Patent Publication Number: US-6219001-B1

Title: Tapered slot antenna having a corrugated structure

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a tapered slot antenna for use in high-frequency transmission, and more particularly, to a tapered slot antenna provided with a corrugated structure, having an improved directivity, for use in mobile communication equipment, small information terminals, etc. 
     2. Description of the Background 
     Tapered slot antennas have a number of potential applications as single elements and focal plane arrays. They have important advantages such as being light in weight, less expensively manufactured with printed circuit board techniques that are capable of accurate replication from unit to unit. 
     FIG. 10 is a perspective view for exemplifying such a known tapered slot antenna. Referring to FIG. 10, the tapered slot antenna  100  includes a substrate  120  and a metal layer  130  disposed thereon. 
     The substrate  120  is made of a dielectric material such as polyimide, having a thickness of 10 to 100 microns. In addition, the metal layer  130  preferably made of copper has a thickness of 2 to 20 microns, with a tapered portion  14  etched away to expose a portion of the dielectric substrate  120 . This tapered portion  14  extends outward toward the aperture  15  of the slot antenna  100 . 
     The tapered slot antenna radiates the electromagnetic wave to the direction parallel to the antenna plane (that is, along a slot line). Since the tapered slot antenna has a structure similar to that of a slot line without requirements for a grounded conductor on the backside thereof unlike the micro strip line, it can be integrated more easily with various components such as, for example, feeder lines and matching circuits in a uniplanar structure. 
     In addition, since the tapered slot antenna has a broad band of transmission frequency and a high antenna gain, it is adequately used in mobile communication equipment, small information terminal and other wireless communication apparatuses. 
     For the known tapered slot antenna  100 , it is found that the D-plane cross-polarized component generally has a magnitude of as large as −6 dB to −9 dB. Referring to FIG. 11, the E- and H-planes of the tapered slot antenna are defined, respectively, as the planes which each includes the direction of radiation, and which is in parallel and in perpendicular to the surface plane of the antenna substrate  120 . In addition, the D-plane is defined as that which makes a 45° angle between both the E- and H-planes. Further, a cross-polarized wave is the electromagnetic wave which is polarized perpendicular to the polarization direction of the antenna. 
     A method of reducing the magnitude of a D-plane cross-polarized component is described by P. R. Acharya, J. Johansson and E. L. Kollberg, entitled SLOTLINE ANTENNA FOR MILLIMETER AND SUB-MILLIMETER WAVELENGTH, Proc. 20th Euro. Microwave Conf., Budapest, Hungary, pp. 353-358, September, 1990. 
     According to this publication, the magnitude of D-plane cross-polarized component can be reduced to −11 dB by fabricating a broken linearly tapered slot antenna (BLTSA). The tapered portion of this slot antenna has the shape composed of three compositional linear portions, each having consecutively different gradient. 
     This publication, however, does not detail further concerning the reasons why and how the D-plane cross-polarized component can be reduced. It is difficult, therefore, to fabricate such a slot antenna as above according to the description. 
     As indicated above, the D-plane cross-polarized component is large for the known tapered slot antenna. This gives rise to disadvantages for use in receiving more than one component, as exemplified hereinbelow: An antenna system is consisted of two tapered slot antennas with each slot line directed to the same direction and each antenna plane in perpendicular to each other to receive two polarized components separately. Because of the above-mentioned large intensity of D-plane cross-polarized component, one antenna in this system is sensitive not only to the component parallel to its own plane but also to the component perpendicular to its plane. Therefore, two polarized components can not be satisfactorily separated with the above antenna system. 
     In addition, the reasons for the aforementioned large magnitude of the D-plane cross-polarized component are yet to be clarified. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present disclosure to provide an improved tapered slot antenna, which overcomes the above-noted difficulties. 
     It is another object of the present disclosure to provide a tapered slot antenna, having a reduced magnitude of its D-plane cross-polarized component and an improved directivity, with clarifying the reasons for a large magnitude of the D-plane component. 
