SHORTED MICROSTRIP ANTENNA

Abstract: A low and broadband shorted microstrip antenna is disclosed which is mainly applicable to a mobile body in a mobile communication system. A first grounding conductive sheet (42) which faces a radiating conductive sheet (48) is provided at both ends thereof with a second (44) and a third (46) grounding conductive sheets which are perpendicular to the first grounding conductive sheet, whereby a beam tilt characteristic of the antenna is improved. A passive element (52) and a conductive stub (62) which is provided on the grounding conductive sheet, which faces the radiating conductive element, serve to improve an impedance matching characteristic.

Description: BACKGROUND OF THE INVENTION The present invention relates to a low and broad bandwidth shorted microstrip antenna which is shorted at one side thereof and may be mounted on a mobile body in a mobile communication system and provided with improved beam tilting and impedance matching characteristics. A shorted microwave strip antenna (SMSA) is a half-sized version of an ordinary patch antenna and provided with a miniature, light weight and low height construction. Due to such advantages, an SMSA is suitable for use as an antenna which is mounted on a mobile body in a mobile communication system. Generally, an SMSA includes a grounding conductive sheet on which a feed connector is mounted, a radiating conductive sheet which faces the grounding conductive sheet with the intermediary of air or like dielectric material, and a connecting conductive sheet positioned at the shorted end of those two conductive sheets perpendicular to the surfaces of the latter in order to connect them together. In the above-described type of SMSA, assume an X and a Y axes in a general plane of the emitting and the grounding conductive sheets (the Y axis extending along the general plane of the connecting conductive sheet), and a Z axis in the general plane of the connecting conductive sheet which is perpendicular to the X and Y axes. Then, emission occurs in the SMSA due to a wave source which is developed in the vicinity of a particular side of the radiating conductive sheet which is parallel to the Y axis and not shorted. If the size of the grounding conductive sheet is infinite, the SMSA is non-directional in the X-Z plane on condition that Z is greater than zero; if it is finite, the SMSA obtains the maximum directivity in the vicinity of the Z axis. When the radiating conductive sheet is positioned at, for example, substantially the center of the grounding conductive sheet, the directivity is such that the maximum emission direction is tilted from the Z direction, resulting in a decrease in the gain in the Z direction. This is accounted for by the fact that the wave source of the SMSA is not located at the center of the grounding conductive sheet. A prior art implementation to eliminate such beam tilts consists in dimensioning the grounding conductive sheet substantially twice as long as the radiating conductive sheet in the X direction. This kind of scheme, however, prevents the SMSA from being reduced in size noticeably, compared to an ordinary microstrip antenna (MSA). It therefore often occurs that it is difficult for an SMSA to be installed in a mobile body such as an automotive vehicle. Further, as regards an SMSA having a relatively small connecting conductive sheet, current is allowed to flow into the jacket of a cable which is joined to a feed connector. This would render the impedance matching characteristic of the antenna unstable while disturbing the directivity. In a French patent specification, which was made available to the public under number 2,552,938 on April 5 1985, there is described a microstrip antenna arrangement including dielectric material separating an intermediate rectangular metallised layer from both a rectangular ground plate and an upper rectangular metallised layer. A co-axial cable core passes through the dielectric layers to the upper metallised layer, and short circuiting connections are provided between faces of the intermediate layer and the adjacent metallised layers respectively at different relative positions from the edges thereof, which together with the non-alignment of any edge of the layers enables the elements to be matched. In a United States patent specification No. 4,123,758 which was published on October 31 1978, there is described a circular disc antenna having a circular conductor disc parallel to a circular ground plate, a feed plate, to which the core of a co-axial cable is connected, the core of the cable passing through a grounding plate extending between corresponding points on the circumferences of the conductor disc and the ground plate, and the feed plate being attached to an opposite point on the circumference of either the conductor disc or the ground plate. A feature of an embodiment to be described is the provision of an SMSA which is small in size and stable in directivity. Other features of an embodiment to be described are that it has improved beam tilt and impedance match characteristics. In a particular embodiment to be described a microstrip antenna which is shorted at one side