REFLECTOR ANTENNA WITH A SELF-SUPPORTED FEED

Abstract: A reflector antenna with a dish-shaped main reflector (10), and a self-supporting feed (11) for the transmission or reception of polarized electromagnetic waves. The feed (11) consists of a tube (12) which is attached to the middle of the main reflector (10) and is terminated by a subreflector (13) so that an intermediate space (14) is formed between the subreflector and the end of the tube. The part of the tube that is nearest the intermediate space (14) contains a cylindrical waveguide (15), or is the waveguide itself, and has an approximately circular or quadratic cross-section. Externally, the intermediate space (14) is bounded by a circular, cylindrical surface (16) with the same diameter as the outer diameter of the tube (12) this being called the aperture surface. The surface of the subreflector (13) which is located just outside the surface of the aperture (16) has circular corrugations (17), or other means of creating a reactive, anisotropic surface impedance, to ensure that the electromagnetic waves are propagated along the surface disregardless of whether the electrical field is tangential to the surface or is normally on it. The part of the subreflector (13) that is located within the aperture surface (16) is shaped as a central conical element (18) with reflecting characteristics and which is inclined towards the tube (12).

Description: REFLECTOR ANTENNA WITH A SELF-SUPPORTED FEED The invention consists of a reflector antenna with a self- supported feed of the type indicated in the introduction to Claim of Patent 1, for the transmission or reception of polarized electromagnetic waves. The antenna is principally intended for the reception of TV signals from sattelites, however it can be used as a radio link, and as an ground station for sattelite communications. These types of reflector antennas are chiefly used because they are straightforward and inexpensive to manufacture. They also provide greater antenna efficiency and lower side lobes in the radiaton pattern than is the case when the feed has to be supported by liagonal struts. The drawback whit the latter configuration is that the main reflector becomes blocked. A self-supported feed is also easely accessible from the back of the reflector, thus is frequently selected when it is best to locate the transmitter and/or the receiver there. This also reduces the loss that occurs when the waves have to be led in a cable along one at the support struts. A. Chlavin, "A New Antenna Feed Having Equal E and H-Plane Patterns", IRE Trans. Antennas Propagat., Vol. AP-2, pp. 113-119, July 1954, describes a reflector antenna with a self-supporting feed. However since this antenna uses a waveguide with a rectangulcer cross-section, it can only transmit or receive waves with one particular linear polarization. C.C-Cutler, "Parabolic-antenna design for microwaves", Proc.IRE, Vol. 35, pp. 1284-1294, Nov. 1947, describes a dual polarized reflector antenna with two variants of a self-supporting feed, called the "ring focus and the "waveguide cup" feeds respectively. A circular waveguide is used in these two feeds whit a reflecting object in front of the waveguide opening, this reflector being respectively shaped as a flat disc and a cup. Both these feeds unfortunately produce high cross-polarization within the main lobe the radiation pattern. The main purpose of the present invention is to design a reflector antenna which has dual polarization with low cross-polarization within the main lobe of the radiation pattern. Dual polarization means that the antenna is capable of receiving or transmitting two waves with ortogonal linear or circular polarization simultaneously. Here, the waveguide must have an almost circular or quadraticcross-section. This objective can be achieved by a design which is in accordance with the charaterizing part of Claim of Patent 1. Further details about the invention are given in Claims of Patent 2-10. The surface of the subreflector is treated so that the electromagnetic waves are reflected from and propated along the surface in approximately the same way disgardless of whether the electric fields is normally on the surface or is tangential to it. Furtfer ore, the design of the other geometries of the feed ensures that the cross-polarization remains low within the main lobe the radiaton pattern. It should be mentioned that a dual polarized reflector antenna with a self-supporting feed is already know from among other sources P.Newham, "The Searc for an Efficient Splashplate Feed", Proceedings of the Third International Conference on Antennas and Propagation (ICAP 83), IEE Conference Publication No.219, pp. 348-352, April 1983, as in previous publications by the same author. In this design the subreflector has a smooth surface. However, it is also possible to obtain low cross-polarization when the subreflector is positioned at a distance from the waveguide aperture so that the waves are prevented from becoming radial and cannot propagate along the surface of the subreflector. This avoids the polarization-dependent reflection coeffiaent for radial waves found in the smooth subreflector. The present invention, on the other