US 2976533 A
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Description (OCR text may contain errors)
March 21, 1961 w. w. SALISBURY 2,976,533
RADIO ASTRONOMY ANTENNA HAVING SFHERICAL REFLECTOR FORMED INTEGRAL WITH EARTH'S SURFACE 2 Sheets-Sheet 1 Filed Nov. 12, 1954 :I: I E 3- Z/ Winfield BY Jaw Anna/5V5 March 21, 1961 w w. SALISBURY 2 976,533
RADIO ASTRONOMY ANTENNA HAVING SPHERICAL REFLECTdR Filed Nov. 12, 1954 FORMED INTEGRAL WITH EARTH'S SURFACE 2 Sheets-Sheet 2 WinfieidhLSa/iabary I]: E INVENTOR RADIO ASTRONOMY ANTENNA HAVING SPHER- ICAL REFLECTOR FORMED INTEGRAL WITH EARTHS SURFACE Winfield W. Salisbury, Lafayette, Calif., assignor, by
mesne assignments, to Zenith Radio Corporation, a corporation of Delaware Filed Nov. 12, 1954, Ser. No. 468,237
4 Claims. (Cl. 343-755) This invention relates generally to a radio astronomy or radar antenna apparatus, and more particularly to antennas which may be used in radar and radio astronomy.
In radio astronomy, it is common practice to use a parabolic reflector with a dipole antenna at the focus to form a telescope. The accuracy with which the arrival of the electromagnetic waves from the source can be determined with such a telescope is limited by the ratio between the width of the reflector and the wave length of the waves received. This is usually referred to as resolving power of the telescope. The larger the aperture of the reflector, the larger the resolving power. In general, prior types of radio telescopes have a poor resolving power in comparison to that of optical telescopes. One of the largest of such radio telescopes has an aperture of 80 meters, and its importance in radio astronomy may be as great as the 200 inch telescope at Mount Palomar in visual astronomy.
In radar, it is important to have an antenna which has a large reflector to thereby intercept a large amount of the energy being reflected by the object being sighted. The larger the aperture of the antenna, the lessreflected power is necessary for operation of the radar system. This makes it possible to view smaller objects at a greater distance, or on the other hand, to transmit less energy and yet retain the sensitivity of the system.
In present parabolic antennas, it is necessary to mount the parabolic antenna on a structure which is movable to enable focusing on different objects. This limits the size of the reflector because weight and mobility become important factors.
In general, it is an object of the present invention to provide an antenna apparatus which has a large eflfective aperture.
It is a further object of this invention to provide an antenna apparatus having a spherical reflector and a means for correcting the aberrations introduced by the said reflector.
It is still a further object of the present invention to provide an antenna apparatus in which it is unnecessary to move the main reflector to sight different objects.
It is a further object of this invention to provide an antenna apparatus easily and inexpensively constructed.
Additional objects and features will appear from the following description in which the preferred embodiment has been set forth in detail in conjunction with. the. ac.- companying drawings.
In the drawings:
Figure 1 shows a schematic view of a spherical antenna together with a correcting lens of the reflecting type;
Figure 2 shows a schematic view of a spherical antenna together with correcting lens of the transmitting p Figure 3 shows an antenna having its reflector formed in the earths surface together with means for mounting the correcting lens and a section of reflector mounted on tracks for rotation about the periphery of the portion formed in the earths surface;
Figure 4 shows an enlarged view of the portion of the reflector of Figure 3 formed in the earths surface;
Figure 5 shows an antenna as in Figure 4 mounted on a stationary mount above the earths surface together with means for mounting the correcting lens and receiver; and
Figure 6 shows an antenna as in Figure 3 with means for mounting the correcting lens and receiver.
In general, apparatus constructed in conformity with the present invention comprises a stationary spherical reflector together with a correcting lens and receiver which are movable with respect to the reflector.
