Abstract:
The invention relates to an antenna in stripline technology, in which the dipoles and the feeder network are etched in one single process. The connections of the dipoles are realized as two-wire transmission lines, fed by Schiffman couplers. The polarization of the antenna is selectably chosen by twisting the transmission lines.

Description:
This application is a Continuation of application Ser. No. 08/516,762, filed on Aug. 18, 1995, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a stripline antenna provided with a feeder network connected to a linear array of dipole antennas. 
     Stripline antennas of this type are for instance used in two-dimensional antenna arrays in which a stack of receive-antenna beams are generated by means of digital beam forming networks. A single antenna array will usually comprise some tens of stacked stripline antennas, each provided with for instance fifty dipole antennas. It is of major importance then to realise the feeder networks and the dipole antennas as lightweight and low-cost constructions, without impairing the quality. 
     SUMMARY OF THE INVENTION 
     The stripline antenna according to the invention is thereto characterized in that the feeder network and the dipole antennas have been realized by etching of a single plated sheet of synthetic material. This particularly enhances the reproducibility of the production process, which minimizes the percentage of rejects and greatly simplifies calibration procedures. 
     A favourable embodiment of the stripline antenna according to the invention is characterized in that the feeder network is disposed insulated between two ground planes. This yields a functional stripline antenna in which the antenna array can subsequently be obtained by stacking a required number of stripline antennas. 
     A favourable special embodiment is obtained by using synthetic foam as insulating material. This is favourable because of its low weight and low dielectric losses; moreover, the two ground planes will protect the vulnerable synthetic foam from damages incurred during storage and transport. 
     A dipole antenna is provided with two connections to be preferably fed in phase opposition. According to the state of the art, two separate distribution networks will usually be provided, each of which feeds a connection of the dipole antennas and which are themselves fed in phase opposition. According to a further favourable embodiment of the invention, the feeder network comprises only a single distribution network and per dipolel antanna a phase-shifting network, for feeding both dipole antenna connections in phase opposition. To this end, a balun well-known in the art may be employed, for instance implemented as a Schieffman coupler. 
     Since the dipole antennas are required to radiate unobstructed, they have been positioned outside the ground planes, the connection to a phase-shifting network being effected via a two-wire transmission line having an impedance that matches the impedance of a dipole antenna. This has the unexpected effect that at least substantially no reflection occurs in the area where a two-wire transmission line leaves the two ground planes, provided that at that position the characteristic impedance of the two-wire transmission line is adapted in a manner known in the prior art. This is all the more surprising since, within the ground planes, the electromagnetic field surrounding the transmission lines is in the stripline mode, whereas outside the ground planes, it is in the two-wire transmission line mode. This mode transition evidently proceeds smoothly. 
     An exceptionally favourable embodiment of the stripline antenna is obtained by removing the superfluous parts of synthetic material surrounding the dipole antennas and the transmission lines. This will cause the dipole antennas to be loosely suspended from the transmission lines which, by the incorporation of a mechanical support, allows them to be set to any required angle, resulting in an antenna radiation field with an adjustable polarization. In a feeder network that is in the horizontal position during its standard mode of operation, it is for instance possible to place the dipole antennas in a vertical position, which yields a vertically polarized radiation field. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be explained in more detail with reference to the following figures, of which: 
     FIG. 1 schematically represents a stripline antenna according to the invention; 
     FIG. 2 represents a part of the stripline antenna according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 schematically represents a stripline antenna 1 according to the invention in which a sheet of synthetic material 2, for instance Kapton, is provided with a conductor pattern 3 on the basis of which RF energy, supplied via a feed point 4, is distributed and is transmitted to dipole antennas 7 via phase-shifting networks 5 and connections 6. Conductor pattern 3, phase-shifting networks 5, connections 6 and dipole antennas 7 have all been realized in a single process by etching a plated, in general copper-plated, sheet of synthetic material 2. The stripline antenna 1 is disposed insulated between two ground planes 8, usually made of aluminium, the dipole antennas 7 and part of the connectors 6 protruding beyond the ground planes. The insulation is preferably realized by inserting, between the aluminium ground planes 8 and on both sides of the sheet of synthetic material, a layer of synthetic foam 18 of a type that is characterized by low dielectric losses and possesses non-hygroscopic properties. Within the frequency range of the stripline antenna, phase shifters 5 have an at least substantially constant phase shift, such that the connections 6 of dipole antenna 7 are powered in phase opposition. Additionally, phase-shifting networks 5 provide for the transformation of an asymmetric stripline mode in conductor pattern 3 to a symmetric stripline mode in at least that part of the connection 6 located between the ground planes 8. Furthermore, the impedance of the stripline is matched to the impedance of the dipole. Such networks are known in the art and are also referred to as baluns. 
     Stripline antenna 1 can of course also be used for reception in which case the RF radiation received by dipole antennas 7 is concentrated within the frequency range of the stripline antenna 1 and is subsequently supplied to feed point 4. 
     FIG. 2 shows a part of the stripline antenna according to the invention, which part can be regarded as a stripline antenna incorporating two dipole antennas 7. RF energy is supplied to feed point 4 after which it is distributed by means of a splitter 9. This distribution need not be symmetrical, which enables a certain tapering across stripline antenna 1. The RF energy is subsequently supplied to phase-shifting networks 5 implemented as Schiffman couplers in which the energy via a symmetrical splitter 10 and two different path lengths and subsequently via connections 6 is transmitted to dipole antennas 7. The connections 6 between phase-shifting networks 5 and dipole antennas 7 are partially positioned between the ground planes 8 and partially extend beyond the ground planes 8. In view of this, impedance matching is required, which is effected in transition 11 by adjusting the width of the print track. Surprisingly, this transition 11 is found to introduce at least substantially no reflections or losses, in spite of the mode patterns between and outside the ground planes being totally different. 
     The removal of superfluous parts of the sheet of synthetic material 2, as shown in FIG. 2, results in dipole antennas that are freely suspended from the connections 6. Moreover, it surprisingly appears that any twisting or bending of the connections 6 has practically no adversely affect on the behaviour of the combination of connection 6 and dipole antenna 7. It is therefore possible, for instance by means of the through-holes 12 in the sheet of synthetic material 2, to mount the dipole antennas at a predetermined angle on a support structure not shown here, which yields a stripline antenna with a predetermined polarization direction. 
     The stripline antenna according to the invention can be employed in a wide frequency range, where the dimensions of the component parts and the thickness of the layer of synthetic foam will have to be selected in accordance with the selected operating frequency, according to methods well-known in the prior art.