hand, has conceived of an antenna where this distance is so small that some of the waves are able to propagate along the surface of the subreflector. Low cross-polarization is then only ensured by a surface where the reflection coefficient for radial waves is independent of the polarization. The main advantage of the present invention over P.Newham's solution is that the diameter of the subreflector can be reduced so that the blockage in the centre of the main reflector is also smaller. It should also be nnoted that a dual polarized antenna that radiates around a cylinder is described by A.W. Love, "Scale Model Development of a High Efficency Dual Polarized Line Feed for the Arecibo Spherical Reflector", IEEE Trans. Antennas Propagat., Vol. AP-21, pp. 628-639, Sept. 1973. This antenna is however a linear array antenna consisting of numerous elements, which feed a main spherical reflector antenna. Further, this antenna has no subreflector. Mention should also be made of a dual polarized element which radiates around a smooth conductor cylinder. This is reported in P.S. Kildal. "Study of Element Patterns and Excitions of the Line Feed of the Spherical Reflector Anrenna in Arecibo". IEEE Trans. Antennas Propagat., Vol. AP-34, pp. 197.207, Feb. 1986. Section 11 of this paper provides a theoretical analysis of such an element. Once again there is no subreflector, and the element does not feed a main reflector. One result of this theoretical work is in fact the present invention. In US Patent 3162858 a dual polarized reflector antenna is described with a self-supporting feed element which mainly consists of a radial waveguide shaped as two plane surface or two coaxial conical surfaces whit a common apex. In the present invention there are no such radial waveguides, a subreflector is employed instead. Since the tube in the present invention is cylindrical rather than conical, the subreflector and the outside of the tube are unable to form radikal waveguides. Consequently, the waves are not propagated in the form of radial wave modes in this area, as is the case in the US Patent mentioned above. The US Patent decribes an antenna with a ring-shaped focus (the equivalent to the phase centre of the feed element) in the opening or aperture of the radial waveguide, and there is no subreflector outside this phase-centre. In the invention however, the feeds ring-shaped phase centre is close to the cylindrically-shaped aperture surface betweean the end of the tube and the middle of the subreflector. Consequently, in the invention the subreflector is mainly outside the phase centre. In the US Patent both walls in the radial waveguide have circular corrugations which are approximately 0.25 ^ wavelengths deep. These corrugations give the walls an anisotropic surface impedance which results in the radial waves being propagated so that they are independent of the polarization in the waveguide. In the invention, it is first and foremost only the subreflector which issupplied with such an anisotropic, reactive surface impedance. Using the investigations derived from the formule in the paper already mentioned in IEEE Trans. Antennas Propagat., Vol. AP-34, Feb.1986, it has been found that in most cases it is unnecessary to treat the outside of the tube with such a surface impedance. This consequently makes the invention cheaper to manufacture than the existing antenna where two surfaces have to be corrugated. There is no reason why the outside of the tube described in the present invention cannot be given an anisotropic reactive surface impedance, this may even be advantageous since in some applications particulary strict demands regarding cross-polarization may be required. The invention is based on a theoretical model concerning the way which radiation is released from a circumterencial slot in a cylinderical tube (cf. the paper mentioned in IEEE Trans. Antennas and Propagat., Vol. AP-34, Feb.1986). The bandwidth problem in the invention is solved by the means mentioned in Claims of Patent 6 and 10. This means that the central part of the subreflector is designed as a cone that is aimed in the direction of the main reflector. This cone reflects the incidence waves from the waveguide in a radial direction so that only small amplitude waves are reflected back to the waveguide. This minimizes return loss. At the same time a correct balance is achieved between the axial and the circular E-fields over the cylindrical aperture, thus ensuring low cross-polarization. This can be achieved over a relative bandwidth of about 10%. All mechanical dimensions between the middle of the subreflector and the end of the tube are critical, nevertheless there are a good number of dimension combinations which provide satisfactory results. The invention will be explained in more detail by making 0 reference to the diagrams, where: Fig. 1 illustrates an example of a reflector antenna with a self-supporting feed. Fig. 2 shows av axial cross-section through a feed designed in accordance with the invention, 5 Fig. 3 shows an axial cross-section through a subreflector which has a corrugated surface, Fig. 4 shows an axial cross-section through a tube with ciecular corrugations on the surface, Fig. 5 shows