Antenna systems

Abstract

 A method of providing a desired coverage area for an antenna system is disclosed in which the reflector surface of the system is modified so as to optimise the radiation levels and or characteristics of the surface The optimisation method may be carried out by a computer and the resulting reflector surface shape, given in the form of mathematical function can be converted into a control programm for computer controlled machine tool.

Description

[0001] This invention relates to antenna systems and is more particularly concerned with antenna systems which provide one or more area coverages of specified shapes for separate or simultaneous transmission and reception of signals.

[0002] There are three categories of antenna systems which produce such coverage: the shaped reflector antenna, the multibeam antenna and the phased array. This invention relates specifically to improvements in shaped reflector systems, which have, according to the prior art, been used only for regular elliptical or moderately irregular coverage specifications. For complex coverage shapes the multiple beat antenna or phased array has been the generally preferred solution.

[0003] The multibeam antenna approach uses a parabolic reflector, and produces the shaped area coverage by using a cluster of similar feed elements positioned around the focal point of the reflector, thereby producing overlapping areas of spot coverage within the coverage area. In the phased array approach no reflector is used and the multiple feeds are pointed directly toward the desired coverage region. In both cases the amplitude and phase weightings on the feed elements can be optimised to prodoce continuous area coverage. These antenna concepts are frequently used in spacecraft applications, and extremely large clusters in excess of 100 elements are often required. These large feed arrays also require complex power splitting arrangements known as beam forming networks (BFN's) which require precision manufacture and close thermal control. At low frequencies the losses are relatively small, but at higher frequencies the losses become more significant and it becomes necessary to use a waveguide network which is dimensionally larger. However, problems arise due to the dispersion effects producing incorrect amplitude and phase relationships at the feed elements which reduce the antenna gain within the required coverage area. A secondary problem arises due to mutual interaction between the feed elements which tend to increase the crosspolarisation generated by the antenna assembly.

[0004] The multibeam antenna generally uses the single offset reflector configuration to achieve high efficiency, but this again suffers from the inherent crosspolarisation generated by this geometry for linearly polarised applications. This shortcoming can be overcome by a new development in reflector technology, the gridded reflector. This is a reflector with a polarisation dependent reflective surface on the front and rear faces of a cellular sandwich construction. The reflective surface is comprised of finely spaced conductive lines printed on the outer skins of the reflector, which are aligned to reflect the desired linear polarisation only, and therefore, the undesired polarisation does not contribute to the antenna coverage area. This technology is still in an early development phase and is a complex and expensive solution due to the manufacturing processes used, and the tight control necessary on the reflector thickness.

[0005] In the case of circular polarisation the mutual coupling between the feed elements also generates a high cross-polar content which cannot be compensated for by the use of gridded reflectors.

[0006] According to the prior art the shaped reflector is generally used for a single coverage with a regular elliptical or moderately irregular coverage. The prior art recognises that this antenna system has significant advantages in that only one feed element is used and the shaping of the coverage area is generated by small deviations in the reflector surface profile from an originally parabolic form. The technique can be used with either single, dual or multiple reflector antenna systems. The single reflector approach is limited, however, in the extent to which the coverage area can be shaped and also in that, for single offset reflector systems, the cross polarisation levels obtained in transmission produced by the antenna geometry are degraded from that of the front fed reflector systems for linear polarisation applications. The prior art recognises in addition that for regular elliptical coverage shapes the dual reflector antenna geometry may chosen to produce low cross polarisation in a linearly polarised system by choosing the feed angle in the offset plane to satisfy the Mizugutch condition ('Offset Dual Reflector Antenna" by Mizugutch Y, Akagawa U and Yokoi U; Proc. APS Symposium Amhurst Mass. 1976). 7his angular constraint guarantees low cross polarisation over a wide bandwidth.

[0007] There are three aspects to this invention.

[0008] According to one aspect of the invention, there is provided a method ofproducing an antenna system which is capable of passing radiation either to or from a shaped coverage area, or both simultaneously, the system including a three dimensional reflector surface positioned to transmit radiation to or receive radiation from said area, the method including:-

defining desired levels and/or characteristics of radiation incident upon or received from selected regions of said coverage area, and

optimising actual radiation levels and/or characteristics for said regions by modifying said reflector surface,

the optimisation being achieved by iteratively determining levels and/or characteristics of radiation incident upon or received from each of said regions and obtaining the least favourable value of level and/or characteristic and modifying said reflector surface to obtain an improved least favourable value of level and/or characteristic.

[0009] The antenna system naturally includes radiation emitting or accepting means positioned for directing radiation onto or receiving radiation from the reflector surface.

