Patent Publication Number: US-9431712-B2

Title: Electrically-small, low-profile, ultra-wideband antenna

Description:
REFERENCE TO GOVERNMENT RIGHTS 
     This invention was made with government support under MSN141269 awarded by the Office of Naval Research and MSN139974 awarded by the National Science Foundation. The government has certain rights in the invention. 
    
    
     BACKGROUND 
     A classical monopole antenna is a type of radio antenna that consists of a straight rod-shaped conductor that is typically mounted perpendicularly over some type of conductive surface, called a ground plane. In some cases, the ground plane is the earth&#39;s surface, while in other cases, the ground plane is formed of a conductive material. The classical monopole antenna has an omnidirectional radiation pattern meaning that it radiates equal power in all azimuthal directions perpendicular to the antenna resulting in a donut shaped radiation pattern. The height of monopole antennas is inversely related to the transmission frequency because operation at low frequencies results in a very large electromagnetic wavelength. As a result, a traditional monopole antenna operating at low frequencies is also physically very large. The physically large size makes the monopole antenna challenging to use in low-profile applications at low frequencies. 
     SUMMARY 
     In an illustrative embodiment, an ultra-wideband, low profile antenna is provided. The antenna includes, but is not limited to, a ground plane substrate, a feed conductor, a top hat conductor, a shorting arm, and a ring slot. The feed conductor includes, but is not limited to, a first end and a second end. The first end is configured for electrical coupling to a feed network through a feed element extending from the ground plane substrate. The top hat conductor includes, but is not limited to, a generally planar sheet mounted to the second end of the feed conductor in a first plane approximately parallel to a second plane defined by the ground plane substrate. The shorting arm includes, but is not limited to, a third end and a fourth end. The third end is mounted to the top hat conductor, and the fourth end is mounted to the ground plane substrate. The ring slot is formed in the ground plane substrate around the feed element. 
     In another illustrative embodiment, a transmitter is provided. The transmitter includes, but is not limited to, a matching network circuit and an antenna. The matching network circuit is coupled to receive a signal through a port and to form a matched signal output through a feed element. The antenna includes, but is not limited to, a ground plane substrate, a feed conductor, a top hat conductor, a shorting arm, and a ring slot. The feed conductor includes, but is not limited to, a first end and a second end. The first end is configured for electrical coupling to the matching network circuit through the feed element to receive the matched signal. The top hat conductor includes, but is not limited to, a generally planar sheet mounted to the second end of the feed conductor in a first plane approximately parallel to a second plane defined by the ground plane substrate. The shorting arm includes, but is not limited to, a third end and a fourth end. The third end is mounted to the top hat conductor, and the fourth end is mounted to the ground plane substrate. The ring slot is formed in the ground plane substrate around the feed element. The matching network circuit is configured to impedance match the antenna. 
     Other principal features and advantages of the invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description, and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Illustrative embodiments of the invention will hereafter be described with reference to the accompanying drawings, wherein like numerals denote like elements. 
         FIG. 1 a    is a perspective view of a top-loaded conical antenna in accordance with an illustrative embodiment. 
         FIG. 1 b    is a top view of the top-loaded conical antenna of  FIG. 1 a    in accordance with an illustrative embodiment. 
         FIG. 1 c    is a side view of a feed conductor of the top-loaded conical antenna of  FIG. 1 a    in accordance with an illustrative embodiment. 
         FIG. 2 a    is a graph showing a voltage standing wave ratio (VSWR) determined by simulating the performance of the antenna of  FIG. 1 a    with different dimensions for a top edge of a conical structure. 
         FIG. 2 b    is a graph showing an input resistance determined by simulating the performance of the antenna of  FIG. 1 a    with different dimensions for the top edge of the conical structure. 
         FIG. 2 c    is a graph showing an input reactance determined by simulating the performance of the antenna of  FIG. 1 a    with different dimensions for the top edge of the conical structure. 
         FIG. 3 a    is a perspective view of a top-loaded conical antenna including a plurality of shorting arms in accordance with an illustrative embodiment. 
         FIG. 3 b    is a top view of the top-loaded conical antenna of  FIG. 3 a    in accordance with an illustrative embodiment. 
         FIG. 3 c    is a perspective view of the top-loaded conical antenna of  FIG. 3 a    zoomed to show a shorting arm in accordance with an illustrative embodiment. 
         FIG. 4 a    is a graph showing a VSWR determined by simulating the performance of the antenna of  FIGS. 1 a    and  3   a.    
