Patent Publication Number: US-8537066-B2

Title: Truncated biconical dipole antenna with dielectric separators and associated methods

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of antennas, and, more particularly, to dipole antennas with broad pattern bandwidth and related methods. 
     BACKGROUND OF THE INVENTION 
     A particular type of antenna may be selected for use in an electronic device based upon a desired application. For example, a different type of antenna may be used for terrestrial communications versus satellite communications. The type of antenna used may also be based upon a desired operating frequency of the antenna. 
     One example of a type of antenna is a broadband antenna. A broadband antenna is an antenna that operates over a wide range of frequencies. The broadband antenna may be formed to provide increased gain along the horizon, for example, during terrestrial communications. 
     One type of broadband antenna is a biconical dipole antenna. A biconical dipole antenna has inherent broadband characteristics as it functions as a self excited horn. The conical horn walls fit different sized waves. However, the diameter of a biconical antenna becomes increasingly large at lower operational frequencies. A larger diameter or size may be restricted in a mobile wireless communications device as the size of the housing carrying the biconical antenna may be limited in size. To reduce the size of the biconical antenna, the biconical antenna may be truncated. As a result, a dipole-type structure is formed. 
     Increased antenna performance at lower frequencies may correspond to increased antenna length. However, at higher frequencies the increased length may result in the formation of lobes in the antenna pattern, thus resulting in relatively low gain on the horizon. In fact, another radiation pattern lobe is formed for every half wavelength of wire dipole length. Electrically large thin wire dipole antennas have nearly endfire radiation rather than broadside radiation. 
     For example, referring now to the biconical antenna  170  in  FIG. 1   a , and the graphs in  FIGS. 1   b - 1   c , the biconical antenna has relatively satisfactory performance at the horizon both for low ( FIG. 1   b ) and high ( FIG. 1   c ) frequencies. However, the biconical antenna has a relatively large diameter, for example, 15.5″ tall by 15.3″ in diameter, for a desired operating frequency range. 
     Additionally, referring to the truncated biconical antenna  180  (i.e. dipole with biconical feed) in  FIG. 2   a , and the graphs in  FIGS. 2   a - 2   c , the truncated biconical antenna feed has relatively satisfactory performance at the horizon at low frequencies ( FIG. 2   b ). The dominate dipole structure may be too long for the higher frequencies, which illustratively causes a lobe to form at the horizon ( FIG. 2   c ). Example dimensions for the truncated biconical dipole are 15.5″ tall×4″ in diameter for the desired operating frequency range. 
     U.S. Pat. No. 7,221,326 to Ida et al. discloses a biconical antenna. More particularly, the biconical antenna includes a columnar dielectric member having frustum-shaped cavities extending respectively from an upper and lower surface toward the center of the columnar member. Flat surfaces of apex portions of the frustum-shaped cavities are parallel and in opposition to one another. 
     U.S. Pat. No. 2,175,252 to P. S. Carter discloses conical monopole and conical dipole antennas with a coaxial cable feed. The cone forms a sleeve over the coaxial cable. 
     U.S. Pat. No. 7,339,542 to Lalezari. discloses an ultra-broadband antenna system that combines an asymmetrical dipole element and a biconical dipole element to form a monopole. The asymmetrical dipole element includes upper and lower asymmetrical dipole elements. The antenna system also includes a plastic expander ring coupled to the lower asymmetrical dipole element. The expander ring is also coupled to a canister sub-assembly. A choke sub-assembly is provided within the canister sub-assembly. 
     However, there may still be a need for a reduced diameter antenna profile, such as a cylindrical “whip” profile, including an increased radiation pattern bandwidth and a satisfactory pattern on the horizon. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing background, it is therefore an object of the present invention to provide an increased radiation pattern bandwidth and a satisfactory pattern on the horizon in a biconical-type antenna. 
