US 3178713 A
Description (OCR text may contain errors)
R. F. H. YANG April 13, 1965 PARABOLIC ANTENNA FORMED OF CURVED SPACED RODS Filed March 8, 1961 3 Sheets-Sheet 1 WWW m 5 Z M 4 mm @W 2 a ,w\ Sm QM, z 5 WW N% N N \l. QM, NN NW RN aw NW Q NW IFHH A ril 13, 1965 R. F. H. YANG 3,178,713
PARABOLIC ANTENNA FORMED 0F CURVED SPACED RODS Filed March 8. 1961 3 Sheets-Sheet 2 FIEEI United States Patent Illinois Filed Mar. 8, 1961, Ser. No. 94,268 Claims. (Cl. 343-840) This invention relates to reflector-type antennas, particularly to parabolic antennas.
The invention is primarily concerned with the reduction of undesired sidelobes in the radiation patterns of parabolic antennas designed to have similar patterns when employed in vertically polarized and horizontally polarized transmission. (It will, of course, be understood that the principles of construction herein discussed are applicable to receiving antennas, so that terms such, as transmission are not limitative in this respect.) However, in addition to the features of construction which produce this result, the invention provides further features which may be more generally applied.
The type of parabolic antenna in most general use employs a reflector of round aperture, i.e., a parabolic surface subtending a circle in the direction of focus. From mere gross or elementary optical principles, the radiation pattern of such an antenna appears to be fully concentrated in the desired direction. In fact, however, as is well known, actual measured radiation patterns have numerous sidelobes at angles to the desired direction. These sidelobes have a number of explanations, both from the practical and theoretical standpoints. From the practical standpoint, deviation from the performance predicted from elementary optical theory is due to a number of factors which vary from the ideal conditions assumed by such elementary theory. In the first place, of course, the feed is not the theoretical point source which elementary theory assumes. Nor is it possible to achieve the ideal conditions of perfectly even distribution of illumination of the reflector. The pattern is further affected by such factors as the presence of the earth and a variety of similar phenomena. Additionally, the dimensions of practical reflectors are far from infinite as compared with the wavelength, so that considerations of phase of elementary components of the signal which contribute to its strength at any given point become of great importance.
A large amount of theoretical work has been done on the matter of the operation of parabolic and other types of reflector antennas, taking into account at least the major ones of these variables. In recent years, it has been demonstrated that substantial improvement can be made in sidelobe reduction by suitable shaping of the reflector of a parabolic antenna. For example, there appears in the literature the so-called cosine-squared reflector aperture, which is shown both theoretically and experimentally to give substantial improvement. There has also been described (Brueckmann, Proceedings of the institute of Radio Engineers, Volume 46, Page 1510) a form of rhombic aperture in which the lower corner is flattened to become a part of the ground plane.
' For a number of reasons, the use of specially shaped apertures for sidelobe reduction has heretofore not been considered practical in most commercial applications, particularly for VHF and UHF transmission. At these high frequencies, the elevation required for efficient operation imposes a requirement of low wind loading which is best met by employing a reflector of the type constructed with parallel rods. Such a reflector operates properly only where the rods are in the direction of polarization produced by the feed for the reflector. In order to 3,178,713 hatented Apr. 13, 1%65 ice minimize interference and cross-talk between communication systems in the same general vicinity, it is customary to use different directions of polarization in such systems, in addition to such precautions as may be taken to cut down sidelobes in the patterns of the antennas employed. It is thus desirable that an antenna for commercial use be capable of the desired characteristics of operation with either vertical or horizontal polarization. Thus, reflectors of circular aperture continue in common use for such purposes, since the rod-type reflectors heretofore proposed as improvements on the circular aperture are not satisfactory for operation in both types of polarization, the difference in aperture aspect upon the rotation of the entire antenna which is necessary for polarization change destroying the desired sidelobe pattern when the reflector is designed for either of the two possible polarizations.
The present invention provides a parabolic antenna which better meets the requirements for commercial use at VHF and UHF than those heretofore proposed or used. With the present construction, the sidelobes are greatly reduced as compared with a reflector antenna of circular aperture, without loss of substantial uniformity of the sidelobe pattern when the antenna is oriented for vertical or horizontal polarization. In addition, the efiiciency in confinement of radiation to the desired direction is aided by substantial reduction of spil -over (radiation from the feed which does not strike the reflector) and also by reduction of the back radiation (primarily leakage through the grid forming the surface of the reflector) with a minimum of addition to the wind loading of the reflector and to the cost of the antenna.