a normal section on a tube with longitudinal 0 corrugations on the surface, Fig. 6 shows an axial cross-section through a means of designing a feed element in accordance with the invention, and, Fig. 7 indicates which dimensions for the design in Fig. 6 25 must be trimmed and are critical. The antenna in Figure 1 consists of a dish-shaped main reflector 10. In the middle of this there is a self-supporting tubular feed element 11. This consists of a cylindrical tube 30 12, and a subreflector 13. The tube and the subreflector are separated by a space 14 which is bounded on the outside by a circular, cylindrical aperture surface 16 which will henceforth be termed the aperture surface or the aperture. Fig. 2 shows an axial section through the feed. The tube 35 12 contains a cyllindrical waveguide 15 which preferably has a sircular cross-section. The tube can also be such a waveguide itself. The waveguide is constructed to propagate the basic mode. This is the TE11 mode when the internal cross-section is circular with smooth, conducting walls. The waveguide must have a larger diameter than 0,6 (approx.) wavelengths _/* * and be smaller than 1.2 ^ (approx.). The tube and the waveguide are mostly made of conducthing materials. Though a smooth surface is shown, it could also be manufactured so that the surface impedance is anisotropic and reactive. The thickness of the walls measured between the inside of the waveguide and the outside of the tube is under 1.0 -^-(approx.). The wall can also be extremely thin. Figure 2 shows a case where the intermeditate space 14 extends slightly into the tube so that a circular waveguide is formed with a larger diameter than waveguide 15. The intermeditate space can also have another design. The subreflector is drawn as a plate with a conical element in the middle, it can also be shaped otherwise. The part of the subreflector's surface that is located outside the aperture surface 16 is drawn to appear smooth, however in fact it is treated so that the surface impedance is anisotropic and reative. This ensures that the electromagnetic waves are reflected from and propagate along the surface in approximately the same way disregardless of whether the electric fields is normally on the surface or is tangential to it. This is important to achieve low cross-polarization. The best results come from making the surface impedance so that there si only a minor amount of radiation in a radial direction along the subreflector both when the fields is normally on the surface and when it is tangential to it. The diameter of the subreflector is always larger than the diameter of the tube, typical values are between 3 ^ and 6 Λ . The aperture surface 16 is indicated in Figure 2 by a broken line. The cross-section of the aperture 16 is under 1.0, l preferably 0,5 /(.(approx.). In the same way the end of the waveguide 15 is marked by a broken line. There is an intermediate space 14 betwen the aperture and the end of the waveguide, this is bounded by the subreflector and the tube. Both the intermeditate space and the aperture are drawn so that they appear to be filled be air. In practice they would be partly or totally filled with dielectric matter, or they could be partially sealed with metallic or dielectric rods or discs that are respectively located in a plane with the axis of symmetry. Though this is necessary to attach the subreflector to be the tube, this is also a means of controlling the radiation characteristics. Fig. 3 shows an axial section of a subreflector 13 where the other part that lies outside the aperture 16 has circular corrugations or grooves 17 in the surface. This grooves are about 0,25 ,/l.deep. This is one way of realizing the anisotropic and reactive surface impedance. The objective is as mentioned before to obtain as little radiation as possible in a radial direction along the subreflector both when the fields is normally on the surface and also when it is tangential to it. This is important to obtain low cross-polarization. This objective can also be achieved by a surface with other characteristics. Fig. 4 shows an axial cross-section of a tube 12 where there are circular corrugations 18 in the surface. These corrugations are about 0,25 _Λ deep and produce an anisotropic, reactive surface impedance. The purpose is to obtain as little radiation as possible along the tube when the fields is orthogonal to the surface and when it is tangential to it. This can also be achieved by a surface with different characte istics. Fig. 5 shows a cross-section of a tube 12 where the surface has longitudinal corrugations 19, these are filled with dielectrium with a relative permittivity of £, • The depth of the corrugations 0,25 / t~ l - These corrugations provide an anistropic, reactive surface impedance. The objective is to produce powerful radiation along the tube both when the field is normally on the surface and when it is tangential to it. This can also be managed by using a surface with other characteristics. Fig. 6 shows a normal means of designing the feed element. The space 14 is filled with a dielectric plug 21 which is glued or screwed into both the tube and the subreflector by means of an extra corrugation 23 inside the