has a generally rectangular radiating conductive sheet for supplying power to be radiated, a first grounding conductive sheet located to face and parallel to the radiating conductive sheet, a generally rectangular second grounding conductive sheet located at one side of and perpendicular to the first grounding conductive sheet and connected to the radiating conductive sheet, and a third grounding conductive sheet located to face and parallel to the second grounding conductive sheet and provided at one side of and perpendicular to the first grounding conductive sheet which opposes the one side. There will now be described prior art arrangements, together with embodiments of the invention, which are given by way of example, with reference to the accompanying drawings in which:- Figs. 1A and 1B are a plan view and a side elevation, respectively, of a prior art ordinary MSA; Fig. 1C is a chart explanatory of the directivity of the MSA as shown in Figs. 1A and 1B; Figs. 2A and 2B are a schematic plan view and a side elevation, respectively, of a prior art SMSA; Fig. 2C is a chart similar to Fig. 1, showing the directivity of the MSA of Figs. 2A and 2B; Fig. 3A is a perspective view of an SMSA embodying the present invention; Fig. 3B is a side elevation of the SMSA as shown in Fig. 3A; Fig. 4 is a perspective view of another embodiment of the present invention; Fig. 5 is a Smith chart comparing the embodiment of Figs. 3A and 3B and that of Fig. 4 in terms of values of impedance characteristic actually measured; Figs. 6A and 6B are a perspective view and a side elevation, respectively, of still another embodiment of the present invention; Fig. 7 is a plot comparing the embodiment of Fig. 4 and that of Figs. 6A and 6B in terms of a reflection loss characteristic; Fig. 8 is a perspective view of a modification to the embodiment of Figs. 6A and 6B; and Fig. 9 is a chart showing the directivity of the SMSA of Fig. 8 together with that of the prior art SMSA for comparison. DESCRIPTION OF THE PREFERRED EMBODIMENTS To facilitate an understanding of the present invention, a brief reference will be made to a prior art MSA and a prior art SMSA, as shown in Figs. 1A, 1B and 2. Referring to Figs. 1A and 1B, a prior art ordinary MSA 10 includes a grounding conductive sheet 12 on which a feed connector 14 is mounted, and a radiating conductive sheet 16 located to face the sheet 12 with the intermediary of air or like dielectric material 18. The reference numeral 20 designates a feed pin. Assuming that the length of the conductive sheet 16 along an X axis is L₁, it is expressed as L₁ = λo/2√ εγ , where λo is the free space wavelength at a frequency used and εγ, the specific relative dielectric constant of the dielectric 18. The conductive sheet 12 is assumed to have a length L₂ in the X direction. In this type of MSA 10, emission is developed by a radiating source which is produced in the vicinity of two sides of the conductive plate 16 which are parallel to a Y axis. Eventually, the emission is such that the maximum emission direction occurs along a Z axis. Figs. 2A and 2B show a prior art SMSA 30 consisting of a grounding conductive sheet 32 carrying the feed connector 14 therewith, a radiating conductive sheet 34 located to face the sheet 32 with the intermediary of air or like conductive material 36, and a connecting conductive sheet 38 located at the shorted end of the sheets 32 and 34 perpendicular to the latter in order to connect them together. Assuming that the length of the conductive sheet 34 in the X direction is L₃, it is produced by L₃ = λo/4√ εγ, where λo is the free space wavelength at a frequency used and εγ, the specific relative dielectric constant of the dielectric 36. The length of the conductive sheet 32 in the X direction is assumed to be L₄. It will be understood that the length of the SMSA 30 is half the MSA 10 in terms of the length of the radiating conductive sheet, allowing the entire antenna to have considerably small dimensions. Such an antenna is desirably applicable to a mobile body of a mobile communication system. In the SMSA 30, emission occurs due to a radiating source which is developed in the vicinity of that side of the radiating conductive sheet 34 which is parallel to the Y axis and not shorted. If the size of the grounding conductive sheet 32 is infinite, the SMSA 30 is non-directional in the X-Z plane on condition that Z is greater than zero; if it is finite, the SMSA 30 obtains the maximum directivity in the vicinity of the Z axis. When the radiating conductive sheet 34 is positioned at, for example, substantially the center of the grounding conductive sheet 32, the directivity is such that, as shown in Fig. 2C, the maximum emission direction is tilted from the Z direction, resulting in a decrease in the gain in the Z direction. This is accounted for by the fact that the wave source of the SMSA 30 is not located at the center of the grounding conductive sheet 32. A prior art implementation to eliminate such beam tilts consists in dimensioning the grounding conductive sheet 32 to Figs. 2A and 2B substantially twice as long as the radiating conductive plate 34 in the X direction, i. e. L₄≈2 × L₃. As previously discussed, the problem with the prior art SMSA 30 is that the radiating conductive plate 34 inclusive of the grounding conductive sheet is not noticeably smaller than that of the MSA 10 of Figs. 1A and 1B, although halved in size. Such often makes it difficult for the antenna to be built in an automotive vehicle and other mobile bodies. Referring now to Figs. 3a and 3B, an SMSA embodying the present invention is shown and generally designated by the reference numeral 40. As shown, the SMSA 40 comprises a first grounding conductive sheet 42, a second and a third grounding conductive sheets 44 and 46 which are mounted on the conductive sheet 42 perpendicular thereto, a radiating conductive sheet 48 connected to the conductive sheet 44, a feed pin 50, and a feed connector 51. The second grounding conductive sheet 44 bifunctions as a connecting conductive sheet which connects the first grounding conductive sheet 42 and the radiating conductive sheet 48 to each other. The SMSA 40 shows the maximum directivity in a Z direction if the dimensions of the second and third grounding conductive sheets 44 and 46 are selected adequately. The SMSA 40 which uses the second and third grounding conductive plates is greater than the prior art SMSA 30 with respect to the area of the entire grounding conductive plate. This allows a minimum of current to flow into the jacket of a feed cable which is connected to the feed connector 51, thereby freeing the impedance and directivity from substantial influence of the feed cable. As described above, in accordance with this particular embodiment, a miniature antenna with a minimum of beam tilt in the Z direction is attained by virtue of a second and a third grounding conductive sheets which are located at both ends of and perpendicular to a first grounding conductive sheet, which faces a radiating conductive sheet. Further, the antenna of this embodiment reduces current which flows into the jacket of a feed cable, compared to a prior art SMSA, whereby the impedance characteristic and the directivity are little susceptible to the influence of the feed cable and, therefore, stable operation is insured. Meanwhile, as shown in Fig. 4, an SMSA 40a which is provided with a passive element 52 is broader in bandwidth than the SMSA 40 of Figs. 3A and 3B which lacks it. Specifically, the SMSA 40a is provided with a several times broader bandwidth than the SMSA 40 by adequately selecting the dimensions of the passive element 52, the distance between the passive element 52 and the radiating conductive sheet 48, and the distance between the passive element 52 and the grounding conductive sheet 42. Referring to Fig. 5, the SMSA 40a having the passive element 52 located close to the radiating conductive sheet 48 as shown in Fig. 4 and the SMSA 40 without a passive element as shown in Figs. 3A and 3B are compared in terms of impedance values which were measured actually. In Fig. 5, a curve A is representative of the impedance characteristic of the SMSA 40a and a curve B, that of the SMSA 40. The curves A and B were attained by setting up a center frequency f₀ of 900 MHz. Further, assuming that the lengths of the SMSA 40a are L₅ to L₁₃ as indicated in Fig. 4, then L₅ = 92 mm, L₆ = 16 mm, L₇ = 50 mm, L₈ = 105 mm, L₉ = 85 mm, L₁₀ = 76 mm, L₁₁ = 67 mm, L₁₂ = 28 mm, and L₁₃ = 8 mm. As described above, an SMSA with a passive element achieves a comparatively constant impedance characteristic by virtue of the effect of the passive element. However, the impedance of such an SMSA involves a part which is derived from a reactance and cannot be desirably matched to a 50-ohm system. Another drawback with this antenna is that the matching situation cannot be improved even if the feed position is changed. Referring to Figs. 6A and 6B, another embodiment of the present invention is shown which is provided with an improved impedance matching characteristic. In Figs. 6A and 6B, the same or similar structural elements as those shown in Fig. 4 are designated by like reference numerals. As shown, the SMSA 60 comprises a conductive stub 62 in addition to the grounding conductive sheet 42, radiating conductive sheet 48, passive element 52, connecting conductor 44, and feed pin 50. The SMSA 60 can serve as a broad bandwidth antenna which well matches itself to a 50-ohm system, only if the dimensions and position of the conductive stub 62 is selected adequately. Fig. 7 shows a reflection loss characteristic of the SMSA 60 of Figs. 6A and 6B as represented by a solid curve and that of the SMSA 40a of Fig. 4 with a passive element as represented by a dotted curve. The solid and the dotted curves were attained with the same center frequency and the same dimensions as those previously described. As shown, hardly any power reflection less than -14 dB (VSWR = 1.5) is attained by the SMSA 40a. In contrast, the SMSA 60 of this embodiment maintains power reflection which is less than -14 dB over a very broad bandwidth, i.e. 16 %. Thus, the embodiment of Figs. 6A and 6B realizes an antenna which shows good matching to a 50-ohm system. Specifically, because the conductive stub 62 serves as an impedance compensating element which