WO2009024996A2

WO2009024996A2 - Shaped and segmented multi beam reflector antenna

Info

Publication number: WO2009024996A2
Application number: PCT/IN2008/000204
Authority: WO
Inventors: B Sharma S; Sood Khagindra; Jyoti Rajeev; Nalinkant Pandya Bhargav
Original assignee: Indian Space Research Organisation
Priority date: 2007-08-22
Filing date: 2008-03-28
Publication date: 2009-02-26
Also published as: WO2009024996A3

Abstract

A shaped, segmented multi beam reflector antenna comprising a single reflector segmented into separate surfaces, upper segments having higher focal lengths than the corresponding lower segments, said segments are shaped according to geophysical optics based interactive optimize operating in conjunction with mini max algorithm for simultaneous gain optimization and side lobe cancellation and individual clusters of feed horns for illuminating each of said segments to generate contiguous but inter based beams.

Description

Shaped and segmented multi beam reflector antenna

This invention relates to shaped and segmented multi beam reflector antenna.

Multiple beam antennas find extensive applications in on-board communication satellites worldwide. Antenna technology aims to provide very high gain beams to reduce user terminal size and power requirements. High gain antennas known in the art produce narrow beams that cannot cover large areas. Hence, multiple beams become necessary with a potential for interference among various beams. Spatial isolation by suppressing side lobes interfering the used beam is one solution and the proposed invention aims at achieving this objective.

Existing multiple beam antennas use single or multiple offset reflectors in conjunction with one or more feed clusters. The geometrical parameters of the configuration viz, the focal length, feed spacing and illumination taper control the sidelobe structure of the beams. The level of sidelobe in the direction of the frequency-reused beam determines the co-polarized interference caused by it. Considering two alternate beams in a coverage zone, the separation between nearest edges of the two beams is equivalent to one beamwidth. The rate of decay of sidelobe energy within this angular spacing will determine the highest interfering sidelobe level. With typical illumination tapers of 6 to 10 dB, the reduction in radiation intensity within this spacing is 16 to 18 dB, which equals the achievable beam-to- beam isolation. Increasing the taper to above 18 dB may increase this isolation to above 25 dB, which is desirable operating condition for frequency-reused beams. However, the feed spacing does not permit this, as a larger feed aperture is required.

This imposes a limitation on the achievable inter-beam isolation with the conventional multi-beam antenna. Note further that this approach attempts to increase the rate of gain fall-off in a symmetrical sense, without concentrating on the exact zone where it is required.

The proposed invention overcomes these limitations by using a series of expedients. The eight beams are divided into three interleaved sets consisting of three, three and two beams respectively. Each set has an alternate beam removed and assigned to another segment. This frees a greater amount of space in the focal plane for an improved illumination taper. The coverage area is split into eight contiguous zones ensuring that the resulting solid angle of coverage is nearly identical while providing the maximum spacing between the reused beams. Frequency reallocation is carried out with four frequency sub-bands to have the same number of beam pairs at mutually alternate locations. With these as the starting point, four parabolic surfaces are defined for each segment. It was necessary to separate the four focal regions without changing the look direction from the common boresight. This has been achieved by using a higher focal length for the upper two segments and by rotating the feed cluster about the secondary coordinate axis of the respective segment. Subsequent to this, the surfaces have been shaped using a physical-optics based iterative optimizer operating in conjunction with the minimax algorithm. The surfaces were represented by a truncated Zernike series of orthogonal polynomials. During optimization, the Zernike coefficients representing the current surface are iteratively varied till the required antenna pattern shape and the sidelobe isolation has been achieved. In the final step, illumination from each feed is considered on all four segments to compute the radiation pattern of the assembly to confirm that the required isolation has been met. Corrugated horns have been employed as the feed element for the segments.

Although presently designed with the Indian coverage in mind, the concept may find use wherever frequency reuse employing identical polarization is required in a small number of beams with close spacing. The shaped and segmented multi beam reflector antenna according to this invention comprises a single reflector segmented into separate surfaces, upper segments having higher focal lengths than the corresponding lower segments, said segments are shaped according to physical - optics based interactive optimizer operating in conjunction with mini max algorithm for simultaneous gain optimization and side lobe cancellation, and individual cultures of feed horns for illuminating each of said segments to generate contiguous but interleaved beams.

This invention will now be described in detail with reference to the accompanying drawings.

Figs. Ia and Ib represent the antenna wherein reference numerals 1 - 4 indicate the segmented reflector, feed born clusters, feed brackets, which mount the feed born clusters and the common mounting surface respectively.

