Abstract:
A dual polarized broadband, lightweight, low cost tapered slot antenna which has first and second radiating tapered slot antennas which are co-located and positioned perpendicular to one another. Each antenna includes a relatively thin dielectric substrate and a radiating metallic antenna element mounted on the upper surface of the dielectric substrate. A tapered notch area, which is centrally located, is etched away to expose the dielectric substrate. The tapered slot antennas allow for linear polarization, elliptical polarization and circular polarization.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to microstrip fed, tapered slot antennas. More specifically, the present invention relates to a dual polarization microstrip fed, tapered slot antenna which provides dual vertical and horizontal polarizations and which operates over a continuous frequency range of 1.5 GHZ (gigahertz) to 18 GHZ. 
   2. Description of the Prior Art 
   There is currently a need by the military for a relatively inexpensive, lightweight antenna which will operate over a frequency range of 1.5 GHZ to 18 GHZ. There is a requirement that the antenna also provide for dual vertical and horizontal polarizations. Ideally, the cost of the antenna should not exceed two hundred dollars to manufacture in relatively small quantities. 
   Broadband antennas, which operate in the 1.5 to 20 GHZ range, and weigh up to 2 pounds are available from several manufacturers and normally perform quite well for their intended function, i.e. test and evaluation of high frequency military communications and weapons systems. These broadband antennas are very expensive often costing more than $5000.00. When a user needs a significant quantity of broadband antennas for test and evaluation or is operating on a limited budget, $5000.00 per antenna is a cost which may be prohibitive. This, in turn, may result in either a limited test and evaluation of a communications or weapons system which is critical to the military, or a cancellation of a military weapons development program because of cost which exceed funds allocated to the program. If a lightweight, broadband antenna is required, no commercial antenna currently available may be satisfactory to the user. 
   Accordingly, there is an urgent need for an inexpensive antenna which costs approximately $200.00 to manufacture, operates over a broad frequency range and provides for dual vertical and horizontal polarizations. 
   SUMMARY OF THE INVENTION 
   The present invention overcomes some of the difficulties of the prior art broadband antennas including those mentioned above in that it comprises a compact, lightweight, low cost antenna providing dual vertical and horizontal polarizations and a continuous operational frequency range of 1.5 GHZ to 18 GHZ. 
   The present invention includes first and second radiating tapered slot antennas which are co-located, orthogonally polarized and positioned perpendicular to one another. Each antenna includes a relatively thin dielectric substrate and a radiating metallic antenna element mounted on the upper surface of the dielectric substrate. A tapered slot area, which is centrally located, is etched away to expose the dielectric substrate. The tapered slot area includes a slot line positioned at the narrow end of the taper. 
   Mounted on the lower surface of the dielectric for each antenna is a microstrip feed line which electrically excites the slot line. The transition from the microstrip feed line to the slot line is a Y to Y transition. The Y to Y transition from the feed line to the slot line transforms electrical current to an electric field, while maintaining a 50 ohm to 100 ohm impedance match. 
   A first antenna of the two antennas has a slot cut down the centerline of the antenna, which allows the second antenna to be inserted perpendicular to the first antenna on the second antenna centerline. 
   The Y to Y transition point location is adjusted in each antenna feed line lengths to maintain phase balance between the antennas. 
   The broadband tapered slot antenna also has four dielectric side walls which surround the two perpendicular antennas and are the support structure for the two perpendicular antennas. 