         FIG. 4 b    is a graph showing an input resistance determined by simulating the performance of the antenna of  FIGS. 1 a    and  3   a.    
         FIG. 4 c    is a graph showing an input reactance determined by simulating the performance of the antenna of  FIGS. 1 a    and  3   a.    
         FIG. 5 a    is a perspective view of a top-loaded conical antenna including a plurality of shorting arms and a rectangular ground plane slot in accordance with an illustrative embodiment. 
         FIG. 5 b    is a top view of the top-loaded conical antenna of  FIG. 5 a    in accordance with an illustrative embodiment. 
         FIG. 6  is a perspective view of the top-loaded conical antenna of  FIG. 5 a    showing equivalent circuit elements for each part in accordance with an illustrative embodiment. 
         FIG. 7 a    is a graph showing a VSWR determined by simulating the performance of the antenna of  FIGS. 3 a    and  5   a.    
         FIG. 7 b    is a graph showing an input resistance determined by simulating the performance of the antenna of  FIGS. 3 a    and  5   a.    
         FIG. 7 c    is a graph showing an input reactance determined by simulating the performance of the antenna of  FIGS. 3 a    and  5   a.    
         FIG. 8  depicts a lumped matching network of a feed network of an antenna in accordance with an illustrative embodiment. 
         FIG. 9 a    is a graph showing a VSWR determined by simulating the performance of the antenna of  FIG. 5 a    using the lumped matching network of  FIG. 8 . 
         FIG. 9 b    is a graph showing a realized gain determined by simulating the performance of the antenna of  FIG. 5 a    using the lumped matching network of  FIG. 8 . 
         FIG. 9 c    is a graph showing an antenna efficiency determined by simulating the performance of the antenna of  FIG. 5 a    using the lumped matching network of  FIG. 8 . 
         FIGS. 10 a  and 10 b    depict graphs showing directional radiation patterns in the azimuth plane at different frequencies obtained by simulating the performance of the antenna of  FIG. 5 a    using the lumped matching network of  FIG. 8 . 
         FIGS. 11 a  and 11 b    depict graphs showing directional radiation patterns in the x-z elevation plane at different frequencies obtained by simulating the performance of the antenna of  FIG. 5 a    using the lumped matching network of  FIG. 8 . 
         FIGS. 12 a  and 12 b    depict graphs showing directional radiation patterns in the y-z elevation plane at different frequencies obtained by simulating the performance of the antenna of  FIG. 5   a.    
         FIG. 13  is a perspective view of a top-loaded conical antenna including a plurality of shorted arms and a circular ground plane slot in accordance with an illustrative embodiment. 
         FIG. 14  is a graph showing a VSWR determined by simulating the performance of the antenna of  FIGS. 5 a    and  13 . 
         FIG. 15  is a graph showing a comparison between a VSWR determined by simulating the performance of the antenna of  FIG. 5 a    and measuring a VSWR using a fabricated prototype of the antenna of  FIG. 5   a.    
         FIG. 16 a    is a perspective view of a top-loaded conical antenna including a plurality of shorting arms and a ground plane slot in accordance with a second illustrative embodiment. 
         FIG. 16 b    is a top view of the top-loaded conical antenna of  FIG. 16 a    in accordance with an illustrative embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 a   , a perspective view of an antenna  100  is shown in accordance with an illustrative embodiment. Antenna  100  may include a ground plane substrate  102 , a top hat conductor  104 , a feed conductor  106 , and a feed element  108 . Ground plane substrate  102  is electrically grounded and may be formed of any material suitable for forming an electrical ground for antenna  100 . For example, ground plane substrate  102  may be formed of a metal sheet alone or with a dielectric or magnetic material or a magneto-dielectric material on a top surface of the metal sheet. Ground plane substrate  102  is generally planar and defines a first plane. To describe the orientation of the components of antenna  100 , a coordinate reference system x-y-z is included in  FIG. 1 a   . Based on the defined coordinate reference system x-y-z, the first plane is the x-y plane. 
     Though the assumption is made that ground plane substrate  102  is an infinite ground plane, in general, if ground plane substrate  102  is just slightly larger than top hat conductor  104 , antenna  100  is still effective as a radiator. For example, ground plane substrate  102  larger by a factor of 1.5 times than top hat conductor  104  is still effective as a radiator. In illustrative embodiment, ground plane substrate  102  is a metal sheet. 
