Plane slot antennas and their use in motor vehicles

Abstract: A plane antenna for automobiles including a peri­ pheral conducting surface (10), a first inner conducting surface (11) provided in the peripheral conducting sur­ face with a space (21) in between, a second inner con­ ducting surface (12) provided in the first inner conducting surface with a space (22) in between. A core conductor (31) of a first coaxial line (30) is connected to the first inner conducting surface and an outer skin (32) of the first coaxial line is connected to the peripheral conducting surface, and a core conductor (41) of a second coaxial line (40) is connected to the second inner conducting surface and an outer skin (42) of the second coaxial line is connected to the first inner con­ ducting surface. The second coaxial line can be divided into two sections (40a, 50) so that these sections are inductively connected to each other by coils.

Description: The present invention relates to a plane antenna which is installed in a horizontal portion such as the roof, trunk lid, etc. of an automobile. Antennas which show little projection are in demand as antennas for mobile units such as automobiles, etc. in order to eliminate wind noise and to improve the external appearance of the vehicle. From this standpoint, plane or flat plate form antennas are desirable. One type of plane antenna is a slot antenna. This is an antenna in which a portion of the conductor is cut out in the form of a circle (generally, cut-out slots are formed at regular intervals to form such a circle). In conventional plane antennas, the mean diameter of the central conductor, i.e. the island-form conductor in the center, reaches approximately 1 m in the case of FM reception, so that the length of the circle (i.e. the circumference) is about wavelength, that is, 3.3 m when the wave received has a frequency of 90 MHz. Accordingly, when an attempt is made to install another plane antenna whose frequency (e.g. the 900 MHz band used for automobile telephones) differs from that of the first plane antenna installed (e.g., the 90 MHz band used for FM reception), and it is attempted to accomplish this antenna installation with the loss of any performance, such installation is very difficult in areas where the amount of space available is limited, as in the roof of a vehicle, etc. Accordingly, the object of the present invention is to provide a plane antenna which is free from performance loss in cases where signals of different frequencies are received, even when the space for antenna installation is relatively small. GB-A-1546571 discloses a planar antenna including first and second closed-figure slots, one square and one circular, for respectively transmitting and receiving signals. The circular slot is almost as wide as the square slot and therefore suitable for handling signals in the same frequency range. One of the slots is used for receiving signals and the other is used for transmitting signals. The present invention provides a plane antenna for automobiles comprising: a first closed-figure slot formed as a gap between a peripheral conductor and a first inner conductor, said first inner conductor being located inside the peripheral conductor, said peripheral conductor having a peripheral length corresponding to a first frequency band in which a first wavelength is present; a second closed-figure slot formed as a gap between the first inner conductor and a second inner conductor located in a centre of the first inner conductor, the second inner conductor (12) having a peripheral length corresponding to a second frequency band in which a second wavelength shorter than said first wavelength is present; a first coaxial line having conductors connected to the first inner conductor and the peripheral conductor respectively; and a second coaxial line having a central conductor connected to the second inner conductor and an outer surface connected to the first inner conductor; characterised by further comprising a first coil member connected to the second coaxial line and a third coaxial line having a second coil member connected thereto, the first and second coil members being arranged to face each other, and to be inductively coupled, across a portion of the first closed-figure slot. Furthermore the antenna may be designed so that the output cable from the second closed-figure slot is interrupted when communication signals are received by the first closed-figure slot, and the central conductor surrounded by the first closed-figure slot is used as a peripheral conductor as a ground plane when communication signals are received by the second closed-figure slot. With such a structure, since the second closed-figure slot is installed in the central area of a first closed figure slot, the performance of the antenna can be maintained when signals of different frequencies are received, even if the amount of space available for antenna installation is relatively small. This 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 plan view of a first arrangement described to assist understanding of the antenna