Patent Publication Number: US-2012032869-A1

Title: Frequency scalable low profile broadband quad-fed patch element and array

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     FIELD OF THE INVENTION 
     The present disclosure relates to a patch antenna, and more particularly, to a patch antenna for transmitting and receiving circularly polarized radio signals for use in a broadband mobile communication system. 
     BACKGROUND OF THE INVENTION 
     In wireless communication systems, a radiofrequency signal is created or received by generating or receiving an electromagnetic field in an antenna. Depending upon the requirements of the communication system, the antenna may be formed in one of a plurality of configurations. 
     In communications systems utilizing satellites, the polarization of the antenna is a significant consideration. Linear polarization requires that a receiving station tightly align its frame of reference with that of a satellite in order to achieve acceptable communications. In particular, signal reception may be affected by relative movement of the transmitting station (i.e. the satellite) with respect to the receiving station, or by differences in orientation between the transmitting and receiving station antennas. Additionally, as linearly polarized signals propagate through the atmosphere, the signals tend to rotate, changing the signal orientation and making it difficult to maintain alignment between the transmitter and the receiver. As a result, satellite communications usually require antennas that transmit and receive circularly polarized signals. Circular polarization of the signals minimizes the geometric and atmospheric effects. However, to achieve satisfactory communication, the degree of circular polarization as measured by axial ratio should be relatively high over a relatively broad bandwidth. 
     Patch antennas are relatively inexpensive to manufacture, and have certain desirable characteristics (such as ease of manufacture, simplicity, small size, and low weight) but patch antennas are inherently linearly polarized, which results in at least a 3 dB gain loss for circularly polarized signals. Nevertheless, patch antennas that transmit and/or receive circularly polarized signals, as opposed to linearly polarized signals, are particularly useful in satellite communication systems due to their desirable characteristics. 
     Several methods are known to obtain circular polarization from a square or rectangular patch antenna. In one method, a single feed is placed at the corner of an approximately square patch. In a second method, a corner truncated square patch is provided with a single feed located at the edge midpoint. In a third method, a slotted square patch with a small slot located at its center along the diagonal may be provided with a single feed along the edge midpoint. These single feed methods are relatively simple and may be moved inboard to change the input impedance seen by the patch. 
     It is also known to provide two feeds to create circular polarization by placing the feeds at the midpoints of adjacent edges, or at adjacent corners of the patch edge, thereby separating the feeds physically by 90 degrees. It is also known to provide signals to the electronic feeds located on adjacent edges or corners that are 90 degrees out of phase with respect to each other, known generally as phase quadrature, to obtain narrow-band circularly polarized signals. However, the circular polarization quality, as measured by axial ratio, degrades over a large bandwidth. 
     In certain frequency band having a wideband frequency range, such as for certain global positioning arrangements, for communications on the move, or for certain military uses, it is desirable that an antenna be able to transmit and receive high quality circularly polarized signals having a relatively large bandwidth (i.e. broadband) and having a relatively large beamwidth. As a non-limiting example, certain communication systems that enable communications on the move may implement various code division multiple access (CDMA) protocols over a given frequency band to provide mobile users with voice, data, and video communication beyond line of sight and while in motion. The frequency band utilized may range from higher frequency GHz bands to lower frequency UHF bands to provide communications to users in environments, such as heavily-forested areas, where higher frequency signals are heavily attenuated. Communications on the move also require a high quality circularly polarized signal having an axial ratio below about 2.0 dB over a relatively large bandwidth, on the order of 150 MHz or more. 
     It is therefore desirable to provide a patch antenna having a low axial ratio over an extended bandwidth (broadband) that is easy and inexpensive to fabricate, and is relatively small and lightweight. 
     SUMMARY OF THE INVENTION 
     Compatible and attuned with the present invention, a patch antenna having a low axial ratio over an extended bandwidth that is easy and inexpensive to fabricate, and is relatively small and lightweight has been surprisingly discovered. 
