US2703842A

US2703842A - Radar reflector

Info

Publication number: US2703842A
Application number: US148454A
Authority: US
Inventors: Willard D Lewis
Original assignee: Individual
Current assignee: Individual
Priority date: 1950-03-08
Filing date: 1950-03-08
Publication date: 1955-03-08
Anticipated expiration: 1972-03-08

Description

March 8, 1955 w. D. LEWIS 2,703,842

RADAR REFLECTQB Filed March 8. 1950 2 Sheets-Sheet 1 INVENTOR. W. D. Lewis 'ATTO'RNEY March 8, 1955 w. D. LEWIS RADAR REFLECTOR 2 Sheets-Sheet 2 Filed March 8, 1950 INVENTOR. W. D. Lewis ATTORNEY llnited States Patent RADAR REFLECTOR Willard D. Lewis, Little Silver, N. J., assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Application March 8, 1950, Serial No. 148,454

4 Claims. (Cl. 25033.65)

ticular antenna reflector depends upon the method of supplying power to the antenna and upon the physical characteristics of the reflector itself. Since both the antenna feed arrangement and the reflector itself are fixed at the time of manufacture, it is diflicult to change the construction of the installation, and changes made under field conditions are usually unreliable.

For many purposes, the radiation pattern of an antenna should be specially shaped to cover a desired area and the ordinary sharp lobe pattern of a conventional antenna system is not suitable. broad pattern in a vertical plane while retaining a sharp pattern in a horizontal plane as is required in an antiaircraft search installation, it has been possible to use a plurality of feed points, all or some of which are displaced from the focal point of the reflector in the plane which is to be broadened from the normal pattern. Somewhat similar results can be obtained by employing a specially shaped antenna reflector. The desired beam pattern may be symmetrical about the axis of the reflector as is the case with a height-finding radar installation, or it may be unsymmetrical; i. e., it may radiate more power above the axis than below it in the case of an anti-aircraft search radar. However, such installations are also fixed in the manufacture, and the equipment is not flexible in application.

The present invention overcomes these objections in presently available apparatus by providing a simple inexpensive means for changing the effective radius of predetermined portions of a parabolic reflector by attaching a phase advancing means over the reflector.

The phase advancing means consists of a series of parallel conducting plates spaced from each other by a distance between a half and a full wavelength at the operating frequency and parallel to the polarization axis of the electromagnetic wave to be reflected. The portion of the reflector not covered by the phase advancing means remains unaffected while the efiective radius of the covered portion is reduced, so that the reflector now behaves as though it were fed from a plurality of points or as though its physical shape had been changed. Since the phase advancing means is easily removable, the installation may be changed by only changing or removing the phase advancing means.

An object of the present invention is to provide a reliable method of modifying the radiation pattern from an existing reflector system.

Another object of the present invention is to provide a method of modifying the radiation pattern from an existing reflector which does not otherwise affect the operation of radar equipment.

A further object of the present invention is to provide an inexpensive and rugged method of modifying the radiation pattern of an existing antenna reflector system.

A still further object is to provide a means for shaping the radiation pattern of a conventional radar antenna reflector to provide a broad beam in a desired plane.

An additional object of the present invention is to provide an easily changed means for producing a preselected antenna radiation pattern from a parbolic reflector.

In order to provide a ice Other objects and advantages of the present invention will be made more apparent by reference to the following specification and by reference to the appended drawings in which:

Fig. l is a perspective view of the phase advancing unit of the present invention applied to the reflector of a search radar installation;

Fig. 2 is a view showing the construction of a section of the phase advancing unit shown in Fig. 1;

Fig. 3 is a diagram illustrating the effect of conducting plates on an electromagnetic wave;

Fig. 4 is a diagram showing the vertical pattern of the reflector shown in Fig. 1 before and after modification; and

Figs. 5 and 5A are a diagrammatical showing of the operation of the phase advancing means.

Referring now to Fig. 1, the symmetrically cut paraboloidal reflector is composed of a frame 11 and a reflecting covering 12 which forms a section of a paraboloid. The frame 11 may be composed of any material which is strong and light, such as steel or aluminum tubing, and the reflecting covering 12 may be of conducting material such as metallic mesh or sheet metal, as is well known. It will be appreciated that the type of reflector may vary widely, and that the type shown is only for purposes of illustration. The reflector is mounted on a pintle 13 and is revolved at a suitable rate of speed by any convenient source of power, the exact arrangement used being a matter of design, and since it forms no part of the present invention it will not be further explained.

The reflector 10 is fed by a horizontally polarized electromagnetic wave through a wave guide 15 from the transmitter, the wave guide being equipped with directing means 14 to distribute the radiated energy over the reflector. The energy is reflected in a predetermined pattern which is controlled by the shape and size of the reflector, which are fixed characteristics of the particular reflector. The pattern is therefore fixed for a particular installation, and a typical example is shown by the wave 40 in Fig. 4.