The features of construction by which these advantages are obtained are incorporated in the embodiment of the invention hereinafter to be described. It has been found through theory, and verified by experiment, that a square aperture, i.e., a paraboloidal surface subtending a square in the direction of focus with a feed polarized in the direction of a diagonal of the square, will produce a much lower first sidelobe amplitude in the horizontal plane than a circular aperture, irrespective of whether vertical or horizontal polarization is used, the difference in patterns obtained in the two polarizations being relatively minor as compared with the difference obtained with aperture shapes heretofore suggested for sidelobe reduction. This finding has been employed to produce a parabolic antenna which may be readily oriented for either polarization in the same manner as a circular aperture antenna in which the reflector surface is formed of parallel conductors.
In the embodiment to be described, a dipole type of feed is employed. In addition to the reflector element or grounded rod usually employed with such a feed, spaced forwardly from the dipole, there are provided two additional reflector rods at the sides of the dipole, confining the sideward radiation from the dipole. As will hereinafter be pointed out, this geometry of the reflectors appears on first glance to be in the nature of a skeleton corner reflector, but is found to produce greatly superior results to those obtained by employing a larger number of rods to fill in the gaps in the corner, while at the same time substantially reducing the cost and complexity as compared with a corner so constructed. An additional feature of construction of the embodiment to be described utilizes the recognition that the necessary spacing of the rods from which the reflector dish is constructed in order that the surface may act as a continuous conductor to any desired degree is not constant on all portions of the surface. The effective or apparent spacing at the edges of the dish is smaller than the actual physical spacing because of the difference in angle of incidence of the radiation from the feed, and this fact is utilized to reduce the weight, wind loading, and cost'of the antenna by in' creasing the spacing with distance from the center of the dish.
The embodiment of the invention to be described also includes further features of construction which are novel and contribute substantially to the provision of an overall construction of'high performance, convenience of transportation and'use, and relatively low cost. These addi tional features will best be understood from the description of a single embodiment, which has been selected for illustration in accordance with the patent laws, and is shown in the attached drawing, in which:
FIGURE 1 is a view in front elevation of a parabolic FIGURE 3 is a sectionalview taken along the offset line 33 of FIGURE 1;
FIGURE 4 is a view in orthogonal perspective of the feed of the antenna;
FIGURE 5 is an experimentally obtained radiation atisosceles right triangles (as viewed from the front) formed by pairs of perpendicular side frame members 18 and 20 and cross frame members 22 and 24 forming the V hypotenuses. These frame members are of suitable angle material curved in such a manner as to appear as right triangles in front aspect. The reflecting surfaces or bodies of the segments 14 and 16 are formed of spaced parallel rods 26 bent in planes which are parallel with each other and with the cross frame members 22 and 24, thebending of the triangular frames and the rods forming a'reflecting surface in the general form of a paraboloid of revolution cut away to form a square aperture, i.e., subtendinga square in the direction of focus. The spacing of the rods is relatively small as compared with the wavelength of the tuned frequency of the feed 10 atrall points. .Howeveiyin order to minimize both cost and wind resistance, the spacing is gradually increased with distance from the center-line of the reflector, i.e., from the joint between the cross frame members 22'and 24 of the triangular, sections'or segments; The inner or longer rods 26 are se-- support 62 onthe other side supports a reflector rod 64. It will, of course, be understood that all of the elements of, the feed visible in the drawing are conducting, all being grounded to the outer coaxial line conductor, except that the feed dipole element is brought out through the block 52 from its connection to the inner conductor by means of conventionalinsulators (not illustrated). Accordingly, all three of the rods' 56, 60 and 64 are grounded.
The three reflector rods are spaced from the radiating dipole 46 by a quarter wave length at the frequency of the dipole, andare somewhat longer than the half-wave dipole, as is conventional in any reflector rod element.
