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(11) | EP 0 140 598 B1 |
| (12) | EUROPEAN PATENT SPECIFICATION |
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| (54) |
Horn-reflector microwave antennas with absorber lined conical feed Mikrowellen-Reflektorantenne deren Speisehornstrahler mit Absorbermaterial bedeckt ist Antenne micro-ondes du type réflecteur à source cornet tapissé d'absorbant |
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| Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1) European Patent Convention). |
[0001] The present invention relates generally to microwave antennas and, more particularly, to reflector-type microwave antennas having conical feeds.
[0002] Conical feeds for reflector-type microwave antennas have been known for many years. For example, a 1963 article in The Bell System Technical Journal describes the selection of a conical horn-reflector antenna for use in satellite communication ground station (Hines et al, "The Electrical Characteristics of The Conical Horn-Reflector Antenna", The Bell System Technical Journal, July 1963, pp. 1187-1211). A conical horn-reflector antenna is also described in Daw- son U.S. Patent No. 3,550,142, issued December 22, 1970. Conical feed horns have also been used with large parabolic dish antennas.
[0003] One of the problems with a smooth-walled conical horn reflector antenna is that its radiation pattern envelope (hereinafter referred to as the "RPE") in the E-plane is substantially wider than its RPE in the H-plane. When used in terrestrial communication systems, the wide beamwidth in the E-plane can cause interference with signals from other antennas. Also, when a smooth-walled conical horn is used as the primary feed for a parabolic dish antenna, its different beamwidths in the E and H-planes make it difficult to achieve symmetrical illumination of the parabolic dish.
[0004] In European Patent Publication No. 66455A there is described an improved horn-reflector antenna having a lining of absorber material within the conical feed horn. That antenna produces narrower E-plane RPE's, thereby bringing the E-plane and H-plane RPE's closer together, without significantly degrading other performance characteristics of the antenna.
[0005] It is a primary object of the present invention to provide an economical and effective way to achieve further narrowing of the E-plane RPE of a horn-reflector antenna having a conical feed, without significantly degrading the H-plane RPE or any other performance characteristic of the antenna. In this connection, a related object of this invention is to provide an improved conical feed which is capable of bringing the RPE's in both the E and H-planes even closer together.
[0006] It is another important object of this invention to provide an improved horn-deflector antenna which introduces only a small gain drop into the microwave system in which it is used.
[0007] It is yet another object of this invention to provide such an improved horn-reflector antenna which can be efficiently and economically fabricated.
[0008] Other objects and advantages of the invention will be apparent from the following detailed description and the accompanying drawings.
[0009] According to the present invention there is provided a conical horn-reflector antenna comprising the combination of a paraboloidal reflector forming a paraboloidal reflecting surface for transmitting and receiving microwave energy, and a conical feed horn for guiding microwave energy from the focus of said paraboloidal reflecting surface to said reflector, the small end of the horn being located near and with its apex coincident with the focal point of the paraboloidal reflector, the lower portion of said horn having a smooth metallic interior wall defining a conical surface which, when projected onto the reflector, is intercepted by the paraboloidal reflecting surface thereof, the upper portion of said horn being lined with absorber on at least those portions of the interior wall which affect the E-plane field distribution so as to cause the radiation pattern envelope width in the E-plane to approach that in the H-plane, characterised by said absorber having its thickness located outside of the projection of said conical surface defined by said lower portion of said horn and having its exposed surface lying substantially on said conical surface projection.
[0010] The exposed surface of the absorber may be smooth or profiled in the form of multiple pyramids or convoluted cones.
Fig. 1 is a front elevation, partially in section, of a horn-reflector antenna embodying the present invention;
Fig. 2 is a vertical section taken along line 2-2 in Fig. 1;
Fig. 3 is a perspective view of the antenna illustrated in Figs. 1 and 2 with various reference lines superimposed therein;
Fig. 4 is an enlarged end view of one of the pads of absorber material used to form an absorber lining in the conical section of the antenna of Figs. 1-3;
Fig. 5 is a vertical section, similar to Fig. 2 of a modified horn-reflector antenna embodying the present invention; and
Fig. 6 is a section taken generally along line 6-6 in Fig. 5.
