(Z s ) and a tolerance function (zfunc), for an example director as a function of the frequency of the electromagnetic radiation. 
         FIG. 3C  is a graph plotting example design parameter Im(Z s ) as a function of frequency for an example reflector. 
         FIG. 4A  is a cross-sectional schematic of an example director. 
         FIG. 4B  is an example circuit diagram of the reactive components in an example director. 
         FIG. 4C  is a perspective view of an example director showing periodic positioning of the reactive loads in a plurality of unit cells. 
         FIG. 5  is a perspective view of an example reflector. 
         FIG. 6A  illustrates an example antenna system coupled to a wing spar, wherein the wing spar comprises a reflector and the antenna system does not include a director. 
         FIG. 6B  is a graph plotting the gain of the antenna system of  FIG. 6A  as compared to the gain without the reflector. 
         FIG. 7A  illustrates an example antenna system coupled to a spar, wherein the antenna system includes a reflector and a director and the spar includes the reflector. 
         FIG. 7B  is a graph plotting gain of the antenna system of  FIG. 7A . 
         FIG. 7C  is a graph plotting directivity of the antenna system of  FIG. 7A . 
         FIG. 7D  is a graph plotting gain of the antenna system of  FIG. 7A . 
         FIG. 8  illustrates an example antenna system comprising microstrips coupled to a spar. 
         FIG. 9  illustrates an example antenna system comprising two directors and a reflector. 
         FIG. 10A  illustrates the gain of the antenna system of  FIG. 9 . 
         FIG. 10B  illustrates the directivity of the antenna system of  FIG. 9 . 
         FIG. 11  is a schematic of an aircraft comprising the antenna system of any of the examples described herein. 
         FIG. 12  is a flowchart illustrating an example method of making an antenna system. 
         FIG. 13  is a flowchart illustrating an example method of using an antenna system. 
     
    
    
