EP0268635B1

EP0268635B1 - Reflector antenna with a self-supported feed

Info

Publication number: EP0268635B1
Application number: EP87903452A
Authority: EP
Inventors: Per-Simon Kildal
Original assignee: STIFTELSEN FOR INDUSTRIELL OG TEKNISK FORSKNING VED NTH (SINTEF)
Current assignee: STIFTELSEN FOR INDUSTRIELL OG TEKNISK FORSKNING VED NTH (SINTEF)
Priority date: 1986-06-03
Filing date: 1987-06-03
Publication date: 1991-12-27
Anticipated expiration: 2007-06-03
Also published as: NO163928C; DE3775528D1; NO880464L; WO1987007771A1; NO880464D0; NO864563D0; ATE70924T1; EP0268635A1; NO864563L; NO163928B; JPH01500790A

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

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 satellites, however it can be used as a radio link, and as a ground station for satellite 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 radiation pattern than is the case when the feed has to be supported by diagonal struts. The drawback with the latter configuration is that the main reflector becomes blocked. A self-supported feed is also easily 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 of the support struts.
A.Chlavin, "A New Antenna Feed Having Equal E and W-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 wavequide with a rectangular cross-section, it can only transmit or receive waves with one particular linear polarization.
C.C-Cutler, "Parabolic-antenna design for microwaves", Proc.IRE.Vil.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 with 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 crosspolarization within the main lobe of the radiation pattern.
The main purpose of the present invention is to design a reflector antenna which has dual polarization with low crosspolarization 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 square cross-section.
This objective can be achieved by a design which is in accordance with the Claim of Patent 1. Further details about the invention are given in Claims of Patent 2-10.
The surface of the subreflector is treated by e.g. corrugations so that the electromagnetic waves are reflected from and propagated along the surface in approximately the same way regardless of whether the electric field is normal to the surface or is tangential to it. Furthermore, the design of the other geometries of the feed ensures that the cross-polarization remains low within the main lobe of the radiation pattern.
It should be mentioned that a dual polarized reflector antenna with a self-supporting feed is already known from among other sources P. Newham, "The Search 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 sub-reflector has a smooth surface. Low cross-polarization is obtained by locating the subreflector at a distance from the waveguide aperture so that the waves are prevented from becoming radial and thereby cannot propagate along the surface of the sub-reflector. This avoids the polarization-dependent reflection coefficient for radial waves found in the smooth sub-reflector. The present invention, on the other hand, has conceived an antenna where this distance is so small that some of the waves propagate along the surface of the sub-reflector. Then, low cross-polarization is only ensured by a surface where the reflection coefficient for radial waves is made independent of the polarization, e.g. by using corrugations.
The main advantager of the present invention over P. Newham's solution is that the diameter of the sub-reflector can be reduced so that the blockage in the centre of the main reflector is also smaller.
It should be noted that the dual polarized antenna that radiates around a cylinder is described by A.W.Love, "Scale Model Development of a High Efficiency 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 sub-reflector.
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 Antenna 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 sub-reflector and the element does not feed a main reflector. One result of this theoretical work is in fact the present invention.
In US Patent 3.162.858 a dual polarized reflector antenna is described with a self-supporting feed element which mainly consists of a radial waveguide shaped as two plane surfaces or two coaxial conical surfaces with a common apex. In the present invention there are no such radial waveguides, a sub-reflector is employed instead.
Since the tube in the present invention is cylindrical rather than conical, the sub-reflector and the outside of the tube are unable to form radial 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 describes 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 apeture surface between the end of the tube and the middle of the sub-reflector. Consequently, in the invention the sub-reflector is mainly outside the phase centre.
In the US Patent both walls in the radial waveguide have circular corrugations which are approximtely 0.25 λ 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 sub-reflector which is supplied 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.
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 in which radiation is released from a circumferencial slot in a cylindrical 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 sub-reflector 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 sub- reflector 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 reference to the diagrams, where:

Fig. 1 illustrates an example of a reflector antenna with a self- supporting feed.
Fig. 2 shows an axial cross-section through a feed designed in accordance with the invention,
fig. 3 shows an axial cross-section through a sub-reflector which has a corrugated surface,
Fig. 4 shows an axial cross-section through a tube with circular corrugations on the surface,
Fig. 5 shows a normal section on a tube with longitudinal 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 must be trimmed and are critical.