The phase advancing unit is added to the bottom portion of the reflector 10 to redirect the energy striking the bottom portion of the reflector by progressively altering the effective radius of the bottom portion of the reflector 10. The phase advancing unit 20 consists of a plurality of

parallel plates

22, 23, 24 and 25 spaced from each other by a distance of slightly more than a half-wavelength, the width of the plates increasing progressively as the distance of the plate from the center of the reflector increases. The plates are assembled on a plurality of vertical spreaders 27 which support the plates at the desired intervals and also aid in holding the phase advancing means 20 to the reflector 10. Any type of fastening means may be employed to attach the plates to the reflector covering 12 or the reflector frame 11 as desired, such as a plurality of U-bolts equipped with plates and wing nuts. Electrical connection with the reflecting surface is unimportant, but the attachment should be secure so as to prevent shifting of the plates relative to the reflector.

For convenience in mounting and storing the phase advancing unit 20 when not in use, the unit is composed of a plurality of sections 21, which sections may correspond to the sections of the reflector 10. The phase advancing sections 21 need not be removed from the reflector 10 when it is disassembled for movement, if the unit is properly sectionalized. A phase advancing section 21 is shown in Fig. 2.

It is well known that the phase velocity of a wave in a 'wave guide is higher than the speed of the wave in air.

Referring now to Fig. 3, the rectangular tube 30 represents a wave guide for a horizontally polarized radio frequency system, the electrical vector being represented by E. The separation A of the

horizontal surfaces

31 and 32 determines the mode of operation of the Wave guide at a particular frequency and also establishes the minimum frequency at which it may be operated. The

vertical surfaces

34 and 35 serve only to bound the region where power is transmitted and do not otherwise affect the trans mission. In the present case, the width of the

horizontal sides

31 and 32 in the direction of the vector E is increased to the length of the reflector. In Fig. l the only elements corresponding to the

vertical sides

34 and 35 are the spreaders 27. The phase velocity is determined by the separation of the plates and the phase advance produced by the plates is proportional to the width of the parallel plates.

The ratio N of the velocity of a wave in air to the phase velocity in the wave guide is given by the equation:

A=the spacing between the plate, and tt=the operating wavelength.

An inspection of the above equation will show that N will be finite for values of A larger than M2, and will be less than unity. Thus the velocity of a wave passing between the plates will be higher than in open air, so that the insertion of a series of parallel plates between a power source and a reflector has the effect of moving the reflector toward the power source by an amount proportional to the width of the plates, thus changing the effective radius of the reflector.

The addition of the phase advancing unit 20 to the reflector 10 produces the effect of two reflectors radiating energy from a single source, since the portion of the reflector not covered by the unit 10 is not changed in any way and produces the same wave front as before the unit was added. However, the portion of the reflector 10 covered by the phase advancing unit 20 now has a different radius and focal point so thatit reflects power in a lobe which is angularly displaced from the original pattern, so that the combined radiation pattern is considerably widened although its length is somewhat shortened.

The width of the

plates

22, 23, 24 and 25 in the phase advancing unit 20 are proportioned so as to produce a smooth wave front from the portion of the reflector covered thereby. In the present example, the plates increase progressively in width from the plate 25 nearest to the center of the reflector 10 to the plate 22 farthest therefrom. As a result, the wave front is bent so that the lower portion leads the remainder, as shown in Fig. A. However, it will be clear to those skilled in the art that other shapes of wave front can also be readily produced by properly selecting the widths of the plates.

The relative power in the lobes from the covered and the uncovered portions of the reflector may be proportioned by changing the relative areas thereof, since the radiant energy to be reflected is distributed over the area of the reflector in a substantially uniform manner. Since the angular displacement of the lobes from the covered and uncovered portions of the reflector is determined by the width of the several plates, and the power of the respective lobes may also be controlled, it is apparent that a wide variety of radiation patterns may be produced by the proper design of the phase advancing unit.

Figs. 5 and 5A show the elfect of the phase advancing unit applied to a reflector. In Fig. 5, the electromagnetic wave energy from the feed reflector 14 is reflected by the reflector to form substantially straight wave fronts as indicated by 51, and since the direction of radiation is perpendicular to the wave front, the pattern produced will be a sharp beam. In Fig. 5A, a phase advancing unit 20 is added to the lower half of the reflector 10, and the wave fronts 52 which are now radiated are deviated with the lower edge leading the remainder of the wave front. It will be observed that a portion of the radiated energy has been deflected upwardly to produce an unsymmetrical radiation pattern with respect to the axis of the reflector in which the upper portion of the lobe produced contains considerably more power than the lower portion, and that the radiation pattern has been fanned out in a vertical direction.