     DESCRIPTION 
     In the following description, reference is made to the accompanying drawings which form a part hereof, and which is shown, by way of illustration, several embodiments. It is understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present disclosure. 
     Technical Description 
     The present disclosure describes an antenna system comprising an antenna (e.g., a fed array) that is reactively loaded so as to control the directivity of the electromagnetic radiation emitted from and/or received on, the antenna. The reactive loading comprises at least one of an inductive load or a capacitive load comprising one or more parallel circuit elements electromagnetically coupled to the antenna. In some examples, the circuit elements comprise reactive loads having complex impedances tailored to vary the phase of the electric fields or currents experienced on each of the elements in the fed array, so that the sum of the collective electric fields, resulting from destructive and/or constructive interference, is an electric field pattern having the desired directivity (with electric field canceled in undesired directions). 
     Example Antenna System 
       FIGS. 1A-1B  illustrate an example antenna system  100  comprising an array  102  of conductors  104  positioned along a length L 1  of the array  102 . The antenna system  100  further includes a first reactive element (e.g., a director  106 ) positioned on a first side  108  of the array  102  and a second reactive element (e.g., a reflector  110 ) positioned on a second side  112  of the array  102 , so that the array  102  is between the director  106  and the reflector HO. In the example shown, the director  106  and the reflector HO each comprise reactive components that reactively load the array  102  so that the resulting directivity is an electromagnetic field pattern having maximum directivity along the x direction and electromagnetic radiation  113  is directed from or to a sidewall  114  (a “knife-edge”) of the array  102 . In the example shown, the conductors  104  are connected by loads  116  and the conductors  104  are disposed along a line to form the array  102  comprising a linear array. In some examples, the array  102  is designed to operate at a single frequency or a narrow range of frequencies of the electromagnetic radiation  113 . 
     In one or more examples, the director  106  comprises a combination of inductive and capacitive loads controlling the phase of the electric fields at each of the conductors  104  in the array  102 , whereas the reflector  110  mainly comprises an inductive load tailored so that the reflector reflects  119  the electromagnetic radiation  113  toward the array  102  or the director  106 . In some examples, the director  106  comprises a capacitive strip  120  comprising a capacitive load including a first rectangular metal layer on a first dielectric and having its length L 2  extending the length L 1  of the array  102 , the reflector comprises an inductive strip  122  comprising an inductive load including a second rectangular metal layer on a second dielectric and having its length L 3  extending the length L 1  of the array  102 , and the reflector  110  and director  106  both have their lengths L 3 , L 2  longer than their width. 
     In one or more examples, the distance D 1  between the director  106  and the array  102  and the distance D 2  between the reflector  110  and the array  102  are also tailored to control the directivity and the reactive impedance of the reactive load. Example distances include, but are not limited to, D 1  within 10% of λ/4 and D 2  within 10% of λ/8 (wherein λ is the longest wavelength of the electromagnetic radiation  113 ). In one or more examples, D 2  is selected so that the reflector HO comprises an inductive load, and D 1  is selected so that the director  106  comprises a capacitive load. 
       FIG. 1B  illustrates an example antenna system  100  implemented using a printed circuit board  124  comprising microstrips having the sidewall  114 . The array  102  comprises a first microstrip  126  comprising the conductors  104 , a conductive backplane  128 , and a first dielectric  130  between the conductors  104  and the conductive backplane  128 . The director  106  comprises a second microstrip  132  including one or more first components  134  combined with a second dielectric  136  to form a director reactance (comprising a first reactive load  135  or first reactive component) varying as a function of position along the length L 2  of the director  106 . The reflector HO comprises a third microstrip  138  including one or more second components  140  combined with a third dielectric  142  to form the reflector reactance (comprising a second reactive load  141  or second reactive component) varying as a function of position along the length L 3  of the reflector  110 . In various examples, the director reactance and reflector reactance control a phase of the electromagnetic field or current experienced at the different conductors  104  in the array  102  so as to tailor at least one of a destructive interference or constructive interference of the electromagnetic field or current experienced at each of the conductors  104 . In one or more examples, when the array  102  of conductors  104  are reactively loaded over the conductive backplane  128  and the reactive loading makes the additional parasitic elements in the director  106  or the reflector  110  appear either shorter (capacitive) or longer (inductive), thereby tuning the directivity. 
     In various examples, the array  102 , the director  106 , and the reflector  110  are formed on the same substrate or printed circuit board  124 , or they may be formed on different substrates or printed circuit boards  124 . 
       FIG. 1C  illustrates an example directivity  144  achieved using the antenna system  100  of  FIG. 1A  as compared to the directivity  146  without the director  106  and the reflector  110 . In some examples, the directivity  144  is selected to focus the electromagnetic radiation along an elevation (theta) direction (rather than the azimuth), so that the electromagnetic radiation converges or focuses to or from a horizon. 
     Although  FIG. 1A-1B  illustrate the array  102  comprising a linear array of conductors  104 , other configurations (e.g., non-linear configurations) of the conductors  104  are also possible. Examples of the array  102  of conductors  104  include, but are not limited to, a fed array, a TCDA wherein the conductors  104  each comprise dipole elements, a phased array (wherein one or more of the conductors  104  in the array  102  are driven and the different conductors  104  in the array  102  experience electric fields or current with different phases), or a multi-tap antenna, as described in the next section. 
     Example Array 
       FIG. 2  illustrates an example array comprising a multi-tap antenna  200  comprising a plurality of loads  116  (e.g., transmission lines) connecting an array of conductors  104  and a feed line  202  connected to the conductors  104 . The multi-tap antenna  200  is configured to: 
     (1) emit electromagnetic radiation in response to an input signal being input to the multi-tap antenna  200  through the feed line  202 ; or
         (2) output an output signal to the feed line  202  in response to electromagnetic radiation being received on the multi-tap antenna  200 .       

       FIG. 2  illustrates the array of conductors  104  are dipole elements capacitively coupled or coupled by a near field interaction of an electric held, so that the electric field generated by the electromagnetic radiation at one  104   a  of the conductors  104  and experienced at a next adjacent one  104   b  of the conductors  104  has: 
     (1) a near-field amplitude proportional to 1/d 2 ; and 
     (2) a reactive near field amplitude proportional to 1/d 3 , where d is a distance separating the one of the conductors  104   a  from the next adjacent one  104   b  of the conductors. 
     Example dimensions include, but are not limited to, each of the conductors  104  comprising a patch having a patch length L 4  within 10% of λ/10 and the conductors  104  separated by a distance d within 10% of λ/100 (wherein λ is the longest wavelength of the electromagnetic radiation). 
       FIG. 2  further illustrates a module  204  connected to a port  206 . In one receiver implementation, the loads  116  tap or receive energy or power from signals generated by the conductors  104  when exposed to the electromagnetic radiation, the module  204  comprises a combiner combining the power received by the loads  116 , and the port  206  comprises an output port receiving the power. In one receiver embodiment, the loads  116  each have an impedance that is equal to a desired impedance for the output port. In one transmitter embodiment, the module  204  comprises a splitter splitting a signal received on the port  206  which includes an input port, so as to distribute the input signal transmitted to each of the conductors  104 . In this manner, power received by or transmitted to the loads  116  is captured or used in a manner that provides improved gain for the multi-tap antenna  200 . 
     The use of the loads  116  (comprising taps) with the conductors  104  broadens the bandwidth of the TCDA comprising the multi-tap antenna  200 . In one or more examples, the loads  116  comprise resistive elements and/or capacitive elements and increase the bandwidth at which the antenna operates by introducing loss that destroys the resonant characteristics of the multi-tap antenna  200 , lowering the efficiency (or gain) of the multi-tap antenna  200 . 
     Example Director and Reflector Design 
     In some examples, the reactive loading provided by the director and/or the reflector is determined empirically by varying the dimensions, circuit design (including impedance), and spacing of the director and reflector and measuring the impact of the varying on the directivity. In other examples, the reactive loading is determined using electromagnetic simulation and modeling software. 
       FIG. 3A  is a flowchart illustrating a method of designing the director reactance and reflector reactance (referring also to elements of  FIGS. 1A-1C  and  FIG. 2 ). 
     Block  300  represents obtaining an expression for a two dimensional (2D) scattering cross section (e.g., radar cross section (RCS)) of the director  106  or reflector  110 , comprising an echo width in units of decibels relative to a knife edge (sidewall  114  of a flat strip), as a function of surface impedance of the director  106  or reflector  110 . In one or more examples, the 2D RCS of a single unit cell of the director  106  or reflector  110  is given by: 
     
