GB2231203A

GB2231203A - An antenna system for shaped beam

Info

Publication number: GB2231203A
Application number: GB9005614A
Authority: GB
Inventors: Shinichi Nomoto; Fumio Watanabe; Yoshihiko Mizuguchi
Original assignee: Kokusai Denshin Denwa KK
Current assignee: KDDI Corp
Priority date: 1989-03-14
Filing date: 1990-03-13
Publication date: 1990-11-07
Anticipated expiration: 2010-03-13
Also published as: GB9005614D0; GB2231203B

Abstract

A shaped beam antenna system which provides a desired shape of beam, comprises a reflector (11) and at least one primary radiator located essentially at a focus of the reflector. The reflection surface (12) of the reflector is formed by a dense set of parabolas in which the vertices of the parabolas lie on a predetermined locus which is a space curve, but not a plane curve. Preferably, the space curve is three dimensional. The primary radiator may be composed of a plurality of primary radiators positioned close to each other. <IMAGE>

Description

An Antenna System for Shaped Beam The present invention relates to a shaped beam antenna system, in particular, relates to such an antenna system which illuminates the desired shape of area.

A shaped beam is obtained by a paraboloidal reflector with a primary radiator. When a primary radiator is positioned so that the phase center of the same coincides with the focus of the paraboloidal reflector, the wave-front of the reflected wave is in plane, thus a sharp beam is radiated in the principal axis direction of the parabola. On the contrary, a paraboloidal reflector focuses a plane wave incoming from the principal axis upon the focus. In fact, the radiation pattern of an antenna is the same whether it is used as a transmitting antenna or a receiving antenna according to the reciprocity theorem of an antenna pattern. So, the following description is directed to a transmit antenna, but it should be appreciated of course that a receive antenna is possible in the similar manner.

Conventionally, a reflector has been used so that a wave is radiated sharply in a desired direction, and minimal wave is radiated in an undesired direction. A pencil beam, or a sharp beam, has been obtained by using a paraboloidal reflector.

On the other hand, when a shaped beam which illuminates a desired shaped service area is requested, a fan-shaped beam is necessary.

An example of-prlor-art for providing a fan-shaped beam is the use of a plurality of pencil beams each of which illuminates related different areas independently.

Fig.ll shows the prior shaped beam antenna system, in which Fig.llA is a perspective view, Fig.llB is a cross section of the array of the primary radiators, and Fig.llC is the equi-level contour pattern in which shape of a beam footprint is shown on a specified surface formed by the intersection of the surface. In the figure, the numeral 101 is a reflector, 102a through 102e are primary radiators, 103a through 103e are feeders for feeding said primary radiators, 104 is a beam forming network, 105 through 105e are element beams, 106a through 106e are equi-level contour pattern of element beams, and 107 is the equi-level contour pattern of the combined shaped beam. The reflector 101 is a paraboloidal reflector, - and 5x5(=25) primary radiators are used in the embodiment.Each of the element beams 105a through 105e is a pencil beam, and provides the small circle of equi-level pattern as shown by the numerals 106a through 106e. When the primary radiators 1OSa through 105e are excited simultaneously through the beam forming network 104, the whole shaped beam 107 which is the sum of the element beams 106a through 106e is obtained. When the beam forming network 104 adjusts the amplitude and the phase of the exciting signal applied to each primary radiator, a desired shaped beam is obtained.

The antenna system of Fig.ll has been used as a satellite antenna for illuminating a desired area on the earth.

However, the prior antenna system of Fig.ll has the disadvantage that so many primary radiators (25 radiators in Fig.ll) must be used, and therefore, the same number of feed lines must be used, and the structure of the beam forming network 104 must be complicated. Further, the minimum spacing between the adjacent element beams is restricted by the physical size of the primary radiators.

When the spacing between two adjacent element beams is large, the electric power flux density on the earth is not uniform, but said flux density is weak at the gap area between two adjacent element beams. Therefore, it is difficult to provide a shaped beam uniform flux density.

Another prior art for providing a shaped beam is the use of a reflector and a single primary radiator in which said reflector is a cylindrical paraboloid; a dense set of parabolas shifted parallel along a predetermined straight line. That prior art is described in accordance with Figs.8 through 10.

Fig.8 shows the reflector of the second prior art, in which the numeral 1 is a reflector, 2 is a reflection plane which is a part of said reflector 1, 2a is an edge of said reflection plane, 3 is a cylindrical paraboloid which composes said reflection surface 2, 4 is a straight line -on -whictr-wertese3 4a -o-f- parabolas - 3a -lncate-.