     With reference to  FIG. 1 b   , a top view of antenna  100  is shown in accordance with an illustrative embodiment. In an illustrative embodiment, top hat conductor  104  is generally planar and oriented in a second plane that is approximately parallel to the first plane defined by ground plane substrate  102 . Thus, top hat conductor  104  is oriented parallel to the x-y plane at a height  118  above ground plane substrate  102 . Top hat conductor  104  may be formed of any conducting material suitable for forming a radiator of antenna  100 . 
     In an illustrative embodiment, height  118  is approximately 100 millimeters (mm). In the illustrative embodiment, top hat conductor  104  has a rectangular shape when projected into the x-y plane. In alternative embodiments, top hat conductor  104  may form other polygonal, circular, or elliptical shapes when projected into the x-y plane. In the illustrative embodiment, top hat conductor  104  has a length  112  in the y-direction and a width  114  in the x-direction. Length  112  and width  114  define a diagonal  116 . In an illustrative embodiment, length  112  and width  114  define are approximately 200 mm though other dimensions may be used depending on the application environment for antenna  100 . 
     With reference to  FIG. 1   c , a side view of feed conductor  106  is shown in accordance with an illustrative embodiment. Feed conductor  106  is electrically connected to feed element  108 . Feed element  108  is positioned approximately at a center of ground plane substrate  102  as shown with reference to  FIG. 1 b   . In an illustrative embodiment, feed element  108  is a short length of coaxial cable including an inner connector  109  electrically coupled to a point on feed conductor  106  and an outer conductor  110  electrically coupled to ground plane substrate  102 . 
     In the illustrative embodiment of  FIG. 1   c , feed conductor  106  is generally planar and oriented in a third plane that is approximately perpendicular to the first plane defined by ground plane substrate  102 . Feed conductor  106  may be formed of any conducting material suitable for forming a radiator of antenna  100 . 
     Feed conductor  106  includes a top edge  120 , a first side edge  122 , a second side edge  124 , a third side edge  126 , a fourth side edge  128 , and a bottom edge  130 . Top edge  120  of feed conductor  106  is electrically coupled to top hat conductor  104  along diagonal  116  of top hat conductor  104  as shown with reference to  FIGS. 1 a  and 1 b   . In an illustrative embodiment, top edge  120  of feed conductor  106  is shorter than diagonal  116  of top hat conductor  104  though the difference is not readily visible in  FIG. 1 b   . As a result, feed conductor  106  is positioned between ground plane substrate  102  and top hat conductor  104 . 
     Top edge  120  and bottom edge  130  are generally parallel. First side edge  122  extends generally perpendicularly from a first end of top edge  120 . Second side edge  124  extends between first side edge  122  and a first end of bottom edge  130 . Third side edge  126  extends generally perpendicularly from a second end of top edge  120 . Fourth side edge  128  extends between third side edge  126  and a second end of bottom edge  130 . Thus, first side edge  122  and second side edge  124  form a first side of feed conductor  106 , and third side edge  126  and fourth side edge  128  form a second side of feed conductor  106 . In the illustrative embodiment, feed conductor  106  is primarily cone shaped. In alternative embodiment, feed conductor  106  may not include first side edge  122  or third side edge  126  and/or bottom edge  130  resulting in a triangular shape. In an illustrative embodiment, feed conductor  106  forms essentially a monopole antenna and can be used to tune and adjust the resonances that result from the monopole structure. These resonances can be optimized such that they merge with the other resonances to form an ultra-wideband antenna. Thus, the shape of feed conductor  106  can be optimized to increase the bandwidth of antenna  100 . 
     With reference to  FIG. 2 a   , a graph is provided that shows a voltage standing wave ratio (VSWR) at feed element  108  determined by simulating the performance of the antenna of  FIG. 1 a    with different dimensions for top edge  120  of feed conductor  106 . A first VSWR curve  200  shows a VSWR as a function of transmit frequency that results for top edge  120  having a value equal to 55 mm. A second VSWR curve  202  shows a VSWR as a function of transmit frequency that results top edge  120  having a value equal to 140 mm. A third VSWR curve  204  shows a VSWR as a function of transmit frequency that results for top edge  120  having a value equal to 255 mm. 
     With reference to  FIG. 2 b   , a graph is provided that shows an input resistance (real part of the impedance) determined by simulating the performance of the antenna of  FIG. 1 a    with different dimensions for top edge  120  of feed conductor  106 . A first resistance curve  210  shows a resistance as a function of transmit frequency that results for top edge  120  having a value equal to 55 mm. A second resistance curve  212  shows a resistance as a function of transmit frequency that results for top edge  120  having a value equal to 140 mm. A third resistance curve  214  shows a resistance as a function of transmit frequency that results for top edge  120  having a value equal to 255 mm. 