       
         
           
             
               
                 
                   
                     2 
                     ⁢ 
                     D 
                     ⁢ 
                         
                     RCS 
                   
                   = 
                   
                     
                       E 
                       s 
                     
                     = 
                     
                       
                         2 
                         ⁢ 
                         χ 
                       
                       
                         
                           χ 
                           ⁢ 
                           α 
                         
                         + 
                         
                           Z 
                           s 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where 
     
       
         
           
             
               α 
               = 
               
                 
                   
                     ( 
                     
                       1 
                       - 
                       
                         
                           
                             2 
                             ⁢ 
                             i 
                           
                           π 
                         
                         ⁢ 
                         
                           ln 
                           ⁡ 
                           ( 
                           
                             τ 
                             4 
                           
                           ) 
                         
                       
                     
                     ) 
                   
                   ⁢ 
                       
                   τ 
                 
                 = 
                 
                   
                     
                       k 
                       0 
                     
                     ⁢ 
                     
                       η 
                       0 
                     
                     ⁢ 
                     w 
                   
                   4 
                 
               
             
             , 
             
               χ 
               = 
               
                 
                   
                     k 
                     0 
                   
                   ⁢ 
                   γ 
                   ⁢ 
                   w 
                 
                 2 
               
             
             , 
           
         
       
     
     and γ=1.781, and Z s  is the surface impedance of the single unit cell, k 0  is the frequency dependent wavevector of the electromagnetic radiation, and γ 0  is the resistive impedance. 
     Block  302  represents finding solutions of E s  that have the desired directivity of the antenna system comprising the director  106 , the reflector  110 , and the array  102 . In one or more examples, E s  is determined using finite element modeling of the director  106  and/or the reflector  110 . 
     Block  304  represents finding the one or more surface impedances Z s  that match the desired solutions of E s  having the desired directivity. In one or more examples, the step comprises plotting the impedance as a function of the frequency of the electromagnetic radiation, using: 
     
       
         
           
             
               
                 
                   
                     Z 
                     s 
                   
                   = 
                   
                     
                       
                         2 
                         ⁢ 
                         χ 
                       
                       
                         E 
                         s 
                       
                     
                     - 
                     
                       χ 
                       ⁢ 
                       α 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     Block  306  represents selecting the geometry and reactance of the single unit cell that has an acceptable 2D RCS for two extremes of frequencies within the bandwidth of the TCDA. In various examples, the acceptable RCS is determined using variables Zi1 and Zi2 (the imaginary parts of Zs at frequencies f1 and f2, respectively) and by minimizing an impedance tolerance percentage (or selecting the impedance tolerance percentage below a predetermined threshold). In one or more examples, the impedance tolerance percentage is given by: 
       100×|(( z func−im( Zs ))/ z func|,
 