In the present illustration, a large portion of the power in the lower half of the normal radiation pattern is shifted to the upper half, thus producing a substantial increase in the vertical angular coverage above the axis of the reflector without a substantial reduction in the angular coverage below the axis. The power distribution and hence the range of the radar installation in a given direction is, of course, altered and at some elevations it is reduced while at others it is increased. However the tactical value of the unit in covering a substantial area is greatly increased by spreading or shaping the beam in the vertical plane to produce a more desirable radiation pattern for radar search purposes.

The effect of the phase advancing unit 20 on the radia tion pattern in a vertical plane through the center of the reflector 10 may be readily seen by reference to Fig. 4. The radar antenna without the phase advancing unit radiates a pattern similar to that illustrated by the curve 40, which pattern is relatively sharp and symmetrical about the axis 41 of the reflector 10. It will be noted that the beam thus produced covers a chord of a circle of about 19. When the phase advancing unit 20 is added to the reflector 10, the pattern is illustrated by the curve 42, which covers a sector which extends from about 10 below the axis 41 to more than 20 above the axis. The power produced by the radar transmitter is now distributed in a pattern which more closely apprgaches the optimum pattern for an anti-aircraft search ra at.

It will be readily seen that a wide variety of patterns may be produced from a particular type of high frequency equipment by a proper selection of phase advancing units, and since the units are quickly changeable, a selection of the desired radiation pattern may be made for each operating station if it should be so desired. The particular installation shown and described is intended to be only an illustration of the present invention.

Having described the present invention, what is claimed is 1. In a high frequency radio antenna system, a paraboloidal reflector having a horizontal plane of symmetry, a feeding device for supplying thereto electromagnetic wave energy having a polarization axis parallel both to said reflector and to said plane, and a plurality of metallic plates projecting from a section of the reflecting surface of said reflector parallel with said plane, adjacent plates being separated by a distance in excess of one-half of the wavelength of said electromagnetic wave energy and the width of said plates being proportioned to advance the wave front radiated by said section of said reflector increasingly as the distance from said plane increases.

2. In combination, a paraboloidal reflector having a horizontal plane of symmetry which substantially includes the axis of the radiation pattern therefrom, a feeding device for supplying horizontally polarized electromagnetic wave energy to said reflector, and a plurality of parallel metallic plates having their major axis parallel to the polarization of said polarized wave energy and parallel to the axis of the radiation pattern of said reflector, said plates being attached to the section of said reflector adjacent a vertical extremity thereof, adjacent plates being separated by a distance in excess of one-half the wavelength of said wave energy, and the width of said plates increasing progressively as the vertical distance of the plate from said plane increases, whereby the radiation pattern from said reflector is substantially broadened in a vertical plane.

3. In combination, a symmetrically cut paraboloidal reflector having a horizontal major axis and a horizontal plane of symmetry which substantially includes the axis of the radiation pattern therefrom, a feeding device for radiantly distributing horizontally polarized electromagnetic wave energy over the reflecting surface of said reflector, and a plurality of metallic plates removably attached to the reflecting surface substantially parallel to the direction of radiation from said reflector, said plates being coextensive with the horizontal length of said reflector and parallel with said plane, adjacent plates being separated by a distance in excess of one-half the wavelength of said wave energy, the width of each of said plates being in accordance with a function of its vertical displacement from said plane, and the widest of said plates being attached to the reflector adjacent the lower extremity thereof, whereby energy is deflected from the lower to the upper portion of the normal radiation pattern of said reflector when said plurality of plates is attached thereto.

4. In an ultra-high frequency antenna system, a paraboloidal reflector having a plane of symmetry which substantially includes the axis of the radiation pattern therefrom, a feeding means for supplying thereto electromagnetic wave energy having a polarization axis parallel both to said reflector and to said plane, and a plurality of parallel plates parallel to the axis of the radiation from said reflector separated from each other by a distance in excess of one-half the wavelength of the wave energy, said plates being adapted to be removably attached to the reflecting surface of said reflector adjacent an extremity thereof and parallel with said plane, and the width of 5 each of said plates being selected in accordance with a function of the displacement of the plate from said plane, whereby the radiation pattern of said reflector is broadened from the normalradiation pattern when said plates are attached to said reflector. 10

References Cited in the file of this patent UNITED STATES PATENTS McClellan June 17, 1947 15 6 Iams June 8, 1948 Becker Oct. 26, 1948 De Vore Aug. 23, 1949 Cutler Nov. 29,1949

OTHER REFERENCES Metal Lens Antennas by W. E. Kock, Proc. 1. R. E., November 1946, pages 828 to 836.