It will, of course, be understood that the illustrated antenna is designed to be employed with a suitable mount attached to the back for securing the antenna to a tower, building, or other'elevated structure.
a One commercial version of the antenna illustrated is designed fiOr operation in the 460 megacycle band. This embodiment employs a squareaperture of 12 /2 foot diagonalspan giving an aperture area substantially the same as a circular reflector of 10 foot diameter. The focal length employed is 44 inches. Three-quarter-inch aluminum tubes are employed, being welded to the angle framework, also of aluminum. .The spacing of the bars is tapered from 5 inches at the extreme H-piane tips of the triangular sections to 2 /2 inches at the center, corresponding respectively to 0.195 Wavelength and half of that value. It will, of course, be understood that the permissible degree of total spacing taper increases with aperture size and decreases with'fooal length for any given frequency. The 2 to 1 taper in spacing was experimentally found to give very satisfactorily low back radiation with the dimensions just' discussed. For best utilization of this construction, thetaper in spacing is preferably continuous, each successive rod outward from the center being increasingly spaced from its neighbor, thus obtaining the maximum effective simulation of a. solid conductor with a minimum of added weight, expense, and wind-loading. Placing of the rods in parallel planes greatly simplifies assembly asv compared with, for example, mounting the rods in planes converging on the feed, also offering the cured in relative position by pairs of tie rods 28 and 30 and a central strap 32 and 34. The spacing of these tying members is large, so that the reflecting surface consists substantially entirely of therods 26. V t, a The bases or crosslmembers 22 and 24 of the triangular sections are formed with rectangular notches or offsets 36 and 38 with suitable'fra ming members forming amount for a feed support plate' iti, the innermost rods 26 having; their central portions cut away for this purpose.
- The plate 40 supports a mounting tube, 42which in turn supports a rigid coaxial line'44' on the end of which the radiating portion of the feed is mounted. The-latter con-' sists of a dipole 46,- consisting of dipole elements 48 and 50, one being connected to the inner conductorof the coaxial line 44 and the other to the grounded outer conductor in conventional manner. A block 52- serving forthe mounting of the dipole 46 is traversed by anaperture from which extends. an extension54 of the outer conductor of the coaxial'line 44. A reflector rod 56 is mount-' ed on this extending end in conventional fashion. A com dipole was reached after substantial experimentation indicated the superiority of this arrangement over arrangements with further added refleotor'rods' For example, the filling in of the skeleton corner reflector with two or four rnore rods symmetrically placed was found to degrade, rather than aid, the performance of the antenna.
It is believed that the superiority'of the construction illus- Ctrated as regards performance is due to phase interferences-of the'additional reflectors with the phase phenomena introduced in the parabolic focusing action.
FIGURE 5 of the'drawing shows the H-plane' radiation pattern of the antenna described above, i.e., the pattern obtained at the median frequency with the antenna oriented for vertical polarization. It will be observed that the maximurnlfirst), sidelobes are down almost 25 The sidelobe amplitude as a function offrequency, over the band from 450 to 470 megacycles is shown in FIGURE 6' for both polarizations;
It will be observed that the E-plane sidelobe attenuation is not as good as that [obtained in the H-plane. Somewhat similar effects,
differences in sidelobe' amplitude in the two orienta Hons, are obtained in any reflector antenna in which the directivity is not identical in'both planes because of the presence of one or more reflectors on the feed which do larly true where the feed is not balanced. 'In the present instance, the H-plane superiority is somewhat increased by the fact that the reflectors employed on the feed substantial-ly reduce spill-over only in the H-plane. Either of the characteristics shown in FIGURE 6 is, however, substantially better than the corresponding characteristics obtained with a standard antenna as described.
FIGURE 7 shows the half-power beamwid-th of the antenna described above as a function of frequency. Here again, due to the difference in H-plane and E-plane directivity of the feed, the E-plane beamwidth is substantially constant, and the H-plane beam-width, although considerably narrower, varies very slightly with frequency. Here again, the characteristics in both instances compare very favorably with those of a circular parabolic aperture with a single-reflector feed. 1
Measurements of the gain of the antenna demonstrate that despite the saving of weight and wind-loading and the improvement of sidelobe characteristics, as compared with a solid circular standard antenna of the same reflector area, there is no sacrifice of gain. Thus it will be seen that the slight back radiation observable at the extremes in FIGURE 5 represents energy which is extracted from sidelobes, rather than from the main beam, in comparing the performance of the illustrated antenna with that of a conventional solid dish. Obviously, the employment of the square apenture with reflector dish surfaces more closely approaching a solid surface, at some sacrifice of increase of wind resistance, etc., enables the present invention to be employed in structures which pnoduce even higher gain.