     where zfunc=Zi1+(f−f1)*(Zi2−Zi1)/(f2−f1). 
       FIG. 3B  plots Im(Zs) and zfunc for the single unit cell of a director  106  and  FIG. 3C  plots Im(Zs) for the reflector  110 , for one example range of frequencies and for the directivity in a narrow cone toward a waterline or horizon. A typical director  106  or reflector  110  includes a plurality of unit cells arranged (e.g., periodically) along a length L 2 , L 3  of the director or reflector, respectively. 
     Example Director and Reflector Structures 
       FIG. 4A  illustrates an example unit cell  400  in the second microstrip  132  (comprising the director  106 ) including the first reactive components implemented as a transmission line or circuit elements  401 . The circuit elements  401  comprise reactive loads C 1 , C 2 , L including conductive components  134  separated by one or more dielectric layers  402 ,  404 , wherein C 1  forms a first capacitive reactance comprising a first conductive pad, C 2  forms a second capacitive reactance comprising a second conductive pad, and L comprises an inductive reactance comprising a wire or conductive track.  FIG. 4B  is a circuit diagram of the unit cell  400 , illustrating the second capacitive reactance (capacitor C 2 ) is in parallel with an inductive reactance (inductor L) and the first capacitive reactance (capacitor C 1 ) is in series with the combination of the second capacitive reactance C 2  and the inductive reactance L. 
       FIG. 4C  illustrates an example wherein the second microstrip  132  comprises an array of the unit cells  400  positioned along the length L 2  of the microstrip with the period P (defined by the spacing d of the conductors  104  in the array  102  or with a positioning commensurate with a positioning of the conductors  104  in the array  102 , as illustrated in  FIG. 1A  or  FIG. 2 ). In one or more examples, each unit cell  400  comprises the circuit elements  401  of  FIGS. 4A and 4B . 
       FIG. 5  illustrates an example third microstrip  138  (comprising the reflector  110 ) wherein the second components  140  comprise a conductive track  502  (e.g., an inductive wire  503 ) having at least one of a meander  504  or a varying thickness  506  along a length of the reflector  110 . Decreasing thickness  506  of the wire increases inductance. Increasing the meander  504  of the wire  503  or conductive track  502  also increases inductance. 
     Example Antenna Assemblies and Performance 
       FIG. 6A  illustrates an antenna system  600  comprising an array  102  and a wing spar  602 , wherein the wing spar  602  comprises a metal ground plane comprising a reflector  110  or acting as a reflector  110 . 
       FIG. 6B  illustrates the gain of an array  102  (a linear array) without a director  106  and without a reflector  110  (omni in elevation), as well as the gain of the array  102  with a reflector  110  but no director  106  (omni-over half space or cardiodal). The efficiency of the array  102  is given by: 
     
       
         
           
             Efficiency 
             = 
             
               
                 
                   g 
                   6 
                 
                 
                   2 
                   ⁢ 
                   kp 
                 
               
               ⁢ 
               
                 ∫ 
                 
                   d 
                   ⁢ 
                   θ 
                   ⁢ 
                   
                     Γ 
                     ⁡ 
                     ( 
                     θ 
                     ) 
                   
                 
               
             
           
         
       
     
     where g 0  is gain for each fed element in the array  102 , Γ(θ) is the normalized elevation pattern, p is the period of the fed elements, and k is the wavenumber 2λ/λ of the electromagnetic radiation. For an omnidirectional radiation pattern, g 0 =2p/λ. As shown in  FIG. 6B , the antenna system including the wing spar  602  (but no director  106 ) has a gain that is 3 dB higher as compared to the directivity without the wing spar  602 , assuming the array  102  is 100% efficient (such that all the conductors are matched with no ohmic loss). The wing spar  602  enables the antenna system  600  to be omnidirectional over half space (cardiodal). 
       FIG. 7A  illustrates an antenna system  600  including an array  102  (a linear array), a director  106 , and a reflector  110  combined with a wing spar  602  according to another example (dimensions and reactances shown in Table 1). The presence of the director  106  significantly increases the gain and directivity of the antenna system  600 , as shown in  FIG. 7B  and  FIG. 7C .  FIG. 7D  illustrates the gain of the antenna system  600  does not change significantly when the load capacitance (capacitance of the load  116  in  FIG. 1A  and  FIG. 2 ) is changed from 9.3 pF to 8.87 pF and the capacitive reactance of the director is reduced from 6.7 pF per square to 6.67 pF per square. 
       FIG. 8  illustrates another example of the antenna system  600  comprising the array  102  (a linear array), a director  106 , and the wing spar  602  comprising the reflector  110 , wherein the director  106  comprises the unit cells  400  comprising circuit elements  401  and components  134  illustrated in  FIGS. 4A, 4B, and 4C . 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Performance of various antenna configurations 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                   
                   
                 FIG. 9 (two 
               
               
                 Configuration 
                 FIG. 7A 
                 FIG. 7A 
                 FIG. 8 
                 directors) 
               
               
                   
               
               
