Patent Publication Number: US-4929959-A

Title: Dual-polarized printed circuit antenna having its elements capacitively coupled to feedlines

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a dual-polarized printed circuit antenna whose elements are capacitively coupled to feedlines. More specifically, the invention relates to a printed circuit antenna employing dual-polarization geometry having feedlines and radiating elements stacked one above the other with feedlines which are capacitively coupled to the radiating elements, such that no RF interconnection is required. By employing electromagnetic coupling in the power transfer from the distribution networks to the radiating elements, a high-performance, light weight, compact, low-cost dual polarized planar or conformal antenna is achieved. 
     Examples of previous work in the field of printed circuit antennas employing capacitive coupling made be found in currently co-pending U.S. Pat. Applications Ser. No. 748,637, filed June 25, 1985, now U.S. Pat. No. 4,761,654 and Ser. No. 930,187, filed November 13, 1986. These applications disclose printed circuit antennas employing capacitive coupling and enabling either linear or circular polarization, depending on the shape of the radiating and feeding elements (which may be patches or slots) which are used. 
     As shown in FIG. 1a, the ground plane 10, feedline 12, and feeding patch 14 are capacitively coupled. Alternative structures, employing radiating slots 16b, are shown in FIGS. 1b and 1c as well. The resulting structure is a light weight, low-cost, singly-polarized planar or conformal antenna capable of operating with either linear or circular polarization. 
     One limitation of this structure is that the antenna constructed according to the techniques disclosed in these co-pending applications can receive only one sense of polarization, either linear or circular, from a satellite. It is desirable to have a compact antenna structure which is capable of receiving both senses of polarization, so that twice as much information can be received. 
     One technique for achieving this desired result involves the provision of a dual-polarized antenna structure. However, because of problems inherent in the interaction among the various radiating elements and the power dividers in different layers in such a structure, it has not previously been possible to provide such an antenna. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing deficiencies, it is an object of the present invention to provide a dual-polarized printed circuit antenna which has its elements capacitively coupled to feedlines, and which minimizes cross-coupling between the arrays. 
     It is a further object of the present invention to provide a printed circuit antenna which is capable of receiving both senses of polarization. 
     It is yet further object of the present invention to provide a dual-polarized antenna which does not require a direct probe to each radiating element for any of the senses of polarization provided in the antenna. 
     It is a still further object of the present invention to provide a dual-polarized printed circuit antenna which uses capacitive coupling from each power divider (corresponding to each polarization) to a respective radiating element. 
     In view of these objects, the present invention provides structure wherein two or more planar arrays of radiating elements are stacked one on top of the other, with appropriate numbers of power dividers disposed between consecutive layers of radiating elements. The power dividers may be disposed orthogonally with respect to each other, such that the antenna can receive two signals with opposite senses of polarization. The shape of the radiating elements may be such as to enable either linear or circular polarization to be achieved for each sense of polarization. Alternatively, a quadrature hybrid or other directional coupler may be employed with a dual polarized linear array to provide an equal power split and 90° phase between the two respective ports to develop a dual circularly polarized array. 
     The construction format of the present invention yields much lower dissipative loss than has been observed previously in most conventional flat planar arrays which incorporate a transmission medium such as microstrip. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention now will be described with reference to the accompanying drawings, in which: 
     FIGS. 1a-1c show cross-sections of structure for a known single-polarized antenna disclosed in the above-mentioned co-pending applications; 
     FIG. 2 shows a blown-up view of the dual-polarization geometry in the printed circuit antenna of the present invention; 
     FIGS. 3a-3l show examples of shapes of radiating elements which may be used in the antenna of FIG. 2 to achieve linear polarization; 
     FIGS. 4a-4f show examples of shapes of radiating elements which may be used in the array of the antenna of FIG. 2 to achieve circular polarization; 
     FIGS. 5a-5f show alternative structures for the feedline which feeds the radiating elements of the array of the antenna of FIG. 2; 
     FIG. 6 shows a view of a quadrature hybrid which may be used in conjunction with the inventive antenna to provide dual circlar polarization with a dual-polarized linear array; and 
     FIGS. 7-12 show examples of the results achieved with the antenna of the present invention. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     FIG. 2 is a blown-up depiction of an example of the dual-polarization geometry of the antenna of the present invention. Shown in the figure are ground plane 100, a first power divider 200, a first sheet of radiating elements 300, a second power divider 400 which may be disposed orthogonally to the first power divider, and a second sheet of radiating elements 500. The radiating elements on the sheets 300 and 500 may comprise patches or slots. Examples of suitably-shaped radiating elements are shown in FIGS. 3a-3l and 4a-4f. 
     The elements shown in the antenna of FIG. 2 are linear elements. These also may be used in a circularly-polarized array by means of a quadrature hybrid 250, which is shown in FIG. 6. Alternatively, the elements may be intrinsically circularly polarized and configured as shown in FIG. 4a-4f, wherein notches 18a or tabs 18b are provided on the elements. 
     In construction, the layers shown in FIG. 2 are appropriately spaced and stacked one over the other with no interconnects between the radiating elements. Spacing is in accordance with the wavelength of electromagnetic radiation λ which is being received. One such spacing may be, for example, λ/10; other spacings may be provided as appropriate, but of course would require different optimization of the elements in the various layers, as is known to those working in the relevant art. 
     All feeding to all the elements in the array of FIG. 2 is done by capacitive coupling. Essentially two arrays are formed. A first array having a first sense of polarization is formed by ground plane 100, power divider 200 having power divider elements 20, and element board 300. In this array, the layers 100 and 300 form the ground plane for the power dividers, and layer 300 also contains the printed radiating elements. 