     With reference to  FIG. 2 c   , a graph is provided that shows an input reactance (imaginary part of the impedance) determined by simulating the performance of the antenna of  FIG. 1 a    with different dimensions for top edge  120  of feed conductor  106 . A first reactance curve  220  shows a reactance as a function of transmit frequency that results for top edge  120  having a value equal to 55 mm. A second reactance curve  222  shows a reactance as a function of transmit frequency that results for top edge  120  having a value equal to 140 mm. A third reactance curve  224  shows a reactance as a function of transmit frequency that results for top edge  120  having a value equal to 255 mm. 
     Antenna  100  is a potentially broadband antenna that is primarily a capacitive antenna in which the parallel capacitance between top hat conductor  104  and ground plane substrate  102  is the dominant factor. The magnitude of the parallel capacitance is directly related to the area of top hat conductor  104 . To achieve a low frequency of operation, the dimensions of top hat conductor  104  are maximized in view of the dimensional constraints that result based on the application environment for antenna  100 . The performance of antenna  100  is examined using full-wave electromagnetic wave (EM) simulations, and the side dimensions of feed conductor  106  are optimized to achieve the lowest VSWR possible over as wide a frequency band as possible. 
     With reference to  FIG. 3 a   , a perspective view of a second antenna  300  is shown in accordance with an illustrative embodiment. Second antenna  300  may include ground plane substrate  102 , top hat conductor  104 , feed conductor  106 , feed element  108 , a first shorting arm  302 , and a second shorting arm  304 . A greater or a fewer number of shorting arms may be included in alternative embodiments. In the illustrative embodiment, first shorting arm  302  and second shorting arm  304  are generally planar sheets and rectangular in shape when projected into the x-y, y-z, or x-z planes though other shapes may be used. For example, first shorting arm  302  and second shorting arm  304  may form other polygonal, circular, or elliptical shapes when projected into the x-y, y-z, or x-z planes. First shorting arm  302  and second shorting arm  304  further need not be formed of generally planar sheets. 
     First shorting arm  302  is electrically coupled to top hat conductor  104  and to ground plane substrate  102  as shown with reference to  FIG. 3 b   . Second shorting arm  304  is also electrically coupled to top hat conductor  104  and to ground plane substrate  102  as shown with reference to  FIG. 3 b   . First shorting arm  302  and second shorting arm  304  may be formed of any conducting material suitable for forming a radiator of antenna  100 . The material used to form first shorting arm  302  and second shorting arm  304  may be the same or different from each other. The material used to form first shorting arm  302  and second shorting arm  304  may be the same or different from that used to form top hat conductor  104  and/or feed conductor  106 . The material used to form top hat conductor  104  and feed conductor  106  may be the same or different from each other. In an illustrative embodiment, first shorting arm  302  and second shorting arm  304  may carry relatively strong current densities. To avoid ohmic losses that could adversely impact the performance of antenna  100 , good conductors may be used to form first shorting arm  302  and second shorting arm  304 . 
     First shorting arm  302  includes a top edge  306 , a first side edge  308 , a second side edge  310 , and a bottom edge  312 . First shorting arm  302  is electrically coupled to top hat conductor  104  along top edge  306 . Top edge  306  is positioned in a first corner of top hat conductor  104 . First shorting arm  302  is electrically coupled to ground plane substrate  102  along bottom edge  312 . Top edge  306  and bottom edge  312  of first shorting arm  302  are generally parallel. 
     Second shorting arm  304  includes a top edge  316 , a first side edge  318 , a second side edge  320 , and a bottom edge  322 . Second shorting arm  304  is electrically coupled to top hat conductor  104  along top edge  316 . Top edge  316  of second shorting arm  304  is positioned in a second corner of top hat conductor  104 . Second shorting arm  304  is electrically coupled to ground plane substrate  102  along bottom edge  322  of second shorting arm  304 . Top edge  316  and bottom edge  322  of second shorting arm  304  are generally parallel. Feed conductor  106  extends between the remaining corners of top hat conductor  104 . Thus, first shorting arm  302  and second shorting arm  304  are positioned in opposite corners of top hat conductor  104  on either side of feed conductor  106 . 