Persons skilled in the art will readily recognize that although the novel combination of features described above as being incorporated in the illustrated embodiment is highly advantageous for the particular purpose for which the present device was intended, many of the features may be used individually or combined in structures far different in general appearance and detail than that herein illustrated. Further, many modifications may be made, some obvious and others apparent after study, which nevertheless embody the teachings of the invention; as one example, the square aperture may be replaced, for certain purposes, with an aperture of some other non-circular shape which has equal diagonals respectively parallel and perpendicular to the direction of polarization and possesses mirror symmetry with respect to both diagonals. Accordingly, the protection to be afforded the invention should not be limited by the particular embodiment herein described, but should extend to all structures described in any one or more of the annexed claims, and to equivalents thereof.
What is claimed is:
1. In a parabolic antenna, a reflector dish comprising a substantially square frame and spaced parallel rods extending in the diagonal direction between pairs of adjacent sides of the frame, the spacing between adjacent rods increasing with distance from the center, the rods and the frame members being bent to form a paraboloidal segment subtending a square in the direction of focus, and a feed at the focus adapted to produce transmissions linearly polarized in the direction parallel with the rods.
2. In a parabolic antenna, a reflector dish comprising a pair of similar halves each having a frame in the general form of an isosceles triangle, and spaced parallel rods extending between the two equal sides of the frame parallel with the third side, the two halves being joined with said third sides in abutment, the rods and the frame members being bent to form a paraboloidal segment subtending a square in the direction of focus, and a feed at the focus adapted to produce transmissions linearly polarized in the direction parallel with the abutting third sides and with the rods.
3. In a parabolic antenna, a reflector dish comprising a pair of similar halves each having .a frame in the general form of an isosceles triangle, and spaced parallel rods extending between the two equal sides of the frame parallel with the third side, the spacing between adjacent rods increasing with distance from said third side, the two halves being joined with said third sides in abutment, the rods and the frame members being bent to form a paraboloidal segment subtending a square in the direction of focus, and a feed at the focus adapted to produce transmissions linearly polarized in the direction parallel with the abutting third sides and with the rods.
4. A reflector dish for parabolic antennas comprising a substantially square frame, and spaced parallel rods extending in the diagonal direction between pairs of adjacent sides of the frame, the spacing between adjacent rods increasing with distance from the center, the rods and the frame members being bent to form a paraboloidal segment subtending a square in the direction of focus.
5. In an antenna comprising a generally paraboloidally curved reflector dish and a feed at the focus of the reflector dish adapted to produce linearly polarized transmissions, the improved construction wherein the reflector dish comprises a pair of similar halves each comprising a frame in the general form of an isosceles triangle, and spaced parallel rods etxending between the two equal sides of the frame parallel with the third side, the spacing between adjacent rods increasing with distance from said third side, the two halves being joined with said third sides in abutment, the rods and the frame member being curved to form a paraboloid segment subtending a square in the direction of focus, the feed comprising a dipole in the plane of the abutment of said third sides and three reflector rod elements parallel with the dipole and equally spaced therefrom in the direction away from the dish and in the respective opposite sidewise directions perpendicular to the dipole.
References Cited by the Examiner UNITED STATES PATENTS 1,928,645 10/33 Dow 343818 X 2,061,508 11/ 36 Dallenbach 343837 X 2,270,314 l/42 Kraus 343834 2,530,098 11/50 Van Atta 343766 2,850,735 9/58 Harris 343916 X 2,928,087 3/60 Parker 343818 X 2,992,429 7/61 Hagaman 343786 X OTHER REFERENCES IRE Transactions on Antennas and Propagation, vol. AP-7, July 1959, No. 3, TK 7800, 12, pages 223, 224, 225, 226.
HERMAN KARL SAALBAC'H, Primary Examiner.
GEORGE N. WESTBY, ELI LIEBERMAN, Examiners.