                 Load 
                 50 ohms in 
                 25 Ohm per 
                 25 Ohm per 
                 50 ohms in 
               
               
                 Reactance (of 
                 series with 9.3 
                 square in 
                 square in series 
                 series with 9.3 
               
               
                 load 116 in 
                 pF capacitance 
                 series with a 
                 with a 36.03 pF 
                 pF capacitance 
               
               
                 FIG. 1A or 
                   
                 8.87 pF per 
                 per square 
               
               
                 FIG. 2) 
                   
                 square 
               
               
                 Director 
                 6.7 pF per 
                 6.67 pF per 
                 FIG. 3A 
                 Both directors 
               
               
                 Reactance 
                 square 
                 square 
                 FIG. 3B 
                 9.78 pF per 
               
               
                   
                   
                   
                 C1 = 10.1 pF per 
                 square 
               
               
                   
                   
                   
                 square 
               
               
                   
                   
                   
                 C2 = 59.3 pF per 
               
               
                   
                   
                   
                 square L = 
               
               
                   
                   
                   
                 comprises 39 
               
               
                   
                   
                   
                 nanohenries per 
               
               
                   
                   
                   
                 square 
               
               
                 Spar to Fed 
                 7-8 inches 
                 7-8 inches 
                 See FIG. 8 
               
               
                 Array 102 
               
               
                 distance 
               
               
                 Spar to 
                 10.5-11.5 
                 10.5-11.5 
                 See FIG. 8 
                 14 inches from 
               
               
                 Director 
                 inches 
                 inches 
                   
                 spar to second 
               
               
                 distance 
                   
                   
                   
                 director 
               
               
                 Gain 
                 FIG. 7B 
                 FIG. 7D 
                   
                 FIG. 10A 
               
               
                 Directivity 
                 FIG. 7C 
                   
                   
                 FIG. 10B 
               
               
                   
               
            
           
         
       
     
       FIG. 9  illustrates an example wherein the antenna system  600  comprises an array  102 , multiple directors  106   a ,  106   b  positioned in front (on the first side  108  of) the array  102 , and the wing spar  602  comprises the reflector  110 .  FIG. 10A  and  FIG. 10B  illustrate the gain and directivity of the antenna system of  FIG. 9  when the second director  106   b  is 14 inches from the wing spar  602  and the array  102  comprises a linear array, showing both the gain and directivity are increased as compared to an antenna system without directors. In some examples, different directors  106   a ,  106   b  are tailored to increase directivity and gain at different frequencies in the bandwidth of the array  102  (e.g., one director  106   a  tailored for higher gain and directivity at high frequencies and the other director  106   b  tailored for higher gain and directivity at lower frequencies). 
       FIG. 11  illustrates an example aircraft  1100  including a fuselage  1102 , a wing  1104 , and aircraft structures  1150 . Example aircraft structures comprising or coupled to the antenna system include various structural parts of the aircraft  1100 , including but not limited to, a bulkhead  1101 , an aircraft skin  1103  (e.g., skin panel), a wing spar  602 , or a leading edge  1152  of the wing  1104 . One or more components of the antenna system (e.g., the reflector  110 ) are integrated or combined with the aircraft structure in various configurations. In some examples, the antenna system  100  is entirely mounted on a surface of the aircraft structure  1150 , and in other examples the antenna system  100  is mounted within an interior of the aircraft structure.  FIG. 11  further illustrates the antenna system is configurable and positioned so that the desired directivity is toward a waterline  1106  or horizon  1108 . 
     Example Process Steps 
     Method of Making an Antenna System 
       FIG. 12  illustrates a method of making an antenna system, comprising the following steps. 
     Block  1200  represents obtaining or fabricating an array of elements (e.g., a multi-tap antenna, a TCDA, a linear array, or a fed array). In one or more examples, the elements comprise conductors. Example conductors include a metal layer on a dielectric. In one or more further examples, the elements each comprise dipole elements. 
     Block  1202  represents coupling a feed line to the array. The array is configured to:
         emit radiation in response to an input signal being input to the dipole elements through the feed line; or   output an output signal to the feed line in response to electromagnetic radiation being received on the multi-tap antenna.       

