Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q19/00—Combinations of primary active antenna elements and units with secondary devices, e.g. with quasi-optical devices, for giving the antenna a desired directional characteristic
  • H01Q19/10—Combinations of primary active antenna elements and units with secondary devices, e.g. with quasi-optical devices, for giving the antenna a desired directional characteristic using reflecting surfaces
  • H01Q19/12—Combinations of primary active antenna elements and units with secondary devices, e.g. with quasi-optical devices, for giving the antenna a desired directional characteristic using reflecting surfaces wherein the surfaces are concave
  • H01Q19/13—Combinations of primary active antenna elements and units with secondary devices, e.g. with quasi-optical devices, for giving the antenna a desired directional characteristic using reflecting surfaces wherein the surfaces are concave the primary radiating source being a single radiating element, e.g. a dipole, a slot, a waveguide termination
  • H01Q19/134—Rear-feeds; Splash plate feeds

Definitions

• 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.
• 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.
• the waveguide must have an almost circular or quadraticcross-section.
• 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.
• the subreflector has a smooth surface.
• the present invention 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.
• 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.
• 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.
• 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.
• 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.
• 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 formula 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.
• 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.
• 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.
• 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
• 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.
• 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
• 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.
• 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
• 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 ⁇ .
• 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.).
• the end of the waveguide 15 is marked by a broken line.
• 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 _ ⁇ 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.
• 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.
• 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.
• 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.
• 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.
• 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.

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Citations (3)

• 1986
  • 1986-11-17 NO NO864563A patent/NO864563L/no unknown
• 1987
  • 1987-06-03 EP EP87903452A patent/EP0268635B1/en not_active Expired - Lifetime
  • 1987-06-03 AT AT87903452T patent/ATE70924T1/de not_active IP Right Cessation
  • 1987-06-03 DE DE8787903452T patent/DE3775528D1/de not_active Expired - Lifetime
  • 1987-06-03 WO PCT/NO1987/000044 patent/WO1987007771A1/en active IP Right Grant
  • 1987-06-03 JP JP62503322A patent/JPH01500790A/ja active Pending
• 1988
  • 1988-02-03 NO NO880464A patent/NO163928C/no unknown

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