Antenna device having low side-lobe characteristics

Abstract: An antenna device having low side-lobe characteristics comprising a pair of array antennas having the same construction. Each array antenna has array elements equidistantly arranged. The center-to-center distance between the array antennas is so determined that the angle of the first zero point of the array factor determined by the center-to-center distance equals the angle of the first side lobe point of the pattern of each array antenna. The array antennas are electrically connected so as to become excited in phase.

Description: The present invention relates to an antenna apparatus which reduces the side lobes without increasing the beam width of the antenna pattern. The antenna pattern of many antennas, including receiving antennas, is improved as the beam width and the side lobes thereof (which serve as indices, of a good antenna pattern) are reduced. An array antenna with reduced side lobes is known from document US-A-4 580 141. Another known antenna device comprising two antennas arranged apart from each other utilizes the multiplication principle of the directional characteristics of antennas in order to reduce the beam width of the antenna device. According to this principle, the combined pattern of the antenna device is obtained by multiplying the pattern of the individual antennas by the array factor of the antenna device. Fig. 1 of the accompanying drawings schematically illustrates such an antenna device. The antenna device comprises first and second antennas 101, 102 which are arranged so that the distance a between the centres of the first and second antennas 101, 102 is equal to or greater than the aperture length b of each of the antennas 101, 102. By this arrangement, the angle of the first zero point of the array factor of the antenna device becomes smaller than the angle of the zero point of the pattern of the individual antennas 101, 102, thereby reducing the beam width of the antenna device. However, the conventional art, including the above-described method for reducing the beam width, fails to reduce either one of the beam width and the level of side lobes, that is, the indices of a good antenna pattern, without increasing the other. According to the conventional art, a reduction of the beam width results in an increase of the level of side lobes, and a reduction of the level of side lobes results in an increase of the beam width. This drawback of the conventional art may cause problems. For example, if the side lobe level of a radar antenna is reduced and, therefore, the beam width thereof is inevitably increased, the resolution of the radar deteriorates, thus reducing the object distinguishing power of the radar. In such a case, the radar may fail to distinguish a plurality of objects and, instead, recognize them as a single object. If the beam width of a radar is reduced and, therefore, the side lobe level is inevitably increased, the radar may make an error in determining whether there are any objects in the direction of the beam (the observation direction). More specifically, if no object exists in the observation direction but an object exists in the direction of the thus-enhanced side lobe, the radar may determine that there is an object in the observation direction. Because neither one of the beam width and the side lobe level can be reduced without increasing the other, the conventional art merely provides a compromise solution based on distributions, for example, Chebyshev distribution, in which the minimum beam width is obtained with respect to a certain side lobe level, or in which the minimum side lobe level is obtained with respect to a certain beam width. Accordingly, an object of the present invention is to provide an antenna device which reduces the side lobe level of the antenna pattern without increasing the beam width thereof. To achieve the object of the present invention, the antenna device of the present invention comprises: a pair of array antennas having the same construction and being arranged so that the centres of the array antennas are spaced apart from each other by a center-to-center distance, the center-to-center distance being determined so that the angle of the first zero point of the array factor determined by the center-to-center distance equals the angle of the first side lobe point of the pattern of each of the array antennas; and means for electrically connecting the array antennas in phase, thereby reducing the side lobe level of the combined antenna pattern of the antenna device. The pattern of the antenna device thus constructed becomes a combined pattern obtained by multiplying the pattern of the individual array antennas by the array factor determined based on the distance between the centers of the array antennas, according to the multiplication principle of the directional characteristics of array antennas. Because, according to the present invention, the pair of antennas arrays are so arranged that the angle of the first zero point of the array factor equals the angle of the first side lobe point of the pattern of the individual array antennas, the antenna device achieves a combined antenna pattern in which the first side lobe is eliminated at the