shows a constant reactance characteristic over a broad bandwidth, that part of the impedance which is derived from reactance can be compensated for without disturbing the constant impedance characteristic which is ensured by the passive element 52. It is to be noted that in although the conductive stub 62 is shown as having a rectangular parallelepiped configuration, it may be provided with any other configuration such as a cylindrical one without affecting the characteristic. As described above, this particular embodiment provides an SMSA with a passive element is provided with a conductive stub on a grounding conductive sheet which faces a radiating conductive sheet, so that its matching with a feed line of an SMSA with a passing element which shows a constant impedance is improved. The SMSA, therefore, functions as a broad bandwidth antenna having a physically low structure. Referring to Fig. 8, a modified embodiment of the SMSA 60 of Figs. 6A and 6B, generally 60a, is shown which is provided with an additional conductive sheet 64 which is mounted on the radiating conductive sheet 48 perpendicular thereto and has a length L₁₄. The sheet 64 functions to lower the resonance frequency. Referring to Fig. 9, there is shown a chart for comparing the modified SMSA 60a of Fig. 8 and the prior art SMSA 30 of Figs. 2A and 2B in terms of data actually measured on the directivity the X-Z plane. In Fig. 9, a solid line is representative of the modified SMSA 60a of the present invention and a dotted line, the prior art SMSA 30. Specifically, while the data associated with the prior art SMSA 30 were measured under the conditions of εγ = 1, L₃ = 75 mm, and L₄ = 200 mm, the data associated with the SMSA 60a of the present invention were measured on the conditions of εγ = 1 and L₁₄ = 7 mm. The other dimensions such as L₅ to L₁₃ were the same as those of the SMSA 40a of Fig. 4. It will be seen from the above that the SMSA 60a in accordance with this modification achieves an improved beam tilt characteristic in the Z direction. This leads to an improvement in the gain in the Z direction by 1.0 to 1.5 dB. Various embodiments will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof. It will be understood from the foregoing that there has been described a microstrip antenna which includes a generally rectangular radiating conductive sheet from which power is to be radiated, a first grounding sheet located to face and parallel to the radiating conductive sheet, a generally rectangular second grounding conductive sheet perpendicular to and connected to both the first grounding conductive sheet and the radiating conductive sheet, the second grounding conductive sheet being connected to and along the lengths of respective and corresponding edges of both the radiating conductive sheet and the first grounding conductive sheet, and a third grounding conductive sheet which faces and is parallel to the second grounding conductive sheet, which is perpendicular to the first grounding conductive sheet and which extends from a second edge of the first grounding sheet opposite to the first mentioned edge thereof towards the plane of the radiating conductive sheet.

Claims: A microstrip antenna (40) including a generally rectangular radiating conductive sheet (48) from which power is to be radiated, a first grounding sheet (42) located to face and parallel to the radiating conductive sheet (48), and a generally rectangular second grounding conductive sheet (44) perpendicular to and connected to both the first grounding conductive sheet (42) and the radiating conductive sheet (48), characterised in that the second grounding conductive sheet (44) is connected to and along the lengths of respective and corresponding edges of the radiating conductive sheet (48) and the first grounding conductive sheet (42) and in that there is provided a third grounding conductive sheet (46) facing and parallel to the second grounding conductive sheet (44), perpendicular to the first grounding conductive sheet (42) and extending from a second edge of the sheet (42) opposite to the first mentioned edge thereof towards the plane of the radiating conductive sheet (48). A microstrip antenna as claimed in claim 1, characterised in that there is further provided a second radiating conductive sheet (52) facing and parallel to the radiating conductive sheet (48) and connected to the second grounding conductive sheet (44). A microstrip antenna as claimed in claim 2, characterised in that there is further provided a conductive stub member (62) connected to the first grounding conductive sheet (42) and projecting towards the first mentioned radiating conductive sheet (48). A microstrip antenna as claimed in claim 3, characterised in that the conductive stub member (62) has a rectangular parallelepiped configuration. A microstrip antenna as claimed in claim 3, characterised in that the conductive stub member (62) has a cylindrical configuration.

Inventors: EBINE, YOSHIO, ITO, TOSHIO C/O ANTEN KOGYO CO.,LTD., YOKOYAMA, YUKIO C/O NEC CORPORATION
Applicants: NEC CORPORATION, NIPPON TELEGRAPH AND TELEPHONE CORPORATION
Application year: 1986
Main classification: H01Q9
IPC codes: H01Q9