Abstract:
A Vivaldi-Monopole antenna is a small form ultra-wideband antenna configured for low frequency operation in modern wireless devices. The Vivaldi-Monopole antenna comprises a tapered-slot element and a monopole element, wherein current modes of each element are combined to yield a functional and small form ultra-wideband antenna configured for low frequency resonances.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation in part (CIP) of U.S. Ser. No. 13/932,150, filed Jul. 1, 2013, and titled “VIVALDI-MONOPOLE ANTENNA”; 
     which claims benefit of priority with U.S. Provisional Ser. No. 61/666,795, filed Jun. 30, 2012; 
     the contents of each of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     This invention relates to antennas for wireless communications; and more particularly, to a novel antenna structure herein termed a “Vivaldi-Monopole Antenna” that is configured for ultra-wideband operation. 
     2. Description of the Related Art 
     Those having skill in the art will appreciate the difficulty in forming an antenna that exhibits stable radiation performance across the ultra-wide bandwidth, especially where low frequency communications bands are desired. 
     For this reason, there is a continued need for ultra-wideband antennas having relatively small form factor for integration with a variety of portable wireless devices. 
     In the prior art, an antenna structure known as a “Vivaldi Antenna” is described as having a tapered notch configured to achieve ultra-wide band resonances. Vivaldi antennas are generally understood by those in the art; however, further review of such antennas can be accomplished with an internet search. Accordingly, a detailed review of Vivaldi antennas is not provided herein. 
     In the Vivaldi antenna, current distribution tends to travel at the edges of the tapered element. Because of this, low frequency bands are not achievable with the standard Vivaldi tapered slot unless a very large element is provided. However, because large antennas are not desirable with modern electronics, a large conventional Vivaldi antenna is not a suitable solution for applications where ultra-wideband and low frequency characteristics are desired. 
     There is a need for ultra-wideband antennas capable of low frequency resonances for use in modern communications devices. 
     SUMMARY 
     A modified Vivaldi antenna, hereinafter referred to as a “Vivaldi-Monopole Antenna” is described. 
     The Vivaldi-Monopole antenna is a novel antenna configuration comprising a tapered slot portion and a monopole element for achieving ultra-wideband and low frequency resonance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The Vivaldi-Monopole antenna is herein described with reference to the appended drawings, wherein: 
         FIGS. 1(A-B)  show a Vivaldi-Monopole antenna in accordance with an embodiment. 
         FIG. 2  shows a Vivaldi-Monopole antenna in accordance with another embodiment. 
         FIG. 3  shows a sectional view of the Vivaldi-Monopole antenna in accordance with an embodiment. 
         FIG. 4A  shows a flexible Vivaldi-Monopole antenna fixed at a ninety degree bend within a device housing. 
         FIG. 4B  shows a flexible Vivaldi-Monopole antenna fixed about a curved surface of a device housing. 
         FIG. 4C  shows a flexible Vivaldi-Monopole antenna fixed within a round device housing such as, for example, a utility meter. 
         FIG. 5  shows a plot of return loss associated with the Vivaldi-Monopole antenna of  FIG. 1 . 
         FIG. 6  shows a plot of efficiency associated with the Vivaldi-Monopole antenna of  FIG. 1 . 
         FIG. 7  shows a plot of peak gain associated with the Vivaldi-Monopole antenna of  FIG. 1 . 
         FIG. 8A  shows a current distribution about the Vivaldi-Monopole antenna of  FIG. 1  at 700 MHz. 
         FIG. 8B  shows a current distribution about the Vivaldi-Monopole antenna of  FIG. 1  at 3000 MHz. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     A novel antenna structure, referred to herein as a “Vivaldi-Monopole Antenna”, is suggested for wireless communication across an ultra-wide bandwidth, including the lower cellular bands at 700 MHz, 850 MHz, and 900 MHz, along with higher frequencies in the wireless industry&#39;s electromagnetic spectrum. 
     The Vivaldi-Monopole antenna comprises a Vivaldi-type tapered slot element and a monopole element. By combining the current distribution modes of the tapered slot element with the monopole element as illustrated herein, an ultra-wideband antenna configured for operation at low band cellular frequencies (ex: 700 MHz-900 MHz) is achieved. 
     Now turning to the drawings,  FIGS. 1(A-B)  show a Vivaldi-Monopole antenna  100  in accordance with an embodiment. The Vivaldi-Monopole antenna  100  comprises a thin rectangular conductor volume  107  extending along a longitudinal axis (L′) from a rear edge to a front edge. The conductor  107  further comprises an aperture  109  having a center thereof disposed near the longitudinal axis, and a first slot  110  extending from the aperture toward a center of the rectangular conductor  107 . At least a portion of the first slot  110  is tapered toward a side edge of the conductor, herein termed the “tapered side”  112 . The first slot  110  forms a tapered slot element  102  that is configured for one or more high frequency resonances. The conductor further comprises a monopole element  101  disposed along the front edge, wherein the monopole element comprises a length of conductor extending from the longitudinal axis toward the tapered side along at least a portion of the front edge. The monopole element  101  is separated from first slot  110  by a lateral slot  111  therebetween, wherein the lateral slot is oriented perpendicular with respect to the longitudinal axis. A signal feed pad  103  and a ground feed pad  104 , respectively, are disposed across the first slot  110  at a point adjacent to the aperture  109 . 