The curved surface 3 is a dense set of- 'parabolas 3a which shift parallel so that the locus of the vertexes of the parabolas 3a is the straight line 4. In other words, said curved surface 3 is a cylindrical paraboloid. The reflector 1 can provide a fan-beam on an elongated service area.

Fig.9 shows the shape of the fan-beam of the reflector 1 in the form of the equi-level contour pattern, in which the horizontal axis shows the horizontal angle and the vertical axis shows the vertical angle. The numeral 8 shows the equi-level contour pattern and the numerals in the igure-show the level in dB.

However, the second prior art described in accordance with Figs.8 through 9 has the disadvantage that the elongated contour is only in straight linear shape, and another shape of fan-beam whose footprint is a curvilinear contour is impossible. This is explained in accordance with Fig.10.

In Fig.10, the numeral A(9a) is the position of a reflector, B and C are ends of an target to be illuminated, D is a foot of a perpendicular on the surface which includes the arc BC(9b) from the point A, 7 is a fan-beam having the shape relating to the arc BC, and 9c is a linear line between the points B and C. When a reflector 1 is positioned at the point A(9a), and the object on the arc BC(9b) on the plane with the spacing (h) from the point A is illuminated, the fan-beam must be in the curved shape 7 which is curved similar to the arc (9b). However, the second prior art which has a reflector 1 with the reflection plane by a cylindrical paraboloid can only provide a linear shaped beam for the linear line 9c, but not a curved shaped beam for the arc BC.Therefore, the second prior art cannot provide a curved fan-beam when h is not zero, although it can provide the same when h is zero.

In accordance with the present invention, an antenna system has at least a reflector and at least one primary radiator or receiver positioned substantially at the focus of the reflector, wherein the reflector has a reflection surface which is a part of a dense set of parabolas whose vertices lie on a predetermined locus comprising a space curve, the principal axis of each parabola extending in the direction of a target on a service area to be irradiated by the antenna system or from which radiation is to be received.

The present invention provides an antenna systerr which can provide â desired shape of beam. In a preferred example, the invention provides a desired shape of beam. by using a small number of primary radiators.

Some examples of antennae according to the present invention and conventional antennae are shown in the accompanying drawings, in which: Fig.l shows an embodiment of a reflector according to the present invention, Fig.2 shows the explanatory drawing for explaining a reflector according to the present invention, Fig.3 shows an equi-level contour pattern of a reflector acccording to the present invention, Fig.4 shows another reflector according to the present invention, Fig.5 shows still another embodiment of the antenna system according to the present invention, Fig.6 shows still another embodiment of the antenna system according to the present invention, Fig.7 shows still another embodiment of the antenna system according to the present invention, Fig.8 shows a prior reflector, Fig.9 shows a prior equi-level pattern, Fig.10 shows a required fan-beam, and Fig.ll is a prior antenna system.

Fig.l shows the first embodiment of the present invention, in which Fig.lA is a perspective view of a reflector, and Fig.lB is a front view of a reflector.

The reflection surface 12 of the reflector 11 is a part of the curved paraboloidal plane 13 which is the dense set of the parabola 13a, and said reflector surface 12 is restricted by the edge 12a. The important feature of the present invention is that the locus 14 of the vertexes of the parabola 13a is a space curve (three dimentional curve), but not a plane curve. The electromagnetic wave radiated by the primary radiator 15 is reflected by the reflector 12, and the reflected wave produces an elongated curved fan-beam, since the locus 14 of the vertexes 14a of the paraboloid 13a is a space curve which has double curvature.

The locus 14 is designed as follows.

In a first method, the locus 14 is obtained by solving the ordinary differential equation which uses the geometrical optics approximation which has been used for the design and analysis of a reflector. That is to say, the locus 14 is obtained by using Snell's law of reflection for reflection of a ray composing a beam, and the power conservation law for an input beam energy and an output beam power, and is obtained by the following equation.

Pldw = P2d (1) where P1 is a radiation power pattern of a primary radiator 15, P2 is a secondary power pattern of a reflector 11, dw is a cubic angle element subtended by a part of the parabola 13a within the edge 12a for the primary radiator 15, and dW is the plane angle element relating to the angle of the output beam reflected by the parabola 13a.

The equation (1) is an ordinary differential equation for the locus 14, and therefore, the accurate numerical solution is obtained by using the Runge-Kutla method.

