Abstract:
The invention provides a compact patch antenna having a cavity underneath the driver patch, so that the electromagnetic volume of the antenna is expanded without increasing the overall area of the antenna. More specifically, the compact patch antenna comprises a base layer having a cavity, a ground plane located on the base layer, and having an opening over at least a portion of the cavity, a substrate located on the ground plane, and a driver patch located on the substrate. The invention further provides a method for constructing a compact patch antenna, comprising the steps of providing a base layer having a cavity, providing a ground plane located on the base layer, and having an opening over at least a portion of the cavity, providing a substrate located on the ground plane, and providing a driver patch located on the substrate.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to communications antennas, and more specifically relates to a novel microstrip patch antenna suitable for use in an antenna array.  
       BACKGROUND OF THE INVENTION  
       [0002]     A modern trend in the design of antennas for wireless devices is to combine two or more antenna elements into an antenna array. Each antenna element in such an array should have a small footprint, a low level of mutual coupling with neighboring elements, a low element return loss, a low axial ratio (in case of circular polarization), and a large frequency bandwidth. For a typical antenna element in an antenna array, however, these requirements are typically at odds with each other. For example, the larger the bandwidth and the larger the size of an antenna element, the stronger will be the mutual coupling between the antenna element and its neighboring elements in the antenna array.  
         [0003]      FIG. 1  depicts a conventional patch antenna element  100  for use in an antenna array. Patch antenna element  100  includes a driver patch  110  and a ground plane  130 , separated by a dielectric substrate  120 . An input signal having a given wavelength λ is inserted via a microstrip feed line (not shown) connected to the driver patch  110 . The length L of the patch is typically selected to be ½ of the wavelength, so that the patch resonates at the signal frequency of the signal and thereby transmits the desired wireless signal. At low frequencies, however, the wavelength λ can be very long, and the patch antenna dimension L can become quite large.  
         [0004]     A known technique to reduce the size of the patch antenna element is to select a dielectric substrate  120  with a very high permittivity ∈ S  (e.g., ∈ S =6 to 20 relative to air). The high permittivity substrate reduces the resonant frequency of the patch antenna element  100  and thus allows a smaller driver patch to be used for a given signal frequency f More specifically, for the patch antenna element shown in  FIG. 1 , and for a given signal frequency f, the length of the driver patch is conventionally selected to be inversely proportional to the square root of the permittivity ∈ S  of the substrate  120 . For example, if the length L were nominally 1 cm for a substrate permittivity of 1, the length L could be reduced to 0.5 cm for a substrate having a permittivity of 4 were used, or to 0.33 cm for a substrate having a permittivity of 9.  
         [0005]     The effect of the increased dielectric permittivity is to raise the capacitance between the patch  110  and ground plane  130  and thereby to lower the resonant frequency. Unfortunately, the reduced antenna volume decreases the bandwidth of the antenna element and causes difficulties with impedance matching. Using conventional design methods known to those of skill in the art, the bandwidth may be improved to some extent by increasing the thickness of the substrate. A thicker substrate, however, introduces additional problems by (i) increasing the antenna&#39;s cost; (ii) increasing the antenna&#39;s mass (or weight), which may be unacceptable in space applications; and (iii) exciting unwanted electromagnetic waves at the substrate&#39;s surface, which lead poor radiation efficiency, larger mutual coupling between antenna elements and distorted radiation patterns. Moreover, a very thin substrate is conventionally used for the feed network—including, e.g., the microstrip feed line (not shown)—and it is preferable to build antenna elements with the same substrate as that used for the feed network.  
         [0006]      FIG. 2  depicts another known technique to improve the bandwidth of an antenna element by adding a parasitic patch above the driver patch, resulting in a “stacked patch antenna.” Stacked patch antennas have been described in the article entitled “Stacked Microstrip Antenna with Wide Bandwidth and High Gain” by Egashira et al., published in IEEE Transactions on Antennas and Propagation, Vol. 44, No. 11 (November 1996); and in U.S. Pat. Nos. 6,759,986; 6,756,942; and 6,806,831. As shown in  FIG. 2 , a conventional stacked patch antenna  200  includes a ground plane  250  supporting a dielectric substrate  240 , a driver patch  230 , a foam dielectric  220  having a permittivity similar to air, and a parasitic patch  210  (also known as a “driven patch” or “stacked patch”). A signal to be transmitted is input to the driver patch  230 . The parasitic patch  210  is electromagnetically coupled to the driver patch  230  and therefore resonates with it. The additional resonance provided by the parasitic patch  210  improves the operational frequency of the stacked patch antenna  200  and increases the bandwidth of the antenna. In conventional stacked patch antennas, however, parasitic patch  210  must be fairly large in comparison with driver patch  230 , as reflected in  FIG. 2 , due to the relatively low permittivity of the foam dielectric  220 . As a result, when stacked patch antenna elements are combined in an antenna array, adjacent elements exhibit a strong mutual coupling effect on each other, which negatively impacts antenna element and array gain, radiation patterns, bandwidth and scanning ability of antenna array. Furthermore, in view of recent trends in miniaturization, conventional stacked patch antennas are still too large.  