     To achieve the foregoing and other objects, and to overcome the shortcomings discussed above, a tapered slot antenna is provided including a substrate and a metal layer which is disposed on the substrate and removed in a region to form a tapered slot portion having a closed narrow end and extending outward toward a relatively wider end that is open. The tapered slot portion radiates the electromagnetic wave from the wider end. In addition, the tapered slot antenna also includes an electric field generating means for generating an electric field which counteracts a field component radiated from the end portion of the tapered slot perpendicular to the substrate. The electric field generating means is formed in outside edge portions of the substrate, having the distances of within 0.65 wavelengths between the edge of the wide end and an outside edge of the substrate. 
     According to one aspect disclosed herein, the electric field generating means in the tapered slot antenna comprises a corrugated structure which includes a plurality of slits formed periodically in opposing outer edge portions of the metal layer parallel to the direction of the radiation, having a slit depth ranging from 0.04 wavelengths to 0.12 wavelengths. 
     According to another aspect disclosed herein, the electric field generating means in the tapered slot antenna comprises a corrugated structure and a plurality of linear slits. The corrugated structure includes a plurality of slits formed periodically positioned in opposing outer edge portions of the metal layer parallel to the direction of the radiation, having a slit depth of the corrugate structure of at least 0.15 wavelengths. In addition, the linear slits are additionally formed in the metal layer between the tapered portion and the corrugate structure parallel to the direction of the radiation. 
     With these structures of the tapered slot antenna disclosed herein including the corrugated structure, an electric field component can adequately be generated so as to reduce the cross-polarized D-plane component and the electric field intensity at the substrate edge portions as well. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments will be described in detail with reference to the following drawings in which like reference numerals refers to like elements and wherein: 
     FIGS. 1A and 1B are perspective and top views illustrating a tapered slot antenna, respectively, according to one embodiment disclosed herein; 
     FIG. 2 is an enlarged view illustrating a portion of a tapered slot antenna according to one embodiment disclosed herein; 
     FIG. 3A shows the intensity measurement results of cross-polarized component in the D-plane of tapered slot antennas according to one embodiment disclosed herein; 
     FIG. 3B shows the intensity measurement results of cross-polarized component in the D-plane with various distances between the aperture edge and slot edge, for a tapered slot antenna according to one embodiment disclosed herein and for a prior tapered slot antenna; 
     FIGS. 4A,  4 B and  4 C show the directivity measurement results of the radiation intensity in the E-, H-, and D-planes for a tapered slot antenna according to one embodiment disclosed herein; 
     FIG. 4D shows the directivity measurement results of cross-polarized component in the D-plane for a tapered slot antenna according to one embodiment disclosed herein; 
     FIGS. 5A,  5 B and  5 C show the directivity measurement results of the radiation intensity in the E-, H-, and D-planes for a prior tapered slot antenna; 
     FIG. 5D shows the directivity measurement results of cross-polarized component in the D-plane for a prior tapered slot antenna; 
     FIGS. 6A and 6B are perspective and top views illustrating a tapered slot antenna, respectively, according to another embodiment disclosed herein; 
     FIG. 7 is an enlarged view of a portion of a tapered slot antenna according to a second embodiment disclosed herein; 
     FIG. 8 is a side view illustrating a tapered slot antenna of FIGS. 6A and 6B facing an antenna aperture  15 , wherein various field components are shown; 
     FIGS. 9A,  9 B and  9 C show the directivity measurement results of the radiation intensity in the E-, H-, and D-planes for a tapered slot antenna according to another embodiment disclosed herein; 
     FIG. 9D shows the directivity measurement results of cross-polarized component in the D-plane for a tapered slot antenna according to another embodiment disclosed herein; 
     FIG. 10 is a perspective view illustrating a prior tapered slot antenna; 
     FIG. 11 is a side view illustrating an antenna, wherein the E-, H-, and D-planes are shown; and 
     FIG. 12 is a side view illustrating a prior tapered slot antenna of FIG. 10 facing the aperture along the slot line. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 10 is a perspective view of a known tapered slot antenna  100  and FIG. 12 is a side view of the slot antenna of FIG. 10 facing the aperture  15  along the slot line. 