Alternatively, the locus 14 is obtained by using the computer-aided optimization method through numerical simulation for providing the optimum solution. In that case, the initial coordinates of the locus 14 is determined to be analogous to the central curve (19 in Fig.3) of the footprint of the fan-beam, and the shape of the fan-beam is calculated for the provisional locus 14.

Next, the coordinates of the locus 14 are slightly modified to see if the shape of the fan-beam changes preferably by the modification. By repeating that process, the final coordinates of the locus 14 which provides the desired fan-beam is obtained.

The present invention is again described in accordance with Fig.2 in detail for the relations of the focus of the parabola 13a and the vertex Q(14a).

Fig.2A(a) shows that the paraboloid 13 on the XY coordinates provides a prior paraboloidal reflector by revoluting a parabola 13m around the Y-axis. The same paraboloidal reflector is obtained as the densed set of the parabolas 13p which are perpendicular to the XY plane, and has the parabolic locus on which the vertexes of the parabolas 13p shift. The direction of the principal axis of the parabolas is constantly directed to a target on the service area to be illuminated during the shift. That is to say, a prior paraboloidal reflector is a dense set of parabolas whose vertexes have a parabolic locus.

On the other hand, a dense set of parabolas 13q which have a locus (on which the vertexes of the parabolas shift) equal to X-axis as shown in Fig.2A(b) is a prior reflector 3 in Fig.8.

The present invention has the locus 14 which is not a plane curve, but a three dimentional space curve with double curvature as shown in Fig.2A(c). The surface of the present reflector is the dense set of the parabola 13a whose principal axes do not necessarily lie on the XY plane in Fig.2A(c) and shift along the locus 14 so that the vertexes of said paraboloids shifts on said locus 14 keeping the principal axis of the parabolas in the direction of a target on the service area to be illuminated by the antenna system.

It should be noted that the prior locuses are plane curve, while the locus 14 of the present invention is not a plane curve, but a three dimentional space curve.

Fig.2B shows the cross section of the present reflector 11 for the explanation of the focus 13c and the vertex Q(14a) of the parabolaid 13a.

The focal length QR satisfies the following equation.

QR= PQ x l+cos e (2) 2 where 6 is an angle (LPQR) shown in Fig.2B and the point P(16) is the fixed point which is common to all the parabolas.

When a primary radiator 15 is positioned so that the phase center of the primary radiator coincides with said point P(16), every reflected ray which is reflected by the parabola 13a has the uniform phase with respect to the principal axis 13b of the parabola 13a, and therefore, no reflected ray in undesired directions occurs in the sense of geometrical optics. Thus a sharp, thin, and curved fan-beam is obtained.

Fig.3 shows the shape of the fan-beam in the form of an equi-level contour pattern on a two dimensional angular space. It should be noted that the footprint of the fan-beam is elongated, and the central curve 19 of the footprint of the fan-beam is curved. In Fig.3, the numeral 18 is an equi-level contour, and 19 is a curved ridgeline of the contours 18.

It should be noted that the present invention can provide the fan-beam which has the desired curved ridgeline 19 of the equi-level contours. The shape of the curved ridgeline 19 is almost analogous to the projection of the locus 14. Therefore, when the locus 14 is obtained by using a computer-aided optimization method, the initial profile of the locus 14 is determined so that it is analogous to the desired curved ridgeline 19, and is adjusted so that the desired shape of fan-beam is obtained.

A reflector surface is not restricted to a solid surface, but a wire grid reflector, or a mesh reflector are also possible.

Fig.4 shows the second embodiment of the present invention, in which Fig.4A is a perspective view and Fig.4B is a front view of a reflector.

The feature of the embodiment of Fig.4 is that a reflector is an offset reflector in which the locus 14 which is a set of the vertexes 14a of the parabolas 13a is not included in a part enclosed by the edge 12a.

Therefore, it is possible in Fig.4 to position a primary radiator 15 so that aperture blockage by the primary feed 15 is reduced or eliminated. Of course, the embodiment of Fig.4 can provide the desired shape of the fan-beam.

Fig.5 shows another embodiment of the present shaped beam antenna system, in which Fig.SA is a perspective view, Fig.SB shows the array of the primary radiators, and Fig.5C shows the shape of the beams on a two dimentional plane in the form of the equi-level contour pattern.

In the figure, the numeral 111 is a reflector which can provide a fan-shaped beam for each single primary radiator, 112a through 112n are primary radiators. In the embodiment, five radiators are shown. The numerals 113a through 113n are feeders, 114 is a beam forming network, 115a through 115n are element beams, 116a through 116n are equi-level contours of the element beams, and 117 is the resultant equi-level contour which is the combination of the element equi-level contours 116a through 116n.

