US2643339A

US2643339A - Reflector scanning antenna

Info

Publication number: US2643339A
Application number: US616396A
Authority: US
Inventors: Chu Lan Jen
Original assignee: US Department of Navy
Current assignee: US Department of Navy
Priority date: 1945-09-14
Filing date: 1945-09-14
Publication date: 1953-06-23
Anticipated expiration: 1970-06-23

Description

June 23, 1953 LAN JEN CHU REFLECTOR SCANNING ANTENNA Filed Sept. 14, 1945 FIGJA FIG.3

INVENTOR LAN JEN CHU ATTORNEY Patented June 23, 1953 UNITED STATES r orlci;

REFLECTOR. SCANNING ANTENNA Application September 14, 1945, Serial. No. 616,396

18 Claims. (01. 250-3335) The present invention relates in general to directive radio antennasfor producinga beam having a. plane wave front and in particular to such an antenna comprising a radiator combinedwith a bifocal paraboloid. reflector to produce a beam having a plane'wave front.

In antennas of the type-under discussion, it has in the past been customary to arrangea relatively small source of radiation at the focal point of a parabolic reflector, radiation being directed at the surface of the said parabolic reflector. Energy reflected from the reflector has. a substantially plane wave front. when the source of radiation approaches a point in size. An objection to. such a system arises from the fact that the system takes up a considerably large amount of. space, inasmuch as it is necessary to place the source of radiationat the focal point of the parabolic reflector.

It is an object of my invention to reduce the size of an antenna of the type under discussion by providing a source of radiation that may be positioned between the focal point of a reflector and the reflector itself to produce a reflected beam having a plane wave front.

It is another object of my invention to provide a. radiating array of antenna elements that will produce a beam having a circular wave front. and to position this array between a focal point. and the. surface of a bifocal paraboloid. reflector to produce a reflected beam having a plane wave front.

It is a further object. of my invention to reduce the weight of. such an antenna by reducing the size of the supporting elements required.

In Figs. 1 and 1A, a linear radiating element l is placed in front of the concave of reflecting surface 3 of a bifocal paraboloid reflector The refiecting surface 3 has two different parabolic curvatures 3' and 3". The flrst curvature 3 is a gradual curve of relatively long focus having a focal point 5, while the second curvature is relatively tight and has a shorter focus and a focal point I. The axes of these two curvatures 3 and 3" passing through the feel are substantially coincident. The radiator I produces a beam having a wave front with two different mutually per-- pendicular substantially circular curvatures 2- and It is a still further object of my invention to I provide facilities. for rapidly scanning the plane wavefront beam of the antennaof my invention.

Other objects and features of my invention will become apparent upon a careful consideration of the following. detailed description when taken together with the. accompanying drawings, the figures of which illustrate. typical embodiments of the. invention.

Fig. 1 illustrates in scheme, the broad concept of my invention;

Fig. 1A is a cross section. along line II of Fig. 1;

Fig. 2:il1ustrates the manner in which an array of. antennas for producing a beam having aplurality of circular wave front may be arranged with a bifocal paraboloid reflector to produce the antenna of my invention; and.

Fig. 3 illustrates a suitable manner in which the beam of the antenna of. my invention. may be scanned.

8. As seen in Fig. l, the first wave front curvarture 2, having an instantaneous radius 7', appears to come. from a point a distance removed from the radiator I itself. The second circular wave front curvature 8, seen in Fig. 1A, appears to emanate directly from the radiator I as a point. The radiating element I is so spaced and oriented with respect to the bifocal paraboloid reflector 4 and its two focal points 5 and I that the center of the first wave front curvature 2 lies coincident with the long focal point 5. Thus the wave front curvature 2 appears to be coming from the long focal point, 5. Under this condition, the first circular wave front curvature 2 will be reflected from the first parabolic curvature 3 of the surface 3 of the reflector 4 as a plane wave 6 moving away from that surface 3 when the radiator I is orientated as shown in Figs. 1 and 1A. Likewise the element I lies coincident with the shorter focalpoint I, so thatthe center of curvature of the second circular wave front curvature 8 lies coincident with the short focal point I. Therefore, the second circular wave front curvature 8 is reflected from the second parabolic curvature 3" ofthe surface 3 as a plane wave 6', as shown in Fig. 1A. Thus the reflected beam has a substantially uniformly plane wave front.

