Patent Publication Number: US-6335703-B1

Title: Patch antenna with finite ground plane

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to improvements to antennas, and more particularly to advantageous aspects of a patch antenna with a finite ground plane. 
     2. Description of the Prior Art 
     In a microstrip patch antenna, the radiator is typically provided by a metallic patch element that has been fabricated, using microstrip techniques, onto a dielectric substrate above a ground plane. Because of their low profile, low cost, and compact size, microstrip patch antennas are suitable for various microwave antenna and antenna array applications. Microstrip patch antennas are used, for example, as the radiating elements of designs based on a microwave integrated circuit (MIC) or monolithic microwave integrated circuit (MMIC) such as those used in aircraft and satellite communications, in missile and rocket antenna systems, as well as personal communication system (PCS) wireless applications. However, one problem associated with microstrip patch antennas is that they typically have a limited beamwidth, compared with, for example, antenna designs employing a dipole element. In addition, current microstrip patch antenna designs do not provide for a compact, cost-efficient mechanism for adjusting the antenna beamwidth. 
     The prior art can be better understood with reference to FIG. 1, which shows a cutaway perspective view of a microstrip patch antenna  10  according to the prior art. As shown in FIG. 1, the antenna  10  comprises a square patch element  12 , a ground plane  14 , and a microstrip feed line  16 , lying on parallel planes defined by the top and bottom surfaces of a pair of dielectric substrates  18  and  20 . The patch element  12  is fabricated onto the top surface of the upper substrate  18 , the ground plane  14  is fabricated between the bottom surface of the upper substrate  18  and the top surface of the lower substrate  20 , and the feed line  16  is fabricated onto the bottom surface of the lower substrate  20 . A fixed metal plate reflector  22  is provided at the bottom of the antenna  10  to reflect radiation towards the top of the antenna  10 . Coupling between the feed line  16  and the patch element  12  is provided by a small rectangular aperture  24  in the ground plane  14  that lies across the feed line  16 . Because of this coupling technique, the design shown in FIG. 1 is known as an “aperture-coupled patch antenna.” Other designs are also used, employing different techniques to couple the feed line to the patch element. 
     In current aperture-coupled patch antenna designs, the ground plane  14  is significantly larger than the aperture  24  such that, from an electromagnetic perspective, the ground plane  14  functions as an infinite surface relative to the aperture  24 . This helps the isolation between the feed line  16  and the patch element  12 . In addition, the use of an infinite ground plane makes analysis of the antenna much easier because the equivalence theorem can be applied. 
     An antenna&#39;s radiation pattern is important in antenna applications. It includes several parameters to characterize the antenna performance, including gain, 3 dB (half-power) beamwidth, side-lobe level, front-to-back (F/B) ratio, polarization, cross-polarization level, and the line. The 3 dB beamwidth parameter is the main parameter to show the coverage of radiated energy. The beamwidth of a conventional patch antenna is approximately 60° to 70°. 
     Because of their high level of integration, patch antennas have been used successfully to form large arrays for highly directional applications. However, other applications require a beam width of greater than the currently available 60° to 70°. For example, a typical three-section cellular system needs to cover a 120° geographic area. In a time division multiple access (TDMA) system, the base station requires an antenna with a 3 dB beamwidth of 105° to 110°, and a code division multiple access (CDMA) system requires a 3 dB beamwidth of 90°. Because of the beamwidth limitations of conventional patch elements, a dipole element is typically used instead in these applications. 
     In addition, it is desirable for the beamwidth of an antenna to be adjustable in certain applications. A dipole element with an angular reflector can be employed to provide beamwidth control by mechanically adjusting the reflector angle. However, this approach requires sophisticated mechanical structures which may not be cost effective, and which may also result in an undesirably large package size to accommodate these structures. 
     SUMMARY OF THE INVENTION 
     One aspect of the invention provides a microstrip patch antenna with enhanced beamwidth characteristics. In a first embodiment, the antenna comprises a patch element and a ground plane separated from the patch element by a first dielectric layer. The antenna further includes a signal feed line separated from the ground plane by a second dielectric layer, the signal feed line being shielded from the patch element by the ground plane. The signal feed line is electromagnetically coupled to the patch element through an aperture in the ground plane lying across the signal feed line, the ground plane functioning as a finite surface relative to the aperture. According to a further aspect of the invention, the beamwidth of the antenna is adjusted by adjusting the position of a reflector behind the signal feed line. Thus, the present invention provides an efficient way to achieve adjustable wide-beamwidth that may be used, for example, in wireless systems in a three-sector configuration. 
