Patent Publication Number: US-6342866-B1

Title: Wideband antenna system

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to the field of radio frequency antennas, and more particularly to an antenna system that incorporates a stack of overlying dual element antennas in a single structure so that the bandwidth of the antenna system is the sum of the bandwidths of all the individual antennas. 
     A dipole antenna generally has about 20% bandwidth, depending on its actual configuration. Multiple bandwidth performance is conventionally achieved by employing separate dipole antennas that each cover a specific portion of the radio frequency spectrum. However, separate dipole antennas collectively tend to be bulky. Shipboard communications systems generally require multiple bandwidth performance. However, multiple antenna systems on board ships must compete for a very limited amount of space. Therefore, there is a strong need for an antenna system that provides multiple bandwidth performance in a compact package. 
     SUMMARY OF THE INVENTION 
     The present invention provides a wideband antenna system incorporates a stack of m antennas, A i , where i is an index from 1 to m, m and i are positive integers, and m≧2. Each antenna A i  includes: an electrically insulating substrate; opposed radio frequency elements mounted to the electrically insulating substrate such that the radio frequency elements of the antennas A 2  through A m  provide ground planes for antennas A 1  through A m−1 ; and a ground plane mounted to the substrate for antenna A m . In other words, each underlying antenna A i  provides a ground plane for the immediately overlying antennas. The bandwidth of the antenna system is generally the sum of the bandwidths of the individual antennas, thereby providing the antenna system with wideband performance characteristics in a compact package. However, it is to be understood that some of the bandwidths of the individual antennas may be continuous, overlapping, spaced apart, or some combination of the foregoing. The antenna system may also incorporate a frequency selective surface so that the antenna system is limited to detecting RF signals having particular bandwidth characteristics. 
     The invention may also be characterized as a wideband antenna system that comprises a stack of m antennas, where m is a positive integer, and m≧2. Each antenna includes: a) an electrically insulating substrate: b) opposed first and second radio frequency elements mounted to the substrate: c) a ground feed electrically connected to the first radio frequency element: d) an excitation feed electrically connected to the second radio frequency element: and e) a ground plane mounted to the substrate of the m th  antenna. The radio frequency elements of each antenna collectively have a unique total area and are mounted to the electrically insulating substrate. The radio frequency elements of the i th  antenna provide a ground plane for the k th  antenna, where i and k are positive integers 1≦k≦(i−1) and 2≦i≦m. The total area of the first and second radio frequency elements of the i th  antenna is greater than the total area of the first and second radio frequency elements of the k th  antenna. 
     In another embodiment of the invention, antenna stacks may be radially distributed about an arcuate shaped structure such as a tube so that each stack has a unique field of view. This configuration allows the antenna system to detect or transmit RF signals to some or all of a broad region without having to rotate the antenna. 
     These and other advantages of the invention will become more apparent upon review of the accompanying drawings and specification, including the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates an exploded view of a wideband antenna embodying various features of the present invention. 
     FIG. 2 illustrates a cutaway view of the wideband antenna shown in FIG.  1 . 
     FIG. 3 is a side view of the wideband antenna shown in FIG.  1 . 
     FIG. 4 is a perspective view of an omnidirectional antenna the incorporated multiple wideband antennas of the type shown in FIG.  1 . 
     FIG. 5 is a top view of the omnidirectional antenna of FIG. 4 showing the angular distribution of the stacked antenna systems. 
     FIG. 6 shows a frequency selective surface incorporated into the antenna system of FIG.  1 . 
     FIG. 7 is a cross-sectional view of the a wideband antenna system that includes a feed to one of the stacked antennas. 
    
