1. Hanyang University 1/15 Antennas & RF Devices Lab. MODERN ANTENNA HANDBOOK by CONSTANTINE A.BALANIS ch. 5.6 ~5.6.5 Jeong Gu Ho

2. Hanyang University 2/12 Contents 5.6 Multiple-beam antennas with examples 5.6.1 Wide angle scan 5.6.2 Contour beam antennas 5.6.3 Low cross-polarization systems 5.6.4 Spherical reflectors 5.6.5 Torus antenna

3. Hanyang University 3/12 5.6 Multiple-beam antennas with examples There is a need for reflector systems with multiple-beam and wide-scan capabilities for both satellite and ground system applications. For satellites, there is the increasing demand for multiple and contour-beam (footprint) applications. There is a considerable savings because one antenna can service multiple satellites. A suitable range of component beamwidths for satellite antennas θ c, is 0.2°< θ <1°. The field of view (FOV) from synchronous altitude for regional coverage such as CONUS is ±4°. Note, however, that INTELSAT systems might require the full Earth FOV of ±9 °. These values imply beams scanned up to 45 beamwidths.

4. Hanyang University 4/12 Applicable reflector concepts can be summarized by the diagram.

5. Hanyang University 5/12 5.6.1 Wide angle scan Several methods have been used to enhance the beamwidth scan. One method Reshaping the reflector surfaces from the classical conic-sections paraboloid or paraboloid and hyperboloid shapes to reflector. Generally this approach has required two reflector surfaces to obtain the degrees of freedom required in the design. The first technique is the bifocal dual-reflector design. A second technique is optimize the analytic surface description for wide angle scanning. although this can be a very time-consuming process but good results have been obtained.

6. Hanyang University 6/12 Second method To utilize a more complex multihorn feed system with a paraboloid. The complex-feed approach generally attempts to combine a number of adjacent pencil beams into one well-formed beam. Applied to a paraboloid, used a triangular lattice of feeds, cluster to improve the pattern of a beam that was scanned by several beamwidths. The principal object in this type of approach is to choose the excitation coefficients of each radiation element in the cluster so that the coherently summed resultant pencil beam has an “optimum” shape cluster overlapsidelobeHorn size

7. Hanyang University 7/12 The important reason of excitiation coefficient. array element Excitiation coefficient Array factor switch

8. Hanyang University 8/12 “Optimum” as the minimum mean-square difference between the actual power pattern and the ideal unscanned power pattern. Four significant elements 1.The search algorithm 2.The initial point in space 3.The object function : in general, a function of 2N real varaiables when there are N feeds. The object function is usually set up to be minimized in the 2N dimensional space. 4.The integration time for the radiated fields. The definition of the object function to be minimized is critical. Several object functions are considered that make use of a mini- max algorithm

9. Hanyang University 9/12 Very significant is the initial set of coefficients used. The set of complex C n that gives the maximum value of G( θ 0, φ 0 ) at one point under condition of P IN = constatnt That is, it is relative field strength at the feed when the antenna has a plane wave incident from ( θ 0, φ 0 ) direction. Use this set of C n as our initial condition for more complicated object functions wherein sidelobe level, cross polarization, contour areas, and so on Conjugated matched field

10. Hanyang University 10/12 The case that the field needs to be known at many points of observation for use in the object function to be optimized The cost functions described above are to maximize either MINI-Max // MINI-RMS

11. Hanyang University 11/12 The conjugate-match function, which only considers the direction of the main beam and produces the maximum gain solution Mini-Max, which maximize the difference between the gain and maximum side lobe level and at the respective grid points. Mini-RMS, which maximize the gain between the gain and the RMS value of all the side lobe grid-level points. Comparisons are made between the conjugate-match solution, the mini-max optimization, and mini-RMS optimization Optimization technique can be successfully used to control the sidelobe levels for large angles of scan in parabolic reflector system Improvement in sidelobe level

12. Hanyang University 12/12 5.6.2 Contour beam antennas A multibeam antenna can also be used to provide a contour beam over a sector of the earth as seen from a satellite antenna. To design system it is necessary to relate positions on the earth to directions in reflector coordinates.

13. Hanyang University 13/12 Plot a coverage map Select a set of contiguous squares or circles that represent a -3dB or -4dB beamwidth for each feed in the offset reflector system Arrange these squares or circles in lattice structure a rectangular If sidelobe suppression is desired, add one extra square or circle along the exterior of the desired coverage area Determine the optimum reflector parameters Determine the optimum amplitude and phase distribution for each individual horn by numerical optimization technique

14. Hanyang University 14/12 5.6.3 low cross-polarization system Dragonian antenna might provide the required improved performance. the relatively large offset distance and focal length of the main reflector and the avoidance of caustics between the two reflector surfaces Excellent polarization properties and wide field of view capability. Use of the concave hyperboloidal subreflector

15. Hanyang University 15/12

16. Hanyang University 16/12 5.6.4 Spherical reflectors This wide angle property results from the symmetry of the surface. Arecibo observatory

17. Hanyang University 17/12 5.6.5 Torus antenna A dual-curvature reflector also capable of multibeam operation when it is fed with multiple feeds. Torus antenna has less phase aberration than the spherical antenna because of the focusing in the parabolic plane. Because of the circular symmetry, feeds placed anywhere on the feed arc form identical beam. Therefore no performance degradation is incurred when multiple beams are placed on the focal arc.

18. Hanyang University 18/12

19. Hanyang University 19/15 Antennas & RF Devices Lab. Thank you for your attention Antennas & RF Devices Lab.