   
     BRIEF WRITTEN DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a view illustrating the support structure for the dual polarized broadband tapered slot antenna comprising the present invention; 
       FIGS. 2–5  are views illustrating the four side walls which form the support structure for the dual polarized broadband tapered slot antenna of  FIG. 1 ; 
       FIG. 6  is a view illustrating the first tapered slot antenna of the two radiating tapered slot antennas which form the dual polarized broadband tapered slot antenna of  FIG. 1 ; 
       FIG. 7  is a view illustrating the second tapered slot antenna of the two radiating tapered slot antennas which form the dual polarized broadband tapered slot antenna of  FIG. 1 ; 
       FIGS. 8–9  are views illustrating the feed lines for first and second tapered slot antennas of  FIGS. 6 and 7 ; and 
       FIG. 10  is a perspective view of the support structure for the dual polarized broadband tapered slot antenna of  FIG. 1  and the placement of the microstrip antenna boards within the support structure. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIGS. 1–5 , the antenna support structure, designated generally by the reference numeral  20  for the dual polarized broadband tapered slot antenna comprising the present invention includes four side walls  22 ,  24 ,  26  and  28  which are fabricated from dielectric boards commercially available from multiple manufactures. A dielectric material which may be used to fabricate the four side walls  22 ,  24 ,  26  and  28  of support structure is a high frequency laminate commercially available from Rogers Corporation of Rogers, Conn. 
   Each of the side walls  22 ,  24 ,  26  and  28  has the shape of a trapezoid. Sides walls  22  and  24  have six tabs  30 ,  32 ,  34 ,  36 ,  38  and  40  which extend from their non-parallel edges  42  and  44 . Tabs  30  and  36  are located at the upper end of side walls  22  and  24 ; tabs  32  and  38  are located at the center of side walls  22  and  24 ; and tabs  34  and  40  are located at the lower end of side walls  22  and  24 . 
   Similarly, side walls  26  and  28  have six rectangular shaped openings  50 ,  52 ,  54 ,  56 ,  58 , and  60  which are positioned adjacent their non-parallel edges  62  and  64 . Rectangular shaped openings  50  and  56  are located at the upper end of side walls  26  and  28 ; rectangular shaped openings  52  and  58  are located at the center of side walls  26  and  28 ; and tabs  54  and  60  are located at the lower end of side walls  26  and  28 . 
   Each of the six tabs  30 ,  32 ,  34 ,  36 ,  38  and  40  on side walls  22  and  24  aligns with one of rectangular shaped opening  50 ,  52 ,  54 ,  56 ,  58 , or  60  within side walls  26  and  28  to receive the tab  30 ,  32 ,  34 ,  36 ,  38  or  40 . The tab  30 ,  32 ,  34 ,  36 ,  38  or  40  when inserted into the aligned rectangular shaped opening  50 ,  52 ,  54 ,  56 ,  58 , or  60  secure side walls  22  and  24  to side walls  26  and  28  to form the antenna support structure  10  illustrated in  FIG. 1 . Antenna support structure  10  provides support for the microstrip antenna boards  66  and  68  ( FIGS. 6 and 7 ) of the dual polarized broadband microstrip antenna comprising the present invention. 
   Referring to  FIGS. 6 and 7 , the dual polarized broadband tapered slot antenna includes two microstrip antenna boards  66  and  68  illustrated respectively in  FIG. 6  and  FIG. 7 . As shown in  FIG. 6 , microstrip antenna board  66  has a planar upper surface  70  with a radiating metallic antenna element  72  (sometimes referred to as a Vivaldi antenna) and a dielectric substrate  74 . The radiating metallic antenna element  72  is fabricated by electrochemical deposition of copper on the dielectric substrate  74 . The dielectric substrate  74  may be any dielectric or ceramic material composite, fiberglass reinforced material and the like 
   Referring to  FIG. 10 ,  FIG. 10  illustrates the antenna support structure  20  for the dual polarized broadband tapered slot antenna of  FIG. 1  and the placement of the microstrip antenna boards  66  and  68  within antenna support structure  20 . As shown in  FIG. 10 , the antenna boards  66  and  68  are co-located within support structure  20  and affixed to the side walls  22 ,  24 ,  26  and  28  of the support structure  20 , with the antenna boards  66  and  68  being positioned perpendicular to one another. 
