US2795783A - Microwave lens antenna - Google Patents

Description

June 11, 1957 A. s. DUNBAR 2,795,783

MICROWAVE LENS ANTENNA Filed Aug. 19, 1953 2 Shets-Sheet 1 J c 0' I 1f i i INVENTOR,

ALLEN s. DUNBAR A TTORNE X June 11, 1957 A. s. DUNBAR 2,795,733

MICROWAVE LENS ANTENNA Filed Aug. 19, 1953 2 Sheets-Sheet 2 ENERGY SOURCE INVENTOR, ALLEN s. pu/vam iz 7 dvmi g United States Patent MICROWAVE LENS ANTENNA Allen S. Dunbar, Palo Alto, Calif., assignor to the United States of America as represented by the Secretary of the Army Application August 19, 1953, Serial No. 375,586

Claims. (Cl. 343754) This invention relates to antennas for electromagnetic energy at microwave frequencies, and particularly to a lens-type of antenna at such frequencies.

In the antenna art, it is frequently desired to vary the direction of radiation of a beam of microwave electromagnetic energy over a wide angle. Further, it is desirable that this be accomplished as simply and as expeditiously as possible. Both of these aims are accomplished by the present invention.

It is, therefore, one object of this invention to provide a very wide angle microwave lens antenna.

Still another object of this invention is to provide a lens antenna which may be focused in any direction by a simple motion of the feed system.

These objects are accomplished by providing a spherically symmetrical lens antenna having a phased line source feed system disposed at its center, which feed system is rotatable, the lens serving to focus microwave energy from said source in any desired direction.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, in which:

Fig. 1 is a sketch of the geometry of the invention;

Fig. 2 is a perspective view of an antenna according to the present invention;

Fig. 3A is a cross-sectional view of one embodiment of the invention, along line 33 of Fig. 2;

Fig. 3B is a cross-sectional view of another embodiment of the invention taken along line 3-3 of Fig. 2; and

Fig. 4 is a cross-sectional view of a feed system for the embodiment of the invention shown in Fig. 3A.

Referring now to Fig. 1, numeral 1 refers to a portion of a spherically symmetrical lens made of a suitable refractive material. The outer portion of lens 1 has a radius equal to R1, the inner portion of the lens having a radius equal to R2, the point 0 being the point of origin of these radii as well as being the center of the lens. The line O--X designates a plane perpendicular to the plane of the drawing and passing through the center point 0, while the line O-Z designates a line at right angles to OX. Points S and S along line OZ are presumed to be sources of electromagnetic, microwave energy directed, as shown by the arrows, toward the lens 1 and striking the inner surface of the lens at points I and I. The line SI makes an angle a with a line drawn through S and parallel to OX. Only the geometry of the invention above plane OX will be discussed in detail, but it should be understood that the same geometry exists below plane OX. The energy directed along lines SI and S is refracted by the lens 1 and leaves the lens at points I and I on its outer circumference, as indicated by the arrows. The line I-I l kes an angle e with the radius R2 to point I, and makes an angle 1 with the radius R1 to point I. The energy leaving the lens at point I, which is a height Y above plane OX, is bent upon leaving the lens and forms an angle 0 with the radius R1 to point I. The angle between lines SI and I0 is equal to an angle b, and the angle between R1 and R2 is equal to an angle d. When the line I-] is extended backwards in a direction generally toward the point 0, it is found that a perpendicular toward this extension from the point 0 strikes the extension at a point K. Further, a dashed line drawn parallel to OX through point I forms an angle m with line I-K. The angle between this dashed line and line SI is equal to an angle a. The angle between line K-I and IO is equal to an angle e. The angle between line J-O and OX is equal to an angle 0.

It will be obvious from an inspection of Fig. 1 that:

a=a' c=c' e=e' and a'=b(m-|-e) But,

m+e'=c'+d Therefore,

a'=b-c'-d and therefore,

a=bc'd (1) In triangle 01], the external angle +f Therefore,

Substituting (2) in (1), we obtain a=bc'e+f Now, by Snells law,

sin c=n sin f=sin c Where n is the index of refraction of the material of lens 1. Therefore,

I sin f 4 However, by inspection sin f=OK/R1 (5) Equating (4) and (5),

I =0K R..

