High-gain single- and dual-polarized antennas employing gridded printed-circuit elements
BACKGROUND OF THE INVENTION 
     The present invention relates to a printed antenna whose elements are capacitively coupled to feedlines. More specifically, the invention relates to a high aperture efficiency, high polarization purity antenna element is transparent to orthogonally polarized radiation. The invention is applicable to printed circuit antennas employing single-polarization and dual-polarization geometries. 
     Printed circuit antennas employing capacitive coupling are known. A single-polarization version of such an antenna is disclosed in U.S. application Ser. No. 748,637, filed June 25, 1985, now U.S. Pat. No. 4,761,654. In that application, the disclosure of which is incorporated herein by reference, both linear and circular polarization are achieved in an antenna which employs capacitive coupling between the feedlines the feeding elements, and also between the feeding elements radiating elements. 
     Copending application Ser. No. 930,187, filed Nov. 13, 1986, discloses an improvement on the techniques disclosed in the first-mentioned application. The contents of that application also are incorporated herein by reference. 
     A dual-polarized printed circuit antenna is disclosed in copending application Ser. No. 165,332, filed Mar. 8, 1988, entitled &#34;Dual-Polarized Printed-Circuit Antenna Employing Patches or Slots Capacitively Coupled to Feedlines&#34;, the named inventors being Robert M. Sorbello, John E. Effland, and Amir I. Zaghloul. In the last-mentioned application, a technique is provided wherein two senses of polarization, orthogonal to each other, may be achieved with appropriate isolation between the arrays of radiating elements. That application also is incorporated herein by reference. 
     An example of the dual polarization geometry is shown in FIG. 1, in which elements 1a and 1b are shown in element arrays 30, 50. These arrays are separated by an array of power dividers 40 which is associated with the element array 50, another power divider array 20 being associated with the element array 30. Details of implementation of this structure are provided in the last-mentioned copending application. 
     Previously-known configurations for radiating elements have employed either a patch or slot geometry, wherein circular or rectangular patches or slots, with or without perturbation segments provided thereon, may be provided. While the antennas as disclosed in the three above-mentioned applications have yielded good results over a relatively large bandwidth, the present inventors have discovered that it is possible to provide yet further improvement in performance. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, it is an object of the present invention to provide a radiating element for use in printed circuit antennas employing either single or dual polarization geometries, yielding improved performance over a wide bandwidth. 
     To this end, a radiating element of the present invention is formed based on a gridded structure which is transparent to one polarization while acting as a solid conducting plane to the other polarization. One form of this structure also makes the field distribution more uniform across the aperture. As a result, there is higher gain and a higher degree of isolation between the two orthogonal polarizations. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 of the application shows an example of dual polarization geometry with two arrays of radiating elements; 
     FIGS. 2a to 2d show examples of gridded structures in accordance with the present invention; 
     FIGS. 3a and 3b show examples of ungridded structures for array elements; and 
     FIG. 4 shows a dual polarization geometry similar to FIG. 1, but with the elements of FIGS. 2a and 2b; and 
     FIGS. 5 to 7 show the gain, polarization isolation, and port-to-port isolation of a 16-element array using the structure shown in FIG. 4. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As will be described in greater detail below, the arrays of gridded radiating elements of the present invention may be achieved by removing additional metal from the metallization layer from which the individual radiating slots are formed. Alternatively, some metal may be left in the slots selectively, so as to provide a gridded structure within the slots themselves. 
     As shown in FIG. 3a an ungridded radiating element 32 comprises a pair of parallel slots 32a surrounding a single interior metallization region 32b. Such an element has a cosine distribution function for the aperture electric field, having a maximum at the slot center and tapering to zero at the edges. 
     FIG. 2a differs from FIG. 3a in that two gridded regions 32c are provided. To form this structure, when the metallization is removed to form slots, the removal is more selective, so that thin metallization areas remain. By providing such structure, each wide slot 32a effectively is divided into an array of narrow slots 32a with thin metal regions 32b. The array of narrow slots has a more uniform distribution across the aperture, so as to increase the element gain. 
     The just-described structure may be implemented as shown in FIG. 4 in the lower element array 30. The gridding enables the element array 30 to appear more like a ground plane for the upper array, which operates in an orthogonal polarization, depending on the orientation of the feedline (e.g. 22, shown in outline) with respect to the grid structure of the radiating element (e.g. 32). 
     FIG. 2b shows an orthogonal feedline configuration which differs from FIG. 3a in that the radiating element 52 has an interior metal region 52b with additional metal removed, so that thin strips 52b are left. These strips are separated by spaces 52a where metallization has been removed. When used in a dual polarized array, the element in FIG. 2b is gridded in a direction orthogonal to the polarization of the second array and hence, is essentially transparent to this polarization. For example, FIG. 2b contains gridded elements that are orthogonal to the radiation associated with FIG. 3a. 
     Similarly, FIG. 3b shows an ungridded element 54 with interior metallization region 54a and continuous surrounding slot 54b, and FIG. 2c shows a corresponding gridded element with remaining metal strips 54a and intervening spaces 54b, again with the feedline parallel to the grid as in FIG. 2b. If such an element is used in the top layer 50 of radiating elements shown in FIG. 4, the element will radiate at a polarization perpendicular to that of layer 30, with the orthogonally polarized radiation of layer 30 propagating through without being attenuated. 
     FIG. 2d shows another example of grid structure which is essentially a gridded version of FIG. 2a. Such structure yields an element 56 with central metallization portions 56b and outer metallization portions 56b within the slot regions. The gridded portions appear electrically continuous to a polarized signal parallel to the grids, and transparent to signals orthogonal to the grid. 
     In the foregoing embodiments, the strip width and spacing should be a small fraction of a wavelength of received radiation. 
     Pursuant to the foregoing description, the structure of the dual-polarized structure yields orthogonally polarized radiating element arrays 30, 50 which are completely isolated from each other, with each array performing in the same manner whether operated alone or in the dual-polarization environment. This is so despite the fact that the two arrays use the same overall projected aperture area. FIGS. 5 to 7 show the superior gain, polarization isolation, and port-to-port isolation achieved in a 16-element array using the FIG. 4 structure. 
     As described in the last-mentioned copending application, for a dual-polarization configuration, it is desirable to have the radiating elements of one array be of a slightly different shape from the other array, in order to provide superior isolation of the arrays. However, with the grids in the respective arrays being mutually orthogonal according to the present invention, the shapes may be similar, and either regular (as shown in FIG. 2c) or non-regular (as shown in FIGS. 2a and 2b), so that when placed perpendicular to each other, superior gain and isolation characteristics may be achieved. 
     The elements shown in FIGS. 2a to 2d may be connected to the power dividers at a single feedpoint, as described in application Ser. No. application now U.S. Pat. No. 4,761,654, and Application Ser. No. 930,187, for a linearly polarized array. By connecting a quadrature hybrid at the input, the array may be operated so as to achieve dual circular polarization. An example of a quadrature hybrid is shown in FIG. 6 of the above-mentioned copending application Ser. No. 165,332, filed Mar. 8, 1988. 
     Also, while FIGS. 2a to 2d show generally rectangular- or square-shaped elements, it is considered to be within the scope of the invention that the techniques described may be applicable to elements of any arbitrary but predefined shape, such as a circular element or a rhomboid element. Thus, the invention is not to be considered as limited to the particular embodiments disclosed above, but rather is to be considered as limited only with respect to the scope of the appended claims which follow immediately.