     Additional features and advantages of the present invention will become apparent by reference to the following detailed description and accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a partial cutaway perspective view of a microstrip patch antenna according to the prior art. 
     FIG. 2 shows a partial cutaway perspective view of a first embodiment of a microstrip patch antenna according to the present invention. 
     FIGS. 3A through 3D show, respectively, top, side, front, and bottom views of a further embodiment of a microstrip patch antenna according to the present invention. 
     FIG. 4 shows a bottom view of the top substrate layer of the antenna shown in FIGS. 3A through 3D. 
     FIGS. 5A through 5C show, respectively, top, bottom, and side views of the bottom substrate layer of the antenna shown in FIGS. 3A through 3D. 
    
    
     DETAILED DESCRIPTION 
     One aspect of the present invention provides a microstrip patch antenna with enhanced beamwidth capabilities. The antenna has a patch element, a ground plane separated from the patch element by a first dielectric layer, and a signal feed line separated from the ground plane by a second dielectric layer. The signal feed line is shielded from the patch element by the ground plane, and the signal feed line is electromagnetically coupled to the patch element through an aperture in the ground plane lying across the signal feed line. As explained below, according to the present invention, the ground plane functions as a finite surface relative to the aperture. 
     FIG. 2 shows a partial cutaway perspective view of a first embodiment of a patch antenna  30  according to the present invention. The FIG. 2 patch antenna  30  includes a patch element  32 , a finite ground plane  34 , and a microstrip feed line  36  lying on parallel planes defined by upper and lower substrates  38  and  40 . A reflector  42  is provided to reflect radiation towards the top of the antenna  30 . The patch element  32  is coupled to the microstrip feed line  36  by a rectangular aperture  44  in the finite ground plane  34 . 
     The dimensions of the finite ground plane  34  are chosen such that it behaves as a finite surface relative to the aperture  44 . The upper limit of the ground plane width is dictated by the edge diffraction conditions which, in the present embodiment of the invention, are derived from the distance of the edges of the ground plane  34  to the radiation caustic, namely, the patch element  32 . Therefore, in the present embodiment of the invention, the definition of the “finite” ground plane  34  is that the width of the ground plane  34  is less than one-half wavelength of the operation frequency (0.5λ) to allow measurable beamwidth variation due to variant reflector positions. Also, the width of the ground plane  34  is more than 1.5 times the width of the patch element  32  to allow a good voltage standing wave ratio (VSWR) performance. 
     Although the use of a finite ground plane complicates the analysis of the antenna  30 , it has been found that the finite ground plane  34  significantly enhances the beamwidth of the antenna  30 . As addressed further below, it has been found that by using a suitably dimensioned finite ground plane, the beamwidth of the antenna can be increased to 85°. 
     It has also been found that the beamwidth capabilities of the antenna  30  are further improved by modifying the shape of the patch element  32 . In current patch antennas, the patch element is typically square. However, it has been found that with a finite ground plane  34 , it is advantageous to use a rectangular patch element  32 , where the width of the patch element  34  is 60 percent of its length or narrower. (It should be noted that, in a wide beamwidth application, the 60 percent width satisfies the above criteria for a finite ground plane.) The use of the rectangular patch element  32  in combination with the finite ground plane  34  has been shown to increase the beamwidth of the antenna  30  to 90°. 
     Further, the FIG. 2 antenna  30  provides a system for adjusting the antenna beamwidth. Using a finite ground plane  34 , it has been found that it is possible to adjust the beamwidth of the antenna  30  by adjusting the position of the reflector  42  relative to the microstrip feed line  36 . Moving the reflector  42  away the feed line  36  increases the “spill” of radiation around the reflector, thereby resulting in an increase in beamwidth. By carefully adjusting the reflector height, the beamwidth can be adjusted to any value in the range of 80° to 110°, without de-tuning the antenna&#39;s impedance matching. In the present embodiment of the invention, shown in FIG. 2, adjustment of the reflector is accomplished by mounting the reflector  42  to a digital stepper motor  46  that is operated by a microprocessor controller  48 . It will be recognized that other spacing control adjusters may be devised and suitably utilized. 