     One drawback of adding first shorting arm  302  and second shorting arm  304  to antenna  100  to form second antenna  300  is that the shorting arms are solely responsible for the radiation characteristics at low frequencies, while at higher frequencies they act as an array antenna and can produce undesirable nulls in the radiation patterns. To ensure the antenna maintains consistent omnidirectional radiation patterns across its entire frequency band, the shorting arms are positioned so that the shorting arms are rotationally symmetric. Thus, first shorting arm  302  and second shorting arm  304  extend from top hat conductor  104  and from ground plane substrate  102  at an angle  324  and are positioned to be rotationally symmetric. In an illustrative embodiment, angle  324  is between 10 and 90 degrees. In an alternative embodiment, angle  324  may be approximately zero if first shorting arm  302  and second shorting arm  304  are curved. Considering the currents on shorting arm  302  and second shorting arm  304 , this method distributes the currents more symmetrically around antenna  100  and improves the omindirectionality at higher frequencies. 
     Top edge  306  and bottom edge  312  of first shorting arm  302  and top edge  316  and bottom edge  322  of second shorting arm  304  have a width  326 . First side edge  308  and second side edge  310  of first shorting arm  302  and first side edge  318  and second side edge  320  of second shorting arm  304  have a length  328 . As a result, first shorting arm  302  and second shorting arm  304  have a projected length  330  when projected into the x-y plane as shown with reference to  FIGS. 3 b  and 3 c   . Of course, first shorting arm  302  and second shorting arm  304  may be oriented in other directions. For example, bottom edge  312  of first shorting arm  302  and bottom edge  322  of second shorting arm  304  may be rotated from zero to 90 degrees in the x-y plane. In an illustrative embodiment, width  326  is approximately 30 mm and length  328  is approximately 122 mm though other dimensions may be used depending on the application environment for antenna  100 . 
     With reference to  FIG. 4 a   , a graph is provided that shows a VSWR at feed element  108  determined by simulating the performance of the antenna of  FIG. 3 a   . A fourth VSWR curve  400  shows a VSWR as a function of transmit frequency that results by including first shorting arm  302  and second shorting arm  304  with the illustrative dimensions and with top edge  120  having a value equal to 255 mm. Third VSWR curve  204  is included in the graph for comparison. 
     With reference to  FIG. 4 b   , a graph is provided that shows an input resistance determined by simulating the performance of the antenna of  FIG. 3 a   . A fourth resistance curve  410  shows a resistance as a function of transmit frequency that results by including first shorting arm  302  and second shorting arm  304  with the illustrative dimensions and with top edge  120  having a value equal to 255 mm. Third resistance curve  214  is included in the graph for comparison. 
     With reference to  FIG. 4 c   , a graph is provided that shows an input reactance determined by simulating the performance of the antenna of  FIG. 3 a   . A fourth reactance curve  420  shows a reactance as a function of transmit frequency that results by including first shorting arm  302  and second shorting arm  304  with the illustrative dimensions and with top edge  120  having a value equal to 255 mm. Third reactance curve  224  is included in the graph for comparison. 
     The addition of one or more shorting arms results in addition of a parallel inductance. The value of the parallel inductance increases by increasing length  328  or decreasing width  326  of first shorting arm  302  and second shorting arm  304 . The parallel inductance due to first shorting arm  302  and second shorting arm  304  and the parallel capacitance due to top hat conductor  104  and ground plane substrate  102  provide a potential parallel resonance below the minimum frequency of operation of antenna  100 . The placement, the size, and the shape of the shorting arms have a significant effect on the antenna impedance (resistance and reactance). The shorting arms are designed and optimized such that the introduced additional resonance is close to the minimum desired operating frequency of antenna  100 , so they can merge together to achieve an ultra-wideband (UWB) structure. 
     With reference to  FIG. 5 a   , a perspective view of a third antenna  500  is shown in accordance with an illustrative embodiment. Third antenna  500  may include ground plane substrate  102 , top hat conductor  104 , feed conductor  106 , feed element  108 , first shorting arm  302 , second shorting arm  304 , and a ring slot  502 . In the illustrative embodiment, ring slot  502  is a rectangular slot formed in ground plane substrate  102 . For example, ring slot  502  may be etched or milled into ground plane substrate  102 . Ring slot  502  is symmetrically positioned to surround feed element  108 . In alternative embodiments, ring slot  502  may be positioned asymmetrically relative to feed element  108 . Ring slot  502  may form other polygonal, circular, or elliptical shapes in the x-y plane. 
     For simplicity in fabrication, a dielectric material with a top surface formed of a metal sheet is used as ground plane substrate  102 , and ring slot  502  is formed by etching of ground plane substrate  102 . The dielectric constant of ground plane substrate  102  can change the value of capacitance formed by ring slot  502 . To minimize the effect of the material, a low dielectric material can be used as ground plane substrate  102 . 