   A tapered notch area  76  is formed on the planar upper surface  70  of antenna board  66  by etching away a tapered portion of the metallic antenna element  72 . The tapered notch area  76  extends from the rounded edges  78  of metallic antenna element  72  to one end of a slot line  80  located at the narrow end of the tapered notch  76 . The opposite end of slot line  80  terminates in a Y connection which includes a slot line short circuited stub  82  formed from one arm of the Y connection and a slot line open circuited stub  84  formed from the other arm of the Y connection. 
   Referring to  FIGS. 6 and 8 , a microstrip copper feed line  86  is mounted on the lower surface  88  of dielectric substrate  74  in the manner illustrated in  FIG. 8 . One end of microstrip copper feed line  86  is terminated by a 50 ohm coax cable connector  90  and the opposite of microstrip copper feed line  86  also terminates in a Y connection. The Y connection for microstrip copper feed line  86  includes a microstrip open circuited stub  92  and a microstrip short circuited stub  94  which connects to the radiating metallic antenna element  72  via a copper plated through hole  96 . The connection of feed line  86  to slot line  80  is referred to as Y—Y microstrip to slot line transition. The electrical length of each arm of the Y for feed line  86  is the same to allow for proper operation of the Y—Y microstrip to slot line transition for the tapered notch antenna  72  at the high end of the frequency range which is approximately 18 gigahertz. The physical length of the arms differs because the open circuited stub  92  has capacitance on its end which requires that the open circuited stub  92  be shorter in length than the short circuited stub  94 . 
   In a like manner, the electrical length of each arm of the Y for slot line  80  is the same and is also the same as the electrical length of each arm of the Y for microstrip feed line  86 . 
   The impedance of the mcirostrip line  86  tapers to 100 ohms. 
   The metallic antenna element  72  radiates when the width of the notch as manifested by the taper  76  becomes excessively wide. The radiation is controlled by the taper with frequency of an RF signal being from 1.5 GHZ (gigahertz) at the wide end  78  of the taper  76  to 18 GHZ at the narrow end  79  of the taper  76 . The antenna is designed to transmit and receive RF signals. The dielectric substrate  74  helps to confine electric fields to the region of the taper  76 . 
   Referring to  FIGS. 7 and 9 , a microstrip copper feed line  106  is also mounted on the lower surface  108  of dielectric substrate  104  in the manner illustrated in  FIG. 9 . One end of microstrip copper feed line  106  is terminated by a 50 ohm coax cable connector  91  and the opposite end of copper feed line  106  terminates in a Y connection. The Y connection for microstrip copper feed line  106  includes a microstrip open circuited stub  112  and a microstrip short circuited stub  114  which connects to the radiating metallic antenna element  126  via a copper plated through hole  116 . The connection of feed line  106  to slot line  110  is also a Y—Y microstrip to slot line transition. The electrical length of each arm of the Y for feed line  106  is the same to allow for proper operation of the Y—Y microstrip to slot line transition for the tapered notch antenna  126  at the high end of the frequency range which is approximately 18 gigahertz. The physical length of the arms differs because the open circuited stub  112  has capacitance on its end which requires that the open circuited stub  112  be shorter in length than the short circuited stub  114 . 
   In a like manner, the electrical length of each arm of the Y for slot line  110  is the same and is also the same as the electrical length of each arm of the Y for microstrip feed line  106 . 
   The impedance of the mcirostrip line  106  tapers to 100 ohms. 
   Referring to  FIGS. 6 and 7 , the dielectric substrate  74  of antenna board  66  has a centrally located slot  98  which extends from the wide end  78  of taper  76  to near the end of slot line  80 . Antenna board  68  is inserted into slot  98  of dielectric substrate  74  such that antenna boards  66  and  68  are co-located, orthogonally polarized and positioned perpendicular to one another. Antenna board  68  also has a centrally located slot  120  at the upper end of antenna board  68 . At the bottom end of antenna board  68  is a cutout/opening  122 , which approximates a trapezoid. Slot  120  and cutout  122  are used to facilitate insertion of antenna board  68  into the slot  98  of antenna board  66  and position the antenna boards perpendicular to one another. 