[0011] Turning now to the drawings and referring first to Figs. 1 and 2, there is illustrated a conical horn-reflector microwave antenna having a conical section 10 for guiding microwave signals to a parabolic reflector plate 11. From the reflector plate 11, the microwave signals are transmitted through an aperture 12 formed in the front of a cylindrical section 13 which is attached to both the conical section 10 and the reflector plate 11 to form a completely enclosed integral antenna structure.
[0012] The parabolic reflector plate 11 is a section of a paraboloid representing a surface of revolution formed by rotating a parabolic curve about an axis which extends through the vertex and the focus of the parabolic curve. As is well known, any microwaves originating at the focus of such a parabolic surface will be reflected by the plate 11 in planar wavefronts perpendicular to the axis 14. Thus, the conical section 10 of the illustrative antenna is arranged so that its apex coincides with the focus of the paraboloid, and so that the axis 15 of the conical section is perpendicular to the axis of the paraboloid. With this geometry, a diverging spherical wave emanating from the conical section 10 and striking the reflector plate 11 is reflected as a plane wave which passes through the aperture 12 and is perpendicular to the axis 14. The cylindrical section 13 serves as a shield which prevents the reflector plate 11 from producing interfering side and back signals and also helps to capture some spillover energy launched from the conical section feed. It will be appreciated that the conical section 10, the reflector plate 11, and the cylindrical shield 13 are usually formed of conductive metal (though it is only essential that the reflector plate 11 have a metallic surface).
[0013] To protect the interior of the antenna from both the weather and stray signals, the top of the reflector plate 11 is covered by a panel 20 attached to the cylindrical shield 13. A radome 21 also covers the aperture 12 at the front of the antenna to provide further protection from the weather. The inside surface of the cylindrical shield 12 is covered with an absorber material 22 to absorb stray signals so that they do not degrade the RPE. Such absorber materials are well known in the art, and typically comprise a conductive material such as metal or carbon dispersed throughout a dielectric material having a surface in the form of multiple pyramids or convoluted cones.
[0014] In keeping with the present invention, the lower end portion of the inside surface of the conical feed horn is formed by a smooth metal wall, and the balance of the inside surface of the horn is formed by a layer of absorber material, the surfaces of the metal wall and the absorber material defining a single continuous conical surface. Thus, in the illustrative embodiment of Figs. 1-3, the bottom section 10a of the conical feed horn 10 has a smooth inside metal surface. The balance of the inside surface of the conical horn 10 is formed by an absorber material 30, with the innermost surfaces of the metal section 10a and the absorber material 30 defining a single continuous conical surface. To support the absorber material 30 in the desired position and shape, the metal wall adjoining the lower horn section 10a forms an outwardly extending shoulder 10b at the top of the section 10a, and then extends upwardly along the outside surface of the absorber 30. This forms a continuous conical metal shell 10c along the entire length of the absorber material 30. At the top of the absorber material 30, the metal wall forms a second outwardly extending shoulder 10d to accommodate the greater thickness of the absorber material 22 which lines the shield portion of the antenna above the conical feed horn.
[0015] This recessed arrangement of the absorber material 30 permits further narrowing of the E-plane RPE and/or reductions in the gain drop of the antenna as compared with the structure shown in the aforementioned European Patent Publication No. 66455A. More specifically, for a given gain drop, the structure of the present invention permits the absober material to be extended farther down into the throat of the conical feed horn 10, thereby further narrowing the E-plane RPE. On the other hand, for a given RPE (in other words, if it is desired to minimise the gain drop of the antenna), the metal surface of the section 10 can be extended farther up from the bottom of the conical feed horn so that the narrowness of the E-plane RPE is essentially the same as that produced by the structure described in European Patent Publication No. 66455A, but at the same time reducing the gain drop relative to that of the structure described in said copending Application.