     In an illustrative embodiment, ring slot  502  has a slot width  504 , a width  506  in the x-direction, and a length  508  in the y-direction. In an illustrative embodiment, slot width  504  is approximately 7 mm, width  506  is approximately 203 mm, and length  508  is approximately 203 mm though other dimensions may be used depending on the application of antenna  100 , and of course, the other dimensions of the components of third antenna  500 . Ring slot  502  does not radiate in the band of interest; instead, ring slot  502  acts as a series capacitance. The value of the series capacitance increases by decreasing width  506  of ring slot  502  or by decreasing slot width  504  of ring slot  502 . The values for slot width  504  and width  506  may be chosen by examining the effect of these two parameters on VSWR, input impedance, and input reactance of antenna  100  to reduce the quality factor of the additional resonance and achieve an impedance match across the entire band. 
     With reference to  FIG. 6 , the effect of top hat conductor  104  is modeled as a parallel capacitance  600 , the effect of first shorting arm  302  and second shorting arm  304  is modeled as a parallel inductance  602 , and the effect of ring slot  502  is modeled as a series capacitance  604 . Third antenna  500  can be designed using the equivalent circuit model illustrated in  FIG. 6  and full wave EM simulation. The placement, mounting angle, and shape of first shorting arm  302  and second shorting arm  304  and the placement and shape of ring slot  502  have a significant effect on the impedance of third antenna  500 . These characteristics are designed and optimized using full wave EM simulation such that the impedance is well-matched and centered on the Smith chart used for analysis of impedance matching. 
     With reference to  FIG. 7 a   , a graph is provided that shows a VSWR at feed element  108  determined by simulating the performance of the antenna of  FIG. 5 a   . A fifth VSWR curve  700  shows a VSWR as a function of transmit frequency that results by including ring slot  502  with the illustrative dimensions and with top edge  120  having a value equal to 255 mm. Fourth VSWR curve  400  is included in the graph for comparison. 
     With reference to  FIG. 7 b   , a graph is provided that shows an input resistance determined by simulating the performance of the antenna of  FIG. 5 a   . A fifth resistance curve  710  shows a resistance as a function of transmit frequency that results by including ring slot  502  with the illustrative dimensions and with top edge  120  having a value equal to 255 mm. Fourth resistance curve  410  is included in the graph for comparison. 
     With reference to  FIG. 7 c   , a graph is provided that shows an input reactance determined by simulating the performance of the antenna of  FIG. 5 a   . A fifth reactance curve  720  shows a reactance as a function of transmit frequency that results by including ring slot  502  with the illustrative dimensions and with top edge  120  having a value equal to 255 mm. Fourth reactance curve  420  is included in the graph for comparison. 
     As shown in  FIGS. 7 a -7 c   , series capacitance  604  helps to decrease the quality factor of the additional resonance, which results in achieving an impedance match across the entire band. The placement and the width of the slot have a significant effect on the capacitance value. The value of the series capacitance increases by decreasing the radius or width  506  of ring slot  502  or by decreasing the radius or slot width  504  of ring slot  502 . 
     To obtain the maximum bandwidth available, the transmission and reflection coefficients of third antenna  500  should be unity inside and outside of the band of interest, respectively. With reference to  FIG. 8 , a feed network  812  is shown in accordance with an illustrative embodiment. Feed network  812  may include a first inductor  802 , a first capacitor  804 , a second inductor  806 , and a second capacitor  808 . First inductor  802  and first capacitor  804  are mounted in series between a port  800  and ground plane substrate  102 . Second inductor  806  and second capacitor  808  are mounted in series between first inductor  802  and ground plane substrate  102 . Feed element  108  is electrically coupled between second inductor  806  and second capacitor  808 . 
     Feed network  812  forms a lumped matching network circuit designed to match the transmission and reflection coefficients of third antenna  500 . The values of first inductor  802 , first capacitor  804 , second inductor  806 , and second capacitor  808  are designed and optimized to achieve an impedance match to third antenna  500  across the entire frequency range. Thus, feed network  812  is coupled to receive a radio frequency (RF) alternating current (AC) signal and to form an impedance matched signal output on feed element  108  for radiation from third antenna  500 . 