[0010] The radiation emitting or accepting means can be, for example, a feed or receiving horn arrangement either use alone or in conjunction with a further reflector surface.

[0011] The further reflector surface may or may not require development in a similar and simultaneous manner to that of the first-mentioned reflector surface.

[0012] According to another aspect of the invention, there is provided a means of producing an antenna system which is capable of passing radiation either to or from (or simultaneously to and from) a shaped coverage region simultaneously in two orthogonal polarisations of the same frequency. The said orthogonal polaristions may be either linear or circular in nature. The system includes two or more three dimensional reflector surfaces one or more of which is shaped according to the procedure defined in the first aspect of this invention.

[0013] The antenna system naturally includes radiation emitting or accepting means for directing radiation onto or receiving radiation from the main reflector surface. The said radiation emitting or accepting means can be, for example, a feed or receiving horn arrangement with suitable means for combining or separating the two orthogonally polarised radiation signals, used in conjunction with one or more further reflector surfaces.

[0014] The two or more reflector surfaces are chosen initially to be undistorted conics arranged relative to the feed or receiving horn to satisfy approximately or exactly the Mizugutch angular contraint I11 for low linear cross polarisation. The surface of the main reflector, and subsequent reflectors is so desired, are then distorted according to the procedure of the first aspect of this invention to optimise the far field radiation.

[0015] The subreflector may then need to be adjusted in angle to improve the crosspolar performance.

[0016] The optimisation process is then repeated until a satisfactory design emerges from this iterative procedure.

[0017] In a third aspect of this invention, a procedure is provided for generalisation of the previously stated aspects to provide multiple simultaneous shaped coverage regions from the a single antenna. In this aspect the single feed or receiving horn is replaced by an array of such radiating devices. One or more feeds or receiving horns is required for each beam and the radiation pattern produced by the shaped reflector must be chosen as a compromise between the differing requirements of the different beams. This multibeam shaped reflector antenna may be designed, for example, so that each beam is flat-topped and approximately hexagonal in shape. The geometry may be chosen so that neighbouring feeds produce neighbouring beams. By choosing beams with low sidelobes, signals, with identical frequencies may be used in non-neighbouring beams without undue interference one to the other.

[0018] By switches or variable power divide network mens the beams of the antenna may be reconfigured in the usual way for a multifeed antenna. The advantage of the shaped reflector mulifeed antenna described in aspect three of this invention is the potential reduction in the number of feeds required and thus a reduction in losses and complexity in the associated power divide network.

[0019] For a better understanding of this invention in its first two aspects, a transmitting antenna system will now be described by way of example, and with reference to the accompanying drawings in which:-

Figure 1 illustrates a desired coverage area in the northern hemisphere;

Figure 2 illustrates the far-field pattern obtained for a standard parabolical reflector in the desired coverage area;

Figure 3 illustrates the m far-field points at which maximisation of the minimum directivity is obtained;

Figure 4 illustrates the distorted far-field pattern of the standard parabolical reflector obtained by adding the distortion terms;

Figure 5 illustrates the optimised far-field plot;

Figure 6 illustrates a contour plot of minimum directivity inside the regions of Figure 1;

Figures 7 and 8 illustrate plots of the basic paraboloid and the distorted surface in the offset plane (^y = 0) and the plane x = x ;and

Figure 9 illustrates the Fourier distortions in three-dimensional form.

[0020] Although the invention will be described with reference to a transmitting antenna system mounted on a satellite, it will become apparent that the method used to shape the reflector surface can be applied to receiving systems irrespective of where they are mounted.

[0021] For ease of description, the method of shaping a single main reflector to obtain the required coverage will be described. Naturally, the method may also be applied to a sub-reflector or both reflectors of a dual reflector system.

[0022] It is necessary to decide on the coverage area required eg part of the northern hemisphere as shown in Figure 1, the coverage area being usually defined by the user. As shown there are three regions defined in the figure. Each of these regions are required to have a given directivity specification ie the ratio of power transmitted in a given direction to the total power transmitted by the antenna system - these values being X_dB for region 1, (X-3)_dB for region 2, and (X-6)dB for region 3 (where X is greater than 30.3dB after losses). The antenna system is then defined by the basic unshaped parameters, for example, for a basic unshaped paraboloidal reflector, these are:-















The feed born offset at an angle of 30.55°, directs two orthogonally polarised radiation beams at a frequency of 14.5GHz onto the reflector which has a basic far-field pattern as shown in Figure 2, each area receiving a different strength output of the beams.