     A second sense of polarization is formed by layers 300, 400 and 500, wherein the layers 300 and 500 provide the ground plane for the power divider 400 having power divider elements 20, and layer 500 contains the printed radiating elements. 
     The element designs on layers 300 and 500 are selected appropriately to minimize both radiation interaction between the lower and upper arrays, and cross-talk between the two power distribution networks. It should be pointed out that there tends to be a natural interaction between the networks in layers 200 and 400, shown in FIG. 2. The metal portion of the layer 300 thus acts as isolation to prevent the two networks from &#34;talking to each other&#34;, a phenomenon known as cross-talk. It is important to minimize cross-talk in order to maximize the independence of operation of the arrays. 
     To improve this isolation, the elements in the layer 300 may differ slightly from the elements in the layer 500. More specifically, additional metallization is provided along a line in each of the elements in the layer 300, so that the radiating slots 16a which are shown in the layer 300 essentially comprise two U-shaped slots. In the limit, the radiating slots may comprise two parallel slots, as shown in FIG. 3d. 
     Another consideration is that the size of the inner portion of the slots 16b in the elements of layer 500 affects how much energy is blocked to the bottom array. If the layer 500 has shapes that are too big, the first array comprising layers 100, 200 and 300 may not be able to &#34;see through&#34; the layers 400 and 500, so that those layers would not be transparent with respect to energy transmitted to that bottom array. 
     More specifically with respect to the particular construction of the elements in the layer 500, the squares in the slot 16b are similar to what is disclosed in copending application Ser. No. 930,187. The layer 300 also may have shapes similar to that in application Ser. No. 930,187, but as mentioned above, there is a little additional metallization as shown to form two U-shaped shapes out of the square. 
     Basically, the elements in the sheet 300 are essentially the same as those in the sheet 500 to start with. However, the lines in the power divider sheet 400 need metal underneath where those lines go underneath the elements in the sheet 500. Accordingly, part of the slot or layer in the elements 300 is covered up with metal, resulting in the two U-shaped pieces shown in FIG. 2. The dual-polarization geometry of the present invention enables the two arrays to operate substantially independently of each other. 
     The feedlines 12 which feed the radiating elements in the sheets 300 and 500 may have any suitable shape. For example, as shown in FIG. 5a-5c, the end of the feedline 12 which is capacitively coupled to a respective radiating element may be paddle shaped (FIG. 5a); wider at one end than at the other (FIG. 5b); or simply straight (FIG. 5c). 
     All the layers shown in FIG. 2 are separated by a suitable dielectric. Air presently is preferred as a dielectric, with a suitable honeycomb structure being provided among the layers to provide physical separation, as is well known to those of working skill. Polyethylene, Duroid™, nomex, or Teflon™ also may be used. However, it should be noted that, depending on the dielectric used, efficiency of the antenna could be degraded, as dielectrics tend to be lossy at microwave frequencies. 
     The operation of the dual-polarized array shown in FIG. 2 is as follows. As mentioned above, what is shown is dual linear polarization, which is dictated by the radiating elements. The two arrays of elements are fed orthogonally, such that one array will radiate either vertical or horizontal polarization, and the other array will radiate correspondingly horizontal or vertical polarization. One way of obtaining circular polarization was described above, with reference to FIG. 4a-4f. However, as shown in FIG. 6, it may be possible to achieve dual circular polarization by having a quadrature hybrid at the input of the array. A quadrature hybrid 250, as shown in FIG. 6, is essentially a directional coupler which is well-known in the art, and need not be described in detail here. However, it should be noted that the quadrature hybrid is connected to the arrays such that the two output ports of the hybrid feed the vertical and horizontal ports of the array, respectively. The input ports of the hybrid then would correspond to right-hand and left-hand polarization, respectively. Such a quadrature hybrid provides inherent isolation so as to allow both senses of polarization to operate simultaneously. The hybrid 250 may be implemented as an external component, or may be integrated directly into the array. 
     FIGS. 7-11 show results achieved with an example of the inventive dual polarized linear array employing 16 elements. FIG. 7 shows the input return loss for both senses of polarization. It should be noted that the figure shows very good input match over a broad band. 
     FIG. 8 shows the corresponding radiation gain for each polarization, and shows very efficient radiation over a broad band for both senses of polarization. The radiation efficiency of each of the arrays appears comparable. 
     FIG. 9 shows array network isolation. The two arrays are virtually decoupled, and operate as required in an independent manner, as shown in this graph. 
     FIGS. 10 and 11 show corresponding radiation patterns for each sense of polarization. The figures demonstrate the efficiency of the radiating array, and the low radiated cross-polarization. 
     FIG. 12 shows an example of the mapping of the dual-polarization linear to dual-polarization circular by a quadrature hybrid. To achieve the results shown in this figure, the 16 element array which was the subject of the experiment was converted to circular polarization by placing an external quadrature hybrid on the vertical and horizontal ports of the array. FIG. 12 shows the resultant measured axial ratio, and demonstrates that good circular polarized performance can be achieved over a large bandwidth. 
     It should be understood that although the data shown in FIGS. 7-12 was achieved for a specific frequency band, the invention is not so limited. Rather, what has been described as a dual-polarized antenna design that can be implemented at any frequency and for any size array, or for any number of elements. Thus, it should be understood that the invention is not to be limited by the description of the foregoing embodiment, but should be considered as limited only by the appended claims which follow immediately.