ANTENNA ELEMENT FOR TWO ORTHOGONAL POLARIZATIONS

Abstract: An antenna element intended for transmission and reception of orthogonal polarisations having a good polarisation purity of electromagnetic radiation is described. It comprises a ground plane (2) and at least one radiation disc (1) connected to two ports (4, 5) for the supply of a desired electromagnetic radiation, preferably in the microwave range. The distance between the ground plane (2) and the radiation disc (1) is a fraction of the wavelength lambda of the radiation. The antenna element is in particular characterized in that the ports (4, 5) are located at an electric distance of 90 DEG from each other and that a quarterwave transformer (6) is added between the ground plane (2) and the radiation disc (1) that is shaped so that the transmission line impedance between the ports (4, 5) is reduced in order to make the ports radiationally independent of each other.

Description: ANTENNA ELEMENT FOR TWO ORTHOGONAL POLARIZATIONS The present invention relates to an antenna element intended for transmission and reception of two orthogonal polarisations having a good purity of polarisation, of electromagnetic radiation, comprising a ground plane and at least one radiation disc connected to two ports for supplying a desired electromagnetic radiation, preferably within the microwave range, the distance between the ground plane and the radiation disc being a fraction of the wavelength of the radiation. Antenna elements of this kind are called micro strip antennas or patch antennas and constitute an all round class of antenna elements. Hence, the basic building block is a radiation disc in the shape of a flat, metallic plate having a shape that in principle is arbitrary, i.e. a patch, separated from a ground plane by means of air or a dielectric. By means of a correct choice of the dimensions of the plate it is made resonant for a chosen electromagnetic radiation frequency, usually in the microwave range, and can be used for absorbing or radiating er^rgy at this frequency. It is often desirable that the antenna absorbs or radiates radiation sciely having a particular polarisation. It is required that the antenna has a good purity of polarisation (a low cross polarisation, a low axial ratio). It is often required that the antenna can obtain two orthogonal polarisations corresponding to two antenna ports, i.e. two linear or opposed elliptical, in particular circular polarisations, having a good purity of polarisation. It is clear that deviations from a geometrical symmetry can cause degradation of the polarisation purity. The supply itself to the radiation disc can cause such an asymmetry. The problem related to polarisation purity then becomes pronounced in the case when the antenna element is made type broad band, since the effect from the supply in this case becomes marked. A number of different solutions of the problem relating to polarisation purity in antenna elements of the above type has been suggested. The problem of geometrical asymmetry caused by the supply of the radiation disc can be solved by means of balanced supply, but this will add supply points. This is a complication that can not always be accepted. Balanced supply results in a doubling of the number of supply points for the antenna element, an increased complexity and the problem with potential build-up. Cutting-outs and tongues at the edges of the antenna elements are often used to obtain a circular polarisation using only one supply point. In the case of two polarisations they can, in principle, be used in order to improve the polarisation purity. In practice it seems to be hard to design such radiation discs having cutting-outs and tongues since they also modify the electric dimensions of the antenna. If the antenna element is to generate a circular, single or double (i.e. right and left rotating) polarisation and a quadrature hybrid is used, unbalance in the hybrid provides a method of improving the polarisation purity. Unless circular polarisation is required and the hybrid already is a part of the antenna the use of unbalanced hybrids seems to be of little value. Sequential rotation of and supply to elements can be used as a method of improving the polarisation purity in an array of antenna elements. It is a supplement to other methods and only works in the case when the antenna elements are used together with other elements in an array where cross-polarisation of different antenna elements can be made to cancel each other at angles close to the broad side direction. The object of the invention is to provide an antenna element of the kind mentioned in the introduction, which has a high polarisation purity but still has a simple construction. According to the invention such an antenna element is primarily characterised in that the ports are located at an electrical distance of 90° from each other and that a quarterwave transformer is added between the ground plane and the radiation disc, that is so designed that the transmission line impedance between the ports is reduced to make the ports radiationally independent of each other. The quarterwave transformer can be designed in different ways. It can be located as one unit inside the 90° sector between the ports or inside the 270° sector opposite to the ports. It can also consist of two parts, symmetrically located within the 90° sector between the ports and within the 270° sector opposite the ports. In a preferred embodiment the quarterwave transformer is formed of steps in the ground plane at a smaller distance from the radiation disc than the rest of the ground plane. The quarterwave transformer can also consist of steps in the radiation disc located at a smaller distance from the ground plane than the rest of the radiation disc. The principle underlying the invention can be described in terms of a transmission model of the radiation disc, for example for a circular radiation disc, which is fed at two points, that are located geometrically 90° apart from each other. At resonance of the base mode TM11 (Hll) the two ports are also electrically separated 90° due to the circumferential &-*- ) " dependence of the electrical field. The two ports, matched to their transmission lines result in high impedance resistive loads on the low impedance loss radiation disc transmission line. The impedance at one port is higher as seen in a direction towards an adjacent port than away therefrom, since the load impedance is transformed via the quarterwavelength, which separates the ports. This has the effect that less energy is distributed to the adjacent port than what is distributed in the opposite direction. The e + - θ and e ~ J components are excited to different amplitudes, i.e. an elliptical polarisation is excited by one port, which in an ideal case would provide a linear polarisation. As a non-limiting exemplary embodiment of an antenna element according to the invention, an antenna element having a circular radiation disc is described below, but in principle the radiation disc in an antenna element according to the invention can have other shapes, such as for example rectangular, in particular quadratic shapes, the quarterwave transformer being adapted to the shape of the radiation disc. In Figure 1 an antenna element is shown in a perspective view, in Figure 2 the same antenna element is shown in a plane view and in Figure 3 in a side view. In the figures a circular radiation disc is denoted 1, a ground plane is denoted 2 and a post of a dielectric or metallic material is denoted 3. The latter one maintains the radiation disc at a distance from the ground plane, which is a fraction of the wavelength λ of the radiation. Two probes 4 and 5 constitute ports for the supply of the present signal to the radiation disc 1. These ports are located geometrically and thereby electrically at a 90° distance from each other. In the sector between them, the ground plane is elevated to form an impedance reducing step 6. By supplying and receiving the corresponding signals respectively through the two ports 4 and 5 a very high polarisation purity in all desired orthogonal polarisations is obtained. An important advantage of the quarterwave transformer, designed as a step in the region between the radiation disc and the ground plane, is that the radiation disc can be adapted and optimized with regard to other parameters before the antenna element is optimized with regard to the requirements of polarisation purity. The reason for this is that the impedance steps in general do not effect the electrical magnitude of the radiation disc. However, if instead tongues and cutting-outs were used on the radiation disc in order to achieve polarisation purity, all other parameters must be optimized simultaneously.