     Being radiated from a tapered slot antenna  100 , the electromagnetic wave is transferred from a traveling wave mode T to a free space mode F, as shown in FIGS. 10 and 12. 
     Referring to FIG. 12, the traveling wave mode T may be decomposed into components T 1 , T 3 , T 4  and T 6  perpendicular to, and components T 2  and T 5  parallel to the substrate  120 . 
     Since these perpendicular components are included as above, they are radiated also into free space to result in cross-polarized components. 
     Since the perpendicular components are highly symmetric with respect to either the E-plane (horizontal direction in FIG. 12) or the H-plane (vertical direction in FIG.  12 ), they offset one another. 
     With respect to the E-plane, for example, since the components T 1  and T 4  (or T 3  and. T 6 ) have the same magnitude with the opposite sense and are located at the same distance, the radiation field is therefore canceled along the E-plane. With respect to the H-plane in a similar manner to above, since the components T 1  and T 3  (or T 4  and T 6 ) also have the same magnitude with the opposite sense and are located at the same distance, the radiation field is canceled along the H-plane. The cross-polarized components in the E- and H-planes are, therefore, not greatly intensified, thereby causing no appreciable effects. 
     In contrast, since the perpendicular components are less symmetric with respect to the D-plane (45° tilted direction in FIG.  12 ), cross-polarized components may have an appreciable intensity along the H-plane. 
     Therefore, means for generating an electric field are disclosed in the following embodiments, for generating an electric field so as to counteract the perpendicular electric field component of the electromagnetic wave radiated from the aperture, to thereby adequately reduce the cross-polarized D-plane intensity. 
     FIGS. 1A and 1B are perspective and top views of a tapered slot antenna, respectively, according to a first embodiment of the invention. 
     Referring to FIGS. 1A and 1B, a tapered slot antenna  10  comprises a substrate  12  and a metal layer  13  disposed thereon. The substrate  12  is preferably made of a dielectric material such as polyimide (Kapton™ from Du Pont de Nemours, for example), having a thickness of 10 to 100 microns. In addition, the metal layer  13  preferably made of copper, as major ingredients, has a thickness of 2 to 20 microns, with a tapered portion  14  etched away to expose a portion of the dielectric substrate  12 . This tapered portion  14  extends toward the aperture  15  of the slot antenna  10 . 
     On the opposing edge portions of the metal layer  13 , which are located in the outer edges of the substrate  12  and not including the aperture  15 , a corrugated structure  16  is formed as shown in FIGS. 1A and 1B, having a plurality of slits periodically provided on these edge portions. 
     The use of such a corrugated structure for controlling the phase and intensity of the electric field is described by the present inventors and others (i.e., S. Sugahara, Y. Mita, K. Adachi, K. Mori and K. Mizuno, IEEE MTT-S International Microwave Symposium Digest, 1998, pages 533-536). It is described in this publication that an electric field component can adequately be radiated with this corrugated structure of FIGS. 1A and 1B, so as for the electric field component to counteract on a perpendicular component (i.e., component perpendicular to the substrate) of the electromagnetic wave radiated from the aperture portion  15  of the antenna  10 . 
     FIG. 2 is an enlarged view illustrating a portion of a tapered slot antenna  10  according to a first embodiment disclosed herein, provided with a corrugated structure  16 . Referring to FIG. 2, the corrugated structure  16  includes a plurality of slits  17  cut into the opposing outer edge portions of the metal layer  13 . These slits are formed periodically by removing rectangular portions of the metal layer  13 , having predetermined depth, width and pitch. As also shown in FIG. 2, the parameters d, w and c represent the depth, width and pitch of the slit  17 , respectively. In addition, the parameter L represents the distance between the edge of the aperture  15  and the outer edge of the slot antenna  10 . 