     With reference to  FIG. 8 , a transmitter and/or receiver or transceiver  810  includes port  800 , feed network  812 , and third antenna  500  in accordance with an illustrative embodiment. The RF AC signal is provided to port  800  from a signal processor (not shown). Feed network  812  is coupled to port  800  to receive the RF AC signal and to form a matched signal output through feed element  108  for radiation from third antenna  500 . Feed network  812  is coupled to feed element  108  to receive a second RF AC signal received by third antenna  500  and to form a matched signal output through port  800  to the signal processor. 
     With reference to  FIG. 9 a   , a graph is provided that shows a VSWR at feed element  108  determined by simulating the performance of third antenna  500  with the illustrative dimensions (top edge  120  having a value equal to 255 mm) and using feed network  812 . In the illustrative embodiment, an inductance value for first inductor  802  was 5.25 nanoHenry (nH), a capacitance value for first capacitor  804  was 2.2 picoFarads (pF), an inductance value for second inductor  806  was 4.4 nH, and a capacitance value for second capacitor  808  was 1.6 pF. A sixth VSWR curve  900  shows a resulting VSWR as a function of transmit frequency. 
     With reference to  FIG. 9 b   , a graph is provided that shows a realized gain of third antenna  500  determined by simulating the performance of third antenna  500  with the illustrative dimensions (top edge  120  having a value equal to 255 mm). A gain curve  902  shows a resulting realized gain as a function of transmit frequency. 
     With reference to  FIG. 9 c   , a graph is provided that shows an efficiency of third antenna  500  determined by simulating the performance of third antenna  500  with the illustrative dimensions (top edge  120  having a value equal to 255 mm). A first efficiency curve  904  shows a radiation efficiency as a function of transmit frequency using feed network  812 . A second efficiency curve  906  shows a total efficiency as a function of transmit frequency. As shown, third antenna  500  achieves a 3.8 dBi realized gain at the lowest frequency of operation and 5 dBi over most of the operating band. Over most of the operating band, the total efficiency remains above 90% though the total efficiency is approximately 65% at lower frequencies. 
     With reference to  FIGS. 10 a  and 10 b   , graphs are provided that show directional radiation patterns in the x-y (azimuth) plane in the frequency range of 0.2-1.4 gigahertz (GHz). The results were obtained by simulating the performance of third antenna  500 . A first curve  1000  shows the representative response at a frequency of 0.2 GHz; a second curve  1002  shows the representative response at a frequency of 0.4 GHz; a third curve  1004  shows the representative response at a frequency of 0.6 GHz; a fourth curve  1006  shows the representative response at a frequency of 0.8 GHz; a fifth curve  1008  shows the representative response at a frequency of 1.0 GHz; a sixth curve  1010  shows the representative response at a frequency of 1.2 GHz; and a seventh curve  1012  shows the representative response at a frequency of 1.4 GHz. 
     With reference to  FIGS. 11 a  and 11 b   , graphs are provided that show directional radiation patterns showing directional radiation patterns in the x-z elevation plane in the frequency range of 0.2-1.4 gigahertz (GHz). A first curve  1100  shows the representative response at a frequency of 0.2 GHz; a second curve  1102  shows the representative response at a frequency of 0.4 GHz; a third curve  1104  shows the representative response at a frequency of 0.6 GHz; a fourth curve  1106  shows the representative response at a frequency of 0.8 GHz; a fifth curve  1108  shows the representative response at a frequency of 1.0 GHz; a sixth curve  1110  shows the representative response at a frequency of 1.2 GHz; and a seventh curve  1112  shows the representative response at a frequency of 1.4 GHz. 
     With reference to  FIGS. 12 a  and 12 b   , graphs are provided that show directional radiation patterns showing directional radiation patterns in the y-z elevation plane in the frequency range of 0.2-1.4 gigahertz (GHz). A first curve  1200  shows the representative response at a frequency of 0.2 GHz; a second curve  1202  shows the representative response at a frequency of 0.4 GHz; a third curve  1204  shows the representative response at a frequency of 0.6 GHz; a fourth curve  1206  shows the representative response at a frequency of 0.8 GHz; a fifth curve  1208  shows the representative response at a frequency of 1.0 GHz; a sixth curve  1210  shows the representative response at a frequency of 1.2 GHz; and a seventh curve  1212  shows the representative response at a frequency of 1.4 GHz. 