         [0007]     Thus, in conventional designs, the performance of a patch antenna is compromised in order to reduce the size of the antenna. Accordingly, there is a need for a patch antenna that requires a smaller volume than existing antennas without compromising the performance of the antenna. The present invention fulfills this need among others.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention provides for a compact broadband patch antenna in which a cavity is etched in a substrate under the driver patch. The inventors have discovered that the cavity expands the electromagnetic volume of the antenna element and greatly enhances the efficiency and bandwidth of the antenna by reducing the capacitive loading of the driver patch. Indeed, the efficiency of the antenna may be increased from about 45% (for very thin substrates) to 95% (for thicker substrates).  
         [0009]     More specifically, the broadband patch antenna according to the invention comprises: (1) a base layer having a cavity; (2) a ground plane located on the base layer, and having an opening that allows electromagnetic coupling between the patch and the cavity; (3) a thin substrate located on the ground plane; and (4) a driver patch located on the thin substrate. The inventors have found that the use of the cavity in this manner greatly increases the capacitive loading of the parasitic patch, which in turn significantly improves the resonant frequency characteristics of the patch antenna. As a result, for a given resonant frequency, the broadband patch antenna in accordance with the invention takes up a significantly smaller surface area on an integrated patch antenna die and has a much smaller mass than a conventional patch antenna having the same resonant frequency.  
         [0010]     Advantageously, the size, location and/or shape of the opening in the ground plane may be adjusted during the design of the antenna in order to obtain a desired capacitive loading from the patch to the ground plane. Because the capacitive loading largely determines the resonant frequency of the driver patch, a desired resonant frequency of the driver patch can be set during the design of the antenna simply by selecting an appropriate geometry (size, shape and/or location) for the opening in the ground plane.  
         [0011]     In still further embodiments, the broadband patch antenna may include a parasitic patch, located over and separated from the driver patch by a radome or a layer of foam or other dielectric material. The driver patch and/or the parasitic patch may also include one or more slots, which further reduce the size of the antenna element and improve the performance of the antenna element and the associated antenna array.  
         [0012]     The invention further provides a corresponding method for constructing a compact broadband patch antenna, comprising the steps of: (1) providing a base layer having a cavity, (2) providing a ground plane located on the base layer, and having an opening over at least a portion of the cavity; (3) providing a substrate located on the ground plane; and (4) providing a driver patch located on the substrate. The method may further include the steps of providing one or more parasitic patches located over and separated from the driver patch by a radome or a dielectric material, such as foam or substrate. The method may still further include the step of providing one or more slots in the driver patch and/or the one or more parasitic patches. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a cross-sectional view of a patch antenna in accordance with the prior art.  
         [0014]      FIG. 2  is a cross-sectional view of a stacked patch antenna in accordance with the prior art  
         [0015]      FIG. 3A  is a cross-sectional view of a broadband patch antenna in accordance with the present invention.  
         [0016]      FIG. 3B  is a top view of the broadband patch antenna in accordance with the present invention.  
         [0017]      FIG. 3C  is a bottom view of the broadband patch antenna in accordance with the present invention.  
         [0018]      FIG. 4  is a cross-sectional view of a broadband patch antenna having a parasitic patch mounted on a radome in accordance with the present invention.  
         [0019]      FIG. 5  is a cross-sectional view of a broadband patch antenna having a parasitic patch mounted on a foam layer in accordance with the present invention.  
         [0020]      FIG. 6  is an isometric view of a broadband patch antenna having a parasitic patch with slots in accordance with the present invention.  
         [0021]      FIG. 7  is an isometric view of an antenna array including two broadband patch antenna elements in accordance with the present invention, coupled in the H-Plane.  
