Planar array antenna

Abstract: A planar array antenna comprises an upper plate (34) having a plurality of holes, a lower plate (30), and a circuit board (32) having printed patterns of a plurality of array elements (32′) and located between the upper plate (34) and the lower plate (30), wherein the lower plate (30) has concave regions (30′) formed at positions corresponding to the positions of the plurality of holes of the upper plate (34).

Description: This invention relates to planar array antennae for use in, for example, receiving satellite broadcasts. In a suspended line feed type planar microwave antenna in which a substrate is sandwiched between metal or metallized plastics plates having a number of openings forming parts of radiation elements, a circular polarized wave planar array antenna has been proposed. In this previously-proposed antenna, a pair of excitation probes which are perpendicular to each other, the number of which corresponds to the number of openings, are formed on a common plane and signals fed to the pair of excitation probes are mixed in phase within the suspended line. Thus, the above-mentioned planar antenna can be reduced in thickness as compared with the existing one, and also its mechanical configuration can be simplified. Moreover, an inexpensive substrate now available on the market can be employed for high frequency use, achieving antenna gain equal to or greater than that of a planar antenna using an expensive mircostrip line substrate. The suspended line achieves such advantages in that it for: a low loss line as a circuit for feeding the planar antenna, and also in that it can be formed on an inexpensive film shaped substrate, and so on. Since this planar antenna utilizes a circular or rectangular waveguide opening element as a radiation element, it is possible to construct an array antenna which has a small gain deviation over a relatively wide frequency range. A so-called patch-slot array antenna has been proposed, which effectively utilizes features of the suspended line and thin radiation elements to provide high efficiency and wide bandwidth. Also, this type of array antenna can be reduced in thickness and weight (see our european patent application EP-A-301580). In the suspended line feed-type planar array antenna in which the substrate is sandwiched between the pair-of metal or metallized plastics plates, a number of resonance type printed patch radiators are formed on the substrate at positions corresponding to openings formed through one of the metal or metallized plastics plates. However, the planar array antenna described in the above patent application has flanges, formed around a number of resonance type printed patch radiators thereof, as supporting portions, so that difficult cutting work cannot be avoided, which makes the efficient mass-production of the antenna impossible. Also, this makes the antenna expensive. In order to solve the aforenoted problems, a suspended line feed type planar array antenna has been proposed (see our european patent application EP-A-312 989), in which a substrate is sandwiched between an upper plate having a number of openings and a lower plate opposing the upper plate. Specifically, in this previously-proposed suspended line feed type planar array antenna, protrusions are formed on the upper and lower plates at their corresponding positions by a press-treatment, and the substrate is supported by these protrusions. With this antenna, difficult cutting work is not needed, and only the simple press-treatment is required, which permits efficient mass production. This can also make the antenna inexpensive. Figure 1 shows a circuit arrangement in which a plurality of circular polarized wave radiation elements fed in phase by a suspended line, form the array. In that case, the circular polarized wave radiation elements are such as described in european patent application EP-A-312 989. The solid line in Figure 2 illustrates that a second metal plate 2 covers the top of the arrangement shown in Figure 1. A plurality of protrusions 11 are formed on a first metal plate 1 between conductive foils 8 and the suspended lines, in order to support a substrate 3. The protrusions 11 are further provided on the first metal plate 1 around the outer peripheral portion of the planar array antenna, as shown. Other portions of the first metal plate 1 form cavity portions 7. There is then the substantial risk that the outputs from the plurality of conductive foils 8 may be supplied through the same cavity portion 7 and hence the above-mentioned outputs will be coupled with each other. If, however, the spacing between the neighbouring conductive foils 8 and the spacing between the upper and lower walls of the cavity portions 7 are properly selected, necessary isolation can be established, this eliminating the risk of mutual coupling. Since the electric lines of force are concentrated on the upper and lower walls of each cavity portion 7, the electric field along the substrate 3 supporting the conductive foils 8 is substantially removed, thus lowering the dielectric loss. As a result, the transmission loss of the line is reduced. Protrusions and cavity portions are also formed on the second metal plate 2 in correspondence with those of the first metal plate. More specifically, protrusions 12 are formed on the second metal plate 2 around slots 5 therethrough, and around the periphery of the feeding portion positions between the conductive foils 8 and