     Block  1204  represents positioning a director in front of the array, wherein the director has a reactance that increases a directivity of the antenna system. In one or more examples, the director comprises a printed circuit board or circuitry comprising metal pads or tracks combined with dielectric to form a first reactive load. 
     Block  1206  represents positioning a reflector behind the array, wherein the reflector is configured to cause reflection of the radiation toward the director or the array. In one or more examples, the reflector comprises a printed circuit board or circuitry comprising metal pads or tracks combined with dielectric to form a second reactive load. 
     Block  1208  represents the end result, an antenna system. Illustrative, non-exclusive examples of inventive subject matter according to the present disclosure are described in the following enumerated paragraphs (referring also to  FIG. 1A ,  FIG. 1B ,  FIG. 2 ,  FIGS. 4A-4C ,  FIG. 5 , and  FIGS. 6A ,  FIG. 8 ,  FIG. 9 , and  FIG. 11 ): 
     A1. An antenna system ( 100 ), comprising:
         an array ( 102 ) of conductors ( 104 ) coupled to a feed line ( 202 ), wherein the array ( 102 ) is configured to:
           emit electromagnetic radiation ( 113 ) in response to an input signal being input to the array ( 102 ) through the feed line ( 202 ); or   output an output signal to the feed line ( 202 ) in response to electromagnetic radiation ( 113 ) being received on the array ( 102 ); and   
           a director ( 106 ) disposed in front of the array ( 102 ), wherein the director ( 106 ) has a first reactive load ( 135 ) having a first complex impedance that is tailored to increase a directivity ( 144 ) of the antenna system ( 100 ) by reactively loading the conductors ( 104 ).       

     A2. The antenna system ( 100 ) of paragraph A1, further comprising a reflector ( 110 ) disposed behind the array ( 102 ), wherein the reflector ( 110 ) is configured to cause a reflection ( 119 ) of a portion of the electromagnetic radiation ( 113 ), received on the reflector  110  and comprising received electromagnetic radiation, toward the director ( 106 ). 
     A3. The antenna system ( 100 ) of paragraph A2, wherein:
         the reflector ( 110 ) comprises a second reactive load ( 141 ); and   the second reactive load ( 141 ) has a second complex impedance that tailors the reflection of the received electromagnetic radiation ( 113 ) toward the director ( 106 ).       

     A4 The antenna system ( 100 ) of paragraph A3, wherein:
         the reflector ( 110 ) comprises a printed circuit board ( 124 );   the printed circuit board ( 124 ) comprises a conductive track ( 502 ); and   the conductive track ( 502 ) comprises at least one of a thickness ( 506 ) or meander ( 504 ) varying as a function of position along a length (L 3 ) of the reflector ( 110 ) so as to tailor the second complex impedance.       

     A5. The antenna system ( 100 ) of any of the paragraphs A1-A4, wherein:
         the director ( 106 ) comprises a printed circuit board ( 124 );   the printed circuit board ( 124 ) comprises circuitry; and   the circuitry has one or more reactive impedances that form the first reactive load ( 135 ).       

     A6. The antenna system ( 100 ) of paragraph A5, wherein:
         the circuitry comprises circuit elements ( 401 ) configured to control a phase of the electromagnetic radiation ( 113 ) at different positions along a length (L 1 ) of the array ( 102 ) so as to increase the directivity ( 144 ) by tailoring at least one of a destructive interference or constructive interference of the electromagnetic radiation ( 113 ) at the different positions.       

     A7. The antenna system ( 100 ) paragraph A5 or A6, wherein the one or more reactive impedances comprise a capacitive reactance and an inductive reactance. 
     A8. The antenna system ( 100 ) of any of the paragraphs A1-A7, wherein the first reactive load ( 135 ) comprises an array of circuit elements ( 401 ), and wherein each of the circuit elements ( 401 ) comprises:
         a first capacitor (C 1 ); and   a second capacitor (C 2 ) in parallel with an inductor (L);   wherein the first capacitor (C 1 ) is in series with the combination of the second capacitor (C 2 ) and the inductor (L).       

     A9. The antenna system ( 100 ) of any of the paragraphs A1-A8, wherein:
         the conductors ( 104 ) are periodically positioned along the array ( 102 ) with a period P; and   the first reactive load ( 135 ) comprises the array of circuit elements ( 401 ) positioned along a length (L 2 ) of the director ( 106 ) with the period P.       

     A10. The antenna system ( 100 ) of any of the paragraphs A1-A9, further comprising:
         a first microstrip ( 126 ) comprising the array, wherein the first microstrip ( 126 ) further includes:
           the conductors ( 104 );   a conductive backplane ( 128 );   a first dielectric ( 130 ) disposed between the conductors ( 104 ) and the conductive backplane ( 128 ); and   a plurality of loads ( 116 ), wherein each of the loads ( 116 ) connects one of the conductors ( 104   a ) to an adjacent one of the conductors ( 104   b ); and   
           a second microstrip ( 132 ) comprising the director ( 106 ), wherein:
           the second microstrip ( 132 ) further comprises the first reactive load ( 135 );   the first reactive load ( 135 ) comprises a plurality of conductive components ( 134 ) separated by one or more dielectric layers ( 402 ,  404 ); and   the plurality of conductive components ( 134 ) comprise at least one of a capacitive pad or a wire having an inductance.   
               