     A quad-fed patch antenna comprises a ground plane having an upper surface substantially within a first plane, and a substantially tetragonal radiating patch spaced from the ground plane. An upper surface of the substantially tetragonal radiating patch includes a first side, a second side, a third side, and a fourth side defining a first corner, a second corner, a third corner, and a fourth corner located within a second plane substantially parallel to the first plane. A first signal feed is associated with the first side of the substantially tetragonal radiating patch. A second signal feed is associated with the second side of the substantially tetragonal radiating patch. A third signal feed is associated with the third side of the substantially tetragonal radiating patch. And a fourth signal feed is associated with the fourth side of the substantially tetragonal radiating patch. The first signal feed, the second signal feed, the third signal feed and the fourth signal feed are fed progressively in phase quadrature. 
     In one embodiment, the substantially tetragonal radiating patch is substantially square. In another embodiment, the first signal feed is attached to the first corner, the second signal feed is attached to the second corner, the third signal feed is attached at the third corner, and the fourth signal feed is attached at the fourth corner. The signal feeds may be arranged to transceive clockwise circularly polarized signals, or the signal feeds may be arranged to transceive counter-clockwise circularly polarized signals. 
     A plurality of the substantially coplanar tetragonal radiating patches may be arranged in a square tiled configuration to produce an array antenna wherein four of the substantially coplanar tetragonal radiating patches are located proximate each vertex. The first signal feeds are received substantially simultaneously by the plurality of substantially coplanar tetragonal radiating patches, the second signal feeds are received substantially simultaneously by the plurality of substantially coplanar tetragonal radiating patches, the third signal feeds are received substantially simultaneously by the plurality of substantially coplanar tetragonal radiating patches, and the fourth signal feeds are received substantially simultaneously by the plurality of substantially coplanar tetragonal radiating patches. Additionally, the first signal feeds, the second signal feeds, the third signal feeds, and the fourth signal feeds are fed progressively in phase quadrature. The first corners of each substantially coplanar tetragonal radiating patch, the second corners of each substantially coplanar tetragonal radiating patch, the third corners of each substantially coplanar tetragonal radiating patch, and the fourth corners of each substantially coplanar tetragonal radiating patch are geometrically oriented to be in the same location on each of the plurality of substantially coplanar tetragonal radiating patches. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above, as well as other advantages of the present invention, will become readily apparent to those skilled in the art from the following detailed description of the preferred embodiment when considered in the light of the accompanying drawings in which: 
         FIG. 1  is schematic top perspective view of a single patch antenna according to the present disclosure; 
         FIG. 2A  is a graph of axial ratio versus frequency for an exemplary square patch antenna having a single feed at a first corner according to the prior art; 
         FIG. 2B  is a graph of axial ratio versus frequency for an exemplary square patch antenna having a dual feed at a first and a second corner according to the prior art; 
         FIG. 2C  is a graph of axial ratio versus frequency for an exemplary square patch antenna having a dual feed at a first and a third corner; 
         FIG. 2D  is a graph of axial ratio versus frequency for an exemplary square patch antenna having a quad feed at a first corner, a second corner, a third corner, and a fourth corner, constructed and operated according to the present disclosure; 
         FIG. 3  is a schematic top perspective view of an antenna array including a plurality of patch antennas according to the present disclosure; and 
         FIG. 4  is an enlarged schematic top perspective view of the antenna array of  FIG. 3  taken within circle  4  showing a slot intersection between four adjacent patches according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION 
     The following detailed description and appended drawings describe and illustrate various embodiments of the invention. The description and drawings serve to enable one skilled in the art to make and use the invention, and are not intended to limit the scope of the invention in any manner. 