In Figure l, I have shown an antenna which comprises a spherical reflector 11, correcting lens 12, and receiver 13. The correcting lens 12 is placed near the principal focus of the spherical reflector. The lens 12 in this instance is of the reflecting type and corrects for the aberrations introduced by the spherical reflector 11. It brings the electromagnetic energy to a focus on receiver 13, which, for example, may be a dipole antenna coupled to conventional detecting means of the electronic type.
As is well known in optics, a spherical mirror deviates the outer rays to a shorter focus than those near the center of the spherical surface. Further, spherical aberrations are reduced by reducing the aperture. In the present invention I can employ a mirror having a large aperture, because the correcting lens 12 performs the important function of correcting for aberrations. The outer rays are deviated toward a shorter focal point than the inner rays, and therefore I form the lens 12 so as to deviate the outer rays in such a manner as to increase their focal length. The exact shape of the mirror may be arrived at by drawing a ray diagram and then forming the contour ofthe mirror 12 so as to properly correct the focal length of the rays. In general, the reflector 12 has a greater curvature at its outer periphery than at the center. As shown, the outer ray 16 strikes a surface which has a greater curvature than the ray 17 which strikes the surface that is relatively flat.
The lens 12 may be of other types well known in the art. For example, the lens 12a shown in Figure 2 may be of the metallic plate type or a metal lens of the delay type, both of which are well known in the art. Lenses of this type are described in Principles and Applications of Waveguide Transmission by Southworth, pages 459 474. The lens shown in Figure 2 increases the focal length of the ray 16 in comparison to that of 17 and brings all of the rays at a focus on the receiver 13.
The receiver 13 was previously described as being of the dipole type, but it may be any antenna which has the proper pattern to receive the energy reflected or trans mitted'by the lenses 12 and 12a.
As shown in Figures 1 and 2, the rays 16 and 17 are entering vertically into the reflector 11. Without the correcting lens 12, these rays Will focus at various points along a line which extends vertically through the center of thereflector 11. Rays near the center of the lens willfocus near the principal of focus of the spherical mirror. If rays enter from a position which lies toward the horizon such as is shown in Figure 3, then the rays will come to a focus along a line which extends perpendicular to the reflector 11 and parallel to the incoming rays. It is seen then that with correction, the various focii will delineate spherical surfaces having diameters which correspond to the various focii. As previously indicated, by means of correcting lenses, these various rays may be brought to a focus at a common focal point. Consequently, with the correcting lens, the rays coming from various directions will be brought to focus and will delineate a spherical surface having a diameter which is characteristic of the diameter of the lens 11.
By moving the combination of correcting lens 12 and receiver 13 in such a manner that they describe a spherical surface wherein the receiver 13 lies on the surface defined by the focus at all times, it is possible to sight the antenna formed by the combination of reflector 11, lens 12 and receiver 13 to any point lying along the horizon or vertically without any motion of the reflector 11.
As a result, the reflector 11 may be made large and mounted on a fixed structure or may be formed into the surface of the earth as shown in Figure 3. In this instance, the earth is removed to form a portion of a sphere 21. The exposed surface is covered with a relatively rigid hard coating 22, such as concrete. Probably the easiest way of applying the surface 22 is by means of Gunite equipment. This type of equipment is used to spray concrete onto a prepared surface. The exposed surface of the concrete 22 is then covered with a suitable reflecting material such as aluminum or copper foil to form the reflector 11 of the antenna system. The surface should be spherical within /a wave length of the energy being received. For radio astronomy, the accuracy varies with the wave length being detected. The ratio window in the atmosphere lies approximately between 1 centimeter and 50 meters, and, therefore, the tolerance varies between A; centimeter and approximately 6 meters depending upon the wave length of the energy being received. In Figure 4 I have shown an enlarged portion of the sphere of Figure 3 to more clearly illustrate the construction.