     This and other objects, features, and advantages in accordance with the present invention are provided by an antenna assembly including first and second adjacent antenna elements each comprising a tapered or conical antenna body having a base and an apex opposite the base, an elongated or cylindrical antenna body extending from the base of the conical antenna body, and including a plurality of adjacent elongated or cylindrical antenna body portions, and a respective dielectric member separating adjacent cylindrical antenna body portions. Each of the first and second adjacent antenna elements are aligned along a common longitudinal axis with respective apexes in opposing relation to define a dipole antenna. 
     Each of the plurality of cylindrical antenna body portions may comprise a tubular cylindrical antenna body portion, a solid cylindrical antenna body portion, a mesh electrical conductor and/or a continuous electrical conductor. Each of the dielectric members may comprise a cylindrical dielectric member, and a diameter of each of the cylindrical dielectric members may be different than a diameter of adjacent cylindrical antenna body portions. The mesh electrical conductor may comprise insulated wire. 
     An antenna feed assembly may be coupled to the first and second adjacent antenna elements. The plurality of adjacent cylindrical body portions may include at least three cylindrical body portions with respective dielectric members therebetween. 
     A method aspect is directed to making an antenna assembly including forming first and second adjacent antenna elements each comprising a conical antenna body having a base and an apex opposite the base, a cylindrical antenna body extending from the base of said conical antenna body, and including a plurality of adjacent cylindrical antenna body portions, and a respective dielectric member separating adjacent cylindrical antenna body portions. The method further includes aligning each of the first and second adjacent antenna elements along a common longitudinal axis with respective apexes in opposing relation to define a dipole antenna. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic view of a biconical antenna in accordance with the prior art. 
         FIGS. 1B and 1C  are respective graphs of low and high frequency radiation patterns of the biconical antenna of  FIG. 1A . 
         FIG. 2A  is a schematic view of a truncated biconical antenna in accordance with the prior art. 
         FIGS. 2B and 2C  are respective graphs of low and high frequency gain patterns of the truncated biconical antenna of  FIG. 2A . 
         FIG. 3A  is a perspective view of an antenna assembly in accordance with the present invention. 
         FIGS. 3B and 3C  are schematic diagrams illustrating various embodiments of portions of the antenna assembly of  FIG. 3A . 
         FIG. 4A  is a schematic diagram illustrating a sample radiation pattern of the antenna of  FIG. 2A  in accordance with the prior art. 
         FIG. 4B  is a schematic diagram illustrating a sample radiation pattern of the antenna assembly of  FIG. 3 . 
         FIGS. 5A and 5B  are respective graphs of low and high frequency gain patterns of the antenna of  FIG. 3 . 
         FIG. 6  is an exploded view of an inductive mesh in the antenna embodiment of  FIG. 3B . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. 
     Referring initially to  FIG. 3 , an antenna assembly  20  includes first and second adjacent antenna elements  21   a ,  21   b . Each of the first and second adjacent antenna elements  21   a ,  21   b  includes a tapered or conical antenna body  22   a ,  22   b  having a base  32   a ,  32   b  and an apex  31   a ,  31   b  opposite the base. 
     Each conical antenna body  22   a ,  22   b  has two-stages defining a step therebetween. As will be appreciated by those skilled in the art, the two-step conical antenna body  22   a ,  22   b  may be used to match a return loss. An approximation of a curve corresponding to a desired return loss at a desired frequency may be accomplished by adding additional stages to form the conical antenna body  22   a ,  22   b . The two-stage conical antenna body  22   a ,  22   b  provides improved return loss performance over a single-plane conical antenna body. Of course, each conical antenna body  22   a ,  22   b  may be formed having a single stage or more than two stages. Moreover, the stages may be formed to define any shape, but an overall spherical shape of the conical antenna body is less desired, for example, for wideband frequency operation. 
     An increase in the size or diameter of each conical antenna body  22   a ,  22   b  advantageously increases performance. For example, an increase in the diameter of the base  32   a ,  32   b  of the conical antenna body  22   a ,  22   b  corresponds to an increase in frequency bandwidth. Thus, the diameter of each conical antenna body  22   a ,  22   b  may be determined based upon a compromise of desired size and desired performance. 
