Multi-mode beam forming networks for multi beam reflector antenna

Abstract

 This invention concerns a partially overlapped beam antenna system, formed by a reflector, an array of radiating elements and a beam-forming network, all in a peculiar configuration which is the most innovative feature of this invention. The configuration consists of a double cascade which exploits all possible degrees of freedom applicable to the optimisation of the radiating element excitation coefficients. The optimisation of such coefficients in a multi-feed reflector system, with two or more overlapping beams, only the orthogonality of the exciters has to be ensured, each for its own coverage. This invention belongs to the electronic antenna field, with a particularly favourable application in space-borne platforms.

Description

[0001] The present invention relates to an antenna system having reflector and feeds fed by one or more multi-mode networks for application to multi-coverage feeds.

[0002] Multi beam antenna till now were based on the optimisation of the reflector (feed) beam-forming network combination which had a prefixed beamforming network - as a result of such optimisation, all system parameters, including the feed network, were defined.

[0003] But this approach limited the number of degrees of freedom left for the optimisation of coverage performance.

[0004] In this antenna system presently proposed, the original network configuration separates the radiating optimisation from that of feed network design.

[0005] The invention is best applied to multi-beam antennae which have partially overlapping beams.

[0006] In this case the overlapping sources are fed by a double cascade-type of network.

[0007] The invention pertains to electronic antennae for space-borne applications.

[0008] The problem which we propose to solve with this invention is that of the generation of overlapping beams which have a given number of common sources using the greatest number of degrees of freedom available. At the present state of the art, the implemenation of multi-mode networks, starting from the excitation coefficients required has taken regard for two input networks and a maximum of four outputs. This set a limit to the number of degrees of freedom availably to optimise the source excitation coefficients of the antenna system when the number of sources common to the two beams was greater than four, this limiting the achievable antenna performances.

[0009] With multiple overlapping beams only particular amplitude and phase distributions could be achieved (e.g. Butler matrices) with loss-less networks, restricting the field of application of this system.

[0010] The invention regards the definition of a peculiar configuration and implementation of a beam forming network which feeds N sources different than M (M less than N) from the network.

[0011] In particular the network generates M orthogonal sets on n output parts previously calculated by optimising the far-field of the antenna on the required coverage, with the only constraint of orthogonality among the calculated feed excitation coefficients.

[0012] This configuration optimises the feed excitation coefficients for different coverages required, independently from the network, with the only constraint of orthogonality of the sets above. This results in improved performance, compared to present state of the art solutions, in terms of gain and flexibility.

[0013] The invention will now be described for illustrative and non limiting purposes, with reference to the tables attached hereto.

[0014] Figure 1 shows a schematic diagram of the antenna system. It shows the following items:

1 reflector (or other focussing element),

2 radiating elements,

3 multi-mode network.

[0015] Figure 2, more significant than the others, is a schematic diagram of the multi-mode network, where:

4 feed output parts,

5 input parts,

6 power splitters,

7 phase shifters.

[0016] Figure 3 is a schematic of possible uses of the proposed antenna system.

[0017] This solution generates two partially overlapped coverages 8 and a global coverage 9. At first, the optical system is optimised, concluding the multi-feed reflector, together with the excitation coefficients of the latter with the only orthogonality constraint. Then by considering the configuration shown in figure 2, the three multi-mode networks 10 are implemented.

[0018] The network is synthesised in a recursive manner by generating the first mode required coefficients with the first row of couplers and phase shifters, the second with the second row and so on for the other sets of coefficients required.

[0019] Figure 4 is the outline schematic of another possible application of the invention. Here we can see output ports U1 - U9 and two input ports I1, I2, but these can vary in number according to the required configuration.

[0020] Now we shall describe the physical build of the invention, with, once again, reference to the tables of figures attached, with non limiting explanatory purposes.

[0021] Figure 2, which shows the beam forming network, highlights outputs U from the network where M sets of N coefficients each are formed; input ports in-1, in-2 ...; the series of double cascaded couplers a1', a2', ..., a1'', a2'' ..., and phase shifters s1', s2' ....

[0022] Going back to figure 4, which shows another possible implementation of this invention, it is worth to emphasise that according to the inventors, this solution has a better frequency response, if possible, compared to that of figure 2.

[0023] For greater accuracy lets briefly digress on the operation of this system.

[0024] Starting from M sets of N excitation coefficients already optimised in the optical system (i.e. in the antenna optics), the first set of N coefficients is taken into examination; a ladderrung network is implemented as shown in figure 2, i.e. the first excuiftion in amplitude and phase is effected by the first coupler 1' and phase shifter 1'. The remaining power is sent to the second coupler 2' through phase shifter 2', and so on till the nth excitation coefficient of the first set is implemented. The remaining M-1 sets of N coefficients are converted into M-1 sets of N-1 coefficients at the ports of the already implemented N-1 couplers, where all four parts of the couplers are utilized.

[0025] The first set of coefficients is implemented by grouping them two by two through the first coupler as shown in figure 4 so as to reduce the order of the multi mode network to be implemented. The process is repeated till an input for each of the M sets of coefficients required is achieved.

[0026] It is worth mentioning that the number of sets physically possible is less or equal to the number of excitation coefficients which are characteristic of each set.

Claims

1. Multibeam antenna system with one or more beam forming networks, consisting of a peculiar configuration of a multi-mode network (figure 2 or figure 4) formed by output ports U 1 ... U N where M sets of N coefficients are synthesised to be then sent to the feeds through M input ports IN - 1, IN - M for the M coverages required, couplers A1', A2' ... A1'', A2'' and phase shifters S1', S2' ... S1'', S2''.

2. Multibeam antenna system with one or more beam forming networks as per claim 1, where feeds (2) are compatible with any type of reflecting or focussing surface.

3. Multibeam antenna system with one or more beam forming networks as per claim 1 or 2, where the number of double or multiple coverage feeds can be any, depending on the requirement.

4. Multibeam antenna system with one or more beam forming networks as per the claims above, consisting of a double cascaded network (figure 2) for multi-mode networks which supply the feeds common to double or multiple coverages.

5. Multibeam antenna system, with one or more beam forming networks, as per the claims above, which can be configured in many different manners (as shown in the diagram of figure 4) so that the network has a symmetrical configuration.

Drawing