[0016] The lining 30 may be formed from conventional absorber materials, one example of which is AAP-ML-73 absorber made by Advanced Absorber Products Inc., 4 Poplar Street, Amesbury, Maine, U.S.A. This absorber material has a flat surface, as illustrated in Fig. 4 (in contrast to the pyramidal or conical surface of the absorber used in the shield), and is about 3/8 inches (9.5 mm) thick. The absorber material may be secured to the metal walls of the antenna by means of an adhesive. When the exemplary absorber material identified above is employed, it is preferably cut into a multiplicity of relatively small pads which can be butted against each other to form a continuous layer of absorber material over the curvilinear surface to which it is applied. This multiplicity of pads is illustrated by the grid patterns shown in Figs. 1-3.
[0017] In accordance with a further aspect of the present invention, the absorber material 30 is provided only on the two diametrically opposed regions of the interior walls of the conical horn 10 that affect the patterns of the antenna in the horizontal plane. In terrestrial communication systems, the only significant patterns of the antenna are those taken in the horizontal plane, which is the Y-Z plane in Fig. 3. That is, for a horizontally polarized signal, the Y-Z plane is the E-plane, and the X-Z plane is the H-plane; for a vertically polarized signal, the Y-Z plane is the H-plane, and the X-Z plane is the E-plane. The portions of the conical feed horn 10 that principally affect the E-plane RPE (of a horizontally polarized signal) are the left and right hand walls of the horn through which the X-Y plane extends. Thus, as illustrated in Fig. 5, the absorber material 30 can be limited to diametrically opposed regions 40 of the inside surface of the feed horn. Restricting the absorber material in this manner reduces the cost of the antenna by reducing both the amount of absorber material required and the labour required to instal the absorber lining within the conical horn.
[0018] When the absorber material 30 does not extend around the entire circumference of the horn 10, the absorber can be recessed (flush mounted) into the horn wall in the two regions 40 so as to maintain a single continuous conical surface on the inside of the horn 10. Alternatively, the metal wall can form the entire conical surface, as in the structure described in the aforementioned European Patent Publication No. 66455A, and the absorber material 30 applied only to the limited regions 40 on the inner surface thereof. These constructions will not offer the full advantages of the recessed absorber arrangement illustrated in Figs. 1-3, but they reduce the manufacturing cost of the antenna.
[0019] As described in the aforementioned European Patent Publication No. 66455A, the absorber material 30 within the conical section 10 causes the field distribution within the cone to taper off more sharply adjacent to the inside surface of the cone, due to the fact that the wall impedance of the absorber lining tends to force the perpendicular E field to zero. Furthermore, it does this while abstracting only a small fraction of the passing microwave energy propagating through the cone.
[0020] There is a substantial difference in the taper or drop-off of the field distributions in the E and H-planes in the absence of the absorber material 30. With the absorber material 30 in the horn, the E-plane field distribution tapers off much more sharply, approaching that of the H-plane field, while there is only a slight degradation in the H-plane taper which brings it even closer to the E-plane field. In the theoretically ideal situation, the profile of the E-plane field distribution would coincide with that of the H-plane. In actual practice, this theoretically ideal condition can only be approximated, though the approximation is closer with the present invention than with the structure described in the aforementioned European Patent Publication No. 66455A.
[0021] Mathematically, the operation of the feed horn can be characterised as follows. If we let E8 (r, 8, 0) and E0 (r, 6, 0) be the polar and azimuthal components of electric field (with the origin at the apex of the cone, and 8 and 0 the polar and azimuthal angles, respectively) then, it can be shown that they can be mathematically expressed as:
where
[0022] Eo―Arbitrary driving constant, k=2n/A, λ=free space operating wavelength and the functions f(w) and g(w) are given by:
with
[0023] One then notes that the fields are uniquely known for the range of 0≤θ≤αo and 0≤0̸≤360° if the parameters E (the Eigen value) and Rs (the spherical hybridicity factor) are known. These parameters are uniquely determined by the nature of the conical wall material.