       FIG. 1  shows a quad-fed patch antenna  10  according to one embodiment of the present disclosure. A generally tetragonal patch  12  is substantially parallel to and spaced from a ground plane  14  that lies within a first plane. It is understood that the patch  12  may have other four-sided shapes, but a substantially square patch  12  will maximize the performance of the patch antenna  10 . The patch  12  includes a first side  16   a , a second side  16   b , a third side  16   c , and a fourth side  16   d . The sides  16   a ,  16   b ,  16   c ,  16   d  cooperate to define a first corner  18   a , a second corner  18   b , a third corner  18   c , and a fourth corner  18   d . The patch  12  is substantially flat so that an upper surface  26  of the patch  12 , including the corners  18   a ,  18   b ,  18   c ,  18   d , lies substantially within a second plane. The patch  12  may have any dimensions capable of broadcasting in any frequency range, and may further be formed as a radiating layer on top of a dielectric layer (not shown). The space between the patch  12  and the ground plane  14  may further be filled partially or completely with a dielectric material (including air), as is known in the art. Spacers  24  may be used to affix the patch  12  to the ground plane  14  while maintaining a desired separation therebetween. 
     In one embodiment, the patch  12  includes four progressively fed signal feeds  20   a ,  20   b  (not shown),  20   c ,  20   d  associated with the respective sides  16   a ,  16   b ,  16   c ,  16   d , resulting in a quad-fed arrangement. The progressively fed signal feeds  20   a ,  20   b ,  20   c ,  20   d  may be attached to the midpoints of the respective sides  16   a ,  16   b ,  16   c ,  16   d  to locate each of the progressively fed signal feeds  20   a ,  20   b ,  20   c ,  20   d  physically 90° out of phase with the preceding signal feed. However, it is often easier to achieve 90° of physical separation between adjacent signal feeds  20   a ,  20   b ,  20   c ,  20   d  by attaching the signal feeds at the corners of the patch. Thus, in another embodiment and as shown in  FIG. 1 , the four progressively fed signal feeds  20   a ,  20   b ,  20   c ,  20   d  are attached to the respective corners  18   a ,  18   b ,  18   c ,  18   d , resulting in a quad-fed arrangement of the patch  12 . Additionally, the progressively fed signal feeds  20   a ,  20   b ,  20   c ,  20   d  provide a signal in phase quadrature, which requires that once the signal fed to the first corner  18   a  is defined by the feed system, the signal fed to the second corner  18   b  lags the signal fed to the first corner  18   a  by 90°, the signal fed to the third corner  18   c  lags the signal fed to the second corner  18   b  by 90° and lags the signal fed to the first corner  18   a  by 180°, and the signal fed to the fourth corner  18   d  lags the signal fed to the third corner  18   c  by 90° and lags the signal fed to the second corner  18   b  by 180° and lags the signal fed to the first corner  18   a  by 270°. In  FIG. 1 , the phase lag of the feed signals is shown at each corner  18   a ,  18   b ,  18   c ,  18   d  with respect to the signal received at the first corner  18   a  of the patch  12 , and the progressively fed signal feeds  20   a ,  20   b ,  20   c ,  20   d  are aligned to provide clockwise circular polarization to the signal. It is understood that the progressively fed signal feeds  20   a ,  20   b ,  20   c ,  20   d  may also be aligned to provide counter-clockwise circular polarization. It is further understood that the feed system providing the signals to each corner  18   a ,  18   b ,  18   c ,  18   d  may be any known feed system capable of providing the required phase delays, including waveguides, hybrid systems, L probes, proximity aperture coupling systems, microstrip feed systems, or the like. 
     The quad-fed patch antenna  10  of  FIG. 1  provides unexpectedly robust circular polarization, as measured by axial ratio. Axial ratio is an important parameter used to measure the quality of the circular polarization. Pure circular would have an axial ratio of 0 dB (i.e. 1) where the orthogonally polarized waves have equal amplitude and 90° phase difference. When the orthogonal components are misaligned the circular polarization tends to an ellipse where the axial ratio is the ratio of the major to minor axis. Therefore, it is desirable to keep the axial ratio as close to 0 dB as possible. Axial ratio, typically measured in dB, measures the differences between the maximum and the minimum peaks of polarization as a circularly polarized wave rotates through 360°. If the axial ratio is near 0 dB, a very small difference between the maximum and the minimum peaks of polarization exists. If the axial ratio is exceeds about 2 dB, the polarization is often referred to as elliptical. As noted previously hereinabove, certain mobile communications systems require that the axial ratio remain below 2 dB over a relatively large bandwidth, on the order of 150 MHz or more. 
