US2922160A

US2922160A - Split paraboloidal reflector

Info

Publication number: US2922160A
Application number: US158582A
Authority: US
Inventors: Lester C Van Atta; Robert J Adams; Kenneth S Kelleher
Original assignee: Individual
Current assignee: Individual
Priority date: 1950-04-27
Filing date: 1950-04-27
Publication date: 1960-01-19
Anticipated expiration: 1977-01-19

Description

Jan. 19, 1960 L. c. VAN ATTA IETAL 2,922,160

SPLIT PARABOLOIDAL REFLECTOR 2 Sheets-Sheet 1 Filed April 27, 1950 INVENTORS LESTER C. VAN ATTA KENNETH S. KELLEHER ROBERT J. ADAMS 26 WAVE FRONT T I. will? Mi, ATTORNEYS Jan. 19, 1960 c, VAN ATTA ETAL 2,922,160

SPLIT PARABOLOIDAL REFLECTOR Filed April 27, 1950 2 Sheets-Sheet 2 [[III II -lO-8'6-4-2 O 2 4 6 8 l0 l2 I416 ELEVATION ANGLE DEG. ELEVATION ANGLE DEG.

INVENTORS LESTER C. VAN ATTA KENNETH S. KELLEHER ROBERT J. ADAMS MM], 41%? 1 ATTORNEYS United States Patent 2,922,160 Patented Jan. 19, 1960 line SPLIT PARABOLOIDAL REFLECTOR Lester C. Van Atta, Chevy Chase, Md., Robert J. Adams, vashington, D.C., and Kenneth S. Kelleher, Alexandria,

Application April 27, 1950, Serial No. 158,582

7 Claims. (Cl. 3'43-8-35) (Granted under Title 35, US. Code (1952}, sec. 266) This invention relates in general to antenna reflectors and more particularly to improvements in paraboloidal reflectors.

Paraboloidal reflectors commonly known in the antenna art are employed to concentrate the radiation from an antenna, located at the focus, to form a beam. The parabola functions to convert a spherical wave originating at the focus into a plane wave, of constant phase, at the mouth of the parabola. The radiation pattern reflected by the parabola is generally characterized by a sharply defined main lobe along its axis of directivity and surrounded by a succession of minor side lobes in which the first and possibly the second side lobes may be relatively large.

In certain antenna applications the side lobes may not be objectionable, so long as directivity is maintained. However, for other antenna applications, such as in an, object locating system, Where a group of radiators are employed to produce overlapping beampatterns, these side lobes present a serious problem especially if the energy level thereof is less than 15 decibels below the main lobe. I

The present invention teaches a modified parabolic reflector intended to give a main beam pattern wherein the first side lobes, normally occurring with the conventional reflectors, are made to appear as shoulders. on the main lobe and the remaining side lobes are reduced to a negligible level.

It is accordingly an object of the present invention to provide a new and improved parabolic reflector.

It is another object of the present invention to provide a new and improved parabolic reflector wherein the Wave fronts reflected thereby have a linear phase deviation from the center line of the reflector.

Still another object of the present. invention is to provide:a new and improved directional antenna.

Further objects and attainments of the present invention will readily become apparent from the following detailed description when taken in conjunction with the drawings in which:

Fig. 1 is one typical embodiment ofJthe present invention illustrating a front elevational view of the modified paraboloidal reflector.

Fig. 2 is a cross-sectional view of a conventional paraboloidal reflector and the wave front generated thereby.

Fig. 3 is a cross-sectional view of a paraboloidal reflector built in accordance with Fig. 1 and: contains an tainable with the conventional paraboloidal reflectorv when fed by a plurality of radiators,

Fig. 7 isa rectangular plot of the antenna patterns obtainable with a paraboloidal reflector shown in Fig. 4 when fed by a plurality of radiators.