     Each of the first and second adjacent antenna elements  21   a ,  21   b  also includes an elongated or cylindrical antenna body  26   a ,  26   b  extending from the base  32   a ,  32   b  of the conical antenna body  22   a ,  22   b . Each cylindrical antenna body  26   a ,  26   b  includes a plurality of elongated or cylindrical antenna body portions  27   a ,  27   b . Also, each of the first and second adjacent antenna elements  21   a ,  21   b  includes a respective dielectric member  29   a ,  29   b  separating adjacent cylindrical antenna body portions. 
     Such dielectric members  29   a ,  29   b  may be formed of foam, graphite or PTFE, for example. The plurality of cylindrical antenna body portions  27   a ,  27   b  and dielectric members  29   a ,  29   b  may be secured together in various ways included being threaded together, slid together and/or adhered together, for example. 
     The longitudinally spaced distance between the cylindrical antenna body portions  27   a ,  27   b  due to the dielectric members  29   a ,  29   b  advantageously affects the performance of the antenna. For example, the longitudinally spaced distance between cylindrical antenna body portions  27   a ,  27   b  affects the radiation pattern and/or return loss by altering the location of lobes in the gain pattern. The gaps created by the separation may control the phasing of current, and the plurality of cylindrical antenna body portions  27   a ,  27   b  may be considered to operate as a series-fed array. 
     Additional dielectric members (not shown) may be included in the cylindrical antenna body  26   a ,  26   b . Thus additional lobe control may be provided. Reduction of “lobing” at other or additional frequencies may be accomplished by adjusting the location and number of the dielectric members  29   a ,  29   b  relative to the center of the antenna assembly  20 . Moreover, the location and number of the dielectric members  29   a ,  29   b  may be chosen based upon a desired operating frequency, bandwidth, return loss, and lobe location, for example. Other factors may be considered in determining the number and location of choke members and thus choke slots. 
     As illustrated in  FIG. 3B , each of the plurality of cylindrical antenna body portions  27   a ′ comprises a tubular cylindrical antenna body portion and/or a mesh electrical conductor. Each of the dielectric members  29   a ′ comprises a cylindrical dielectric member, and a diameter d of each of the cylindrical dielectric members may be different than a diameter D of adjacent cylindrical antenna body portions  27   a ′, as also illustrated in  FIG. 3B . As illustrated in  FIG. 3C , each of the plurality of cylindrical antenna body portions  27   a ″ comprises a solid cylindrical antenna body portion, and/or a continuous electrical conductor. 
     Referring to  FIG. 6  an exploded view of an inductive mesh is provided. The inductive mesh may be used to form individual cylindrical antenna body portions  27   a ,  27   b  and it provides a distributed self inductance that may reduce the physical length requirements, e.g. inductive loading. In  FIG. 6 , metal wires  60 , preferentially a copper metal wire, is coated with an insulation coating  62  such as enamel paint or lacquer. The weave may be a plain or linen weave with the insulated metal wires  60  oriented on a diagonal bias relative the antenna  20  elongate axis. The insulated metal wires are soldered to metal bands  64  to provide to prevent fraying. As can be appreciated by those in the art, the electrical currents should follow a diagonal and weaving path which provides increased conductor electrical length and increased self inductance. This in turn reduces antenna operating frequency and or physical size of the antenna  20 . 
     The conical antenna body  22   a  of the first antenna element  21   a  has an opening  25   a  at the apex  31   a  thereof. A tubular dielectric spacer  24  is positioned in the opening  25   a  for receiving an inner conductor  41  of a coaxial cable  40 , or other conductor, for example. The conical antenna body  22   b  of the second antenna element  21   b  may be similarly configured with an opening  25   b  at an apex  31   b  thereof, and may have a connector (not shown) therein for receiving the inner conductor  41 . 
     The antenna assembly  20  includes a hollow shaft  28   a  in the first antenna element  21   a , and the coaxial cable  40  extends through the hollow shaft  28   a . The inner conductor  41  is coupled to the conical antenna body  22   b  of the second antenna element  21   b . The inner conductor  41  passes through the tubular dielectric spacer  24  in the apex  31   a  of the first antenna element  21   a  to couple with the conical antenna body  22   b  of the second antenna element  21   b . A coaxial cable connector (not shown) may be included in the conical antenna body  22   b  of the second antenna element  21   b  for coupling to the center conductor  41 . 