     As a non-limiting example, a sample patch antenna representative of the patch antenna  10  of  FIG. 1  was built and tested for use in the UHF band. The sample patch antenna  10  included a patch  12  formed of a substantially square copper foil approximately 40 cm (approximately 15.75 inches) in length, approximately 40 cm (approximately 15.75 inches) in width, and having a thickness of about 0.5 mm (about 0.020 inches), and formed on a dielectric substrate having a thickness of about 1.5875 mm (about 0.0625 inches). The patch  12  was spaced from the ground plane  14  by about 5 cm (about 2 inches). It is understood that the physical dimensions of the antenna may be varied or reduced to customize the antenna to any particular frequency range, including UHF, L-band, S-band, C-band, X-band, Ku-band, K-band, Ka-band, or the like. The axial ratio of the sample patch antenna  10  constructed as described was subjected to testing across a 250 MHz frequency band (from 250 to 500 MHz in the UHF band).  FIG. 2A  shows the axial ratio across the tested 250 MHz frequency band where the sample patch antenna  10  is fed only at the first corner  18   a . When fed at a single corner  18   a , the sample patch antenna  10  provides linear polarization only with high axial ratio values that are never less than about 5.0 dB. 
       FIG. 2B  shows the axial ratio across the same 250 MHz frequency band where the sample patch antenna  10  is progressively fed at the first corner  18   a  and at the second corner  18   b , where the dual feeds are offset by 90° in phase quadrature. The axial ratio was measured to be less than 2 dB only between about 288 MHz and 319 MHz, for a narrowband acceptable bandwidth of only 31 MHz, or about 12% of the tested bandwidth. 
       FIG. 2C  shows the axial ratio across the measured 250 MHz frequency band where the sample patch antenna  10  is progressively fed at the first corner  18   a  and at the third corner  18   c , where the dual feeds are phase offset by 180°. Similar to  FIG. 2   a , the sample patch antenna  10  provides linear polarization only, with extremely high axial ratio values that are never less than about 30 dB. 
     Finally,  FIG. 2D  shows the axial ratio across the measured 250 MHz frequency band where the sample patch antenna  10  is progressively fed according to the present disclosure at all four corners  18   a ,  18   b ,  18   c ,  18   d  in phase quadrature, where each feed is phase offset by 90° from the preceding corner as shown in  FIG. 1 . Unexpectedly, the sample quad-fed patch antenna  10  provides an acceptable axial ratio of less than about 2.0 dB across the entire 250 MHz frequency test band, resulting in a broadband capability. Even more unexpectedly, the quad-fed patch antenna  10  provides a broadband and extremely high quality circular polarization having an axial ratio less than about 1.0 dB between 250 MHz and 445 MHz. 
     Advantageously and regardless of the targeted frequency band, the bandwidth of a single quad-fed patch antenna  10  of  FIG. 1  may be further broadened by constructing an antenna array from a plurality of closely spaced quad-fed patch antennas. A representative antenna array  100  including four quad-fed patches  112 ,  212 ,  312 ,  412  arranged to develop clockwise circular polarization is shown in  FIG. 3 . It is understood that an antenna array  100  may have any number of quad-fed patches, and may be formed into any desired overall size and shape to achieve substantially any signal frequency, shape, and polarization, including counter-clockwise circular polarization. Favorable results have been obtained when the quad-fed patches are arranged in square tiling or a square grid, where four substantially square patches are arranged around every vertex. 