In various angle resolving radio equipments, such a radar, the bearing or height of an object is determined by comparing the received signal strength picked up by a pair or more of overlapping beam patterns. The present invention is described in conjunction with and readily adapts itself to a height finding radar but it is to be understood that it is not to be limited thereto. The overlapping beams are produced by a single antenna reflector and the number of beams is in accordance with the number of feed elements radiating into the refiecton. The position of each beam is, of course, dependent on the position of the feed element generating the beam with. respect to the focal point of the reflector.

To render it readily practical to determine the height: of a detected object in a height finding radar system, pred-- icated on the ratio of signals received on adjacent beams,v the signal strength from the detected object, in any one particular beam, must not be below a reference level in: the order of 12 to 15 db when the detected object is in the center of the adjacent beam. With the commonly known parabolic reflectors producting highly directional; beam pattetns, this requirement necessitates extremely close spacing of the beam patterns, so that the space:

within the reflector available for the feed elements is:,

severely limited. Close spacing of the feed elements con-- sequently meansthat the ?taper of illumination, from. the'c enter to the top and bottom of the reflector, is: no more than 2 or 3 db. A taper of illumination of nomore than 2 or 3 db approaches a uniform illumination; "and results in the'gen'eration of a number of very high intensity side lobes that may render the comparison of the signal strength of detected objects impossible. An=- other disadvantage in close spacing of the feed elements,:. is that a considerable loss'of gain is had, due to what is'; commonly known asspillover.. In accordance with 'the spirit andscope of the present. invention a parabolic reflector is employed, wherein: modifications are incorporated in the reflector which over come the heretofore mentioned diificulties. In one em bodiment a paraboloidal reflector is cut or split along a central plane and the halves are separated preferably by a conductive strip, in the order of one wavelength, bent to conform to the curvature of the paraboloidal reflector along the central plane.

When a single or a plurality of" feed elements are placed in the focal plane of such' a reflector, each of the wave fronts generated thereby is gabled, rather than plane; That is, the phase of the wave front varies linearly from-the axis of the parabola; The result obtained isan antenna pattern having a main lobe" with the of the normally occurring side lobes appearing as shoulders thereon, and the remaining side l'obes are reduced to a negligible level.

It may be noted here, that the specially shaped patterns, as more specifically described hereinafter, are obtained only in the plane perpendicular to the plane in which the reflector is cut; The pattern in the plane of the cut is substantially unaffected. In a height finding radar, for which purpose the present inventionwas designed, the elevation patterns are the patterns affected. The azimuth patterns of all beams are the same as they would be from a conventionalparaboloidal reflector having the: same focal length, width, and height.

Referring now in particular to Fig. 1 there is shown a; paraboloidal reflector constructed in accordancewith the teachings of the present invention. In the case where the adjacent beam patterns are stacked vertically, parab= oloidal reflector 1 is centrally divided ina horizontal.

' plane to produce spacially disposed complementary halves I 2 and 3. Halves 2 and 3 are joined by separator 4, conforming to the inside horizontal contour of halves 2 and 3. The separator 4, as illustrated in the preferred embodiment, has a height that approximates one wavelength of the operating frequency. The one wavelength height of the separator 4 is, of course, for the one particular purpose, a height finding radar, however, the height of the separator may be altered, as hereinafter shown, for other antenna applications. Separator 4 is shown here merely as illustrative of the spacing to be maintained between the halves of the reflector, since the conductivity of separator 4 is not of importance, moreover, a separator need not be employed at all and free air space may be maintained between the halves, or a nonconductive material may be inserted therebetween.

With reference to Fig. 2 there is shown the wave front, generated by the conventional paraboloidal reflector. Spherical waves originating at the focus 5 are converted by the reflector 6 to plane waves 8, the fields established at the mouth of the reflector being of the same phase. It is also known in the art, that placing of the efiective center of the radiation of the feed slightly above the focus or slightly below the focus, does not greatly affect the reflector performance but does materially affect the directivity of the pattern. As per example, were the center of radiation displaced from the axis to point 7 of Fig. 2 the phase at 12 would lag the phase at 11, to shift the pattern 13 in a downward direction from the normal axis of directivity to that shown by the dotted line pattern 14.