The antenna in Fig. 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 12, and a sub-reflector 13. The tube and the sub-reflector 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 12 contains a cylindrical waveguide 15 which preferably has a circular 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 λ and be smaller then 1,2 λ (approx.). The tube and the waveguide are mostly made of conducting 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 1.0λ (approx.). The wall can can also be extremely thin. Fig. 2 shows a case where the intermediate space 14 extends slightly into the tube so that a circular waveguide is formed with a larger diameter than waveguide 15. The intermediate space can also have another design.
The sub-reflector is drawn as a plate with a conical element in the middle. It can also be shaped otherwise. The part of the sub-reflector'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 reactive. This ensures that the electromagnetic waves are reflected from and propagate along the surface in approximately the same way regardless of whether the electric fields are normal to the surface or are tangential to it. This is important to achieve low cross-polarization. The best results come from making the surface impedance so that there is only a minor amount of radiation in a radial direction along the sub-reflector both when the fields are normal to the surface and when the fields are tangential to it. The diameter of the sub-reflector is always larger than the diameter of the tube, typical values are between 3 λ and 6 λ.
The aperture surface 16 is indicated in Fig. 2 by a broken line. The cross-section of the aperture 16 is under 1,0 λ , preferably 0,5 λ (approx.). In the same way the end of the waveguide 15 is marked by a broken line. There is an intermediate space 14 between the aperture and the end of the waveguide,. This is bounded by the sub-reflector and the tube. Both the intermediate space and the aperture are drawn so that they appear to be filled with 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 being respectively located in a plane with the axis of symmetry. Though this is necessary to attach the sub-reflector to the tube, this is also a means of controlling the radiation characteristics.
Fig. 3 shows an axial section of a sub-reflector 13 where the other part that lies outside the aperture 16 has circular corrugations or grooves 17 in the surface. These grooves are about 0,25 λ 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 sub-reflector both when the fields are normal to the surface and also when the fields are 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 are orthogonal to the surface and when they are tangential to it. This can also be achieved by a surface with different characteristics.
Fig. 5 shows a cross-section of a tube 12 where the surface has longitudinal corrugations 19, these are filled with dielectric with a relative permittivity of ε . The depth of the corrugations 0,25 λ/ $\sqrt{ε-1}$
. These corrugations provide an anisotropic reactive surface impedance. The objective is to produce powerful radiation along the tube both when the field is normal to 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 sub-reflector 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 sub-reflector 13. The part of the sub-reflector 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 conical element 20 so that it can be screwed into the sub-reflector. 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 of 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 (φ -mode). These two modes radiate out of the aperture 16, the z- mode principally in the E-plane and the φ -mode chiefly in the H-plane. 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 sub-reflector 13 is the reason why the z-mode radiates the same way in the E- plane as the φ -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 φ-mode are excited by the correct amplitude and phase, relatively speaking. The z-mode and the φ - 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 necessary for the application the alternative in Fig. 6 was developed for.
The reactive and anisotropic surface impedance of the sub- reflector is realized by means of circular corrugations 17. These prevent the z-mode radiating strongly in the radial direction. The excitation of the φ -mode and the z-mode are controlled by varying the dimensions of x, y, z and 2a in Fig. 7. The best results are obtained if the external part of the tube forms a waveguide with a larger diameter than the waveguide 15, enabling both the TE11 and the TM 11 modes to 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. It the diameter of the tube is about 1 λ ,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 apparent from the claims of patent that there are numerous other forms of design possible. Common for all is that the part of the sub-reflector's surface which is outside the aperture 16 has an anisotropic and reactive surface impedance, and that the sub-reflector is located as close to the end of the waveguide 15 that the field at the aperture surface is described by two modes. Other common features are that the geometries of the central part 20 of the sub-reflector 13 and the condition of the intermediate space 14 is designed so that the required modes are excited with the correct phase and amplitude, relatively speaking.
This design makes particular allowance on how the modes radiate both along the tube and the surface of the sub-reflector. The ideal shape is when the radiation patterns from both modes are integrated in an optimal manner so that the resultant pattern is in rotational symmetry and has low cross-polarization. Altering the shape of the intermediate space or filling this completely or partially with dielectric, 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 sub-reflector can be manufactured of plastic with a metallic surface coating. The plug 21 in the intermediate space can be combined with the sub-reflector 13 in other ways that those shown, for instance just one of elements 22 or 23 are used. If only element 23 is used, the sub-reflector will not have a central outlet at its point 20. If only element 22 is used, the sub-reflector will not have any corrugations inside the aperture 16.