angle of the first side lobe point. Since the first side lobe is generally the largest of all the side lobes in an antenna pattern, elimination of the first side lobe at the angle of the first side lobe point significantly reduces the total side lobe level. Incidentally, because the distance between the centers of the two antennas must be smaller than the size of the aperture of the antennas in order to equalize the angle of the first zero point of the array factor to the angle of the first side lobe point of the individual antennas, the present invention is not applicable to an antenna having a real aperture, such as a parabola antenna. Thus, the present invention must employ array antennas. The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which:- Fig. 1 illustrates the construction of a known antenna; Fig. 2 is a schematic diagram illustrating the principles of an antenna device in accordance with the present invention; Fig. 3 illustrates an example of the in-phase coupling of an array antenna according to the present invention; Fig. 4 indicates the power pattern of each array antenna of an antenna device according to the present invention; Fig. 5 indicates the pattern of the array factor based on the center distance between the array antennas of an antenna device according to the present invention ; Fig. 6 indicates the combined power pattern of an antenna device according to the present invention ; Fig. 7 indicates the power pattern of the known antenna device shown in Fig. 1 ; Fig. 8 indicates the pattern of the array factor of the known antenna device shown in Fig. 1 ; Fig. 9 indicates the combined power pattern of the known antenna device shown in Fig. 1 ; Fig. 10 illustrates the construction of an antenna device according to the present invention ; and Fig. 11 illustrates an equivalent circuit of the antenna device shown in Fig. 10. Referring to Fig. 2, two array antennas 1, 2 have the same construction in which a number N (13 in Fig. 2) of array elements 3 are arranged leaving intervals d along the x axis indicated by the arrow x in the figure. The two array antennas 1, 2 are arranged so that the center points P1, P2 of the array antennas 1, 2 are slightly apart from each other. More specifically, a distance d' between the center points P1, P2 of the array antennas 1, 2 (hereinafter, referred to as "the center-to-center distance d'") is so determined that the angle of the first zero point of the array factor determined by the center-to-center distance d' equals the angle of the first side lobe point of the pattern of the individual array antennas 1, 2. The array antennas 1, 2 are electrically connected in phase so as to become excited in phase. "The antennas 1, 2 are electrically connected in phase" means that all the feed lines connecting a feed point S to the individual array elements 3 have the same length. This in-phase connection is not illustrated in Fig. 2 because it would complicate the drawings. Fig. 3 illustrates an example of the wiring system for achieving the in-phase connection. Besides the wiring system as shown in Fig. 3, other methods may be employed to achieve the in-phase connection, for example: a method in which phase shifters are provided in the feed lines; and a method in which the lengths of the feed lines of array elements relatively close to the feed point S are increased. The antenna device thus constructed can be used as both a transmitting antenna and a receiving antenna without having to make any change in the construction. Because the combined pattern of the antenna device is obtained by multiplying the pattern of the individual array antennas by the array factor according to the multiplication principle, the above antenna device, in which the angle of the first zero point of the array factor is equal to the angle of the first side lobe point, achieves a combined pattern in which the first side lobe is reduced. Next explained will be determination of the center-to-center distance d' which achieves an optimal reduction of the side lobe level in the case where each of the array antennas 1, 2 has a number N of array elements 3 arranged equidistantly at intervals d and has a uniform electric field distribution. First, the pattern of each array antenna is obtained from the following expression (1): [sin (N·2π/λ·d/2·sin) / {N·sin(2π/λ·d/2·sin)}]·g() where λ is the radio wave wavelength,  is the angle from the antenna beam direction, g() is the pattern of the array elements of the array antenna. Based on the expression (1), the angle  of the first side lobe point approximately satisfies the following expression (2): N·2π/λ·d/2·sin = 3π/2 The array factor is obtained from the following expression (3): cos(2π/λ·d'/2·sin) where d' is the center-to-center distance between the array antennas 1 and 2. The conditions by which the array factor provides the first zero point at the angle  which satisfies the expression (2) are obtained from the following expression (4): 2π/λ·d'/2·sin = π/2 Therefore, based on the expressions (2) and (4), the optimal center-to-center distance d' is written as the following expression (5): d' = N/3·d The expression (5) requires a condition where N ≠ 3n (n being a positive integer) because if N is a multiple of 3, then