Solving for OK,

0K=R1 sin c But by inspection,

sin c'=Y/R1 Substituting (7) into (6), we find Y R; OK: 5 1071 (8) Now, by Snells law,

sin b=n sin e=n sin e' (9) By inspection,

sin e=0K/R2 (10) Substituting (8) into (10),

Y sln e 11 Solving for e, we find e=arc sin=e (12) Substituting 11 into 9 nY Sm Solving for b,

=arc sin Y/Rz (13) From (7), we find c'=arc sin Y/Rr (14) By inspection,

sin f=OK/R1 But, from (8),

K=Y/ n Therefore,

Solving for f,

farc srn (15) Substituting (12), (13), (14), and (15) into (3), we find r 2:- 1 a-are srn are sm arc sm +arc sln (16) Examining Equation 16 and Fig. 1, it will be apparent that a lens which satisfies this equation will produce a beam of electromagnetic energy that is substantially parallel to plane OX. Further, an examination of Equation 16 will disclose that where n is greater than one, the angle a is positive; and that where n is less than one, the angle a is negative. In the case where angle a is positive, the energy from sources S and S' converge toward plane OX, in the manner shown in Fig. 1. In the case where angle a is negative, the energy from sources S and S' will diverge from plane OX. The material of the lenses necessary to produce such results will be discussed in connection with the remaining figures.

Referring now to Fig. 2, there is shown a lens 1 made of material having an index of refraction n. This lens is spherically symmetrical and may have a form such as would be produced by cutting through a hollow sphere along a pair of parallel planes respctively equidistant from the center of the sphere. Such a lens is spherically symmetrical. Disposed withinthe lens 1 is a phased line feed source 3 of electromagnetic microwave energy. Source 3 directs energy toward lens 1 from a straight line, which straight line corresponds to line SOS' of Fig. l, and which line passes through the center point of lens 1. An energy source 7 produces electromagnetic, microwave energy, and a wave guide 5 transmits said energy from energy source 7 to feed source 3. A motor 9 is connected by a shaft 11 to wave guide 5, and serves to rotate feed source 3.

Lens 1 may be composed of any suitable material which will pass electromagnetic energy with a minimum of dissipation. Such materials as Plexiglas or polystyrene, both of which have indices of refraction equal to 1.6, may be used. In such a case, according to Equation 16, since 11 is greater than one, angle a is positive and a convergent beam of energy must be provided. However, if the lens is made up of metal plates, which plates can have an index of refraction of from zero to 0.86, 0.5 being most common, the angle a will be negative, and a divergent beam of energy must be provided. As was noted in connection with Fig. 1, if these conditions for satisfying Equation 16 are met, the energy from feed source 3 will leave the lens 1 and travel in a direction generally perpendicular to the line source.

Referring now to Figs. 3A and 3B, straight line sources 3 respectively, pass through points 0, the centers of the lenses 1 shown in Figs. 3A and 3B. As will be noted from an inspection of these figures, and as was pointed out in connection with Fig. 2, when lens 1 is. made of material having an index of refraction greater than one, feed source 3 must produce a convergent beam of energy, as shown in Fig. 3A. On the other hand, when lens 1 is made up of material having an index of refraction that is less than one, feed source 3 must produce a divergent beam, as shown in Fig. 3B. Line sources for microwave energy are discussed in a number of publications. For example, the book Aerials for Centimetre Wave Lengths by Fry and Coward, Cambridge University Press, 1950, contains chapters 6 and 7 which are primarily devoted to this subject. The subject of line sources is also discussed to a great extent in Microwave Antenna Theory and Design, Radiation Laboratory Series, volume 12, McGraw-Hill Book Co. Inc., 1949. It will be clear from these publications that a phased line source to provide a divergent beam may be provided by employing a microwave linear array or by employing a horn and lens arrangement. As the literature shows, a variety of ways of producing such a source are readily available.

A suitable arrangement for producing the convergent antenna beam shown in Fig. 3A is shown in Fig. 4. In this figure is shown, in cross-section, a metallic, elliptical reflector 13, having a source of energy 15, which may be a dipole or small horn disposed at a point F1, one of the foci of the reflector. It will be apparent from this figure that the energy from source 15, when directed toward reflector 13, will be reflected therefrom to form a convergent beam pattern passing through a point F2, the other one of the foci of reflector 13. The convergent beam pattern produced by the arrangement of Fig. 4 is suitable for use as a line feed source 3 in Fig. 3A.

It should be understood that, in Fig. 2, although motor 9 may rotate feed source 3 through an angle of 360, such a rotation is not essential to the invention. A lesser angular rotation could also be used where it is desired to scan a smaller sector of space. It should also be understood that although an antenna lens having a circular cross-section and being a portion of a sphere, has been depicted in the figures, the present invention is not limited to this feature. For example, a complete sphere could be used to replace lens 1, focusing the energy in any desired direction. Further, an even smaller portion of a sphere could be used than is shown, where it is desired to scan a more limited sector of space.

From the foregoing description of this invention, it is believed apparent that this invention possesses the ability to focus a beam of electromagnetic microwave energy in any desired direction. Further, this is achieved simply and economically, due to the fact that the feed source is on the inside of the lens and can be rotated by simple mechanical devices.