     Thus, the present invention provides an efficient way to achieve adjustable wide-beamwidth (between 80° and 110°) for various wireless systems in a three-sector configuration, which requires coverage of a 120° geographic area. It not only extends the beamwidth of a traditional patch antenna from 60°-70° to over 90°, but also provides a readily adjustable beamwidth. The invention thus allows patch antennas to be used in applications such as three-sector base station radiators. Thus, the conventional dipole antennas can be replaced by these low-cost, low-profile, and highly-integrated patch antennas. 
     Further, using the present invention, it is possible to engineer cell boundaries in a cellular network to be adjustable, such that cell loading can be properly managed and optimized depending upon such variables as the time of day, season, and geographical area. This approach can be realized by employing a base station antenna with the above-described beamwidth control capability. 
     FIGS. 3A through 3D show, respectively, top, right side, front, and bottom views of a further embodiment of an antenna  50  according to the present invention. The antenna includes a patch element  52 , a finite ground plane  54 , and a microstrip feed line  56  that are laid onto upper and lower dielectric substrates  58  and  60 . The patch element  52 , shown in greater detail in FIG. 4, is a relatively narrow rectangle that is fabricated onto the bottom surface of the upper dielectric substrate  58 . The finite ground plane  54 , shown in greater detail in FIG. 5A, is fabricated onto the top surface of the lower dielectric substrate  60 . The microstrip feed line  56 , shown in greater detail in FIG. 5B, is fabricated onto the bottom surface of the lower dielectric substrate  60 . The microstrip feed line  56  is fed by a coaxial feed  62 , the outer conductor  64  of which is electrically connected to the finite ground plane  54  and the inner conductor  66  of which is electrically connected to the microstrip feed line  66 . Finally, a metal reflector  68  is provided to reflect radiation towards the top of the antenna  50 . The reflector  68  includes a first pair of wing members  70  extending upward around the lower substrate  60  and a second pair of wing members  72  extending downward around the coaxial feed  62 . As shown in FIG. 3D, the reflector  68  includes a hole  88  through which the coaxial feed  62  passes. 
     In the present embodiment of the antenna, the upper and lower substrates  58  and  60  are separated from each other by a set of four spacers  84 . This creates a layer of air between the patch element  52  and the ground plane  54 . If desired, the layer of air can be replaced by a solid substrate. A second set of four spacers  86  is used to separate the lower substrate  60  from the reflector plate  68 . In an embodiment of the invention in which the reflector plate  68  is adjustable, the four spacers  84  are replaced by a movable mounting assembly that allows the reflector plate  68  to be moved precisely relative to the upper and lower substrates  58  and  60  while maintaining a parallel relationship with those elements. In that embodiment, the movement of the reflector plate  68  is controlled using a microprocessor-controlled stepper motor, as shown in FIG.  2 . 
     FIG. 4 shows a bottom view of the upper substrate  58  with the metallic patch element  52  fabricated thereon. As discussed above, according to the present invention the shape of the patch element  52  is a relatively narrow rectangle having a width that is 60% or less of its length. However, it would also be possible to practice the present invention using a square patch element  52 . 
     FIG. 5A shows a top view of the lower substrate  60 . The finite ground plane  54  is fabricated onto the substrate  60 , and includes at its center a rectangular aperture  90 . In the embodiment shown in FIG. 5A, the aperture  90  only extends through the ground plane  54 . It does not extend through the substrate  60 , although it would be possible to do so, if desired. As discussed above, the size of the ground plane  54  relative to the aperture  90  is such that the ground plane  54  functions as a finite surface with respect to the aperture  90 . 
     FIGS. 5B and 5C show, respectively, bottom and side views of the lower substrate  60 . The microstrip feed line  56  is fabricated directly onto the bottom surface of the lower substrate  60  and extends across the aperture  90  in the ground plane  54 . As mentioned above, the aperture  90  does not extend all the way through the substrate  60 . The coaxial feed  62  is mounted perpendicular to the lower substrate  60 . Its inner conductor  66  is electrically connected to the microstrip feed line  56 . Its outer conductor  64  extends through the lower substrate  60  and is electrically connected to the ground plane  54  on the other side of the substrate  60 . 
     While the foregoing description includes details which will enable those skilled in the art to practice the invention, it should be recognized that the description is illustrative in nature and that many modifications and variations thereof will be apparent to those skilled in the art having the benefit of these teachings. It is accordingly intended that the invention herein be defined solely by the claims appended hereto and that the claims be interpreted as broadly as permitted by the prior art.