Claims: CLAIMS 1. An antenna element intended for transmission and reception of two orthogonal polarisations having good polarisation purity of electromagnetic radiation, comprising a ground plane (2) and at least one radiation disc (1) connected to two ports (4,5) for the supply of a desired electromagnetic radiation, preferably in the microwave range, the distance between the ground plane (2) and the radiation disc (1) being a fraction of the wavelength λ of the radiation, characterized in that the ports (4, 5) are located at an electrical distance of 90° from each other and a quarterwave transformer (6) is located between the ground plane (2) and the radiation disc (1) that has such a shape that the transmission line impedance between the ports (4, 5) is reduced to make the ports radiationally independent of each other. 2. An antenna element according to claim 1, characterized in that the quarterwave transformer (6) is located inside the 90° sector between the ports (4, 5). 3. An antenna element according to claim 1, characterized in that the quarterwave transformer (6) is located inside the 270° sector opposite the ports (5, 4). 4. An antenna element according to claim 1, characterized in that the quarterwave transformer consists of two parts, symmetrically located inside the 90° sector between the ports and inside the 270° sector opposite the ports. 5. An antenna element according to any of claims 1 to 4, characterized in that the quarterwave transformer is formed by steps (6) in the ground plane (2) located more closely to the radiation disc (1) than the rest of the ground plane (2). 6. An antenna element, according to any of claims 1 to 4, characterized in that the quarterwave transformer (6) is formed by steps in the radiation disc located more closely to the ground plane (2) than the rest of the radiation disc (1).

Inventors: JOSTELL ULF, JOSTELL, ULF
Applicants: JOSTELL ULF, SAAB ERICSSON SPACE AB, JOSTELL, ULF
Application year: 1996
Main classification: H01Q9
IPC codes: H01Q9