of the present invention; Fig. 1B is a sectional view taken along the line I-I of Fig. 1A; Fig. 2A is a plan view of a second arrangement described to assist understanding of the present invention; Fig. 2B is a sectional view taken along the line II-II of Fig. 2A; and Fig. 3 is a plan view which illustrates an embodiment of the present invention. Fig. 1A illustrates one arrangement useful to assist understanding of the present invention and Fig. 1B is a schematic vertical section viewed along the line I-I in Fig. 1A, coaxial cables being omitted from Fig. 1B. A first closed-rectangular slot 21 is provided between a peripheral conductor or peripheral conducting surface 10 and a first central conductor or first central conducting surface 11, and a second closed-circle slot 22 is provided between the first central conductor 11 and a second central conductor or second central conducting surface 12. The core conductor 31 of a coaxial cable 30 is connected to an edge of the first central conductor 11, and the outer skin 32 of the coaxial cable 30 is connected to an edge of the peripheral conductor 10. Meanwhile, a core conductor 41 of a coaxial cable 40 is connected to an edge of the second central conductor 12, and the outer skin 42 of the coaxial cable 40 is connected to a portion that is close to the second closed-circle slot 22 of the first central conductor 11. The coaxial cable 30 is a cable which extracts signals in the FM frequency band and AM frequency band, while the coaxial cable 40 is a cable which extracts signals in the automobile telephone band (900 MHz). Furthermore, the size of the first closed rectangular slot 21 is approximately 1 m x 0.7 m. The frequency band of the signals extracted by the coaxial cable 30 is lower than the frequency band of the signals extracted by the coaxial cable 40; however, it would also be possible to arrange the antenna so that signals of frequencies other than those indicated in the above mentioned example are extracted. Furthermore, in the arrangement, the output cable 40 from the second closed-circle slot 22 is interrupted when communications are received by the first closed-rectangular slot 21, and the central conductor 11 surrounded by the first closed rectangular slot 21 is used as a peripheral conductor (as a ground plane) when communications are received by the second closed-circle slot 22. The coaxial cable 30 extracts signals in the FM frequency band and signals in the AM frequency band. Since signals in the FM frequency band have a high frequency, almost all of the current flows through the edge portions of the first central conductor 11; i.e., there is almost no flow of FM frequency band signals through the central portion of the first central conductor 11. Thus, since the central portion of the first central conductor 11 has no effect on the FM frequency band, signals in a higher frequency band can be extracted by the coaxial cable 40. In other words, the slot antenna utilizes the concentrated current flowing along the conductor gap; thus, the central portion of the central conductor 11 has no great effect. Accordingly, this central portion can be utilized as a ground part for a second antenna. As a result of this feature, the plane antenna can be installed in a small area and used as a multi-band antenna. Furthermore, the coaxial cable 30 also extracts signals in the AM frequency band. In the case of such signals in the AM frequency band, current flowing through the entire area of the first central conductor 11 is extracted. However, since the area of the second central conductor 11 is small compared to the area of the first central conductor 11, there are no practical problems in terms of signal reduction even if such a second central conductor 12 is installed in a portion of the first central conductor 11. Fig. 2A and 2B show another arrangement useful to assist understanding of the present invention. Fig. 2A is a plan view, and Fig. 2B is a longitudinal section viewed from the line II-II in Fig. 2A. The arrangement of Fig. 2 differs as follows from the arrangement of Fig. 1 in that a first antenna is constructed from a peripheral conductor 10, a first central conductor 11 and a first closed-rectangular slot 21 as in Fig. 1, but a strip-form flat-plate antenna is used instead of the second antenna consisting of the second central conductor 12, second closed-circle slot 22 and first central conductor 11. The central conductor 11 surrounded by the first closed-rectangular slot 21 acts as a ground plate for the strip-form flat-plate antenna. In other words, the strip-form flat-plate antenna is constructed from the first central conductor 11 and a flat plate 60 which is provided above the central conductor 11. In actuality, the first central conductor 11 and conductive flat plate 60 are constructed from printed circuit boards and consist of copper foil. Specifically, the arrangement of Fig. 2 is a plane antenna which is equipped with (a) a slot antenna which receives communications by means of a closed-circle slot, and (b) a strip-form flat-plate antenna which uses the central