the suspended lines to support the substrate 3, while other portions between the protrusions form cavity portions 7. Since the substrate 3 is uniformly supported by the protrusions 11 and 12, the substrate 3 can be prevented from being warped downwardly. In addition, since the metal plates 1 and 2 are brought into face-to-face contact with the substrate 3 around the respective radiation elements, the feeding portions and so on as described above, it is possible to prevent any resonance at a particular frequency from being caused. Referring to Figure 1, sixteen radiation elements are arranged in groups of four, to provide four radiation element groups G₁ to G₄. A junction P₁ of the suspended lines in each group is displaced from the centre point of the group by a length λg/2 (λg represents the line wavelength at the centre frequency). Junctions P₂ and P₃ in the suspended lines feeding two radiation elements in each group are connected with a displacement of each of λg/4 from the centre point between these two. Accordingly, in each group of the radiation elements, the lower-right-hand radiation element is displaced in phase from the upper-right-hand radiation element by 90 degrees, the lower-left-hand radiation element is displaced therefrom by 180 degrees and the upper-left-hand radiation element is displaced therefrom by 270 degrees, respectively, which results in the axial ratio being improved. In other words, the axial ratio can be made wide by varying the spatial phase and the phase of the feeding line. In another aspect, any two of vertically or horizontally neighbouring patch radiators have slit directions 90 degrees apart from each other. The junction P₁ in each group and the junctions P₄ to P₆ in the suspended lines feeding the respective groups are coupled to one another in such a fashion that they are distant from the feeding point 10 of a feeding portion 9 by an equal distance. That is, it is possible to obtain various kinds of directivity characteristics, by changing the feeding phase and the power distribution ratio, by changing the positions of the junction P₁ and the junctions P₄ to P₆. In other words, the feeding phase is changed by varying the distances from the feeding point 10 to the junctions PI and to the junctions P₄ to P₆, and the amplitude is varied by varying the impedance ratio by increasing or decreasing the thickness of the lines forming the various branches of the suspended line, whereby the directivity characteristics can be varied over a wide range. In a method in which the substrate is supported by a number of protrusions as shown in Figure 1, the protrusions are formed on the pair of metal plates between the conductive foils, and the patch slot type resonance print elements deposited on the substrate are coaxial with the slots and the suspended lines, so that no problem will arise in a portion where the protrusions are concentrated to some degree. However, in a portion where the protrusions are formed poorly, the substrate cannot be uniformly supported at its intermediate portion. Thus, the positional displacement of the substrate is slackened. There is then the substantial risk that the printed radiation element will touch the metal plate. As a result, there is the substantial disadvantage that deterioration of antenna characteristic such as decrease of antenna gain will occur. Furthermore, since a number of protrusions have to be formed in correspondence on the pair of plates, the number of manufacturing processes for manufacturing the plates is increased, and the productivity is relatively poor. Therefore, in a suspended line feed type planar antenna in which a substrate is sandwiched between an upper plate having a number of openings and a lower plate opposing the upper plate, spacers or distance pieces having a number of corresponding openings are provided between the upper plate and the substrate and between the substrate and the lower plate, respectively thereby supporting the substrate. Thus, the substrate can be positively supported at the intermediate portion between the upper and lower plates with a uniform distance therebetween. As a result, the protrusions formed on the upper and lower plates can be reduced considerably, which makes the manufacturing processes for the upper and lower plates simple, and can increase the productivity (see Japanese patent application no. JP-A-63199513. Figure 3 shows in cross section the structure of a planar array antenna, described in Japanese patent application no. JP-A-63199513, and comprising a rear cover 20, a lower plate 21, a distance piece or spacer 22, a film substrate 23 on which a number of resonance type printed patch radiators (radiation elements) 23′ are printed, a distance piece or spacer 24, an upper plate 25, a support cushion 26 made of low foaming styrol and a radome 27. In that case, the rear cover 20 is 3 mm in thickness, the plates 21 and 25 and the spacers 22 and 24 are 1 mm in thickness, the support cushion 26 is 12 to 14 mm in thickness, and the radome 27 is 1 mm in thickness. The entire thickness of this planar array antenna is about 20 to 22 mm. This previously-proposed planar array antenna shown in Figure 3 cannot avoid the following defects: (1) Since the distance between the radiator 23, and the lower plate 21 provided as the ground plate is 1 mm, the change of element impedance and the ratio in which the operational gain is changed are made large due to slackening of the film substrate 23. (2) Since the