[0023] The method optimises the shape of the reflector surface so that the minimum directivity obtained from a series of m far-field points is maximised - the m points being shown in Figure 3 as a set of black dots indicating the points to which the beam is to be directed. These points are user-defined and are initially specified in terms of longitude and latitude, and are then converted to coordinates in the antenna system. The optimisation of the shape of the reflector surface is carried out on a basic function Z(x,y) (obtained from the shape of the basic reflector) plus an arbitrary function expanded as a two-dimensional Fourier series, that is,



where

ie the Fourier series describes a function with period 2h in the x-direction and period 2k in the y-direction with origin at (x_c,y_c). Variables h, k, x_c. y_c are defined by the user and are related to the antenna system vhich is being used. The variables

etc. are optimisation variables. The user also specifies each individual term to be included, thereby giving freedom to leave out terms due to, for example, symmetry considerations. In the present example, there is no symmetry worth exploiting, so all terms were included up to n_max = m_max = 3, giving 49 optimisation variables. The other parameters were set to:-

[0024] In order to obtain a realistic starting point for the optimisation method, the gross features of the coverage area are obtained by distorting the basic paraboloidal reflector using quadratic distortions in x and y, the distortions being of the form

where x₀ is the x-coordinate of the aperture centre and

[0025] The first two terms generate a roughly elliptical beam shape, and the third term points the beam slightly north. The resulting far-field pattern for the distorted reflector is shown in Figure 4.

[0026] The optimisation method involves a series of iterative steps in which the directivity is calculated at each of the m far-field points. As mentioned previously, the reflector distortions are defined as a two-dimensional Fourier series, the coefficients of the series being the optimisation variables. The directivity for each point is calculated for various sets of coefficients, each calculation or iteration involving only one set of coefficients and utilises physical optics. The method gradually produces an optimum set of coefficients, ie the 49 Fourier coefficients in the optimised state with the minimum directivity value maximised. The resulting far-field plot is shown in Figure 5, with Figure 6 showing a contour plot of the minimum directivity inside each of the regions 1, 2 and 3.

[0027] After the optimisation has been executed the reflector surface function S (x,y) is now described by



where

is the basic paraboloid, and the other terms are as previously denoted.

[0028] Figure 7 shows a plot of the basic paraboloid, ie

together with a plot of the distorted surface tie S(x,y)] in the offset plane (ie in the plane y = 0). Figure 8 shows the same surfaces in the plane x = x_o. On this scale, the visible differences are due to the quadratic distortions - the Fourier distortions being too small to be discerned. This is because the quadratic distortions are of the order of 5 to 6cm, the Fourier distortions, shown in Figure 9 in three-dimensional form, being of the order of 3mn.

[0029] If a sub-reflector is used in the system, at each iteration, its scattered field has to evaluated using geometrical optics and this gives a tabulation of the main reflector incident field on a rectangular grid over the main reflector aperture. This incident field is then used in the calculation of the directivity at each point.

[0030] The optimisation methods disclosed may be carried out by a suitably programmed computer and the resulting optimised reflector surface shapes, given in the form of mathematical function and/or a table of z-direction displacements for a series of x, y coordinate positions over the reflector surface can be converted by suitable software, of which examples are known, into a control program for computer controlled machine tool. This tool then cuts the required reflector surface shape from a workpiece or, more likely, cuts that shape in a metal or graphite block which is then used as a mould for forming the reflector using known composite technology.

Claims

₁. A method of producing an antenna system which is capable of passing radiation either to or from a shaped coverage area, the system including a three dimensional reflector surface positioned to transmit radiation to or receive radiation from said area, the method including:-

defining desired levels and/or characteristics of radiation incident upon or received from selected regions of said coverage area, and

optimising actual radiation levels and/or characteristics for said regions by modifying said reflector surface,

the optimisation being achieved by iteratively determining levels and/or characteristics of radiation incident upon or received from each of said regions and obtaining the least favourable value of level and/or characteristic and modifying said reflector surface to obtain an improved least favourable value of level and/or characteristic.

2. A method according to claim 1, wherein the antenna system incloses radiation emitting or accepting means positioned for directing radiation onto or receiving radiation from the reflector surface.

3. A method according to claim 2, whrein the radiation emitting or accepting means is a feed or receiving horn arrangement.

4. A method according to claim 2, wherein the radiation emitting or accepting means is a feed or receiving born arrangement used in conjunction with a further reflector surface.

₅. A method according to claim 4, wherein the further reflector surface is also developed to optimise said radiation levels and/or characteristics.

Drawing