     A tapered slot antenna radiates the electromagnetic wave to the direction parallel to the antenna plane (that is, along the slot line). This capability is adequately utilized, when the antenna is incorporated in mobile communication equipment, small information terminal and other wireless communication apparatuses. 
     A variety of tapered slot antennas were fabricated and then subjected to various measurements of characteristic parameters in order to reduce cross-polarized components in the D-plane. Namely, a copper metal layer  13  was disposed having a thickness of about 5 microns, on a Kapton™ substrate  12  having a thickness of about 50 microns. Subsequently, three kinds of antennas d 4 cw 04 , d 5 cw 04  and d 5 cw 08 , were fabricated having fundamental antenna parameters such as an antenna length of 20 millimeters and an antenna aperture of 5 millimeters. 
     As to the aforementioned parameter L which represents the distance between the aperture edge and the outer edge of the antenna, the following values were selected such as, 2 millimeters (i.e., 0.4 wavelengths) for the antenna d 4 cw 04 , having the symbol prefixed by d 4 , and 2.5 millimeters (0.5 wavelengths) for the antennas d 5 cw 04  and d 5 cw 08 , each having the symbol prefixed by d 5 . 
     In addition, the slit width w and slit pitch c were selected, respectively, to be 0.2 millimeters (0.04 wavelengths) for the antennas d 4 cw 04  and d 5 cw 04 , each having the symbol suffixed by cw 04 , for the antenna dScw 08 , having the symbol suffixed by cw 08 . Further, the slit depth d was varied for respective antennas d 4 cw 04 , d 5 cw 04  and d 5 cw 08 . 
     The measurements related to the intensity were carried out at 60 GHz (5 mm in wavelengths). The results from the measurements are shown in FIG. 3A, in which the intensity (in dB) of cross-polarized D-plane components on the ordinate is plotted versus the pitch depth d, normalized to the wavelengths, on the abscissa. 
     The cross-polarized D-plane intensity was obtained larger than −10 dB for d value of 0 to 0.04 wavelengths for all three antennas. The intensity decreases rapidly with the increase in d for the range of 0.04 wavelengths to 0.12 wavelengths, and the lowest intensity of −7.5 dB was obtained for the antenna d 4 cw 04 , in particular. For d value of ranging from 0.12 wavelengths to 0.2 wavelengths, the intensity larger than −10 dB was obtained again for all antennas. 
     As indicated in FIG. 3A, the cross-polarized D-plane intensity is decreased considerably for the d value ranging from 0.06 wavelengths to 0.1 wavelengths. Accordingly, the tapered slot antenna according to the first embodiment is fabricated including a corrugated structure  16  with a slit depth preferably ranging from 0.04 wavelengths to 0.12 wavelengths, more preferably from 0.06 wavelengths to 0.1 wavelengths. With the present structure of the corrugate structure, the reduction in the cross-polarized D-plane intensity can be achieved for the tapered slot antenna  10 . 
     This reduction in the cross-polarized D-plane intensity in the tapered slot antenna is attained as follows. First, by acting with the corrugated structure  16  to reverse the phase, then to enhance the intensity, of the electric field at the edge portion of the substrate; and second, by generating an electric field in the vicinity of the aperture portion  15 , having the phase opposite to that of the perpendicular component (i.e., component perpendicular to the substrate) of the electric field radiated from the aperture  15 ; the cross-polarized D-plane intensity is then adequately reduced by superposing the above two components. 
     Another set of tapered slot antennas were fabricated and subsequently subjected to measurements to determine an optimum value of the distance L between the edge of the aperture  15  and the outer edge of the slot antenna  10 . 
