Abstract:
A method for improving bandwidth and gain of a microstrip patch antenna and a microstrip patch antenna are provided. The method includes forming a highly anisotropic superstrate, and positioning the highly anisotropic superstrate at a predetermined distance away from the ground plane side of the microstrip patch antenna, increasing the bandwidth of the microstrip patch antenna. The antenna provides a microstrip patch antenna having a highly anisotropic superstrate. The highly anisotropic superstrate can include a spacing layer, a dielectric material positioned on the spacing layer and a plurality of conductive strips disposed on the dielectric layer.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     CROSS REFERENCE TO OTHER PATENT APPLICATIONS 
     None. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention provides methods and apparatus of improving both the gain and the bandwidth of a microstrip patch antenna. 
     (2) Description of the Prior Art 
     A patch antenna, also referred to as a rectangular microstrip antenna, is a type of radio antenna with a low profile that can be mounted on a flat surface. The patch antenna includes a flat conductor mounted on a dielectric substrate over a larger conductor, typically referred to as a ground plane. The two metal sheets of the patch antenna form a resonant piece of microstrip transmission line. The patch is designed to have a length of approximately one-half wavelength of the radio waves being transmitted or received. A patch antenna can be constructed using the same technology as that used to make a printed circuit board. 
     An ordinary patch antenna exhibits resonant behavior characterized by a high Q-factor and a relatively narrow impedance bandwidth on the order of 2-6 percent, depending on the losses in the antenna. Some patch antennas are formed from two stacked patches and are designed to have a double resonance, one corresponding to the L1 frequency (1575 MHz) and the other to the L2 frequency (1227 MHz) commonly used in global positioning systems.  FIG. 1  provides an exemplary measured voltage standing wave ratio (VSWR) plot for such an antenna. A first resonance is indicated at L1 and a second resonance is indicated at L2. 
     Typical patch antennas are tuned to the L1 and L2 GPS commercial frequencies, but they lack performance at the operating frequencies of other desirable services, including new and emerging COMMs bands, such as Iridium™, which typically operates between 1616 MHz and 1626.5 MHz. 
     Thus, there is a need for antennas that can receive these new bands. There is a further need for adapting existing patch antennas to accommodate additional services operating at these other frequencies. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a microstrip patch antenna having improved bandwidth and gain. 
     Another object is to provide method for retrofitting an existing microstrip patch antenna to make an antenna having an improved bandwidth and gain. 
     Yet another object is to provide a kit that can be used to retrofit an existing microstrip patch antenna. 
     In view of these objects, there is provided a method for improving bandwidth and gain of a microstrip patch antenna. A highly anisotropic superstrate is formed and positioned at a predetermined spacing away from the ground plane side of the microstrip patch antenna. A cover layer can be mounted over the highly anisotropic superstrate. The highly anisotropic superstrate can includes a plurality of conductive strips regularly disposed over a dielectric material. In further embodiments, the conductive strips can be provided with a capacitive load region at each end of each conductive strip. 
     An antenna is further provided including a microstrip patch antenna for mounting on a ground plane and a highly anisotropic superstrate having a predetermined resonance placed at a specific spacing above said microstrip patch antenna in the direction away from the ground plane. A cover layer can be positioned over the highly anisotropic superstrate in the direction away from the ground plan. A spacing layer can be disposed on said microstrip patch antenna in order to maintain the specific spacing between the microstrip patch antenna and the highly anisotropic superstrate. The highly anisotropic superstrate can include a plurality of conductive strips regularly disposed on a dielectric substrate. Each of the conductive strips can be provided with a capacitive load region at each end of each conductive strip. The microstrip patch antenna can be a stacked patch antenna having at least two patches where the highly anisotropic superstrate is positioned at a specific spacing above one of the patches. A kit is further provided for retrofitting an existing patch antenna. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features and advantages of the present invention will become apparent upon reference to the following description of the preferred embodiments and to the drawings, wherein corresponding reference characters indicate corresponding parts throughout the several views of the drawings and wherein: 
         FIG. 1  is a diagram showing a measured voltage standing wave ratio (VSWR) plot for prior art patch antennas; 
         FIG. 2  is an exploded perspective view of a microstrip patch antenna having a highly anisotropic superstrate added thereto; 
         FIG. 3  is a diagram showing a measured voltage standing wave ratio plot of the patch antenna of  FIG. 2 ; and 
         FIGS. 4A ,  4 B,  4 C and  4 D show exemplary alternative embodiments of highly anisotropic superstrates which can be used for practice of the current invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, and more particularly to  FIGS. 2 and 3 , the present invention provides methods and apparatus for improving both the gain and the bandwidth of a microstrip patch antenna  10 . 
     A microstrip patch antenna  10  includes one or more rectangular conductive surfaces  12 ,  12 ′ printed on a grounded dielectric substrate  14 ,  14 ′ and fed by a coaxial probe (not shown) that penetrates the dielectric substrate  14 ,  14 ′ from beneath. Patch antenna  10 , in use, is mounted on a conducting ground plane  16 . For purposes of this description, the distance away from the ground plane  16  is referenced as being above the ground plane  16 . The patch antenna  10  shown is a stacked patch antenna having two conductive surfaces  12  and  12 ′ and two substrates  14  and  14 ′. This is used so that the antenna can have two resonances such as at the L1 GPS frequency and at the L2 GPS frequency as is commonly known in the art. 