It is to be noted that by using a linear array radiator I that produces a beam having a circular wave front of a curvature 2 that appears to emanate from a point removed from the radiator, my invention provides an antenna of the paraboloid reflector type in which the radiator may be located inside a focus of the paraboloid. However, another circular wave curvature 8 arises out of the fact that the, linear array radiator I behaves like a point with respect to a plane perpendicular to the line of the radiator I. This makes necessary a second curvature 3" on the reflecting surface 3 having its focus I coincident with the line of the radiator I, in order that the reflected beam may have a fully plane wave front. Thus the reflector 4 is preferably of the bifocal. type, as described hereinabove. By making it possible to position the radiator l Within the long focus 5, I provide a substantial reduction in size of this type of antenna.

In Fig. 2, a linear array In of radiating elements H produces a beam having a first substantially circular wavefront curvature [2 of radius R, which appears to come from a point .IB removed from the array I0. The array 10 is arranged to be pivotally mounted at a point l3 near its geometric center. A bifocal paraboloid reflector l4 similar to the reflector 4 of Figs. 1 and 1A having a long focus along a focal line I5l5 on which the long focal point It: lies is placed before the array It. The array l fires its beam at the concave surface I! of the reflector l4. As in the illustration of Fig. 1, the array 10 is so placedor positioned with respect to the reflector I l and the long focal point l6 that the first circular wave front curvature l2 appears to be coming from the long focal point l6, that is, the center of curvature of the circular wave front I2 lies coincident with the long focal point 16. As a result, the wave front 12 is reflected by the concave surface H of the reflector I4 as a substantially plane wave front l8 proceeding away from the reflector M. As in the illustration of Fig. 1A, there is a second circular wave front curvature (not shown) to the radiation produced by the array 10, arising out of the fact that the array In acts substantially as a point source with respect to a plane lying perpendicular to the line of the array [0. This second wave front curvature is rendered plane by reflection by a second parabolic curvature (not shown) of the surface l1, having its focus coincident with the line of the array II], in the same manner as illustrated in Fig. 1A, and substantially as described in the discussion hereinabove in connection therewith.

The beam produced by the antenna illustrated in Fig. 2 may be scanned or changed in direction merely by tilting the array l0 about the oint l3. Thus as illustrated in Fig. 2, if the array I0 be tilted to occupy a new position I9 the newly-directed circular wave front l2, having the radius R, identical in magnitude to the radius R, will appear to be coming from a new point 20, which is its center of curvature. The point 20 will not lie upon the long focal line II5 of .the reflector l4 but will be slightly inside that focal' line, as is apparent from the geometry of the system. Thus the newlydirected circular wave front I2 is identical in curvature to the original circular wave front l2, but has a new direction. This latter circular wave front i2 is reflected from the surface I! of the reflector M as a newly-directed beam having a substantially plane wave front [8. The newlydirected reflected beam is proceeding in a different direction from the original beam but is still going away from the reflector [4. Thus the direction of the beam produced by the antenna 'of my invention may be altered and therefore 4 beam, which will become progressively worse as tilting is increased. This difficulty is circumvented by the apparatus illustrated in Fig. 3, showing an improved antenna of my invention incorporated in a radio system, such as for example, a radio echo detection system, A receiving and transmitting device 2| is connected by means of a wave guide 22 to a scanning apparatus 23 which operates upon a linear array of antenna elements 25 mounted upon a side of a wave guide 27. The antenna elements 25 are preferably so positioned and arranged upon the wave guide 2'! as to produce a beam having a first substantially circular wave front curvature 26 or 26', the center of which is at a point removed from the array. The wave guide 2! upon which the antenna elements 25 are mounted serves both to feed and to support those elements. The depth of penetration into the wave guide 21 of associated probes (not shown) of the -antenna elements 25 and the relative spacing of the antenna elements 25 may be adjusted in a fashion known to the art to produce the first substantially circular

wave front curvature

26 or 26. The scanning apparatus 23 is preferably efiective to change the wave length in the wave guide 21 of energy at the operative frequency carried extremities when tilted. This will introduce distortion in the plane wave front of the reflected therein, and thereby shift the phase of waves feeding the antenna elements 25. As a result, the effective direction of the wave front curvature 26 or 26' may be thereby progressively altered. Scanning apparatus suitable to accomplish this result is disclosed in the copending application of Luis W. Alvarez, Ser. No. 509,790, filed November 10, 1943, now Patent No. 2,605,413, issued July 29, 1952. Such apparatus is not the subject of this invention. The scanning apparatus 23 will permit the direction of the circular