     Namely, a copper metal layer  13  was disposed having a thickness of about 5 microns, on a Kapton™ substrate  12  having a thickness of about 50 microns. Subsequently, the antennas were formed, which are each various in the distances L between the edge of the aperture and outer edge of the antenna, and has the fundamental antenna parameters such as an antenna length of 20 millimeters and antenna aperture of 5 millimeters. In addition, the antennas were provided in both edge portions with a corrugated structure having a slit depth d of 0.4 millimeters (0.08 wavelengths), a slit width w of 0.2 millimeters (0.04 wavelengths, and a pitch of 0.4 millimeters (0.08 wavelengths). 
     With the thus formed antennas, the cross-polarized intensity in the D-plane was measured for each antenna with various L values. The measurements were carried out at 60 GHz (5 mm in wavelength) to obtain the results shown in FIG. 3B, in which the intensity (dB) of cross-polarized D-plane components on the ordinate is plotted versus the distances L between the aperture edge and slot edge, normalized to free space wavelength. 
     There also shown in FIG. 3B for comparison are the results obtained for a prior antenna without a corrugate structure. The latter results are shown by the dotted curve in FIG. 3B, while the results from the measurements of presently embodied antenna are shown by the solid curve, which are related to the antenna provided with the corrugated structure mentioned above. 
     For the tapered slot antenna with the corrugated structure disclosed herein, it is shown in FIG. 3B that the intensity of cross-polarized D-plane component decreases gradually with the increase in the L value ranging from 0.3 to 0.4. This results in a lowest intensity of approximately −17.5 dB at around 0.4 wavelengths, then gradually increases with the increase in the L value ranging from 0.4 wavelengths to 0.7 wavelengths. For the prior antenna noted above, however, the intensity of cross-polarized D-plane component remains almost constant throughout the L values in the above range. 
     In addition, it is shown in FIG. 3B that the intensity of cross-polarized D-plane component for the tapered slot antenna disclosed herein is lower than that of the known antenna throughout the L values in the above range. It is also shown that the intensity of cross-polarized D-plane component for the tapered slot antenna is adequately decreased for L values in the range of 0.6 wavelengths or below. 
     Incidentally, it is known that the electric field radiated electromagnetic wave is, in general, distributed primarily within the spatial range of four tenths of its wavelength toward the outside of the tapered portion. Therefore, it may be assumed that a corrugated structure needs to be formed at least within the distance of one quarter wavelength from the edge of the tapered portion  14  in order for an electric field component to adequately be generated and to counteract on the electric field component. 
     Therefore, it is considered that the corrugated structure in the present embodiment is preferably formed within the distance of 0.65 wavelengths from the edge of the tapered portion  14 . 
     The results shown in FIG. 3B appear to confirm the aforementioned requirements for the desirable tapered slot antenna. Accordingly, the tapered slot antenna  10  in the present embodiment is provided with the corrugated structure  16  which is preferably formed within the distance L of 0.65 wavelengths between the tapered portion  14  and the corrugated structure (that is, between the edge of the aperture and the edge of the slot antenna). The intensity of cross-polarized D-plane component increases for the L value of 0.3 wavelengths or less, and this is indicative of the degradation of the antenna characteristics in this range of L, as also as shown in FIG.  3 B. The lower limit of the L values is, therefore, preferably 0.3 wavelengths. With this structure, the intensity of cross-polarized D-plane component of the tapered slot antenna  10  is adequately decreased. 
     According to the results from the two experiments, it is clearly indicated that the desirable tapered slot antenna  10  can be fabricated by providing the corrugated structure  16  which has a slit depth preferably ranging from 0.04 wavelengths to 0.12 wavelengths, more preferably from 0.06 wavelengths to 0.1 wavelengths, and which is formed within the distance of 0.65 wavelengths from the edge of the tapered portion  14 . With the present structure, an electric field can be generated so as to counteract to a perpendicular field component to the substrate of the electromagnetic wave radiated from the aperture  15 , to thereby be able to reduce the cross-polarized D-plane intensity most efficiently. 