     Above this patch antenna  10 , at a spacing h, is placed a superstrate  18  of a highly anisotropic superstrate. The spacing, h, can be provided by, for example, a layer of foam  20 . Spacing layer  20  can be made from any material that is effectively transparent to electromagnetic radiation at the operating range of the modified antenna. A cover layer  22  can be placed over the superstrate  18  for physical protection. The cover layer  22  can be made from syntactic foam. As used herein, a “highly anisotropic superstrate” is characterized by a relative permittivity tensor: 
                         ɛ   _     r     ⁢     (   ω   )       =     [           ɛ     x   ⁢           ⁢   x           0       0           0         ɛ       y   ⁢           ⁢   y     ⁢                   0           0       0         ɛ     z   ⁢           ⁢   z             ]             (   1   )               
where the superstrate  18  is considered to be highly anisotropic if one of the diagonal elements in the tensor is greater than the other two by a factor of at least eight to ten.
 
     Without the highly anisotropic superstrate  18 , an ordinary patch antenna such as 12 exhibits a resonant behavior characterized by a high Q-factor and a relatively narrow impedance bandwidth on the order of 2-6 percent. As described above,  FIG. 1  shows a VSWR plot for a typical stacked patch antenna having two resonances. 
     The addition of the highly anisotropic superstrate  18  allows for the bandwidth of the antenna to be improved. In one exemplary embodiment of the present invention, the superstrate  18  was implemented as an array of copper stripes  24 , 0.25 inch wide and 2.75 inches long, placed on a 0.25 inch thick piece of syntactic foam as shown in  FIG. 3 . The length-to-width ratio of the stripes  24  gives them a static polarizability of approximately 10 times that of free space, satisfying the definition of a highly anisotropic superstrate. The stripes  24  were placed 1 inch apart. Experimentation with different heights above the patch antenna  12  showed that the significant improvement in bandwidth occurred for a height, h, of 0.625 inch. This spacing was obtained by placing a block of milled polystyrene foam between the patch  12  and the syntactic foam  22  layers. 
     The example VSWR plot for the antenna of  FIG. 2  is shown in  FIG. 3 . L1 indicates the resonance at the L1 GPS frequency, and L2 indicates the resonance at the L2 frequency. A broadened passband is present between about 1425 MHz and 1870 MHz (resulting in approximately a 240 MHz span). This broadened passband allows reception or transmission of frequencies other than those provided by the two microstrip patch antennas  14  and  14 ′. 
     The metallic stripes  24  forming the anisotropic superstrate were designed to be sub-resonant at the frequencies of interest for the above example. They do not achieve resonance until just above 2100 MHz and, therefore, act as polarizing shapes and not as parasitic radiators, as would be the case in a Yagi-Uda configuration or a log-periodic array. 
     In the exemplary embodiment shown in  FIG. 2 , the highly anisotropic layer can be implemented as an array of sub-resonant metallic shapes  24 , resembling the letter “I”. The capital and the base of the “I,” identified as  26 , serve as capacitive loads at the ends of the lengths of the “I”. These regions  26  allow the induced current at the end of the shape  24  to be non-zero which helps the shapes perform as an anisotropic dielectric over a wider range of frequencies. Regions  26  are thus termed “capacitive load regions.” 
     The sub-resonant shapes  24  should be oriented with respect to the microstrip patch antenna  10  relative to the current flowing on the patch antenna  10  to maximize the desired performance. This orientation should be such that induced current in the shapes  24  is maximized. 
     While not limited to any particular theory or mode of operation, in some embodiments, the resulting antenna operates by controlling the flow of current on the patch. The presence of the highly anisotropic superstrate and the alignment of the dominant axis of the permittivity tensor with the fields associated with the resonant mode of the patch cause a near-field interaction effect. This interaction effect alters the current distribution on the antenna, limiting the presence of standing waves on the antenna and improving the bandwidth. 
     The antenna of the present invention allows for a single simple antenna to cover a much wider bandwidth than it would ordinarily be able to, while also providing a modest improvement in gain. This allows the new structure to support more communications channels at greater ranges than is possible with current technology. The highly anisotropic superstrate can be easily retro-fitted to existing microstrip patch antennas to accommodate additional communications channels. 
     Other embodiments of the highly anisotropic superstrate are shown in  FIGS. 4A ,  4 B,  4 C, and  4 D.  FIG. 4A  shows an embodiment having only strips  24 A.  FIG. 4B  shows a highly packed configuration having offset strips  24 B.  FIG. 4C  shows a configuration having enlarged capacitive load regions  26 ′ at the end of strips  24 C.  FIG. 4D  shows another embodiment for strips  24 D. The benefits of each of these configurations can be determined by computer modeling. Thus, it can be seen that the highly anisotropic superstrate can have a variety of configurations within the scope of the current disclosure. 
     It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. 
     The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description only. It is not intended to be exhaustive nor to limit the invention to the precise form disclosed; and obviously many modifications and variations are possible in light of the above teaching. Such modifications and variations that may be apparent to a person skilled in the art are intended to be included within the scope of this invention as defined by the accompanying claims.