distance between the lower plate 21 and the upper plate 25 is 2mm, the feed line loss is large. For example, when the line width was 1.5 mm at a frequency of 12 GHz and the characteristic impedance Z₀ of the line was selected to be 76 ohms the feed line loss was 1.6 to 1.8 dB/m. (3) The element gain is small (about 6.5 dB). (4) The impedance matching band width of the elements is narrow. (5) Since the resonance type printed patch radiator is of the type for feeding at one feed point, the circularly polarized wave band is narrow and the pair of four elements must be fed with a phase difference therebetween. (6) Because of the disadvantages (4) and (5), the excitation balance of elements cannot be made without difficulty. According to the present invention there is provided a planar array antenna comprising: an upper plate having a plurality of holes; a lower plate; and a circuit board located between said upper plate and said lower plate and having printed patterns of a plurality of array elements thereon in alignment with the holes in the upper plate, the circuit board being separated from the upper and lower plates by respective spacers having openings therein corresponding to the holes in the upper plate, wherein said lower plate has concave regions formed therein in alignment with and of a size corresponding to the holes in the upper plate. Embodiments of the present invention can provide a microwave planar array antenna in which various characteristics such as element gain, impedance matching band widths of elements, and excitation balance can be improved while the decreased thickness thereof can be maintained. The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: Figure 1 is a plan view of a feed circuit of a previously-proposed antenna; Figure 2 is a cross-sectional view taken along line II-II in Figure 1; Figure 3 is a cross-sectional view of another previously-proposed antenna; Figure 4 is a graph of a return loss versus frequency characteristic of a previously-proposed antenna; Figure 5 is a cross-sectional view illustrating a first embodiment of planar array antenna according to the present invention; Figure 6 is a plan view of a feed circuit of the first embodiment; Figure 7 is schematic diagram showing a portion of the first embodiment; Figure 8 is a schematic diagram showing a portion of a second embodiment of planar array antenna according to the present invention; and Figure 9 is a graph of return loss versus frequency characteristics of an embodiment. Referring to Figure 5, the first embodiment comprises a lower plate 30 made of a metal or metallized plastics plate, a spacer or distance piece 31 made of dielectric high foaming material having low dielectric ratio and low loss such as polyethylene, polypropylene or polystyrol, and a film substrate (circuit board) 32. On the film substrate 32 there are formed by a printing process a number of resonance type printed patch radiators (radiation elements) 32′, shown in Figure 6. Figure 6 shows a circuit arrangement of a feeding circuit by which a plurality of circular polarized radiation elements forming an array are co-phase fed by suspended lines. The patterns are connected so as to obtain a green circular polarization of the electromagnetic wave. While the diameter of the radiation element of Figure 1 is selected to be 12 mm, the diameter of a radiator 32 of the embodiment of Figure 6 is 9.6 nm. In this embodiment, the radiators 32′ are arranged in pairs, with the members of the pairs oriented at a right angle to each other, are fed at different phases so that parameters are reduced thereby. From a characteristic standpoint, this is advantageous in that excitation balance of elements can be achieved with ease. Turning back to Figure 5, the embodiment also comprises a spacer or distance piece 33 similar to the spacer 31, an upper plate 34 of thin plate type configuration formed of a metal or metallized plastics plate, a support cushion 35 made of, for example, low foaming styrol and a radome 36. A number of openings are formed through the spacers 31 and 33 and the upper plate 34 in correspondence with a number of radiators 32, similarly to the previously-proposed examples. In this embodiment, concave regions 30′, are formed in the lower plate 30 in alignment with a number of openings formed through the upper plate 34. That is, the height from the radiators 32′ to the lower plate 30 is increased to provide a predetermined height d and this predetermined height d is selected to be, for example, 5 mm. The concave regions can be formed by machining the lower plate. In the example shown in Figure 2, the dimension corresponding to the predetermined height d is 1 mm, and the bandwidth in which the voltage standing wave ratio (that is, VSWR) is kept less than 1.4 is about 300 MHz in the region of the 12 GHz band as shown in Figure 4. However, with the predetermined height d selected to be 5 mm, as in this embodiment, the bandwidth in which the voltage standing wave ratio is kept less than 1.4 is about 700 MHz in the vicinity of the 12 GHz band, as shown in Figure 9, which can provide a relatively wide gain. Thus, deterioration of excitation balance of radiation elements due to distribution or the like can be reduced, the change of impedance is reduced, and the change of characteristic due to slackening of the substrate can also be reduced. In addition, the gain of radiators can be increased. In other words, by selecting the height d between the radiator 