     The simulated results demonstrate that third antenna  500  provides monopole-like omnidirectional radiation patterns over the entire frequency band of interest. Additionally, third antenna  500  using feed network  812  of  FIG. 8  achieves a VSWR lower than 2:1 over a 7.5:1 bandwidth. For third antenna  500 , the value of the comparison factor is 27.6, which is more than twice that of the Goubau antenna as a standard small wideband antenna. As a result, third antenna  500  provides a better design in terms of the bandwidth-to-size ratio. At the lowest frequency of operation, third antenna  500  using feed network  812  of  FIG. 8  has electrical dimensions of 0.065λ min ×0.13λ min ×0.13λ min , where λ min  is the free space wavelength at the lowest frequency of operation ˜0.2 GHz. 
     With reference to  FIG. 13 , a perspective view of a fourth antenna  1300  is shown in accordance with an illustrative embodiment. Fourth antenna  1300  may include ground plane substrate  102 , top hat conductor  104 , feed conductor  106 , feed element  108 , first shorting arm  302 , second shorting arm  304 , and a second ring slot  1302 . In the illustrative embodiment, second ring slot  1302  is a circular slot formed in ground plane substrate  102 . Second ring slot  1302  is symmetrically positioned to surround feed element  108 . In an illustrative embodiment, second ring slot  1302  has slot width  504 . In an illustrative embodiment, slot width  504  is approximately 7 mm and a diameter of second ring slot  1302  is approximately 203 mm though other dimensions may be used depending on the application of antenna  100 , and of course, the other dimensions of the components of third antenna  500 . 
     With reference to  FIG. 14 , a graph is provided that shows a VSWR at feed element  108  determined by simulating the performance of fourth antenna  1300  with the illustrative dimensions. A sixth VSWR curve  1400  shows a VSWR as a function of transmit frequency that results by including ring slot  1302  with top edge  120  having a value equal to 255 mm. Fourth VSWR curve  400  is included in the graph for comparison. 
     A prototype of third antenna  500  was fabricated. The prototype was scaled down by a factor of three for simplicity. Thus, the operating frequencies of the antenna scale up by the same factor of three. Feed network  812  was not considered. With reference to  FIG. 15 , a graph is provided that shows a VSWR at feed element  108  generated by the prototype. A seventh VSWR curve  1500  shows a VSWR as a function of transmit frequency generated by the prototype. An eighth VSWR curve  1502  shows a VSWR as a function of transmit frequency determined by simulating the scaled version of the antenna. Eighth VSWR curve  1502  is included in the graph for comparison. The prototype results compare favorably with the simulated results. In the fabricated prototype, Rogers 5880 material with a dielectric constant of 2.2 was used to form ground plane substrate  102 . 
     With reference to  FIG. 16 a   , a perspective view of a fifth antenna  1600  is shown in accordance with an illustrative embodiment. Fifth antenna  1600  may include ground plane substrate  102 , top hat conductor  104 , a feed conductor  106 , a second feed conductor  1602 , feed element  108 , first shorting arm  302 , second shorting arm  304 , a third shorting arm  1604 , a fourth shorting arm  1606 , and ring slot  502 . In the illustrative embodiment, second feed conductor  1604  has the same shape as feed conductor  106 . In the illustrative embodiment of  FIG. 16 a   , first feed conductor  106  is positioned along a center of top hat conductor  104  parallel to the y-z plane, and second feed conductor  1602  is positioned along a center of top hat conductor  104  parallel to the x-z plane. 
     With reference to the illustrative embodiment of  FIGS. 16 a  and 16 b   , first shorting arm  302  extends from top hat conductor  104  adjacent a first end  1608  of feed conductor  106 , second shorting arm  304  extends from top hat conductor  104  adjacent a second end  1610  of feed conductor  106 , third shorting arm  1604  extends from top hat conductor  104  adjacent a first end  1612  of second feed conductor  1602 , and fourth shorting arm  1606  extends from top hat conductor  104  adjacent a second end  1614  of second feed conductor  1602 . 
     Using fifth antenna  1600  additional omnidirectionality can be achieved. However, a drawback of adding more feed conductors and shorting arms is an increase in the corresponding parallel inductance and, as a result, an increase in the minimum frequency of operation. Thus, fifth antenna  1600  can be used in applications in which omnidirectionality is a higher priority than lowest frequency of operation. 
     The word “illustrative” is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as “illustrative” is not necessarily to be construed as preferred or advantageous over other aspects or designs. Further, for the purposes of this disclosure and unless otherwise specified, “a” or “an” means “one or more”. Still further, the use of “and” or “or” is intended to include “and/or” unless specifically indicated otherwise. 
     The foregoing description of illustrative embodiments of the invention has been presented for purposes of illustration and of description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and as practical applications of the invention to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.