The antenna pattern as generated by the conventional system, irrespective of its axis of directivity, is characterized by a single main lobe 13 and a series of minor lobes, that in certain instances, the first of the side lobes, as shown at 15 and perhaps the second of such as shown at 16, may be very large in proportion to the main lobe.

In operation of the split parabolic reflector the principle of displacing the center of radiation from the axis of the reflector is employed in a unique manner. Effectively the wave front as produced by the split para boloidal reflector is a combination of two lower halves of the wave front as illustrated in the lower half of Fig. 2. That is, effectively we are oppositely displacing one half of the reflector above the center of radiation-and the other half belowthe center of radiation, to give a gabled wave front.

With the parabolic reflector modified in accordance with the present invention, illustrated and referred to now in Fig. 3 and bearing the same numerals as in Fig. 1, the wave front 26 generated thereby is not plane at the mouth but is gabled, having a linear phase shift from the center line of the wave front. In this particular instance of an exemplary embodiment, the maximum phase shift is adjusted to approximately 90 to absorb the usual first side lobes, in the vertical pattern, into the sides of the main beam as shoulders.

The phase deviation (represented as (p) from the center line of the wave front extending laterally may be adjusted as represented by the equation:

where h is the height of the added separator 4; the beam factor is the ratio of the angle between the reflected rays (X of Fig. 3a) and the angle olf the axis of the feed displacement (Y of Fig. 3a) in an unmodified reflector; f is the focal length; and H the height of the reflector. It is seen, therefore, by increasing the height of separator 4 of Fig. 3 the phase deviation is adjusted which would have the resultof raising the shoulders on the beam pattern generated by the reflector.

There is shown in Fig. 4 a paraboloidal reflector, as taught by the present invention, wherein there is employed three

conventional feed elements

22, 23 and 24 to feed the reflector, the type of radiator being, of course, merely a matter of choice and may be replaced by any other known radiator. The antenna pattern in a plane perpendicular to the plane in which the reflector is cut, obtainable from the arrangement shown in Fig. 4, is substantially as shown in Fig. 7. The pattern shown in Fig. 6 is that obtainable from a conventional reflector employing the same three feeds to offer a comparison with the pattern of Fig. 7. The patterns a, b and c of Fig. 7 and a, b and c of Fig. 6 are generated by horn feeds 22, 23 and 24 respectively, with the spacing between the individual patterns being dependent on the spacing of the horn feeds 22, 23 and 24. When the maximum phase shift of the wave front (as shown in Fig. 3) is adjusted to approximately 90 the first side lobes in the vertical pattern, those attaining the greatest proportion, are absorbed into the sides of the main beams a, b and c as shoulders 17. The side lobes 31 and 32 are absorbed into main beam a,

side lobes

33 and 34 are absorbed into main beam b and

side lobes

35 and 36 are absorbed into main beam c. The uppermost section of the shoulders 17 are at about the 12 decibel level. The other side lobes, typified by 16 and 19 of Fig. 6 are not absorbed into the main beams but are further decreased in proportion to the order of 30 decibels as shown at 18 in Fig. 7, therefore, becoming insignificant for most practical purposes.

With the pattern generated by the conventional reflector as shown in Fig. 6, it is seen that if a signal were to appear at the center of beam a, as per example in the proximity of point 20, signal comparison, dependent on the ratio of signals in adjacent beams as employed in a height finding radar, would be extremely erratic. The signal strength at this point of beam b is considerably less than 15 db below the main beam level which is normally required for average accuracy of signal comparison. It can further be seen, that if a signal were detested at this level of elevation it may well be appearing in one of the relatively large side lobes attaining this gain level. Of course, if the signal were appearing on one of the side lobes signal comparison for any purpose is impossible.