     A11. The antenna system ( 100 ) of paragraph A10, further comprising:
         a third microstrip ( 138 ) comprising the reflector ( 110 ) positioned behind the array ( 102 ), wherein the third microstrip ( 138 ) comprises a second reactive load ( 141 ) including a wire having at least one of a varying thickness ( 506 ) or a meander ( 504 ) varying an inductance of the wire along a length (L 3 ) of the third microstrip ( 138 ).       

     A12. The antenna system ( 100 ) of paragraphs A10 or A11, wherein two or more of the first microstrip ( 126 ), the second microstrip ( 132 ), and the third microstrip ( 138 ) are parallel, coplanar, and have the same length. 
     A13. The antenna system ( 100 ) of any of the paragraphs A1-A12, wherein:
         a distance (D 1 ) between the array ( 102 ) and the director ( 106 ) is within 10% of λ/4;   a distance (D 2 ) between the array ( 102 ) and the reflector ( 110 ) is within 10% of λ/8; and   λ is the longest wavelength of the electromagnetic radiation ( 113 ).       

     A14. The antenna system ( 100 ) of any of the paragraphs A1-1A13, wherein at least one of the first reactive load ( 135 ) or the second reactive load ( 141 ) are tailored as a function of:
         a frequency of the electromagnetic radiation ( 113 ) in range between 10 MHz and 10 GHz; and   the directivity ( 144 ) of the antenna system ( 100 ).       

     A15. The antenna system ( 100 ) of any of the paragraphs A1-A14, wherein the directivity ( 144 ) comprises the electromagnetic radiation ( 113 ) converging to or from a sidewall ( 114 ) (e.g., edge) of the array ( 102 ) facing the director ( 106 ). 
     A16. The antenna system ( 100 ) of any of the paragraphs A1-A15, wherein the director ( 106 ) is configured so that the directivity ( 144 ) comprises the electromagnetic radiation ( 113 ) focused in an elevation direction from or to a horizon ( 1108 ). 
     A17. The antenna system ( 100 ) of any of the paragraphs A1-A16, wherein the array ( 102 ) comprises a tightly coupled dipole array (TCDA) or a multi-tap antenna ( 200 ). 
     A18. The antenna system ( 100 ) of any of the paragraphs A1-A17, wherein:
         the conductors ( 104 ) each have a length (L 4 ) within 10% of λ/10;   the conductors ( 104 ) are separated by a distance (d) within 10% of λ/100; and   λ is the longest wavelength of the electromagnetic radiation ( 113 ).       

     A19. The antenna system ( 100 ) of any of the paragraphs A1-A18, wherein:
         the conductors ( 104 ) are capacitively coupled or coupled by a near field interaction of an electric field, so that the electric field generated by the electromagnetic radiation ( 113 ) at one of the conductors ( 104   a ) and experienced at a next adjacent one of the conductors ( 1041 )) has:
           a near-field amplitude proportional to 1/d 2 ; and   a reactive near field amplitude proportional to 1/d 3 , where d is a distance separating the one of the conductors ( 104 - a ) from the next adjacent one of the conductors ( 104   b ).   
               

     A20. The antenna system ( 100 ) of any of the paragraphs A1-A19, further comprising an aircraft structure ( 1150 ), wherein:
         the aircraft structure ( 1150 ) comprises or is attached to the reflector ( 110 ); and   the aircraft structure ( 1150 ) further comprises a skin ( 1103 ), a wing spar ( 602 ), a bulkhead ( 1101 ), or a leading edge of a wing.       