     Subsequently, in order to measure directivity in each E-, D-, and H-plane, a tapered slot antenna was fabricated according to the first embodiment. Namely, a copper metal layer  13  was disposed having a thickness of about 5 microns, on a polyimide substrate  12  having a thickness of about 50 microns. Subsequently, the antenna was formed, which had an antenna length of 20 millimeters, antenna aperture of 5 millimeters, and distances L between the aperture and slot edge of 2.5 millimeters (0.5 wavelengths). In addition, at the end of both edge portions of the antenna, a corrugated structure was formed having a slit depth d of 0.4 millimeters (0.08 wavelengths), slit width w of 0.2 millimeters (0.04 wavelengths), and pitch of 0.4 millimeters (0.08 wavelengths). Further, the slot antenna was designed for the electromagnetic wave of 60 GHz. 
     The results from the measurements of the directivity of the radiation intensity in the E-, H-, and D-planes are shown in FIGS. 4A,  4 B and  4 C, respectively. The results of the directivity measurements of cross-polarized component in the D-plane are also shown in FIG.  4 D. Throughout FIGS. 4A through 4D, the intensities in dB on the ordinate are plotted versus radiation angles in degrees on the abscissa. 
     Comparative results from directivity measurements are shown in FIGS. 5A through 5D for a prior antenna without a corrugate structure. These results of the radiation intensity in the E-, H-, and D-planes, and the directivity of the cross-polarized component in the D-plane are shown in FIGS. 5A,  5 B,  5 C and  5 D, respectively in a similar manner to the FIGS. 4A through 4D. 
     The cross-polarized component in the D-plane for the known antenna is as high as B 9  dB as shown in FIG. 5D, while that component for the antenna disclosed herein is obtained as low as B 13  dB, as shown in FIG.  4 D. 
     As described above, the tapered slot antenna  10  was formed according to the first embodiment, having the distances L of within 0.65 wavelengths between the edge of the aperture and the outer edge of the substrate. The tapered slot antenna  10  was provided with a, corrugated structure at the end of both edge portions of the substrate  12  parallel to the direction of the radiation, preferably having a slit depth d ranging from 0.04 wavelengths to 0.12 wavelengths, more preferably ranging from 0.06 wavelengths to 0.1 wavelengths. With this structure of the tapered slot antenna, an electric field can adequately be generated so as to counteract to a perpendicular field component of the electric field radiated from the aperture  15 , and this enables to adequately reduce the cross-polarized D-plane intensity most efficiently. 
     Although the shape of slit of the corrugated structure was assumed to be rectangular in the first embodiment, it is needless to say that other slit shapes may also be adopted as long the slits give rise to the similar results as above on the cross-polarized D-plane intensity. 
     FIGS. 6A and 6B are perspective and top views illustrating a tapered slot antenna, respectively, according to a second embodiment of the invention. 
     Referring to FIGS. 6A and 6B, a tapered slot antenna  20  comprises a substrate  22  and a metal layer  23  disposed thereon. 
     The metal layer  23  was subsequently provided with a plurality of linear slits S 1 , S 2  which were formed parallel to the direction of radiation between a corrugated structure  16  and a tapered portion  15 . Since the overall structure of the slot antenna  20  is similar to that of the aforementioned antenna  10  except for the slit portions S 1 , S 2 , a detailed description on the structure is abbreviated herein. 
     FIG. 7 is an enlarged view of a portion of a tapered slot antenna  20  according to the second embodiment. 
     Referring to FIG. 7, the corrugated structure  16  includes a series of slits  17  cut into the opposing outer edge portions of the metal layer  23 . These slits were formed by periodically removing rectangular portions of the metal layer  23 . In addition, a plurality of linear slits S 1 , S 2  were formed by etching away to expose portions of the dielectric substrate  22 . The linear slits S 1 , S 2  were located between a corrugated structure  16  and a tapered portion  15 , approximately in parallel to the direction of electromagnetic radiation and to the edge of the substrate  22 . 