    
     Throughout the several view, like elements are referenced using like references. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, the present invention is directed to a wideband antenna system  10  that incorporates a stack of dual element antennas A i  each having a particular bandwidth, where i is an index from 1 to m, m is a positive integer, and m≧2. The overall bandwith of antenna system  10  is generally the sum of the bandwidths of each of the individual dual element antennas A i . Each antenna A i  includes an electrically insulating substrate  14   i  and a pair of two diametrically opposed and preferably symmetrical radio frequency elements  12   i  and  13   i  mounted to one side of insulating substrate  14   i . An important feature of the invention is that radio frequency element pairs  12   2 / 13   2  through  12   m / 13   m  of antennas A 2  through A m  ground planes for antennas A 1  through A m−1 . Radio frequency elements  12   i  and  13   i  transform radio frequency (RF) energy into an electrical signal and/or transform an electrical signal into radiated radio frequency energy. A radio frequency (RF) ground plane  16 , preferably made of an electrically conductive metallic material, is mounted to substrate  14   m  of antenna A m  on a side opposite the side on which radio frequency element pairs  12   m / 13   m  are mounted. Substrates  14   i  are preferably implemented as electrically non-conductive materials and/or material systems such as fiberglass, phenolic, S-glass, and E-glass, and may have a thickness in the range of about 0.1 to 20 mm, depending on the desired frequency response. 
     Antennas A i  are stacked as shown in FIGS. 1-3 to form antenna system  10  having an overall bandwidth determined by the bandwidths of each of antennas A 1  through A m . Thus, antenna system  10  may be characterized as a wideband antenna, where a wideband antenna is an antenna system having a bandwidth that is determined by the bandwidths of all the individual dual element antennas A i  that comprise antenna system  10 . Stacked antennas A i  may be held together using conventional methods such as adhesive or mechanical fasteners, not shown. By way of example, in FIGS. 1 and 2, radio frequency element pairs  12   i / 13   i  preferably each are shaped as symmetrically opposed, isosceles triangles such that antennas A i  define bow-tie antennas. However, it is to be understood that radio frequency element pairs  12   i / 13   i  may have other linearly tapered shapes as well to enhance the impedance match of the antenna with respect to feed  21   i  over a broad bandwidth. A broad bandwidth may be in the range of about 100 MHz to 20 GHz. Each feed  21   i  includes an excitation line feed  23   i  electrically connected to each of radio frequency elements  12   i  and a ground feed  25   i  electrically connected to each of radio frequency elements  13   i . Ground feed  25   i  provides a ground with respect to excitation line feed  23   i . Byway of example, each feed  21   i  maybe implemented as coaxial cable. 
     Radio frequency elements  12   i  and  13   i  have an apex  18   i  and  19   i , respectively, and are positioned so that they are diametrically opposed and symmetrical about the intersection of orthogonal axes a—a and axis b i —b i . Radio frequency elements  12   i  and  13   i  are generally made of an electrically conductive material or material system that includes copper, aluminum, gold, or other electrically conductive materials, and are mounted to one side of substrate  14   i . Each substrate  14   i  may have a thickness, for example, in the range of about 0.1-20 mm. 
     As shown in FIGS. 1-3, radio frequency element pairs  12   2 / 13   2  through  12   m / 13   m  of antennas A 2  through A m  overlie and thereby provide ground planes for radio frequency element pairs pairs  12   1 / 13   1  through  12   m−1 / 13   m−1 . FIGS. 2 and 3 are cross-sectional and cut-away views of antenna system  10  that further show antenna elements  12   k / 13   k  underlying antenna elements  12   i / 13   i , respectively, where i and k are positive integer indices, 1≦i≦(m−1), and 2≦k≦m. Exemplary dimensions of one pair of radio frequency elements  12   i  and  13   i  when implemented as isoceles triangles are b=λ/4 and d=λ/4, where λ represents the center frequency of a specific antenna of antennas A i . The thickness of radio frequency elements  12   i  and  13   i  is not critical, but maybe in the range of 0.1 to 20 mm. In general, the bandwidth of a bow-tie antenna such as antenna A 1  is approximately ±10 per cent of the center frequency, c/λ, where c represents the speed of light. If for example, antenna A 1  is to have a center frequency of 200 MHz, then b≈λ/4 (0.375 m) and d≈λ/4 (0.375 m), thereby providing antenna A 1  with a bandwidth of approximately ±10% of 200 MHz, or ±20 MHz. 
     Another embodiment of the invention is an antenna array  30  that incorporates multiple antenna systems  10   j , where 1≦j≦M, and j is an index from 1 to M, M≧2, and j and M are positive integers. Antenna systems  10   j  may be configured into an array radially distributed about axis g—g at an angle θ about an arcuate or circular structure  32  as shown in FIGS. 4 and 5, where θ=360°/M. Each of antennas  10   j  may be constructed as described above with reference to antenna system  10  and affixed to circular structure  32  using well known fabrication techniques such as adhesives, mechanical fasteners, bonding agents, and the like. Circular structure  32  may be implemented as a tube and be made of an electrically non-conductive material such as fiberglass, S-glass, and E-glass. An important advantage in having antennas  10   i  radially distributed about structure  32  is that each individual antenna  10   i  has a unique field of view. Thus, antenna system  30  may detect RF signals from or transmit RF signals to a broad region without having to rotate the antenna. 
     Antenna system  30  is shown, for example, in FIGS. 4 and 5 to include  10  antennas  10   j  (j=1, 2, 3, . . . 10). However, it is to be understood that antenna system  30  may be constructed to include any integral number of antennas  10   i  required to suit the requirements of a particular application. Further, M may be an odd or even integer that is equal to or greater than two. 
     As shown in FIG. 6, antenna system  10  may further include a frequency selective surface (FSS)  40  to filter RF signals so that only signals having particular wavelength characteristics may be received by antenna system  10 . Examples of FSS  40  suitable for use in conjunction with the present invention are described in commonly assigned U.S. Pat. No. 5,917,458, incorporated herein by reference. 
     FIG. 7 illustrates another embodiment of the present invention wherein antenna system  10  includes a feed  21   m  to antenna A m . The other antenna A i , where (1≦i≦m−1) do not have feeds, and serve as parasitic elements to increase the bandwidth of antenna A m . 
     The invention may also be characterized as a wideband antenna system that comprises a stack of m antennas, where m is a positive integer, and m≧2. Each antenna includes: a) an electrically insulating substrate, b) opposed first and second radio frequency elements mounted to the substrate; c) a ground feed electrically connected to the first radio frequency element: d) an excitation feed electrically connected to the second radio frequency element: and e) a ground plane mounted to the substrate of the m th  antenna. The radio frequency elements of each antenna collectively have a unique total area and are mounted to the electrically insulating substrate. The radio frequency elements of the i th  antenna provide a ground  12  plane for the k th  antenna, where i and k are positive integers, 1≦k≦(i−1), and 2≦i≦m. The total area of the first and second radio frequency elements of the i th  antenna is greater than the total area of the first and second radio frequency elements of the k th  antenna. 
     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, the invention may be practiced otherwise than as specifically described.