wave front curvature

26 or 26 to be progressively shifted without physically altering therelative positions of the antenna elements 2 5, or the orientation of the wave guide 21. The linear array of antenna elements 25 is positioned in front of the concave surface 28 of a bifocal rparaboloid reflector 29 and between that reflector 29 and its long focal point 30 in the manner taught hereinabove in connection with the illustration of Figs. 1 and 1A. In the same manner as in the apparatus of Fig. 2 a plane wave front 3| or 3 I going away from the parabolic reflector .29 will be produced for each first

circular wave front

26 or 26 respectively. The first

circular vwave fronts

26 and 26 have the same radius of curvature P and P respectively and appear reach to be coming from a

point

32 or 32 respectively which lies close to the long focal point 30 of the bifocal paraboloid reflector '29. Thus as the scanning mechanism 23 causes the direction of the first circular wave front 26 or 26' to change so also will the direction in which the plane wave front 3| or 3! is going be changed. Therefore the beam of the antenna of Fig. 3 will be scanned over an angle determined by the angle of change produced in the first

circular wave front

26 or 26 by the scanning mechanism 23. Again, a second circular wave front (not shown) similar to that illustrated in Fig. 1A will be produced,

and refiectively rendered plane by the second curvature (not shown) of the reflecting surface 28 as taught in connection with the illustration of Fig. 1A.

It should be obvious that any antenna or array of antenna elements will produce a beam having a substantially circular wave front having its center at a point outside the array may be used with a bifocal paraboloid reflector to produce the antenna of my invention, and thereby accomplish the result of obtaining smaller mechanical size and weight for such an antenna. I am fully aware of the many modifications possible in my invention although I have shown and described only certain specific embodiments thereof.

I claim: 7

l. A radio antenna comprising a bifocal paraboloid reflector having first and second focal points and means for generating a first beam having a wave front having first and second substantially circular curvatures, said first curvature appearing to emanate from a point outside of said means and saidsecond curvature being centered on said means, said means being positioned near the front of said reflector inside said first focal point and at said second focal point so that the center of said first curvature of said wave front is substantially at said first focal point, and the center of said second curvature is substantially at said second focal point, whereby said first beam is reflected by said reflector as a second beam having a substantially plane wave front.

2. A radio antenna comprising, a bifocal paraboloid reflector having first and second focal points, and a linear array of radiators arranged to produce a first beamhaving a wave front having first and second mutually perpendicular substantially circular wave front curvatures, said first curvature having its center outside of said array and said second curvature having its center on said array, said array being positioned between said reflector and said first focal point so that said center of said first curvature of said wave front is substantially at said first. focal point, and on said second focal point so that said center of said second curvature of said wave front is substantially at said second focal point, whereby said first beam is reflected by said reflector as a second beam having a substantially plane wave front.

3. Apparatus in accordance with claim 2 in which said array is pivotally mounted in a manner adapted to facilitate changing the direction of propagation of said second beam,

4. A radio antenna comprising a bifocal paraboloid reflector having first and second focal points and a linear array of radiators so fed and so spaced with respect to each other as to produce a first beam having a wave front having first and second mutually perpendicular substantially circular wave front curvatures, said first curvature having its center located at a point outside of said array and said second curvature having its center on said array, said array being positioned between said reflector and said first focalpoint, and on said second focal point, so that said center of said first curvature of said wave front is substantially at said first focal point and said center of said second curvature of said Wave front is substantially at said second focal point whereby said first beam is reflected by said reflector as a second beam having a substantially plane wave front.

5. The apparatus of claim 4 in which the feed of said array includes means for altering the Wave length of the energy for changing the direction of propagation of said second beam.

6. A radio antenna comprising a reflector, the reflecting surface of which is defined as that surface of a paraboloid which meets first and second, intersecting, mutually transverse planes in first and second parabolas having a common vertex and a common axis, said first and second parabolas defining said first and second planes, respectively, said first and second parabolas having first and second focal points, respectively, in the same direction away from said common vertex, said first focal point being between said vertex and said second focal point, and a linear array of radiators positioned at said first focal point with its axis included in one of said planes and transverse to the other of said planes, said array of radiators being spaced with respect to each other and being adapted to be fed so as to produce a first beam directed at said reflecting surface and having a wave front having first and second mutually perpendicular, substantially circular wave front curvatures, said first curvature having a center on said array at said first focal point, said second curvature having its center displaced from said array and at said second focal point, whereby said first beam is reflected at said reflecting surface as a second beam having a substantially plane wave front.

