Multicoverage shaped reflector antenna designs

Abstract

INTRODUCTIONCurrent trends in satellite communications point to the requirements for antenna systems that can radiate several shaped beams simultaneously, with each beam providing a different function such as telephony, TV, specialised business service etc. It is envisaged that such systems will employ electrically large diameter reflectors to achieve an adequate level of beam contouring and the isolation between the beams. The use of the conventional array-fed parabolic reflector antenna represents one possible approach to this requirement; however, the solution has the drawback of increased mass penalty and the feed system losses. It is evident that these limitations become particularly severe where each shaped beam is synthesised using narrow spot-beams from a large diameter focused reflector. In this context, the use of a shaped reflector rather than a focused reflector can offer potential benefits by reducing the number of feed elements that would be otherwise required. However careful attention must be given to the synthesis of the reflector surface shape which must be optimised for several beams simultaneously.In this paper we report on a study carried out for the European Space Agency to examine the application of shaped reflector antennas to such multicoverage scenarios. The design objective here is to derive two or more contoured beams from a single shaped offset reflector using either a single feed per beam or small array of feeds per beam as appropriate. The synthesis of such an antenna involves simultaneous optimisation of the reflector shape and the excitations of the feed array elements associated with each beam. In this context the geometrical optics based reflectors synthesis techniques are inappropriate and the design must be based on the more elaborate diffraction optimisation technique. SYNTHESIS APPROACHIn the approach adopted here, the reflector is initially described as a sum of two surfaces, viz a parboloid and a perturbation surface represented in terms of bi-cubic spline functions. The radiation field from this arbitary surface is then determined using a physical-optics surface current integration technique. The analysis is embedded in a search procedure (MINIMAX) which continuously monitors the radiation fields from the reflector as the design parameters are varied. In the case of the multi-element fed shaped reflector the design parameters include the coefficients of the bi-cubic spline functions and the excitations of the feed array elements. Where a group of feeds is assigned to a particular beam, only the contribution of that group of feeds is taken into account in calculating the coverage performance of the beam. DESIGN EXAMPLESThe above design approach has been used to deal with different multi-coverage scenarios including dual beam AFSAT coverage (C-band), the seven beam EUROPESAT coverage 1182 CHZ6642/89/000&1182 81.00 01989 IEEE (Ka-band) and the modified five beam EUROPESAT supercoverage. The results of the EUROPESAT coverage are presented below as an illustration. The coverag...

A procedure for the design of wideband slant-polarized shaped reflector antennas using a PO-based near field analysis method

“…Beam shaping is feasible either by shaping the pattern of feed antennas [1][2] or by shaping the reflector surface [3][4]. While a larger cross polarization is usually the result of reflector surface shaping, array feeding often introduces large spill over levels as well as a higher loss due to the feeding network [4]. One category of shaped surface reflectors is the doubly-curved antennas which have a narrow beamwidth in the search direction and a wide radiation pattern in the elevation direction.…”

A Method of Generating Simultaneous Contoured and Pencil Beams From Single Shaped Reflector Antenna

“…This increases mass and volume on the spacecraft. The hybrid approach described in [2] and [3] reduces the number of feeds required by shaping the reflector for multiple beam requirements. However when there is a large difference in required coverage gain and its shapes (one contoured beam and another high gain pencil beam) then limitations are imposed on the performance optimization.…”

“…Evolutionary algorithms (EAs), such as genetic algorithm (GA) [2] and nondominated sorting genetic algorithm (NSGA) [3], and particle swarm optimization (PSO) [4] have been suggested to solve MOPs related to antenna design. In such techniques, a MOP is either converted into a single objective optimization problem (SOP) using some composition approaches [2], [4] or treated as a whole [3]. In the implementation, a weight vector represents one search direction in objective space so that computational cost could be unendurable due to low searching efficiency.…”