     A flexible mini-coaxial cable  105  is shown, wherein the mini coaxial cable comprises a mini-RF connector  106  at a terminal end thereof, and a conductor wire being soldered to each of the ground  104  and signal feed pads  103 , respectively. 
     The conductor can be fabricated on a substrate using any electroplating, electro-depositing, printing, or other method known in the art. Moreover, the substrate can be a dielectric substrate. 
     In various applications as illustrated herein, it is beneficial to form the antenna on a flexible substrate. Flexible substrates include Kapton™ polyimide substrate and other similar substrates known in the art. 
       FIG. 2  shows a Vivaldi-Monopole antenna in accordance with another embodiment. The antenna is similar to the embodiment described above. However, the antenna in this embodiment comprises three conductor portions  207   a ;  207   b ; and  207   c , respectively. The first conductor portion  207   a  is separated from the second conductor portion  207   b  by the first slot  110  of the tapered slot element  205  extending therebetween, and by a first gap  203  extending therebetween at the rear edge. The third conductor portion  207   c  forming the monopole element  206 , is separated from the second conductor portion  207   b  by a second gap  201  extending therebetween at the front edge. 
     In this form, the Vivaldi-Monopole antenna can be tailored to various applications by coupling a component between two adjacent conductor portions. For example, a low pass filter  204  can be coupled between the first conductor portion  207   a  and the second conductor portion  207   b  across the first gap  203 . Moreover, a high pass filter  202  can be coupled between the second conductor portion  207   b  and the third conductor portion  207   c  across the second gap  201 . In this regard, the respective conductor portions can be filtered for configuring the Vivaldi-Monopole antenna for various resonances depending on the application. If filtering is not desired, a conductor, resistor or other passive component may be coupled between two adjacent portions. 
       FIG. 3  shows a sectional view  300  of the Vivaldi-Monopole antenna in accordance with an embodiment. The antenna comprises a substrate layer  304  having a top surface and a bottom surface thereof. A metallized layer  303 , preferably copper, tin, gold, or other conductor metal, is disposed about the top surface of the substrate. A layer of solder mask  301  is applied to the metallized layer in a desirable pattern as would be determined by those having skill in the art. An optional conductive layer  302 , for example, tin, can be formed on a portion of the metallized layer  303  to form one or more solder pads. A bottom solder mask layer  307  is formed on the bottom surface of the substrate. An adhesive layer  306  is formed below the bottom solder mask layer. A removable liner  305  is attached to the adhesive layer. 
     Although the Vivaldi-Monopole antenna can be fabricated in a rigid form, it is preferable to form the antenna on a flexible substrate for certain applications. 
     For example,  FIG. 4A  shows a device housing  405  having an orthogonal bend (or right-angle) corner. In order to attach the antenna  404  at the corner, it is beneficial to form the antenna on a flexible substrate. 
     Similarly,  FIG. 4B  illustrates a wavy device housing. A flexible antenna  402  can conform to the wavy housing  403 . 
     An example of an application suitable for a flexible Vivaldi-Monopole antenna is a utility meter, such as an electric or water utility meter.  FIG. 4C  illustrates the flexible antenna  400  attached to a round utility meter housing  401 . 
       FIG. 5  shows a plot of return loss (dB) of the Vivaldi-Monopole antenna of  FIG. 1  over a wideband spectrum. Both a simulated plot and a measured plot are illustrated. As shown in  FIG. 5 , the Vivaldi-Monopole antenna provides low-band resonances between 700 MHz and 900 MHz, and additional high-band resonances between 1600 MHz and 6000 MHz. 
       FIG. 6  shows a plot of efficiency (%) of the Vivaldi-Monopole antenna of  FIG. 1  over a wideband spectrum. 
       FIG. 7  shows a plot of peak gain (dB) of the Vivaldi-Monopole antenna of  FIG. 1  over a wideband spectrum. 
       FIG. 8A  illustrates the current distribution of the Vivaldi-Monopole antenna according to the embodiment of  FIG. 1  for a first working frequency at 700 MHz. 
       FIG. 8B  illustrates the current distribution of the Vivaldi-Monopole antenna according to the embodiment of  FIG. 1  for a first working frequency at 3000 MHz. 
     Although the above examples illustrate particular embodiments, it should be understood by those having skill in the art that a variety of alternative embodiments can be practiced with little experimentation or deviation from these examples. Accordingly, the spirit and scope of the invention shall not be limited to these descriptions, which are provided as illustrative examples of the various features and embodiments only, but rather, the scope shall be set forth by the appended claims.