No Absorber
[0024] For no absorber present one can show that E=1.84 and Rs=0 thus giving:
where amplitude distributions (in dB normalized to on axis, 0=0) are shown as the solid lines in Fig. 6 (Note: E-plane=-20 log10/f(w)/f(w)/w=0/H plane=-20 log10/g(w)/g(w)/w=0/).
Perfect Absorber
[0025] For the perfect absorber case (also a corrugated horn with quarter wave teeth) it can be shown that E=2.39, Rs=+1, thus giving
perfect absorber where the identity
has been used, with Jo(X)=Bessel function of order zero, argument X. One notes that the dB plot of (11) is virtually identical to that of (10), thus showing that the H-plane of the smooth wall and perfect absorber wall are virtually identical. Also, for this perfect absorber case, we then see that the E-plane is identical to the H-plane.
Actual Absorber
[0026] An actual absorber has E differing from the no absorber case of 1.84 and the perfect absorber case of 2.39, with a hybridicity factor, Rs, neither zero (no absorber) or unity (perfect absorber). In general both will be complex with finite loss in the absorber.
[0027] The RPE improvements described above can be achieved over a relatively wide frequency band. For example, the improvements described above for the antenna illustrated in Figs. 1-3 can be realised over the common carrier frequency bands commonly referred to as the 4 GHz, 6 GHz and 11 GHz bands.
[0028] Absorber materials are generally characterised by three parameters: thickness, dielectric constant, and loss tangent. The absorber used in the present invention must have a thickness and loss tangent sufficient to suppress undesirable surface (slow) waves. Such surface waves can be readily generated at the transition from the metallic portion of the inside surface of the cone wall to the absober-lined portion of the cone wall, but these waves are attenuated by the absorber so that they do not interfere with the desired field pattern of the energy striking the reflector plate 11. The end result is that all the improvements described above are attained without producing any undesirable distortion in the field patterns. The narrowing E-plane effect can, in fact, be achieved with zero loss tangent material, but with no loss the surfaces waves are not attenuated and the operating bandwidth is reduced. Consequently, it is preferred to use an absorber material with some loss.
[0029] Although the invention has been described with particular reference to a horn-reflector antenna, it will be appreciated that the invention can also be used to advantage in a primary feed horn for a dish-type antenna. Indeed, in the latter application the substantially equal main beam widths in the E and H planes provided by the absorber lined feed horn are particularly advantageous because they provide symmetrical illumination of the parabolic dish. The consequent approximately equal second patterns with their reduced sidelobes, over a wide bandwidth, and with negligible gain loss, are also important in this primary feed horn application.
1. A conical horn-reflector antenna comprising the combination of
a paraboloidal reflector (11) forming a paraboloidal reflecting surface for transmitting and receiving microwave energy, and
a conical feed horn (10) for guiding microwave energy from the focus of said paraboloidal reflecting surface to said reflector,
the small end of the horn (10) being located near and with its apex coincident with the focal point of the paraboloidal reflector,
the lower portion (10a) adjacent the small end of said horn (10) having a smooth metallic interior wall defining a conical surface which, when projected onto the reflector, is intercepted by the paraboloidal reflecting surface thereof, and the upper portion of said horn (10) being lined with absorber (30) on at least those portions of the interior wall which affect the E-plane field distribution so as to cause the radiation pattern envelope width in the E plane to approach that in the H plane, characterised by said absorber (30) having its thickness located outside of the projection of said conical surface defined by said lower portion (10a) of said horn (10) and having its exposed surface lying substantially on said conical surface projection.
2. A conical horn-reflector antenna as claimed in claim 1, characterised in that the
exposed surface of said absorber material (30) inside said conical feed horn (10)
is substantially smooth.
3. A conical horn-reflector antenna as claimed in either preceding claim, characterised
in that said layer of absorber material (30) is provided only on the opposed side
walls that affect the E-plane RPE of a horizontally polarized signal.