aperture surface or by means of a central outlet 22 in the conical part 20 of the subreflector 13. The part of the subreflector which lies outside the aperture surface is plane and has circular corrugations. The dielectric plug 21 passes into the tube and forms a cylindrical waveguide with a larger diameter than the waveguide 15. Fig. 7 also shows the design in Fig. 6. The critical dimensions which must be trimmed in the laboratory model are marked x, y, z and 2a. This can be done by making the concial element 20 so that it can be screwed into the subreflector. In addition, the waveguide 15 and the dielectric plug 21 are both to be made so that they can be screwed into the tube 12. The manner in which the design in Fig. 6 works for linear polarization is explained in the next paragraph. In the case circular polarization the design works in an equivalent way because the geometry has rotational symmetry. The manner of operation is explained for transmission, but is equivalent when receiving. A wave in the TE11 mode is propagated in the waveguide 15. This wave is coupled to two modes at the surface of the aperture 16. For one mode the electric fields are directed exclusively in the z-direction (z-mode), and for the other the fields are directed in the azimuth-direction transverse to the z-direction (<p~mode). These two modes radiate out of the aperture 16, the z-mode principally in the E-plane and the (P-mode chiefly in the H-plan. To get a rotationally- symmetrical radiation pattern with low cross-polarization, the radiation patterns in the E and H-planes must be similar in both amplitude and phase. The anisotropic and reactive surface impedance to the subreflector 13 is the reason why the z-mode radiates the same way in the E-plane as the -^-mode radiates in the H-plane. At the same time the internal dimensions of the feed element are controlled so that the z-mode and the -mode are excited by the correct amplitude and phase, relatively-speaking. The z-mode and the c -mode radiate differently along the tube. This can be improved by making the surface impedance along the tube anisotropic and reactive, as described previously. This is an extra cost and was not found to be necessery for the application the alternative in Fig. 6 was develged for. The reactive and anisotropic surface impedance of the subreflector is realized by means of circular corrugations 17. These prevent the z-mode radiating strongly in a radial direction. The excition of the cp-mode and the z-mode are controlled by varying the dimensions of x, y, z and 2a in Figure 7. The best results are obtained if the external part of the tube forms a waveguide whith a larger diameter than the waveguide 15, enabling both the TE11 and the TM 11 modes to be propagated here. The resulting radiation pattern from the feed antenna has low cross-polarization. Unfortunately there are considerable phase errors because the source of radiation, the aperture 16, is a long way from the axis. These phase errors can be compensated for by shaping the main reflector differently from a parabolic surface. If the diameter of the tube is about l / , the optimal reflector shape will deviate by upto 1.6 mm from the best fitted parabola. The resultant radiation characteristics of the whole antenna are excellent and have low cross-polarization. Fig. 6 shows one design of the antenna, it should nevertheless be apparant from the claims of patent that there are numerous other forms of design possible. Common for all is that the part of the subreflector's surface which is outside the aperture 16 has an anisotropic and reactive surface impedance, and that the subreflector is located as close to the end of the waveguide 15 that the field at the aperture surface is descibed by two modes. Other common features are that the geometries of the central part 20 of the subreflector 13 and the sondition of the intermeditate space 14 are designed so that the required modes are excited with the correct phase and amplitude, relatively speaking. This design makes particular allowanse for how the modes radiate both along the tube and the surface of the subreflector. The ideal shape is when the radiation patterns from both modes are intergrated in an optimal manner so that tha resultant pattern is in rotational symmetry and has low cross-polarization. Altering the shape of the inter adiate space, or filling this completely or partially with dielectricum, are two means of influencing the relative excitation of the modes. Finally, it should be mentioned that the self-supporting feed antenna has already been christened, and is known as the hat antenna or the hat feed. The different elements that are illustrated in Figs. 2 and 3 can be combined and modified in various ways. The tube 12 can be a polygonal or square cylinder. The subreflector can be manufactured of plastic with a metallic surface coating. The plug 21 in the intermediate space can be combined with the subreflector 13 in other ways than those shown, for instance just one of elements 22 or 23 are used. If only element 22 is used, the subreflector will not have a central outlet at its point 20. If only element 23 is used, the subreflector will not have any corrugations inside the aperture 16.