         [0022]      FIG. 8  is an isometric view of an antenna array including two broadband patch antenna elements in accordance with the present invention, coupled in the E-Plane. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     Referring to  FIGS. 3A, 3B , and  3 C, an embodiment of the broadband patch antenna  300  is shown in a cross-sectional view ( FIG. 3A ), a top view ( FIG. 3B ) and a bottom view ( FIG. 3C ). The illustrated device comprises a base layer  390  having a cavity  350 , a ground plane  330  having an opening  340  (shown in  FIG. 3C ), a dielectric substrate  320 , and a driver patch  310 . As in conventional patch antenna  100  described above, an input signal is preferably provided to the driver patch  310  via a microstrip line  395  (in  FIG. 3B ) and radiated outward by driver patch  310 . Alternatively, the input signal may be provided via a coaxial probe feed passing upward through the base layer  390 , cavity  350 , and opening  340  to the driver patch  310 .  
         [0024]     The opening of the ground plane  330  may be larger than, coextensive with, or smaller than the cavity or the driver patch  310 . Ground plane  330  is preferably extended beneath driver patch  310 , such that at least a portion of the ground plane  330  overlaps the driver patch  310 . Still more preferably, the ground plane opening  340  is centered over, and smaller than, the cavity  350 , such that the ground plane  330  overlaps the driver patch  310  around the entire perimeter of the ground plane opening  340 . Preferably, the overlap between the ground plane and the driver patch is selected based upon the thickness of the substrate. For thinner substrates, for example, the overlap could be as small as 0.01λ (one-hundredth of a wavelength). This overlap helps to lower the resonant frequency of the broadband patch antenna  300  by capacitively loading the driver patch  310 . It thereby also helps to reduce the overall size of broadband patch antenna  300  without loading the cavity with a dielectric. It should be noted, however, that the broadband patch antenna  300  is suitable for operation without this overlap.  
         [0025]     Base layer  390  is preferably a metal material such as aluminum, steel, silver or gold, milled or machined to form cavity  350 . Alternatively, base layer  390  may be a semiconductive or insulating material formed by conventional photolithographic techniques. If base layer  390  is a semiconductor or insulator (e.g., a dielectric material), however, then the performance of the broadband patch antenna may be improved by lining the surfaces  360 ,  370 ,  380  of cavity  350  with a thin layer of conductive material, preferably a metal such as silver or gold. The metal lining on vertical surfaces  360  and  370  of the cavity may be provided in the form of an array of metal vias (not shown) around the perimeter of cavity  350 , preferably at distances of approximately ⅛ to 1/10 of the wavelength. In this way, the electromagnetic field emitted by the driver patch  310  is contained and reflected back toward driver patch  310 .  
         [0026]     As described above, the cavity  350  serves to improve the radiation efficiency and thereby also to lower the overall dissipation loss of the driver patch. Without the back cavity, the currents in the driver patch  310  tend to be non-uniform, causing a higher resistive loss and thus lower radiation efficiency. In contrast, in the presence of the back cavity, the radiation efficiency is improved, because the effective dielectric thickness (thin substrate plus air cavity) is larger. By way of example, for thin substrates, the cavity helps to improve the radiation efficiency from about 50% to 90%.  
         [0027]     Further, because the bandwidth of a stacked patch antenna is typically proportional to its volume (i.e., the volume below the driver patch), the cavity  350  also serves to improve the bandwidth of the broadband patch antenna by increasing the effective volume of the antenna below the driver patch. In general, the larger the volume, the better will be the resulting antenna bandwidth (until saturation eventually occurs). By expanding the three-dimensional volume of the antenna below the ground plane and into the space formed by the cavity  350 , the bandwidth of the antenna is greatly enhanced. For example, without the cavity, the bandwidth will typically be in the range of about two to five percent of the centre operating frequency. In other words, if the centre frequency is 10 GHz, the bandwidth would be five percent of 10 GHz, or 0.5 GHz, such that the conventional patch antenna would operate from 9.75 GHz to 10.25 GHz. In contrast, with the cavity, a bandwidth in the range from about 10 to 16% may be achieved.  
         [0028]     Dimensionally speaking, the cavity width is preferably slightly larger than that of the driver patch  310 , and the cavity depth is preferably in the range of 0.01 to 0.02 times the signal wavelength. Because the cavity depth may be very small, it adds very little additional volume to the antenna array.  
         [0029]     Cavity  350  in base layer  390  may also be filled or unfilled. Filling the cavity  350  with foam or another suitable dielectric material advantageously provides structural support to driver patch  310 .  
         [0030]     Substrate  320  may be any low loss substrate material conventionally used by those of skill in the art for constructing patch antennas, such as RT Duroid® or a Teflon®—based substrate as manufactured by Rogers Corporation, Taconic® and Arlon, Inc. Such substrates typically have a permittivity of about 2 to about 6.  