     In  FIG. 3 , the four substantially coplanar quad-fed patches  112 ,  212 ,  312 ,  412  are substantially equally spaced from a ground plane  114 . The patches  112 ,  212 ,  312 ,  412  are arbitrarily numbered and will be described starting from the lower left and proceeding clockwise, but it is understood that the following description and features are not dependent upon the location of the patches  112 ,  212 ,  312 ,  412 . 
     The first quad-fed patch  112  includes four sides  116   a ,  116   b ,  116   c ,  116   d  that define four corners  118   a ,  118   b ,  118   c ,  118   d . The second quad-fed patch  212  includes four sides  216   a ,  216   b ,  216   c ,  216   d  that define four corners  218   a ,  218   b ,  218   c ,  218   d . The third quad-fed patch  312  includes four sides  316   a ,  316   b ,  316   c ,  316   d  that define four corners  318   a ,  318   b ,  318   c ,  318   d . And the fourth quad-fed patch includes four sides  416   a ,  416   b ,  416   c ,  416   d  that define four corners  418   a ,  418   b ,  418   c ,  418   d.    
     For each of the patches  112 ,  212 ,  312 ,  412 , the location of each of the respective first corners  118   a ,  218   a ,  318   a ,  418   a  and first sides  116   a ,  216   a ,  316   a ,  416   a  may be arbitrarily chosen. However, the patches  112 ,  212 ,  312 ,  412  are oriented so that the first sides  116   a ,  216   a ,  316   a ,  416   a  are substantially parallel. Similarly, the second sides  116   b ,  216   b ,  316   b ,  416   b  are substantially parallel, as are the third sides  116   c ,  216   c ,  316   c ,  416   e  and the fourth sides  116   d ,  216   d ,  316   d ,  416   d . Additionally, the patches  112 ,  212 ,  312 ,  412  are geometrically oriented so that the first corners  118   a ,  218   a ,  318   a ,  418   a  are in the same location on each of the respective patches  112 ,  212 ,  312 ,  412 . In  FIG. 3 , the first corners  118   a ,  218   a ,  318   a ,  418   a  are shown to be the lower left hand corner of each of the respective patches  112 ,  212 ,  312 ,  412 . Subsequent corners of each of the patches  112 ,  212 ,  312 ,  412  are numbered in a clockwise direction because the antenna array  100  in  FIG. 3  is arranged to transceive signals having primarily clockwise circular polarization, as described hereinbelow. It is understood that subsequent corners may be numbered in a counter-clockwise direction to obtain counter-clockwise circular polarization of transceived signals according to the description hereinbelow. 
     A progressive feed system (not shown) is attached to each of the corners of each of the patches  112 ,  212 ,  312 ,  412  so that each of the respective first corners  118   a ,  218   a ,  318   a ,  418   a , second corners  118   b ,  218   b ,  318   b ,  418   b , third corners  118   c ,  218   c ,  318   e ,  418   c , and fourth corners  118   d ,  218   d ,  318   d ,  418   d  is progressively and substantially simultaneously fed 90° out of phase with the preceding corner. Thus, in one embodiment, each of the first corners  118   a ,  218   a ,  318   a ,  418   a  is fed simultaneously. Thereafter, each of the second corners  118   b ,  218   b ,  318   b ,  418   b  is simultaneously fed 90° out of phase with the first corners  118   a ,  218   a ,  318   a ,  418   a . The third corners  118   c ,  218   c ,  318   c ,  418   c  are then progressively fed 180° out of phase with the first corners  118   a ,  218   a ,  318   a ,  418   a , and are fed 90° out of phase with the second corners  118   b ,  218   b ,  318   b ,  418   b . The fourth corners  118   d ,  218   d ,  318   d ,  418   d  are then progressively fed 270° out of phase with the first corners  118   a ,  218   a ,  318   a ,  418   a , are fed 180° out of phase with the second corners  118   b ,  218   b ,  318   b ,  418   b , and are fed 90° out of phase with the third corners  118   c ,  218   c ,  318   c ,  418   c . The patch antenna array  100  of the present invention may have any shape or number of patch elements, as desired, and any number of the patch antennas may be actively fed, as desired. In one embodiment, in an antenna array  100  having a plurality of patches  112 ,  212 ,  312 ,  412 , all of the patches  112 ,  212 ,  312 ,  412  receive the progressively fed signal feeds in phase quadrature. In another embodiment, the feed system may be controlled to feed less than all of the patches  112 ,  212 ,  312 ,  412 , depending upon the desired signal shape and gain. As a non-limiting example, a first number of the patches may actively transceive signals, while a second number of the patches may be included as replacements for any active patch in the event that the active patch ceases operation for any reason. 