d' becomes a multiple of d, resulting in overlap of array elements of the array antennas 1 and 2. Although the optimal center-to-center distance d' has been thus obtained on the assumption that the array antennas have a uniform electric field distribution, optimal center-to-center distances for antennas having other patterns of electric field distribution can be obtained in generally the same manner. Figs. 4 to 6 show the results of the simulation of an antenna device as shown in Fig. 2 according to the present invention which reduces the side lobe level. The simulation was performed on the assumption that each of the array antennas of the antenna device had a uniform electric field distribution and comprised 31 array elements (N = 31) arranged equidistantly at intervals of 0.5λ (d = 0.5λ) and, further, that the array elements were half-wave dipole antennas with reflectors (the distance between the array elements and the reflectors being λ/4) which were arranged so that the dipole axes were parallel to the y axis perpendicular to the x axis. Fig. 4 shows the power pattern of the individual array antennas 1 and 2. Fig. 5 shows the pattern of the array factor determined based on the center-to-center distance d' (= 31/3·d ≈ 5.17λ) between the array antennas 1 and 2. Fig. 6 shows the combined power pattern of the antenna device constructed as shown in Fig. 2. These figures indicate that the maximum side lobe level can be reduced from about -13 dB in the pattern of the individual array antennas to about -18 dB in the combined pattern of the antenna device according to the present invention. The figures further indicate that even the beam width can be slightly reduced. For comparison, Figs. 7 to 9 shows the results of the simulation of the known antenna device, as shown in Fig. 1, in which the center-to-center distance is greater than the aperture length of each array antenna. The simulation was performed on the assumption that the two array antennas of the known antenna device were the same as those employed in the antenna device according to the present invention but shifted away from each other by a center-to-center distance d' = 20λ. Fig. 7 shows the power pattern of the individual array antennas. Fig. 8 shows the pattern of the array factor determined based on the center-to-center distance between the two array antennas. Fig. 9 shows the combined power pattern of the conventional antenna device. These figures indicate that the conventional antenna device achieves almost no reduction of the maximum side-lobe level. Fig. 10 illustrates the construction of an antenna device according to the present invention. Each of array antennas 11 and 12 comprises patch antennas 13 and 14, respectively, as the array elements. All the patch antennas 13, 14 of the array antennas 11, 12 are connected in phase. The equivalent circuit of this antenna device is shown in Fig. 11. As described above, the antenna device of the present invention achieves a combined antenna pattern in which the first side lobe is eliminated at the angle of the first side lobe point of each array antenna, thus reducing the side lobe level without increasing the beam width. While the present invention has been described with reference to what is presently considered to be the preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments.

Claims: An antenna device having low side-lobe characteristics comprising: a pair of array antennas having the same construction and being arranged so that the centres of said array antennas are spaced apart from each other by a centre-to-centre distance, the centre-to-centre distance being determined so that the angle of the first zero point of the array factor determined by the centre-to-centre distance equals the angle of the first side lobe point of the pattern of each of said array antennas; and means for electrically connecting said array antennas in phase, whereby the side lobe level of the combined antenna pattern of said antenna device is reduced. An antenna device having low side-lobe characteristics according to claim 1, wherein each of said array antennas comprises elements equidistantly arranged so as to achieve a uniform electric field distribution and wherein the centre-to-centre distance is determined by a following formula: d' = (N/3).d where: d' is the centre-to-centre distance; N is the number of elements of each of said array antennas, excluding multiples of 3; and d is the interval between elements. An antenna device having low side-lobe characteristics according to claim 1 or 2, wherein said means includes a feed point and feed lines connecting said feed point individually to elements of each of said array antennas, said feed lines having the same length. An antenna device having low side-lobe characteristics according to claim 1 or 2, wherein said means includes a feed point, feed lines connecting said feed point individually to elements of each of said array antennas, and a phase shifter provided for at least one of said feed lines.

Inventors: SEZAI TOSHIHIRO, SEZAI, TOSHIHIRO, SEZAI, TOSHIHIRO, C/O NATIONAL SPACE DEVELOPMENT
Applicants: NAT SPACE DEV AGENCY, NATIONAL SPACE DEVELOPMENT AGENCY OF JAPAN
Application year: 1993
Main classification: H01Q21
IPC codes: H01Q21