Stripline antenna

Abstract: The invention relates to an antenna (1) in stripline technology, in which the dipoles (7) and the feeder network (3,5,6) are etched in one single process. The connections of the dipoles (7) are realised as two-wire transmission lines (6), fed by Schiffman couplers (5). The polarisation of the antenna (1) is selectably chosen by twisting the transmission lines (6).

Description: The present invention relates to a stripline antenna comprising a plurality of dipole antennas each having two connections, a feeder network including a plurality of phase-shifting networks, one for every dipole antenna, for feeding both dipole antenna connections with opposite phases, a plurality of two-wire transmissions lines, one for each dipole antenna, for connecting the dipole antennas to the phase-shifting networks, the dipole antennas, the feeder network and the two-wire transmission lines consisting of an etched pattern on a sheet of single plated synthetic material; and two layers of insulating material and ground planes for enclosing at least the feeder network. A single stripline antenna of this type has been described in the Conference Proceedings Military Microwaves '88, London, England, pages 293-298 by Mailloux. An linear array of stripline antennas of this type has been described in IEEE Transactions on Antennas and Propagation, vol.41, no. 1, 1993, New York, US,pages 108-115, by Parfitt et all. For both stripline antennas the dipoles are situated in the same plane as the feeder network, which forms a limitation to the applicability of the known stripline antennas. The stripline antenna according to the invention obviates this limitation and is thereto characterised in that the synthetic material is chosen flexible and the dipole antennas have been mounted with a predetermined angle relative to the feeder network, for obtaining a predetermined polarization direction, which angle is obtained by twisting the two-wire transmission lines. A favourable embodiment of the inventive stripline antenna is characterised in that the two-wire transmission lines are implemented such as to have impedances that match the impedances of the dipole antennas. More specifically the two-wire transmission lines are according to the invention provided with transitions where they leave the ground planes. Still more specifically the the two-wire transmission lines are chosen wider outside the ground planes than inside the ground planes. 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. 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. 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. 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 realised 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 realised by inserting, between the aluminium ground planes 8 and on both sides of the sheet of synthetic material, a layer of synthetic foam of a type that is characterised 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 Schieffman 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.

Claims: Stripline antenna (1) comprising a plurality of dipole antennas (7) each having two connections, a feeder network (3,5) including a plurality of phase-shifting networks (5), one for every dipole antenna (7), for feeding both dipole antenna connections with opposite phases, a plurality of two-wire transmissions lines (6), one for each dipole antenna (7), for connecting the dipole antennas (7) to the phase-shifting networks (5), the dipole antennas (7), the feeder network (3,5) and the two-wire transmission lines (6) consisting of an etched pattern on a sheet of single plated synthetic material (2); and two layers of insulating material and ground planes (8) for enclosing at least the feeder network (3,5), characterised in that the synthetic material (2) is chosen flexible and the dipole antennas (7) have been mounted with a predetermined angle relative to the feeder network (3,5), for obtaining a predetermined polarization direction, which angle is obtained by twisting the two-wire transmission lines (6). Stripline antenna (1) according to claim 1, characterised in that the two-wire transmission lines (6) are implemented such as to have impedances that match the impedances of the dipole antennas (7). Stripline antenna (1) according to claim 2, characterised in that the two-wire transmission lines (6) are provided with transitions where they leave the ground planes (8). Stripline antenna (1) according to claim 3, characterized in that the two-wire transmission lines (6) are chosen wider outside the ground planes (8) than inside the ground planes (8). Stripline antenna (1) as claimed in claim 1, characterised in that a layer of synthetic foam is used as insulating material.

Inventors: TEUNISSE PETRUS JOHANNES STEPH, TEUNISSE, PETRUS JOHANNES STEPHANUS
Applicants: HOLLANDSE SIGNAALAPPARATEN BV, HOLLANDSE SIGNAALAPPARATEN B.V.
Application year: 1995
Main classification: H01Q1
IPC codes: H01Q1, H01Q9, H01Q21