Planar array antenna

Abstract: A planar array antenna includes a ground plate (10) formed of metallic material, a plurality of patch antenna elements (11, 12) supported on the ground plate (10) by insulation spacers (13, 14), respectively, and arrayed at a predetermined pitch, and a feed line (17) for coupling adjacent antenna elements of the plurality of patch antenna elements (11, 12).

Description: The present invention relates to a planar array antenna which can be applied to a transmit/receive antenna used for a WLL (wireless local loop) terminal. FIGS. 5A to 5C illustrate one example of a prior art planar array antenna of the above type. Referring to these figures, a plurality of (two in this example) patch antenna elements 101 and 102 are arrayed on a rectangular dielectric substrate 100. The elements 101 and 102 are coupled to each other by a feed line 103, while the element 102 is coupled to a feeding point 105 by a feed line 104. The feed lines 103 and 104 are each constituted of a strip line adhered onto the dielectric substrate 100. In the prior art planar array antenna, an electric power is applied, as a series feed, from the feeding point 105 to the patch antenna elements 101 and 102 through the feed lines 103 and 104. The planar array antenna so constituted is miniaturized as a whole by the dielectric effect of the dielectric substrate 100. Since, however, the antenna is decreased in gain due to a dielectric loss, a usable bandwidth of VSWR (voltage standing-wave ratio) is narrowed. Since, moreover, the plurality of patch antenna elements 101 and 102 are arrayed and an electric power is applied to these elements as a series feed, the following problem arises. The patch antenna elements 101 and 102 are difficult to arrange at the optimum interval under the influence of a so-called contraction rate due to the dielectric of the dielectric substrate 100. This problem will be described more specifically. As illustrated in FIGS. 5A and 5B, the electrical length of the antenna is determined such that the length of each of the patch antenna elements 101 and 102 and the interval between them are both λ/2 when the wavelength of transmitted/received wave is λ. In FIGS. 5A and 5B, it is λ/2 and P = λ that correspond to the electrical length. The contraction rate, which is one of dielectric effects of the dielectric substrate 100, is taken into consideration in order to set the electrical length. Assuming that Teflon (known under the trade name of du Pont) is employed as the dielectric substrate 100 and its effective permittivity is εe, an actual physical distance R between the patch antenna elements 101 and 102 is given by the following equation: R = λ/2(εe) 1/2 ≒ 0.7λ/2 If, as shown in FIG. 5C, the energy area of the patch antenna element 101 is S101 and that of the patch antenna element 102 is S102, these areas overlap each other to cause a region S103 shaded diagonally therein. The overlapped region S103 reduces the antenna efficiency and accordingly the maximum gain cannot be obtained under the influence of a dielectric loss. When Teflon is used as the dielectric substrate 100, the gain falls within a range from 8dBi to 9dBi, which is about 30% lower than the maximum gain in the ideal status or in air. If an electric power is applied to the patch antenna elements 101 and 102 as a parallel feed, the foregoing problem does not arise, whereas the following drawback occurs: since the antenna necessitates an allotter, its structure is complicated and increased in size, and a loss is produced from the allotter. Prior art documents are US5572222A, US4117489A, US4464663A and EP0847101A. A planar array antenna according to the preamble of claim 1 is disclosed in US5572222A. The object of the present invention is to provide a planar array antenna having the following advantages: (a) Even though an electric power is applied to arrayed patch antenna elements as a series feed, the energy areas of adjacent antenna elements can be prevented from overlapping and the antenna elements can be arrayed at ideal intervals, when the length of each of the elements and the interval therebetween are both set to a predetermined electrical length; (b) Since the ideal intervals can be secured and no dielectric loss occurs, the antenna efficiency is remarkably improved and the maximum antenna gain can be obtained; and (c) The antenna can be simplified and miniaturized as a whole, and its costs can be lowered greatly. In order to attain the above object, the present invention provides a planar array antenna as set out in the appended claim. Other preferred features will be clarified in the Description of the Invention. The invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: FIG. 1A is a perspective view of the constitution of a planar array antenna according to an embodiment of the present invention; FIG. 1B is a side view of the constitution of the planar array antenna according to the embodiment of the present invention; FIG. 1C is an illustration for explaining a function of the planar array antenna according to the embodiment of the present invention; FIG. 2 is a graph showing VSWR characteristics of the planar array antenna according to the embodiment of the present invention; FIG. 3 is a radiation-pattern view of the directivity of E-plane of the planar array antenna according to the embodiment of the present invention; FIG. 4 is a radiation-pattern view showing the directivity of H-plane of the planar array antenna according to the embodiment of the present invention; FIG. 5A is a perspective view of the constitution of a prior art planar array antenna; FIG. 5B is a side view of the constitution of the prior art planar array antenna; and FIG. 5C is an illustration for explaining a problem of the prior art planar array antenna. (Embodiment) [Constitution] FIGS. 1A to 1C illustrate a planar array antenna according to an embodiment of the present invention. In FIGS. 1A and 1B, reference numeral 10 denotes a ground plate formed of metallic material such as brass. A plurality of (two in this embodiment) patch antenna elements 11 and 12, which are metal plates formed of the same brass, are supported on the ground plate 10 by means of insulation spacers 13 and 14, respectively. Reference