     The coaxial cable  40  also includes an outer conductor  42  surrounding the inner conductor  41  and coupled to the cylindrical antenna body  26   a  of the first antenna element  21   a . Other types of conductors may extend through the hollow shaft, for example a rigid conductor. Additionally, a second hollow shaft  28   b  may also be included, thus reducing manufacturing costs by reducing the amount of material used and the machining of two different antenna elements  21   a ,  21   b . In some embodiments, the cylindrical antenna body portions  26   a ,  26   b  may not be hollow (e.g. as shown in  FIG. 3C ). 
     Each of the first and second conical antenna bodies  22   a ,  22   b  are illustratively aligned along a common longitudinal axis  23  with respective apexes  31   a ,  31   b  in opposing relation to define a symmetrical biconical-type dipole antenna. 
     The overall height of the first and second adjacent antenna elements  21   a ,  21   b  is typically determined by the desired highest operating frequency. The overall height of the antenna  20  is typically determined by the lowest operating frequency. The height of the antenna may also be determined based upon a size limitation of a device housing, for example. The lengths of the individual cylindrical antenna body portions  27   a ,  27   b  may be unequal so each cylindrical body portion may provide tuning to a portion of the radio frequency spectrum. 
     The antenna assembly  20  may further include a balun (not shown). A balun may be desired based upon how the coaxial cable  40  or conductor is attached to the conical antenna body  22   a ,  22   b . The balun may advantageously balance the RE signals in each of the first and second adjacent antenna elements  21   a ,  21   b.    
     Referring now to the elevation plane radiation patterns in  FIGS. 4A and 4B , the dielectric separators advantageously reduce “lobing” at certain frequencies, thus reducing nulls in the radiation pattern of the antenna assembly  20  that are located on the horizon, for example. The antenna is oriented along the Z axis so the XY plane represent the horizon. The gain patterns in the graphs illustratively have improved performance over the prior art antennas, whose gain patterns are illustrated in the graphs of  FIGS. 1B ,  1 C,  2 B, and  2 C. 
     A method aspect is directed to making an antenna assembly  20  including forming first and second adjacent antenna elements  21   a ,  21   b  each comprising a conical antenna body  22   a ,  22   b  having a base  32   a ,  32   b  and an apex  31   a ,  31   b  opposite the base. A cylindrical antenna body  26   a ,  26   b  extends from the base  32   a ,  32   b  of the conical antenna body, and includes a plurality of adjacent cylindrical antenna body portions  27   a ,  27   b , and a respective dielectric member  29   a ,  29   b  separating adjacent cylindrical antenna body portions. The method further includes aligning each of the first and second adjacent antenna elements  21   a ,  21   b  along a common longitudinal axis  23  with respective apexes  31   a ,  31   b  in opposing relation to define a dipole antenna. 
     For example, an antenna assembly was formed to have a height of 12 inches and a diameter of 1.5 inches. The antenna assembly exhibits operation from 240 MHz to 3 GHz with reduced or no nulls on the horizon, for example as illustrated in the graphs of  FIGS. 4A and 4B . In contrast, a prior art antenna, without the dielectric separators, exhibited nulls between 800 and 900 MHz. 
     The antenna assembly  20  matches to very broad bands, with the lowest band of operation being dictated by the length of the antenna, and the highest band of operation being dictated by the accuracy of the dimensions near the center of the antenna. Desirable radiation pattern performance is achieved while a relatively small diameter. 
     Accordingly, the antenna assembly  20  may be particularly advantageous in a frequency range of about 240 MHz to 3 GHz, and in ultra-wideband applications, for example. Of course, the antenna assembly  20  may be used for other frequency ranges and other applications. 
     Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is understood that the invention is not to be limited to the specific embodiments disclosed, and that modifications and embodiments are intended to be included within the scope of the appended claims.