Fig. 2 represents a close up of the shaped antenna in which reference numerals denote the following parts:

1 - North inboard segment

2 - South inboard segment 3 - North outboard segment

4 - South outboard segment

These form segments are named according to their position when mounted as described herein below:

Fig. 3 shows a rear isometric view of the feed clusters wherein reference numerals 1 - 3 stand for feed born, wave-guide port interface and feed bracket respectively. Fig. 4 shows the front view of the horns which are corrugated. Reference numerals 1 - 4 indicate the corrugated feed born horn tracking feed, mounting plate for mounting the born cluster and the bracket interface to common plane respectively.

Figs. 5 and 6 illustrate sample results obtained when the antenna is put in operation.

In its present form, the invention has a single reflector segmented into four separate surfaces (1, Fig Ia). This may be a deployed reflector from one of the panels of a geostationary spacecraft. Four clusters of feed horns (2, Fig Ib) are used to illuminate the four segments. The horns are interfaced to the common mounting plate (4, Fig Ib) using individual feed brackets (3, Fig Ib). The common mounting surface may be the Earth- Viewing deck of a geostationary spacecraft. For unambiguous nomenclature, the four segments are named according to their position. The segments closer to the spacecraft body are termed inboard while those away are termed outboard. The prefixes North & South denote the orientation on the spacecraft (refer Fig 2.) The electrical interface for each beam is available from the rear of the respective feed horn (1, Fig 3.) In the present case, it is a standard rectangular waveguide for Ka-band. Though not part of the claims in the present inventions, a tracking feed (2, Fig 4) is included as a requirement of the system operation.

The feed horn for any beam operates in this manner it directs its radiated energy on to the specific segment. The shaped segment generates an antenna footprint that provides the desired beam shape while producing sidelobe cancellation into the area represented by the reused beam. As explained earlier, the segment has been shaped using an iterative algorithm using a physical-optics based analysis tool coupled with optimization based on the minimax procedure. The analysis is carried out by computing the induced currents on the reflector surface due to the radiated energy from the feed incident on it. The physical optics integral is computed with a sufficiently dense polar grid to ensure convergence in the far-field. For optimization, a global optimizer like LMS is used for bringing the shaped surface to near the desired value. For fine optimization, the achieved gain and isolation values across a set of field stations are checked against a set of target values specified by the designer. The current worst (maximum) error is minimized i.e. one by one till the desired target values are achieved to an acceptable convergence level.

Sample results of the antenna are illustrated in Figs. 5 & 6. The eight contiguous shaped beams are evident in Fig. 5. As observed, these beams all have different shapes that suit the coverage are identified for them and together provide full Indian mainland coverage. Fig. 6 shows an example of how isolation is achieved between alternate beams. Two of the beams of Fig. 5 are reproduced in Fig. 6 illustrating how the interfering sidelobe contour bypasses the other beam which may be frequency reused on account of this. An inter-beam isolation of better than 25 dB is achieved in the present application.

Through the use of segmented shaped reflector for gain optimization as well as sidelobe cancellation simultaneously, it became possible to realize multiple shaped beams. In addition, we could achieve a high order of inter-beam isolation amongst alternate beams from this set. These beams could maintain the same sense of polarization and still reuse the frequency band. Use of the orthogonal polarization was not required and this additional spectrum remains free for further reuse.

This invention as described above has the following applications:

• Generation of multiple shaped beams for optimum coverage of a geographical zone

• Frequency-reuse in a limited number of beams with collinear polarization assignments • Frequency reuse amongst alternate beams without needing orthogonal polarization

• Realization of a high-capacity communication antenna with a limited spectrum through efficient utilization of the available bandwidth

It is observed that shapes of the segments are not critical and may include circular/elliptical aperture shapes also. The technique of interference suppression generated by this antenna is not restricted to geostationary platforms and may be utilized for other purposes also.

Claims

WE CLAIM:

1. A shaped, segmented multi beam reflector antenna comprising a single reflector segmented into separate surfaces, upper segments having higher focal lengths than the corresponding lower segments, said segments are shaped according to geophysical optics based interactive optimize operating in conjunction with mini max algorithm for simultaneous gain optimization and side lobe cancellation and individual clusters of feed horns for illuminating each of said segments to generate contiguous but inter based beams.

2. The shaped, segmented multi beam reflector antenna as claimed in claim 1 wherein said feed horns are provided with feed brackets and wave-guide port interface.

3. The shaped, segmented multi beam reflector antenna as claimed in claim 2 wherein said feed horns are corrugated and are mounted on a mounting plate.

4. The shaped, segmented multi beam reflector antenna as claimed in claim 3 wherein the said antenna is mounted on a surface much as earth viewing deck of a geostationary space craft.

5. The shaped, segmented multi beam reflector antenna as claimed in claims 1 to 4 wherein individual feed brackets are provided for interfacing with said feed horns and the common mounting surface.