Claims

1. Reflector antenna, consisting of a spherical dish-shaped main reflector (10), and a self-supporting feed element (11) for transmitting or receiving polarized electromagnetic waves, where the feed element (11) consists of a load-carrying straight tube (12), which has one end attached to the centre of the reflector (10), and the other terminated by a sub-reflector (13) in such a way that an intermediate space (14) is formed between the sub- reflector (13) and the end of the tube, where the section of the tube (12) that is closest to the intermediate space (14) either contains a cylindrical waveguide (15), or is the waveguide itself, where the waveguide (15) has an almost circular or square cross-section, where the intermediate 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, characterized in that part of the tube (12) which is closest to the aperture surface has 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 sub-reflector (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 in that part of the sub- reflector's (13) surface which lies outside the aperture surface (16) is 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 regardless of whether the electric field is normal to 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 described in Claim 1, where the main reflector (10) is rotationally symmetrical and has an almost parabolic shape, characterized by the tube (12) having a diameter less than about 1,0 wavelengths λ .

3. Reflector antenna as described in Claim 1, where the main reflector (10) is rotationally symmetrical, characterized 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 λ.

4. Reflector antenna which is in accordance with one of Claims of Patent 1-3, characterized by giving the sub-reflector an anisotropic and reactive surface impedance by using rotationally symmetrical tracks or corrugations (17) in an otherwise smooth electrically conductive surface, where the corrugations have a depth of about 0,25 wavelengths λ and where the corrugations are so close that there are more than two corrugations per wavelength in a radial direction.

5. Reflector antenna in accordance with one of the Claims of Patent 1-4, characterized by the tube (12) having a reflecting surface which is either smooth or where measures have been completely or partially 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 regardless of whether the electric field is normal to the surface or tangential to 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/

\sqrt{ε-1}

wavelengths λ , where ε is the relative pemittivity of the dielectric material.

6. Reflector antenna which is in accordance with one of Claims of Patent 1-5, characterized by the part of the sub-reflector (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 incident 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 frequency range.

7. Reflector antenna as described in Claim of Patent 6, characterized by the conical part (20) of the sub- reflector being manufactured in one piece together with the sub-reflector (13), or as a separate element (20) mounted in a central opening in the sub-reflector (13).

8. Antenna in accordance with one of Claims of Patent 1-7, characterized by having an intermediate space (14) between the waveguide (15) and the sub-reflector (13) which is completely or partly filled with a dielectric element (21), which is preferably interlocked with the waveguide (12) and the sub-reflector (13).

9. Antenna in accordance with Claim of Patent 8, characterized by the filling element (21) with 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 sub-reflector (13).

10. Antenna which is in accordance with one of Claims of Patent 1-9, characterized by an intermediate space (14) which extends into the tube (12) in such a way that the cylindrical waveguide which is formed in this area has a larger diameter than the waveguide (15) so that the two circular wave models TE11 and TM11 are both able in the waveguide (15).