7. An antenna as claimed in claim 6, said antenna further comprising means for pivotally mounting said array in a manner adapted to facilitate the changing of the direction of propagation of said second beam.

8. An antenna as defined in claim 6, said antenna further comprising means for altering the wave length of the energy fed to said array, without altering the frequency thereof, for altering the direction of propagation of said second beam.

9. A radio antenna comprising a bifocal paraboloid reflector having first and second focal points and means for radiating a beam of electromagnetic Wave energy in the direction of said reflector, said beam having a wave front having first and second substantially circular curvatures, said first curvature appearing to emanate from a point outside of said means and said second curvature being centered on said means.

10. A radio antenna comprising a bifocal reflector having first and second focal points and a linear array of radiators for radiating a beam of electromagnetic wave energy in the direction of said reflector, said radiators being arranged so that said beam has a wave front having first and second mutually perpendicular, substantially circular wave front curvatures, said first curvature having its center outside of said array, and said second curvature having its center on said array.

11. Apparatus as defined in claim 10 and means for pivotally mounting said linear array of radiators at a point displaced from said reflector.

12. A radio antenna comprising a bifocal reflector having first and second focal points and a linear array of radiators for radiating a beam of electromagnetic wave energy in the direction of said reflector, said radiators being so fed and so spaced with respect to each other that said beam has a wave front having first and second mutually perpendicular, substantially circular wave front curvatures, said first curvature having its center at a point displaced from said array, and said second curvature having its center on said array.

13. A radio antenna comprising a bifocal reflector having first and second focal points and means for radiating a beam of electromagnetic wave energy in the direction of said reflector, said means comprising a linear wave guide, a linear array of radiators disposed along the length of said wave guide and adapted to be excited by energy carried by said wave guide,

and means coupled to said wave guide for controlling the wave length of said energy in said wave guide without altering the frequency thereof, said radiators being excited and spaced so that, at a particular frequency of said energy, said beam has a Wave front having first and second mutually perpendicular, substantially circular wave front curvatures, said first curvature having its center located at a point outside said array, the position of said point being dependent upon the magnitude of the wave length of said energy in said wave guide, said second curvature having its center on said array independently of the magnitude of said wave length.

14. A radio antenna comprising a bifocalparaboloid reflector having first and second focal points and means for radiating a beam of electromagnetic wave energy in the direction of said reflector, saidmeans comprising, a linear wave guide adapted to carry electromagnetic wave energy, means coupled to said wave guide for controlling the wave length of energy in said wave guide without altering the frequency thereof, and a linear array of radiators disposed along the length of said wave guide and adapted to be excited by energy in said wave guide, said radiators being spaced and excited so that, at a particular frequency of said energy, said beam has a wave front having first and second mutually perpendicular, substantially circular wave fronts, said first curvature having its center on said array independently of the magnitude of said wave length, said second curvature having its center located at a point outside said array, the position of said point being dependent upon the magnitude of said wave length, said array being positioned between said reflector and said second focal point and on said first focal point so that the center of said first curvature is substantially at said first focal point independently of the magnitude of said wave length and the center of said second curvature is substantially at said second focal point at a predetermined magnitude of said wave length.

15. A radio antenna comprising a bifocal paraboloid reflector having first and second focal points, and means for radiating a beam of electromagnetic wave energy in the direction of said reflector, said beam having a wave front having first and second substantially circular curvatures With centers on said first and second focal points, respectively. 7

16. A radio antenna comprising a bifocal paraboloid reflector having first and second focal points, and means at said first focal point for radiating a beam of electromagnetic wave energy in the direction of said reflector, said ,bea'm having a wave front having first and second substantially circular curvatures with centers on said first and second focal points, respectively;

17. A radio antenna comprising a bifocal paraboloid reflector having two distinct focal points, and means for radiating a beam of electromagnetic energy in the direction of said reflector, said beam having a wave front having two distinct curvatures, at. least one of which has a center on one of said focal points.

18. A radio antenna comprising a bifocal paraboloid reflector having two distinct focal points, and means at one of said focal points for radiating a beam of electromagnetic wave energy in the direction of said reflector, said beam having a Wave front having two separate curvatures, at least one of which has a center on one of said focal points.

LAN JEN CHU.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,299,397 Conklin et a1 Apr. 1, 1919 2,276,497 Kroger Mar. 17, 1942 2,407,057 Carter Sept. 3, 1946 2,409,183 Beck Oct. 15, 1946 2,436,380 Cutler Feb. 24, 1948 FOREIGN PATENTS Number Country Date 804,966 France Aug. 17, 1936