numerals 15 and 16 indicate fixing screws for mounting and fixing the patch antenna elements 11 and 12 onto the ground plate 10. The insulation spacers 13 and 14 are each a cylinder (short cylinder in this embodiment) formed of resin such as polyacetal, polycarbonate, and ABS. These spacers each have a considerably small diameter D and an appropriate thickness T, with respect to the areas of the patch antenna elements 11 and 12, such that they can locally support the central parts of the elements 11 and 12. The electrical length is determined such that the length of each of the patch antenna elements 11 and 12 and the interval between them are both λ/2 when the wavelength of transmitted/received wave is λ. In other words, the patch antenna elements 11 and 12 each having a length of λ/2 are arranged in an orderly line at a given interval or with a pitch P = λ. The elements 11 and 12 are connected to each other by means of a feed line 17 constituted of a strip line whose length is λ/2 and whose resistance ranges from 100Ω to 500Ω. The strip line can be formed using a brass- or copper-made wire or plate. To determine the above electrical length, any contraction rate need not be considered in particular since there are no dielectric substrates. Consequently, the length of the feed line 17 or the actual physical distance R between the patch antenna elements 11 and 12 can be set equal to the length L of each of the elements 11 and 12. In other words, both the distance R and length L can be set to λ/2. Points A and B are set on the patch antenna element 12. Since the side lobe of directivity is out of balance at the point B, the point A is regarded as a feeding point. As shown in FIG. 1B, a feeding pin 18 stands on the point A, a portion of the pin 18 which projects toward the back of the ground plate 10, is connected to a matching substrate 19 for correcting a reactance, and the matching substrate 19 is connected to a feeder 20. [Function] As described above, the patch antenna elements 11 and 12 of the present invention are formed on the ground plate 10 of metallic material and their central parts are locally supported by their respective insulation spacers 13 and 14 of short cylinders. The antenna elements 11 and 12 are coupled to each other by means of the feed line 17 of the wire or plate strip line such that the line acts as a bridge in the air. The length of each of the elements 11 and 12 is λ/2, and they are arrayed at a predetermined interval (with a pitch P = λ). Consequently, the dielectric-loss elements of the planar array antenna are only the ultrasmall-sized insulation spacers 13 and 14 supporting the patch antenna elements 11 and 12. In the embodiment of the present invention, therefore, the permittivity is εr related to the antenna gain becomes "1" which is close to that in air, with the result that the dielectric loss is very low and the gain is hardly decreased. Since no dielectric is present between the two patch antenna elements 11 and 12, the physical distance R between them is not influenced by the contraction rate due to a dielectric and, in other words, the distance R can be set to a length corresponding to λ/2. Even though an electric power is applied to the arrayed patch antenna elements 11 and 12 as a series feed, the energy areas S11 and S12 of adjacent elements 11 and 12 can be prevented from overlapping when the element length and the element interval are set to the electrical length of λ/2 as illustrated in FIG. 1C. In other words, the ideal array interval can be secured, so that the antenna efficiency is remarkably increased and the maximum antenna gain can be achieved. In the present invention, the gain of the two patch antenna elements 11 and 12, which was conventionally 8dBi to 9dBi, can be increased up to 12dBi or higher. If the number of patch antenna elements having the same structure is increased, the gain can be improved further. A usable bandwidth of VSWR can be broadened greatly. FIG. 2 is a graph showing VSWR characteristics of the planar array antenna according to the embodiment of the present invention. As is apparent from FIG. 2, the bandwidth W1, which was conventionally 1.5%, is improved to 2.9% when VSWR is 1.5 or less, while the bandwidth W2, which was conventionally 2.8%, is improved to 5.3% when VSWR is 1.8 or less. FIG. 3 is a radiation-pattern view (beam width: 27.75 degrees) of the directivity of E-plane (electric-field plane) of the planar array antenna according to the embodiment of the present invention, while FIG. 4 is a radiation-pattern view (beam width: 61.50 degrees) of the directivity of H-plane (magnetic-field plane) of the planar array antenna. As illustrated in FIGS. 3 and 4, the directivity of both the E and H planes have good characteristics which are sufficiently in practical use. The planar array antenna of the embodiment of the present invention can be simplified and miniaturized as a whole. Since, furthermore, the ground plate 10 of metallic material is used as a base, the materials cost of the antenna becomes 10% to 20% lower than that of a conventional one using a dielectric substrate as a base. The antenna of the present invention can thus be manufactured at very low cost.

Claims: A planar array antenna comprising: a ground plate (10) formed of metallic material; a plurality of patch antenna elements (11, 12) whose central parts are locally supported on the ground plate (10), the patch antenna elements (11, 12) are arrayed at a pitch of λ when the wavelength of transmitted/received wave is λ; a feed line (17) which feeds power between the patch antenna elements (11, 12), characterised in that : the feed line (17) is formed of a strip line which connects adjacent sides of the patch antenna elements (11, 12) to each other extending like a bridge between the patch antenna elements (11, 12); each of the patch antenna elements (11, 12) being supported by a single insulation spacer (13, 14) formed of a cylinder; and the length of each of the patch antenna elements (11, 12) is set to λ /2.

Inventors: KAWASAKI MORIYOSHI, UCHINO SHIGERU, KAWASAKI, MORIYOSHI, UCHINO, SHIGERU
Applicants: HARADA IND CO LTD, HARADA INDUSTRY CO., LTD.
Application year: 1999
Main classification: H01Q21
IPC codes: H01Q21, H01Q9, H01Q13