While there have been described what are at present considered preferred embodimentsof the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention; and it is aimed in the apparent claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A microwave antenna comprising, a spherically symmetrical lens having an outer radius of length R1 and inner radius of length R2 as measured from the center point of said lens, said lens being composed of a refractive material having an index of refraction equal to n, directive means disposed within said lens for producing a phased source of electromagnetic wave energy directed toward said lens, said straight line source passing through said center point and being perpendicular to a plane also passing through said center point, the directivity of said wave energy being predetermined so as to be defined by a ray drawn from each extremity of said straight line source respectively making an angle a with lines respectively drawn through said extremities parallel to said plane, the rays from said extremities respectively leaving the outer circumference of said lens at points at a height Y as measured by perpendiculars to said plane, angle a being equal to are sin Y/Rz-arc sin Y/R1 arc sin Y/nRz-l-arc sin Y/nRi and means for rotating said directive means.

2. The antenna of claim 1, wherein the material of said lens has an index of refraction that is greater than one.

3. The antenna of claim 1, wherein the material of said lens has an index of refraction that is less than one.

4. A microwave antenna comprising, a lens having the form of a portion of a hollow sphere with an outer radius of length R1 and inner radius of length Rz as measured from the center point of said lens, said lens being composed of a refractive material having an index of refraction equal to n, directive means disposed within said lens for producing a phased source of electromagnetic wave energy directed toward said lens, said straight line source passing through said center point and being perpendicular to a plane also passing through said center point, the directivity of said wave energy being predetermined so as to be defined by a ray drawn from each extremity of said straight line source respectively making an angle a with lines respectively drawn through said extremities parallel to said plane, the rays from said extremities respectively leaving the outer circumference of said lens at points at a height Y as measured by perpendiculars to said plane, angle a being equal to are sin Y/Rz-arc sin Y/Riarc sin Y/nRz-i-arc sin Y/nRi and means for rotating said directive means.

5. The antenna of claim 4, wherein the material of said lens has an index of refraction that is greater than one.

6. The antenna of claim 4, wherein the material of said lens has an index of refraction that is less than one.

7. A microwave antenna comprising, a lens having the shape of a spherically symmetrical portion between two parallel planes passing through a hollow sphere at equal distances from the center of said sphere, the cross section at the center of said symmetrical portion being a pair of concentric circles of which the larger circle has a radius of R1 and the smaller has a radius of R2 as measured from the center point of said lens, said lens being composed of a refractive material having an index of refraction equal to n, directive means disposed within said lens for producing a phased source of electromagnetic wave energy directed toward said lens, said straight line source passing through said center point and being parallel to said planes, the directivity of said wave energy being predetermined so as to be defined by a ray drawn from each extremity of said straight line source respectively making an angle a with lines respectively drawn through said extremities parallel to said planes, the rays from said extremities respectively leaving the outer circumference of said lens at points at a height Y, as measured by perpendiculars to a plane that is perpendicular to said straight line and passes through said center point, angle a being equal to are sin Y/Rz-arc sin Y/R1- arc sin Y/nRz-I-atc sin Y/nRr and means for rotating said directive means in a plane perpendicular to said straight line.

8. A microwave lens antenna comprising an electro= magnetic wave lens having a dielectric constant n and outer and inner surface envelopes which are spherical and concentric about a center point, a phased line source of electromagnetic wave energy positioned to intersect said center point, the wave energy radiated from points along said line source having relative phase relations such that the wave energy directed toward the inner surface of said lens is convergent for a lens where n is greater than unity and divergent where n is less than unity, thereby to radiate a focused beam of parallel rays of energy from said outer surface and means for rotating said phased line source about an axis thru said center correspondingly to rotate the direction of said parallel rays.

'9. A microwave lens antenna comprising an electromagnetic wave lens having a dielectric constant n which is greater than unity and outer and inner surface envelopes which are spherical and concentric about a center point, a phased line source of electromagnetic wave energy positioned to intersect said center point, the electromagnetic wave energy radiated from points along said line source having relative phase relations such that the wave energy directed toward the inner surface of said lens is convergent thereby to radiate a focused beam of parallel rays of energy from said outer surface and means for rotating said phased line source about an axis thru said center correspondingly to rotate the direction of said parallel rays.

10. A microwave lens antenna comprising an electro magnetic wave lens having a dielectric constant n which is less than unity and outer and inner surface envelopes which are spherical and concentric about a center point, a phased line source of electromagnetic wave energy positioned to intersect said center point the electromagnetic wave energy radiated from points along said line source having relative phase relations such that the wave energy directed toward the inner surface of said lens is divergent, thereby to radiate a focused beam of parallel rays of energy from said outer surface and means for rotating said phased line source about an axis thru said center correspondingly to rotate the direction of said parallel rays.

References Cited in the file of this patent UNITED STATES PATENTS 2,547,416 Skellett Apr. 3, 1951 2,611,870 Clavier Sept. 23, 1952 FOREIGN PATENTS 501,429 Great Britain 1939

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