     A21. An aircraft ( 1100 ) comprising the antenna system ( 100 ) of paragraph 1. 
     A22. The antenna system ( 100 ) of any of the paragraphs A1-A21, wherein the director ( 106 ) and the reflector ( 110 ) comprise passive elements. 
     A23. The antenna system ( 100 ) of any of the paragraphs A1-A16, wherein the electromagnetic radiation ( 113 ) comprises radio frequencies. 
     A24. A transmitter comprising the antenna system of any of the paragraphs A1-A18, wherein the directivity ( 144 ) focuses energy of the electromagnetic radiation to a sensor at a waterline or horizon. 
     A25. The antenna system ( 100 ) of any of the paragraphs A1-A24, wherein the array ( 102 ), the director ( 106 ), and the reflector ( 110 ) are reactively loaded over a conductive backplane ( 128 ) to provide an improvement of up to 6 Decibels in gain. 
     A26. The antenna system ( 100 ) of any of the paragraphs A1-A25, wherein the array ( 102 ), the director ( 106 ), and the reflector ( 110 ) are reactively loaded so that when an active center dipole element comprising one of the conductors ( 104 ) in the array ( 102 ) is excited, other dipole elements (comprising other conductors ( 104 ) are also excited, but in a given phase in which they excitation fields of the dipole element add in the direction of the horizon and cancel above and below the array (up and down). 
     A27. The antenna system ( 100 ) of any of the paragraphs A1-A26, wherein the directivity ( 144 ) is increased in the elevation direction (angle theta) but not significantly increased in the azimuth direction, so that the electric field pattern comprises an cone having elliptical cross section comprising a long axis along the elevation direction and a short axis along the azimuth direction. 
     A28. The antenna system ( 100 ) of any of the paragraphs A1-A27, wherein the array ( 102 ) comprises a linear array of the conductors ( 104 ). 
     A29. The antenna system ( 100 ) of any of the paragraphs A1-A28, wherein the conductors ( 104 ) comprise dipole elements. 
     A30. The antenna system ( 100 ) of any of the paragraphs A1-A29, wherein the array ( 102 ) comprises a fed array. 
     A31. The antenna system ( 100 ) of any of the paragraphs A1-A29, wherein the array ( 102 ) comprises a TCDA. 
     A32. The antenna system ( 100 ) of any of the paragraphs A1-A29, wherein the array ( 102 ) comprises a plurality of loads ( 116 ) and each of the loads ( 116 ) connects one of the conductors ( 104   a ) to an adjacent one of the conductors ( 104   b ). 
     A33. The antenna system ( 100 ) of paragraph A32, wherein each of the loads ( 116 ) comprises a resistance or a resistance in series with a capacitance. 
     A34. The antenna system ( 100 ) of any of the paragraphs A1-A33, wherein the first reactive load ( 135 ) comprises a capacitive strip ( 120 ) comprising a first metal layer on a first dielectric. 
     A35. The antenna system ( 100 ) of any of the paragraphs A3-A34, wherein the second reactive load ( 141 ) comprises an inductive strip ( 122 ) comprising a second metal layer on a second dielectric. 
     A36. The antenna system ( 100 ) of any of the paragraphs A1-A35, wherein the first reactive load ( 135 ) comprises at least one capacitor (C 1 ) including a dielectric layer  404 . 
     A37. The antenna system ( 100 ) of any of the paragraphs A1-A36, wherein at least one of the first reactive load ( 135 ) or the second reactive load ( 141 ) comprises circuitry on a dielectric layer ( 404 ) and/or a semiconductor. 
     A38. The antenna system ( 100 ) of paragraph A37, wherein the circuitry comprises one or more discrete electrical components, one or more circuit elements  401 , one or more conductive tracks ( 502 ), or one or more conductive pads. 
     A39. A method of making an antenna system, the method comprising:
         obtaining a multi-tap antenna comprising an array of conductors and a plurality of loads coupling the array of conductors;   coupling a feed line to the array of conductors so that the multi-tap antenna is configured to:
           emit electromagnetic radiation in response to an input signal being input to the multi-tap antenna through the feed line; or   output an output signal to the feed line in response to electromagnetic radiation being received on the multi-tap antenna;   
           positioning a director in front of the multi-tap antenna, wherein the director has a director reactance that increases a directivity of the antenna system; and   positioning a reflector behind the multi-tap antenna, wherein the reflector has a reflector reactance that causes reflection of the radiation toward the director.       

     A40. The method of paragraph A39, further comprising:
         varying the reflector reactance as a function of position along a length of the reflector; and   varying the director reactance along a length of the director, thereby controlling a phase of the electromagnetic radiation at different positions along the length of the director so that at least one of a destructive interference or constructive interference of the electromagnetic radiation is tailored at the different positions.       

     Method of Using an Antenna Array 
       FIG. 13  illustrates a method of using an antenna system. 
     Block  1300  represents receiving or transmitting radiation using an antenna array (e.g., a TCDA). 
     Block  1302  represents increasing a directivity of the antenna system using a director positioned in front of the array and a reflector positioned behind the antenna array. In one or more examples, the directivity is toward a horizon or waterline. 
     CONCLUSION 
     This concludes the description of the preferred embodiments of the present disclosure. The foregoing description of the preferred embodiment has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of rights be limited not by this detailed description, but rather by the claims appended hereto.