         [0031]     Ground plane  330  and driver patch  310  may be any conductive material (including copper, aluminum, silver or gold). In practice, ground plane  330  is preferably formed by depositing the conductive material on the bottom surface of the dielectric substrate, while driver patch  310  is formed by depositing the conductive material on the top surface of the dielectric substrate.  
         [0032]     Suitable dimensions for the compact broadband patch antenna shown in  FIGS. 3A-3C  signals may be selected using electromagnetic simulation techniques of the type conventionally used by those of skill in the art in the design of patch antennas. Suitable 3D electromagnetic simulation software packages include CST Microwave Studio® by CST of America, Inc. and HFSS™ by Ansoft Corp.  
         [0033]      FIGS. 4 and 5  illustrate further embodiments of compact broadband patch antennae in accordance with the invention. In addition to the elements of antenna  300 , antenna  400  in  FIG. 4  further includes a parasitic patch  410 , mounted under a radome  405 . As in conventional stacked patch antennas, parasitic patch  410  resonates with the signal emitted by driver patch  310  and thereby improves the radiation characteristics of driver patch  310 .  
         [0034]     Parasitic patch  410  may be supported by a radome  405  (as in  FIG. 4 ) or by a dielectric material  505  (as in  FIG. 5 ). Radome  405  in  FIG. 4  is preferably a polycarbonate material that provides structural support to resonant patch  410  and physical protection to the broadband patch antenna  400 . Dielectric material  505  in  FIG. 5  is preferably dielectric foam but may alternatively be formed from other dielectric materials. Because the permittivity of foam tends to be low (e.g., ∈ FOAM ˜1), however, parasitic patch  410  may need to have a larger area than driver patch  310 , if foam is used to support resonant patch  410 .  
         [0035]      FIG. 6  illustrates a further embodiment of a broadband patch antenna as in  FIG. 3 , to which slots  610  and  620  have been added in the parasitic patch  410 , perpendicular to the direction of the electromagnetic field in the parasitic patch  410 . These slots  610  and  620  provide a longer current path for electrical currents in the parasitic patch  410 , thereby artificially increasing the electrical length of the current paths. Accordingly, the dimensions of the stacked patch antenna  400  may be made smaller without negatively impacting the antenna characteristics. Alternatively, a single slot may also be used.  
         [0036]      FIGS. 7 and 8  illustrate the manner in which the slotted broadband patch antenna of  FIG. 6  may be implemented in an antenna array. In general, the slots are preferably positioned perpendicular to the direction of the electrical field E—i.e., perpendicular to the antenna&#39;s E-plane and parallel to its H-plane. (The “E-plane” of an antenna is defmed as “[f] or a linearly polarized antenna, the plane containing the electric field vector and the direction of maximum radiation,” per IEEE Standard Definitions of Terms for Antennas, Std 145-1993. The “H-plane” lies orthogonal to the E-plane and may be defined as “For a linearly polarized antenna, the plane containing the magnetic field vector and the direction of maximum radiation.”)  
         [0037]     Thus, for example, in  FIG. 7 , where two broadband patch antennas  710  and  720  are located side-by-side and coupled in the H-plane in an antenna array, the slots of each broadband patch antenna should be aligned end-to-end, as shown, parallel to the direction of H-plane coupling. In contrast, in  FIG. 8 , where two broadband patch antennas  810  and  820  are located side-by-side and coupled in the E-plane, the slots for each broadband patch antenna should be placed in parallel as shown, perpendicular to the E-plane coupling.  
         [0038]     Advantageously, the use of slots in the resonant patch element and their arrangement perpendicular to the E-field results as shown in  FIGS. 6 through 8  greatly reduce the size of the patch and hence the mutual coupling between neighboring antenna elements, and thereby improve antenna gain response, radiation patterns, and scanning performance.  
         [0039]     The patch antenna in accordance with the present invention provides several advantages over existing patch antennas. In particular, a smaller antenna with better performance can be achieved. Moreover, because the patch antenna of the present invention does not require a high dielectric constant substrate to get a low resonant frequency, it has a very high efficiency and low mass.  
         [0040]     It should be understood that the foregoing is illustrative and not limiting and that obvious modifications may be made by those skilled in the art without departing from the spirit of the invention. Accordingly, the specification is intended to cover such alternatives, modifications, and equivalence as may be included within the spirit and scope of the invention as defined in the following claims.