     As described earlier, an adequate electric field is generated intentionally by the tapered slot antenna  10  according to the first embodiment. This electric field is generated in the vicinity of the aperture portion  15  so as to have the phase opposite to that perpendicular field component (i.e., component perpendicular to the substrate) of the electric field radiated from the aperture  15 , to thereby offset each other. At the edge of the substrate, however, these two electric field components are in the same phase, thereby being superimposed to increase the intensity. Such increase in the intensity at the substrate edge results in the increase in the crosstalk between neighboring compositional antennas in an antenna array structure. 
     Accordingly, it is an object of the second embodiment to provide a tapered slot antenna  20  and thereby reduce an electric field intensity at the edge of the substrate by forming linear slits S 1 , S 2  between the corrugated structure  16  and tapered portion  15 . 
     In this context, the use of a corrugated structure for suppressing such an electric filed intensity has been reported by the present inventors and others in the aforementioned publication (IEEE MTT-S International Microwave Symposium Digest, 1998, pages 533-536), in which the corrugated structure is suggested to have a slit depth of preferably at least 0.15 wavelengths. 
     FIG. 8 is a side view of the tapered slot antenna  20  of FIGS. 6A and 6B facing an antenna aperture  15 . Referring to FIG. 8, there shown are a transmission mode T in slot line and an electric field G in the slit region. In addition, T 3  represents an electric field component generated by the transmission mode T, and G 1  and G 2  represent each an electric field component perpendicular to the substrate  22 , generated by the electric field G. Although the above description of the field component was made for the slits S 1 , S 2  at one end of the substrate, this is also true for the slits at the other end of the substrate. 
     As indicated in FIG. 8, the components T 3  and G 1  tend to cancel each other, while T 3  and G 2  tend to intensify. Since the components T 3  and G 1  are located spatially close to each other and in the same phase, they cancel each other. In contrast, since the components T 3  and G 2  are located separated to each other, they are not exactly in the same phase. This causes a slight shift in the phase to thereby result for these components to intensify each other to a certain extent but not complete because of the phase shift. Therefore, the electric field intensity at the end of the substrate can be suppressed and the cross-polarized component in the D-plane can be reduced. 
     In the present embodiment, the slits may also be formed in a manner other than described above, as long as the desirable slit characteristics are satisfied. 
     For example, the linear slits may be formed such that the number thereof is more than two, and/or the configuration thereof is not strictly in parallel to the direction of the radiation. In addition, the shape may also be other than linear slit mentioned above, including the combination of various other shapes. Further, the number, and the width and depth, of the slits and the combination thereof may adequately be selected depending on the requirements for the antenna used. Still further, although the shape of the slit  17  of the corrugated structure  16  was described earlier to be rectangular, it is needless to say that other shapes of the slit may also be adopted as long as the slit gives rise to the similar results as indicated earlier. 
     In order to measure directivity in each E-, D-, and H-plane, a tapered slot antenna was fabricated according to the second embodiment. Namely, a copper metal layer  13  was disposed having a thickness of about 5 microns, on a polyimide substrate  12  having a thickness of about 50 microns. Subsequently, the antenna was formed, which had an antenna length of 20 millimeters, antenna aperture of 5 millimeters, and distances L between the aperture and slot edge of 2.5 millimeters (0.5 wavelengths). In addition, at the end of both edge portions of the antenna, a corrugated structure was formed having a slit depth d of 0.8 millimeters (0.16 wavelengths), slit width w of 0.2 millimeters (0.04 wavelengths), and pitch of 0.4 millimeters (0.08 wavelengths). 