     As noted above, the patches  112 ,  212 ,  312 ,  412  are arranged as a square tiling substantially within in the same plane equally spaced from the ground plane  114 . Each of the patches  112 ,  212 ,  312 ,  412  is spaced from adjacent patches by a predetermined gap G. Each of the patches  112 ,  212 ,  312 ,  412  is therefore electrically isolated from the adjacent patches. The gap G is typically on the order of one to two percent of a wavelength so that the array  100  resembles a slot antenna configuration having a vertex or slot intersection  500 , which further increases the bandwidth of the antenna array  100 . However, it is understood that other gap distances G may be used as desired. 
     An enlarged view of the slot intersection  500  between the four adjacent patches  112 ,  212 ,  312 ,  412  of  FIG. 3  is shown in  FIG. 4 . The slot intersection  500  is formed at the vertex of the square tiled patches  112 ,  212 ,  312 ,  412 , and in particular by the third corner  118   c  of the first patch  112 , the fourth corner  218   d  of the second patch  212 , the first corner  318   a  of the third patch  312 , and the second corner  418   b  of the fourth patch  412 . To facilitate attachment of the feed systems (not shown), the various corners of the patches  112 ,  212 ,  312 ,  412  may be beveled. For example, as shown in  FIG. 4 , the fourth corner  218   d  of the second patch  212  is beveled to form an attachment edge  222   d  for receiving a progressive signal feed  220   d . Similarly, the second corner  418   b  of the fourth patch  412  includes a beveled edge  422   b  for receiving a progressive signal feed  420   d . The third corner  118   c  of the first patch  112  and the first corner  318   a  of the third patch  312  are shown without beveling. However, it is understood that any of the corners may be beveled to facilitate attachment of the progressive signal feeds to the corners. 
     Advantageously, because of the square filing geometric arrangement of the substantially square patches  112 ,  212 ,  312 ,  412 , and because of the progressive signal feeds to each respective corner of the substantially square patches  112 ,  212 ,  312 ,  412 , each corner that comprises the slot intersection  500  is progressively fed 90° out of phase with the preceding corner, commencing with the first corner  318   a  of the third patch. The four corners  318   a ,  418   b ,  118   c ,  218   d  are thus physically and electrically 90° out of phase with respect to each other. Because the four corners  318   a ,  418   b ,  118   c ,  218   d  are progressively fed in phase quadrature, the antenna at the slot intersection  500  approximates that of a crossed dipole element, wherein the corners  318   a  and  118   c  form a first dipole, and the corners  218   d  and  418   b  form a second dipole orthogonal to the first dipole, thereby further enhancing the bandwidth of the patch antenna array  100  while maintaining good circular polarization. 
     The quad-fed patch antenna  10  and the quad-fed patch antenna array  100  of the present invention provide extremely robust circular polarization having an axial ratio of less than about 2.0 dB across a very large bandwidth, greater than 150 MHz. Unlike single feed (such as demonstrated with reference to  FIG. 2A ) or dual feed patch antennas (such as demonstrated with reference to  FIG. 2B  or  FIG. 2C ), the quad-fed patch antenna of the present disclosure is not bandwidth limited due to the poor circular polarization, and therefore does not encounter a gain loss due to poor circular polarization. 
     While certain representative embodiments and details have been shown for purposes of illustrating the invention, it will be apparent to those skilled in the art that various changes may be made without departing from the scope of the disclosure, which is further described in the following appended claims.