   Referring to  FIGS. 7  ad  9 , the top side of antenna board  68  includes radiating metallic antenna element  126  and tapered notch area  124  which is formed on the planar upper surface  128  of antenna board  68  by etching away a tapered portion of the metallic antenna element  126 . Antenna board  68  also has slot line  110  which terminates in a Y connection. The Y connection for slot line  110  includes a slot line short circuited stub  130  formed from one arm of the Y connection and a slot line open circuited stub  132  formed from the other arm of the Y connection. 
   Referring to  FIGS. 1–7 , antenna boards  66  and  68  each have two alignment tabs  134  and  136  on the side opposite their feed lines and one alignment tab  138  on the side which includes their feed lines. The alignment tabs  134  and  136  are inserted into rectangular shaped openings  140  and  142 , respectively, in side walls  22  and  26 . The alignment tabs  138  are inserted into the rectangular shaped openings  144  in side walls  24  and  28 . Side walls  24  and  28  each have slot  146  at their upper end which centrally located and extends downward into the side walls  24  and  28 . The portion of antenna boards  66  and  68  which includes their microstrip feed lines  86  and  106  and associated 50 ohm coax cable connectors  90  and  91  passes through slots  146  extending outward from side walls  24  and  28 . Cable connectors  90  and  91  allows a user to connect an external RF signal cable to antenna boards  66  and  68 . 
   At this time it should be noted that the copper trace of the tapered notch antennas  72  and  126  functions as a ground for the microstrip feed lines  86  and  106 . 
   Each antenna board  66  and  68  also has an outer routing path  148  and  150 , respectively. The outer routing paths  148  and  150  are formed around the periphery of the antenna boards  66  and  68 . The routing paths  148  and  150  assist the manufacture of the boards in fabricating the boards  66  and  68  to fit within the antenna support structure  20  formed by side walls  22 ,  24 ,  26  and  28 . 
   The tapered notch antennas/radiating metallic antenna elements  72  and  126  allow for linear polarization, elliptical polarization and right or left circular polarization. Polarization can be either horizontal or vertical. For circular polarization, the signals fed to the microstrip feed lines  86  and  106  will differ to provide for a ninety degree phase shift between the signals transmitted on microstrip feed lines  86  and  106 . For linear polarization only one of the two tapered notch antennas  72  or  126  is excited. 
   Tapered notch antennas  72  and  126  create at an electric aperture at the current frequency of operation. The lowest frequency of operation occurs at the rounded edges  78  of antenna  72  and the rounded edges  105  of antenna  126  which is defined as the mouth of antennas  72  and  126 . As the frequency of operation rises radiation occurs in the narrow widths of the tapered notch areas  76  and  124 . Radiation generally begins at one quarter of wavelength in width at the mouth of antennas  72  and  126  and will continue as long as the slot has a width of one quarter wavelength. The antenna pattern provided by antennas  72  and  126  is a single lobe antenna pattern and the width of the mouth is configured to maintain the pattern. Rounded edges  78  and  105  prevent diffractions in the radiation pattern. 
   The antennas  72  and  126  are designed to radiate at the same phase. This necessitates that the slot lines  80  and  110  for antenna boards  66  and  68  and the microstrip lines  86  and  106  be configured as illustrated in  FIGS. 6 and 8  from the coax cable connector elements  90  and  91  to a like point in the tapered section of the antennas  72  and  126  and have the same electrical lengths. An external antenna coupler can be used to provide a ninety degree phase shift between the signal fed to microstrip feed line  86  and the signal fed to microstrip feed line  106  to achieve circular polarization. For linear polarization only one antenna  72  or  126  is excited. 
   The two copper traces of each antenna  72  and  126  are phase shifted by 180 degrees which creates an electric field across the tapers  76  and  124  of antenna boards  66  and  68 . 
   From the foregoing, it may readily be seen that the present invention comprises a new unique and exceedingly useful dual polarized broadband tapered slot antenna which constitutes a considerable improvement over the known prior art. Obviously many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims that the invention may be practiced otherwise than specifically described.