     Further, between the corrugated structure  16  and a tapered portion  14 , linear slits S 2  and S 1  were respectively formed, having a width of 0.3 millimeters (0.06 wavelengths) located from 1.2 millimeters (0.24 wavelengths) to 1.5 millimeters (0.3 wavelengths) inside the substrate edge, and a width of 0.2 millimeters (0.04 wavelengths) located from 1.8 millimeters (0.36 wavelengths) to 2.0 millimeters (0.4 wavelengths) inside the substrate edge. 
     Still further, the tapered slot antenna was designed for the electromagnetic wave of 60 GHz. 
     The results from the directivity measurements of the radiation intensity in the E-, H-, and D-planes are shown in FIGS. 9A,  9 B and  9 C, respectively. The results from similar measurements of cross-polarized component in the D-plane are also shown in FIG.  9 D. Throughout FIGS. 9A through 9D, the intensities in dB on the ordinate are plotted versus the radiation angle in degrees on the abscissa. 
     The cross-polarized component in the D-plane for the prior antenna is as high as B 9  dB as shown earlier (FIG.  5 D), while that component for the antenna disclosed herein is obtained as low as −13.2 dB, as shown in FIG.  9 D. 
     As described above, the tapered slot antenna  20  was formed according to the second embodiment, further provided with the linear slits formed between the corrugated structure and tapered portion; in addition to the corrugated structure formed at the end of both edge portions of the substrate  22 , preferably having a plurality of slits  17 , each having a depth d of 0.15 wavelengths. 
     With this structure of the slot antenna, an electric field component can adequately be generated to offset the perpendicular field component of the electric field radiated from the aperture  15 . As a result, an undesirable increase in the electric field intensity at the substrate edge portions can be suppressed, and this enables to decrease the aforementioned crosstalk between neighboring compositional antennas. Therefore, the above-mentioned structure can be utilized adequately in an antenna system such as an antenna array composed of a plurality of the antennas disclosed herein. 
     As disclosed earlier, by generating an electric field having an opposite phase from a portion (as the field generating means) formed within the distance of 0.65 wavelengths from the edge of the tapered portion, the cross-polarized D-plane intensity can efficiently be reduced. This is accomplished with the generated electric field having the opposite phase by counteracting to the electric field component perpendicular to the substrate of the electromagnetic wave radiated from the aperture. 
     In addition, the electric field generating means may be formed as the corrugated structure in the tapered slot antenna, which includes a plurality of slits formed periodically positioned in opposing outer edge portions of the metal layer parallel to the direction of the radiation and has a depth of the slit ranging from 0.04 wavelengths to 0.12 wavelengths. The electric field is generated by this corrugated structure, having the opposite phase to counteract a perpendicular field component of the electromagnetic wave radiated from the aperture, to thereby be able to reduce the cross-polarized D-plane intensity. 
     Further, the electric field generating means may also be formed as the combination of the corrugated structure and the plurality of linear slits. As aforementioned, the corrugated structure includes a plurality of slits formed periodically positioned in opposing outer edge portions of the metal layer parallel to the direction of the radiation, having a depth of the slit of the corrugate structure of at least 0.15 wavelengths. In addition, the linear slits are formed in the metal layer between said tapered portion and the corrugate structure parallel to the direction of the radiation. The electric field is generated by this structure, having the opposite phase to counteract the perpendicular field component of the electromagnetic wave radiated from the aperture, to thereby be able to reduce the cross-polarized D-plane intensity. 
     As a result, an undesirable increase in the electric field intensity at the substrate edge portions can be suppressed, and this enables to decrease the aforementioned crosstalk between neighboring compositional antennas, to thereby be adequately utilized in an antenna system such as an antenna array composed of a plurality of the antennas disclosed herein. 
     This document claims priority and contains subject matter related to Japanese Patent Application No. 10-375826, filed with the Japanese Patent Office on Dec. 18, 1998, the entire contents of which are hereby incorporated by reference. 
     Additional modifications to, and variations of, the embodiments described above may be made without departing from the spirit and the scope of the present invention as defined in the appended claims.