Claims: Claim of patent: 1. Reflector antenna, consisting of a spherical dished-shaped main reflector (10), and a self-supporting feed element (11) for transmitting or receiving polarized electromagnetic waves, where the feed element (11) consist of a load-carrying stright tube (12), which has one end attached to the centre of the reflector (10), and the other terminated by a subreflector (13) in such a way that an intermediate space (14) is formed between the subreflector (13) and the end of the tube, where the section of the tube (12) that is nearest the intermediate space (14) either contains a cylindrical waveguide (15), or is the waveguide itself, where the waveguide (15) has an almost circular or quadratic cross- section, where the intermeditate space (14) provides a connection between the waves inside the waveguide and those outside the feed element, and where the intermediate space (14) is externally bounded by a circular, cylindrical surface (16) which has the same diameter as the outer diameter of the tube (12) and is called the aperture surface, c h a r a c t e r i z e d by that part of the tube (12) which is nearest to the aperture surface having an outer surface which is mainly cylindrical with a circular cross-section, so that the other surface of the tube (12) and that part of the subreflector (13) that is located outside the aperture surface (16) does not form a radial waveguide where just one or a small number of elementary radial wave modes can be propagated, and in a way that the phase centre of the feed element is ring-shaped and lies close to the aperture surface (16), and by that part of the subreflector's (13) surface which lies outside the aperture surface (16) being treated so that an anisotropic and reactive surface impedance is created, thus ensuring that the radial cylindrical electromagnetic waves are reflected from and propagated along the surface in approximately the same way disregardless of whether the electric field is normally on or tangential to the surface, so that this, together with the design of the remaining geometry in the feed element, produce a radiation pattern in the feed element with low cross-polarization, which is almost in rotational symmetry around the tube. (12). 2. Reflector antenna as decribed in Claim 1, where the main reflector (10) is rotationally symmetrical and has an almost parabolic shape, c h a r a c t e r i z e d by the tube (12) having a dea eter less than about 1,0 wavelengths - .. 3. Reflector antenna as described in Claim 1, where the main reflector (10) is rotationally symmetrical, c h a r a c t e r i z e d by the main reflector (10) deviating from a parabolic shape in such a way that it corrects the phase errors in the radiation in the pattern of the feed element when the tube (12) has a diameter larger than about 1,0 wavelengths _ - 4. Reflector antenna which is in accordance with one of Claims of Patent 1 - 3, c h a r a c t e r i z e d by giving the subreflector an anisotropic and reactive surface impedance by using rotationally symmetrical tracks or corrugations (17) in an othervise smooth electrically conductive surface, where the corrugations have a depth of about 0,25 wavelengths s\ , and where the corrugations are so close that there are more than 2 currogations per wavelength in a radial direction. 5. Reflector antenna in accordance with one of Claims of Patent 1 - 4, c h a r a c t e r i s e r t by the tube (12) having a reflecting surface which is either smooth or where measures have been completely or partically taken to produce an anisotropic and reactive surface impedance to ensure that waves propagate in approximately the same way along that part of the tube disregardless of wheter the electric field is normally on the surface or tangential it. Where this is preferably achieved by means of surface corrugations, which are either circular corrugations (18),where the depth of the latter is about 0,25 wavelengths Λ , or longitudinal corrugations filled with a dielectric matter (19) in an otherwise smooth electrically-conductive surface, where the corrugations have a depth of about 0,25 v ~ < 7 1 wavelengths >■ , where & is the relative permittivity of the dielectric material. 6. Reflector antenna which is in accordance with one of Claims of Patent 1 - 5, c h a r a c t e r i z e d by the part of the subreflector (13) that lies within the aperture surface (16) being shaped as a central conical or equivalent converging element (20) which has reflecting characteristics and which is inclined towards the tube (12) so that the incidence waves from the waveguide (15) are reflected in such a manner that there is balance between the axial and circular fields at the surface of the aperture (16) and so that the wave that is reflected back into the waveguide has low amplitude over a large freqency range. 7. Reflector antenna described in Claim of Patent 6, c h a r a c t e r i z e d by the conical part (20) of the subreflector being manufactured in one piece together with the subreflektor (13), or as a separate element (20) mounted in a central opening in the subreflector (13). 8. Antenna in accordance with one of Claims of Patent 1 - 7, c a r a c t e r i z e d by having an intermediate space (14) between the waveguide (15) and the subreflector (13) which is completely or partly filled with a dielectric element (21), which is preferably interlocked with the waveguide (12) and the subreflector (13). 9. Antenna in accordance Claim of Patent 8, c h a r a c t e r i z e d by a filling element (21) whith a central outlet (22) pointing towards and connected to a similar outlet in the conical element (20), or preferably with a circular corrugation (23) which is interlocked with a circular corrugation (17) in the subreflektor (13). 10. Antenna which is in accordance with one of Claims of Patent 1 - 9, c h a r a c t e r i z e d by an intermediate space (14) which extends into the tube (12) in such a way that a the cylindrical waveguide which is formed in this area has a larger diameter than the waveguide (15) so that the two circular wave modes TEH and TM11 are both able in the waveguide (15).

Inventors: KILDAL PER-SIMON, KILDAL, PER-SIMON
Applicants: STIFTELSEN IND OG TEK FORSK, STIFTELSEN FOR INDUSTRIELL OG TEKNISK FORSKNING VE
Application year: 1987
Main classification: H01Q19
IPC codes: H01Q19