The reflector 111 is not a conventional paraboloidal reflector, but a doubly curved reflector which is a dense set of parabolas whose vertexes have the locus which is linear or space curve as mentioned in accordance with Fig.2A. Therefore, each of the element beams 115a through 115n by each primary radiator is an elongated fan-beam.

Fig.5A is the embodiment that a plurality of element beams 115a through 115n are positioned laterally by locating the primary radiators 112a through 112n linearly as shown in Fig.5B. The equi-level contour of each fan-beam is shown in Fig.5C, in which each of the patterns 116a through 116n is an equi-level contour of each element fan-beam on the two dimensional space. When the beam forming network 114 excites the primary radiators 112a through 112n which are positioned on a straight line simultaneously, the resultant fan-beam 117 is obtained as the superpose of each of the element fan-beams 116a through 116e.

As described above, according to the present embodiment, each primary radiator provides an elongated fan-beam because of the specific structure of a reflector, and smaller number of primary radiators as compared with that of Fig.ll are enough for providing a wide shaped-beam 117 which illuminates wide rectangular area with a uniform radiation level.

Fig.6 shows still another embodiment of the present invention, in which Fig.6A is a perspective view, Fig.6B shows the array of the primary radiators, and Fig.6C shows the equi-level pattern of the fan-beam.

The feature of the embodiment of Fig.6 as compared with the embodiment of Fig.5 is that the primary radiators 102a through 102n are positioned in a zig-zag fashion as shown in Fig.6B. The central line C connecting the center of the cross section of the primary radiators is not a linear straight line, but is offset for each primary radiator as shown in Fig.6B, and therefore, the spacing (d) between two adjacent primary radiators is smaller than that of the embodiment of Fig.5. The offset positioning of the primary radiators allows the decrease of the essential spacing between two adjacent fan-beams, and provides the constant field in the resultant fan-beam 107.

Fig.7 shows still another embodiment of the present invention, in which Fig.7A is a perspective view, Fig.7B shows the array of the primary radiators 112a through 112n, and Fig.7C shows the equi-level contour pattern of the antenna. In the figure, the numeral 121 is a reflector which provides a curved fan-beam as described in accordance with Fig.3, the numerals 125a through 125n are curved fan-beams, 126a through 126n are equi-level contour patterns of each fan-beams, and 127 is resultant equi-level contour pattern.

The feature of the embodiment of Fig.7 as compared with Fig.5 is that each element fan-beam is curved by using the specific reflector described in accordance with Fig.l or Fig.4. It should be appreciated that the shape of each fan-beam in Fig.7 is curved, and therefore the complicated shape of shaped-beam 127 as shown in Fig.7C is obtained.

It should be appreciated that the embodiments of Figs.5 through 7 are advantageous in that a smaller number of primary radiators as compared with that of Fig.ll are enough for illuminating wide area with uniform flux density, since each beam is a fan-beam, but not a spot beam.

From the foregoing, it will now be apparent that a new and improved antenna system has been found.

Claims

1. An antenna system having at least reflector and at least one primary radiator or receiver positioned substantially at the focus of the reflector, wherein the reflector has a reflection surface which is a part of a dense set of parabolas whose vertices lie on a predetermined locus comprising a space curve, the principal axis of each parabola extending in the direction of a target on a service area to be irradiated by the antenna system or from which radiation is to be received.

2. An antenna system according to claim 1, wherein the antenna system is an offset antenna system in which the reflection surface does not include essentially set of the vertices of the parabolas.

3. An antenna system according to claim 1 or claim 2! wherein said relection surface is composed of a plurality of linear wires.

4. An antenna system according to any of claims 1 to 3, wherein the refection surface is composed of mesh.

5. An antenna system according to any of the preceding claims, wherein a plurality of primary radiators or receivers are positioned close to each other.

6. An antenna system according to claim 5, wherein the primary radiators or receivers are located cn a straight line.

7. An antenna system according to claim 5 or claim 6, wherein said primary radiators or receivers are positioned so that a centre line which connects the centre of the cross section of the primary radiators or receivers is offset from each of the primary radiators or receivers.

8. An antenna system according to any of the preceding claims, wherein the locus is a line other than a parabola.

9. An antenna system substantially as hereinbefore described with reference to any of the examples shown in Figures 1 to 7 of the accompanying drawings.