If only the section which is formed into the earths surface is used and an object which lies on or near the horizon is sighted, the aperture of the antenna is considerably reduced. To sight objects lying along the horizon and yet maintain a large aperture, it is desirable to provide a supplemental spherical surface 25 which forms a continuation of the reflector 11 and lies above the surface of the earth. The surface 25 can be carried by a suitable framework 26 provided with wheels 27 and adapted to ride on the trackway 28. This trackway extends about the periphery of the reflector 11 whereby the framework 26 can be shifted to any position desired through an angle of 360 with respect to the vertical axis of the stationary spherical surface. For convenience, the structure 26 can be driven by suitable motive means, such as an electric motor or internal combustion engine. It will be evident that with this arrangement the surface 25 can be positioned at any location about the periphery of the reflector 11 where it properly supplements the stationary reflecting surface to sight objects on or near the horizon. I
In Figure 3, the structure for mounting the supplemental surface 25 also serves to mount a boom 29. A track 31 is carried by boom 29, and extends laterally from the vertical axis of the spherical surface 11. Suitable means 32 forms a structural connection between the reflector 12 and the receiver 13. This assembly is carried by and engaged with the track 31. The track 31 has a circular contour such that the receiver at all times will lies in the sphere described by the focii. Therefore this arrangement permits focusing of the apparatus on ditferent objects by use of the supplemental surface 25, while at the same time maintaining the reflector 12 and receiver 13 in a proper position to correct for aberrations.
As previously indicated, the reflector 11 preferably re mains in a fixed position and therefore may be mounted on a fixed structure. In Figure 5 the mounting consists of the tubular columns 36 which support the spherical section 11. Supplemental structure 37 serves to mount the lens 12 and receiver 13.
In Figure 6, I have shown another means for mounting the lens 12 and receiver 13. In this instance a structure 38 extends from the earths surface through the center of the reflector 11 and provides a boom 39 for mounting a curved track 40 corresponding to the track 31 of Figure 3.
It is seen that the antenna of my invention may be quickly and inexpensively built. Further, the reflector 11 may have an aperture which is considerably greater than that of antennas presently in use. This is made possible because it is not necessary to build a movable structure to mount the reflector 11. In my invention, it is necessary to move only the relatively small lens 12 and the receiver 13 to focus on different objects. This advantage arises from the fact that I make use of a spherical reflector which is symmetrical in all directions rather than a parabolic reflector.
It is obvious that such a structure can be built on a hillside along a coastline, and with a suitable spherical section 21, could be made to form a radar antenna for receiving energy from aircraft flying off the coast, or from approaching marine craft.
1. In apparatus of the class described, a spherical reflector formed integral with the earths surface for receiving and reflecting electromagnetic energy, a spherical section forming a continuous surface with the said reflector and adapted to be moved about said reflector to thereby permit viewing objects on the horizon, an electromagnetic lens placed near the principal focii of said reflector, said lens intercepting the reflected energy and being aspherically formed to correct aberrations introduced by the reflector, and a receiver located at the focus of the said lens.
2. Apparatus as in claim 1 together with a means for moving the said lens and receiver to intercept energy from various objects.
3. In apparatus of the class described, a spherical reflector for receiving and reflecting electromagnetic energy, a spherical section forming a continuous surface with the said reflector and adapted to be moved about said reflector to thereby extend the viewing angle range, an electromagnetic lens placed near the principal focii of said reflector, said lens intercepting the reflected energy and being aspherically formed to correct aberrations introduced by the reflector, and a receiver located at the focus of the said lens.
4. Apparatus as in claim 3 together with means for moving the said lens and receiver to intercept energy from various objects.
References Cited in the file of this patent UNITED STATES PATENTS 2,419,556 'Feldman Apr. 29, 1947 2,454,144 Epstein Nov. 16, 1948 2,516,453 Dobell July 25, 1950 2,607,009 Aftel Aug. 12, 1952 2,671,853 Lindahl Mar. 9, 1954 2,677,056 Cochrane Apr. 27, 1954