conductor surrounded by the closed-circle slot as a ground plate. The output cable from the strip-form flat-plate antenna is interrupted when communications are received by the slot antenna, and the central conductor surrounded by the closed-circle slot is used as a ground plate when communications are received by the strip-form flat-plate antenna. Fig. 3 is a plan view of an embodiment of the present invention. This embodiment differs from the arrangement illustrated in Fig. 1 in that the coaxial cable 40 of Fig. 1 is divided into two cables, i.e., a coaxial cable 40a and a coaxial cable 50. The coaxial cable 40a and coaxial cable 50 are inductively coupled by coils 43 and 53. When the coaxial cables are inductively coupled by coils 43 and 53, the loss of AM frequency band signals extracted via the coaxial cable 30 is reduced. Specifically, AM frequency band signals use a capacitive antenna, and are generated by capacitive components from the ground. Accordingly, if reactive capacitive components are present, the voltage is divided by the reactive capacitive components so that the reactive capacitive components generate a loss. As a result, AM frequency components extracted via the coaxial cable 30 are reduced. Specifically, in Fig. 1, a large reactive capacitance is generated between the outer skin of the coaxial cable 40 and the peripheral conductor 10 when the two parts face each other over a long distance. The AM frequency band signal components extracted via the coaxial cable 30 are greatly reduced by this reactive capacitance. If, however, the coaxial cable is divided into two coaxial cables 40a and 50, and these cables 40a and 50 are inductively coupled by coils 43 and 53 as shown in Fig. 3, the capacitance between the outer skin of the coaxial cable 50 and the peripheral conductor 10 will be interrupted even if a large capacitance should be generated between such parts. In this case, the capacitance acting as reactive capacitance from the standpoint of the AM signals extracted via the coaxial cable 30 will be only the capacitance generated between the outer cover of the coaxial cable 40a and the first peripheral conductor 10. Since the length of the coaxial cable 40a is only a few tens of cm at most, this length is extremely short from the standpoint of the wavelengths used in the AM frequency band. Accordingly, for all practical purposes, the loss in the AM frequency band can be ignored. Furthermore, it is desirable that the gap between the facing coils 43 and 53 be positioned directly above the first closed-circle slot; and in such a case, the loss is minimal. In addition, it is also possible to apply this method, i.e., to divide the coaxial cable extracting signals from the second central conductor 12 and form an inductive coupling by installing facing coils in the resulting gap as shown in Fig. 3 to the arrangement of Fig. 2. Furthermore, in the above described embodiments, the first closed slot 21 has corner parts; however, the efficiency may be improved by constructing the first closed-circle slot with curved parts instead of the aforementioned sharp corner parts. As described in detail above, according to the present invention any loss of performance in cases where signals of different frequencies are received is prevented even when the space for antenna installation is relatively small.

Claims: A plane antenna for automobiles comprising: a first closed-figure slot (21) formed as a gap between a peripheral conductor (10) and a first inner conductor (11), said first inner conductor being located inside the peripheral conductor (10), said peripheral conductor having a peripheral length corresponding to a first frequency band in which a first wavelength is present; a second closed-figure slot (22) formed as a gap between the first inner conductor (11) and a second inner conductor (12) located in a centre of the first inner conductor (11), the second inner conductor (12) having a peripheral length corresponding to a second frequency band in which a second wavelength shorter than said first wavelength is present; a first coaxial line (30) having conductors connected to the first inner conductor (11) and the peripheral conductor (10) respectively; and a second coaxial line (40a) having a central conductor (41) connected to the second inner conductor (12) and an outer surface (42) connected to the first inner conductor (11); characterised by further comprising a first coil member (43) connected to the second coaxial line (40a) and a third coaxial line (50) having a second coil member (53) connected thereto, the first and second coil members being arranged to face each other, and to be inductively coupled, across a portion of the first closed-figure slot (21). A plane antenna for automobiles according to claim 1, in which said second inner conductive surface is a flat conductive plate positioned above said second inner conducting surface.

Inventors: HARADA TAKUJI, HARADA, TAKUJI
Applicants: HARADA IND CO LTD, HARADA INDUSTRY CO., LTD.
Application year: 1989
Main classification: H01Q9
IPC codes: H01Q21, H01Q9, H01Q13, H01Q1, H01Q5