32 and the lower plate 30 to be 5 mm, it is possible to remove the above defects (1) to (4) and (6). In this embodiment, as shown in Figure 7, a spacing b is maintained between the lower plate 30 and the upper plate 34, on opposite sides of a line (feeder) 32″, and this spacing is selected to be 4 mm, while it is 2 mm in the previously-proposed examples. More specifically, while the feed line loss of the previously-proposed examples are in a range of from 1.6 to 1.8 dB/m, if the line width W of the line 32″ is selected to be 1.5 mm at 12 GHz, the characteristic impedance Z₀ of the line 32″ is selected as about 111 ohms, and the spacing between the lower plate 30 and the upper plate 34 is selected to be 4 mm as in this embodiment, the feed line loss can be improved to about 0.9 to 1.1 dB/m. The reason for this is that dielectric loss of the film substrate is reduced by increasing the spacing b. Although the coupling amount is increased and a higher degree mode tends to occur, these defects can be removed by selecting proper parameters. By increasing the spacing b between the lower plate 30 and the upper plate 34 relative to the line 32″, it is possible to remove the above defect (5). Furthermore, in this embodiment, the element gain can be increased by properly selecting the thickness of the radome 36. According to the experimental results, when the thickness of the radome 36 is 3 mm, the element gain can be increased by + 2.5 to 2.9 dB as compared with the previously-proposed examples, which can relive the above defect (1). When the thickness of the respective portions of Figure 5 are examined, the thickness of the lower plate 30 is 5 mm, the thickness of the spacers or distance pieces 31 and 33 are 2 mm, the thickness of the upper plate 34 is 1 mm, the thickness of the support cushion 35 is 12 to 14 mm and the thickness of the radome 36 is 3 mm. The entire thickness becomes 25 to 27 mm, which is adequate to provide the thin planar array antenna, although the entire thickness is increased a little as compared with the previously-proposed examples. Figure 8 shows the second embodiment of the present invention. While in the first embodiment of Figure 5 the lower plate 30 is thick and the concave regions 30′ are formed thereon by a cutting-process or the like, in the arrangement of Figure 8, the whole of a lower plate 30A is moulded as a thin planar plate having the concave regions 30′ moulded therewith by a press-forming process. In this embodiment the thickness of the upper plate is substantially the same as the thickness of the lower plate. In the case of Figure 5, the lower plate 30 is thick, so that a rear cover is not needed. However, in the case of Figure 8, a rear cover may be attached to the lower plate 30A, if necessary. As described above, since the upper plate is formed as a flat thin plate and the concave regions are formed on the lower plate and the concave regions are formed on the lower plate in alignment with a number of holes of the upper plate, various characteristics such as the element gain, the impedance matching band width of element, the excitation balance or the like can be improved while maintaining the decreased thickness or the planar array antenna.

Claims: A planar array antenna comprising: an upper plate (34) having a plurality of holes; a lower plate (30); and a circuit board (32) located between said upper plate (34) and said lower plate (30) and having printed patterns of a plurality of array elements (32′) thereon in alignment with the holes in the upper plate (34), the circuit board (32) being separated from the upper and lower plates (34, 30) by respective spacers (33, 31) having openings therein corresponding to the holes in the upper plate (34), wherein said lower plate (30) has concave regions (30′) formed therein in alignment with and of a size corresponding to the holes in the upper plate (34). An antenna according to claim 1 wherein the thickness of each of the spacers (31, 33) is 2mm and the distance (d) between the array elements (32′) and the lower plate (30) in said concave regions (30′) is 5mm. An antenna according to claim 1 or claim 2 including a radome (36) having a thickness of 3mm located over the upper plate (34) and separated therefrom by a support cushion (35). An antenna according to any preceding claim wherein the thickness of the upper plate (34) is 1mm. An antenna according to any preceding claim wherein said lower plate (30) is thicker than said upper plate (34). An antenna according to any one of claims 1 to 4 wherein said concave regions (30′) are made by press-forming said lower plate (30). An antenna according to claim 5 wherein said concave regions (30′) are made by machining said lower plate (30). An antenna according to claim 6 wherein the thickness of said upper plate (34) is substantially the same as that of said lower plate (30). An antenna according to any preceding claim wherein said patterns of the plurality of array elements (32′) are connected with each other. An antenna according to claim 9 wherein said patterns are connected so as to obtain a given circular polarization of the electromagnetic wave.

Inventors: FUKUZAWA KEIJI C O PATENTS DIV, ITO FUMIHIRO C O PATENTS DIVIS, TSURUMARU SHINOBU C O PATENTS, FUKUZAWA, KEIJI, C/O PATENTS DIVISION, ITO, FUMIHIRO, C/O PATENTS DIVISION, TSURUMARU, SHINOBU, C/O PATENTS DIVISION
Applicants: SONY CORP, SONY CORPORATION
Application year: 1990
Main classification: H01Q21
IPC codes: H01Q21, H01Q13