On the other hand if a signal were to appear at the center of beam a, of Fig. 7 generated by the split reflector as taught above, as per example in the proximity of 21, the gain level of beam b at this point still exceeds the signal level in the order of 15 db below the main beam level to permit an accurate comparison of signals in adjacent beams. It is also readily apparent from Fig. 7 that the danger of a signal appearing in a side lobe is completely eliminated.

With the form of beam pattern shown in Fig. 7, the angular spacing between beams may be increased and still meet the requirement for a height finding radar as previously mentioned. An increase in the spacing between the beams accordingly permits greaterspacing of the feed elements to provide a more nearly optimum illumination taper across the reflector, which further provides for the reduction of the unabsorbed side lobes to the above mentioned order of 30 db below the main beam level.

In Fig. 5 another paraboloidal reflector is divided in a horizontal plane into spatially disposed complementary halves as described above and embodying further modifications as shown. The outside contours of the parabolic reflector are trimmed along

lines

28, 29, 30 and 37 to form a substantially diamond shaped reflector. The antenna pattern obtained from the diamond shaped reflector uniform blending of the first side lobes into the main beams 11, b and c and the remaining side lobes are reduced to a lower level. A more uniform antenna pattern is derived from this reflector through a greater effective taper of illumination by the elimination of the illumination of the outer edges of the reflector that normally causes the generation of the side lobes.

Although we have shown only certain and specific embodiments of the present invention, it is to be expressly understood that many modifications are possible thereof Without departing from the true spirit of the invention.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

What is claimed is:

l. A parabolic reflector for reflecting radiant energy waves comprising: a paraboloidal reflector divided into spatially disposed complementary half sections, and means for maintaining a spacing between said half sections.

2. A parabolic reflector for reflecting radiant energy waves comprising: a paraboloidal reflector divided along a central plane into spatially disposed complementary half sections and a separator connecting said half sections.

3. A parabolic reflector for reflecting radiant energy waves comprising: a paraboloidal reflector centrally divided along an axial plane into spatially disposed complementary half sections and a separator having a height substantially one wavelength at a given frequency connecting said half sections.

4. In combination; a paraboloidal radio antenna reflector comprising, first and second paraboloidal half sections disposed in spaced relation to one another to form a paraboloidal reflecting surface interrupted in an axial plane by the spacing between the half sections; and a feed system for said reflector, comprising a plural ity of radiators superimposed in a single plane normal to the axis of one of the paraboloidal half sections and intersecting the focal point thereof.

5. In combination, a paraboloidal reflector, said paraboloidal reflector, said paraboloidal reflector divided into spatially disposed complementary half sections, a separator connecting said half'sections, and a radian energy radiator located in a plane passing through the focal point of one of the half sections of the paraboloidal reflector normal to the axis of that half section.

6. In combination, a paraboloidal reflector, said paraboloidal reflector being divided into spatially disposed complementary half sections, a separator connecting said half sections, and a plurality of radiant energy radiators positioned in a plane normal to the axis of one half section of the paraboloid and intersecting the focal point thereof.

7. In a signal comparison object locating system, an antenna comprising, a paraboloidal reflector, said reflector being divided into spatially disposed complementary half sections, a separator connecting said halves whereby the first of the normally generated side lobes are absorbed as shoulders onto said main beams and the remaining normal- 1y generated side lobes are reduced to a minute level, and

a plurality of radiant energy radiators positioned in a. plane normal to the axis of one of the half sections of the paraboloidal reflector and intersecting the focal point thereof.

References Cited in the file of this patent UNITED STATES PATENTS 1,081,215 Coulson Dec. 9, 1913 1,796,694 Silva Mar. 17, 1931 2,160,853 Gerhard et a1 June 6, 1939 2,430,568 Hershberger Nov. 11, 1947 2,489,865 Cutler Nov. 29, 1949 2,516,376 Field et a1 July 25, 1950 FOREIGN PATENTS 410,336 Great Britain May 17, 1934 436,355 Great Britain Oct. 9, 1935 928,399 France Nov. 26, 1947