1. Konische Horn-Reflektor-Antenne mit einer Kombination aus einem Parabolreflektor
(11) mit einer parabolischen Reflektorfläche zum Senden und Empfangen von Mikrowellen
und mit einem konischen Speisehorn (10) zum Führen der Mikrowellen vom Fokus der parabolischen
Refletktorfläche zum Parabolreflektor, wobei das enge Ende des Speisehorns (10) in
der Nähe des Fokus des Parabolreflektors angeordnet ist und die Spitze des Konus mit
dem Fokus zusammenfällt und wobei der dem engen Ende des Speisehorns (10) benachbarte
untere Abschnitt (10a) des Speisehorns (10) eine glatte, metallische Innenwand aufweist,
die eine konische Fläche bildet, die bei Projektion auf den Reflektor von der parabolischen
Reflektorfläche geschnitten wird, und wobei der obere Abschnitt des Speisehorns (10)
wenigstens in den Bereichen der Innenwand mit einem Absorbermaterial (30) ausgekleidet
ist, die die Feldverteilung in der E-Ebene derart beeinflußt, daß die Keulenbreite
in der E-Ebene sich der Keulenbreite der H-Ebene nähert, dadurch gekennzeichnet, daß
die Dicke des Absorbermaterials (30) außerhalb der durch den unteren Abschnitt (10a)
des Speisehorns (10) begrentze konische Fläche liegt, und die freie Oberfläche des
Absorbermaterials im wesentlichen an der Projektion der konischen Oberfläche liegt.
2. Horn-Reflektor-Antenne nach Anspruch 1, dadurch gekennzeichnet, daß die freie Oberfläche
dieses Absorbermaterials (30) innerhalb des konischen Speisehorns (10) im wesentlichen
glatt ist.
3. Horn-Reflektor-Antenne nach einem der Ansprüche 1 oder 2, dadurch gekennzeichnet,
daß diese Schicht des Absorbermaterials (30) nur an den gegenüberliegenden Seitenwänden
vorgesehen ist, die die E-Ebene der Keule eines horizontal polarisierten Signals beeinflussen.
1. Antenne à réflecteur et cornet conique, comprenant la combinaison
caractérisée en ce que l'épaisseur de ladite matière absorbante (30) est située en dehors de la projection de ladite surface conique définie par ladite partie inférieure (10a) dudit cornet (10), et la surface apparente de ladite matière absorbante est située sensiblement sur ladite projection de surface conique.
d'un réflecteur paraboloïdal (11) formant une surface réfléchissante paraboloïdale pour émettre et recevoir une énergie de micro-ondes, et
d'un cornet d'alimentation conique (10) pour guider l'énergie de micro-ondes, à partir du foyer de ladite surface réfléchissante paraboloïdale, vers ledit réflecteur,
la petite extrémité du cornet (10) étant située près du point focal et de sorte que son sommet coïncide avec ce point focal du réflecteur paraboloïdal,
la partie inférieure (10a) dudit cornet (10), adjacente à la petite extrémité de celui-ci, ayant une paroi intérieure métallique lisse définissant une surface conique qui, lorsqu'elle est projetée sur le réflecteur, est interceptée par la surface réfléchissante paraboloïdale de ce dernier, et
la partie supérieure dudit cornet (10) étant revêtue d'une matière absorbante (30) sur au moins les parties de la paroi intérieure qui affectent la répartition du champ dans le plan E, de sorte que la largeur de l'enveloppe du diagramme de rayonnement dans le plan E se rapproche de la largeur dans le plan H,
caractérisée en ce que l'épaisseur de ladite matière absorbante (30) est située en dehors de la projection de ladite surface conique définie par ladite partie inférieure (10a) dudit cornet (10), et la surface apparente de ladite matière absorbante est située sensiblement sur ladite projection de surface conique.
2. Antenne à réflecteur et cornet conique, suivant la revendication 1, caractérisée
en ce que la surface apparente de ladite matière absorbante (30) du côté intérieur
dudit cornet d'alimentation conique (10) est sensiblement lisse.
3. Antenne à réflecteur et cornet conique suivant l'une ou l'autre des revendications
précédentes, caractérisée en ce que ladite couche de matière absorbante (30) est prévue
seulement sur les parois latérales opposées qui affectent l'enveloppe du diagramme
de rayonnement dans le plan E d'un signal horizontalement polarisé.