US2801412A - Radio frequency antenna - Google Patents

Description

y 1957 P. c. MAYBURY ET AL RADIO FREQUENCY ANTENNA 2 Sheefs-Sheet 1 Filed July 22, 1953 INVENTORS 7 PAUL O. MAYBURY GILBERT WILKES I WRNEYS 1957 P. c. MAYBURY ET AL RADIO FREQUENCY ANTENNA 2 Sheets-Sheet 2 Filed July 22, 1953 INVENTORS PAUL G. MAYBURY GILBERT WILKES ATTORNEYS United States Patent RADIO FREQUENCY ANTENNA Paul C. Maybury, Pikesville, Md., and Gilbert Wilkes,

Detroit, M1ch., assignors to the United States of Amertea as represented by the Secretary of the Navy Application July 22, 1953, Serial No. 369,647

4 Claims. (Cl. 343-753) This invention relates generally to devices for radio frequency wave refraction and more particularly to an improved antenna for focusing radio frequency (R. F.) energy emitting from .a radar horn or the like on a desired distant restricted area.

The conventional antenna or radiator, such as the electromagnetic horn, the dipole and the waveguide aperture, transmits energy as a diverging wave front, resulting in a low-energy distribution at a distance from the radiating element. In examining objects or phenomena in a restricted region, the low-energy distribution of such a diverging front complicates a determination of the direct effect of the objects or phenomena on the radiations. Moreover, the incidence of reflected energy, because of the divergent character of the wave front, will alter the effect caused by the objects or phenomena in the region under examination, making a determination of such effect impossible or at least unreliable.

In studying the attenuation, reflection and phase effects of the exhaust stream of a reaction engine on the transmission of R. F. energy using a conventional radiator, for example, it had been impossible to assign a direct relationship between the measured effect on the transmitted energy and the effect caused by the exhaust stream. That is, the energy passing through the exhaust stream constituted but a small proportion of the total energy transmitted from the radiator. A disturbance of the energy passing through the exhaust stream, being intrinsically small, therefore affected only a small fraction of the total energy transmitted.

In addition, the divergent wave front produced by the conventional radio frequency (R. F.) energy radiators used for this purpose gave rise to reflected energy from nearby equipment and structures. This reflected energy combined with the transmitted energy passing through the propellant flame to further complicate and render unreliable the determination of the exhaust stream effects.

Devices for focusing microwave radiations have been constructed heretofore. However, because of their complex structure and the close structural tolerances involved, the successful manufacture of these devices was extremely difficult. In certain instances the machining involved in manufacturing the device was prohibitive by virtue of the character and size of the material used. In other cases a casting process was inappropriate in the interests of homogeneity in the final product, and the final assembly of other devices to the close tolerances required was diflicult. In addition, such prior devices were inflexible in application, each being designed with a single, unchangeable focal length.

The principal object of the present invention, therefore, is to provide an improved antenna principally constituted by a dielectric lens, for use with a R. F. energy source to converge the R. F. energy radiations emitting from said source on a distant restricted area.

Another object of this invention is to provide ,a dielectric lens for converging R. F. energy radiations on a "ice distant restricted area, said lens having a focal length which may be easily altered.

A further object of this invention is to provide a dielectric lens for refracting R. F. energy radiations which is easily constructed and ruggedly built.

Further objects and attendant advantages of the present invention will become evident from the following detailed description, taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a diagrammatic view illustrating the method for deriving the geometry of the dielectric lens constituting this invention;

Fig. 2 is an elevation showing the profile of a dielectric lens constructed in accordance with this invention;

Fig. 3 is an exploded perspective view of the lens shown in Fig. 2 and an electromagnetic horn with which said lens may be used;

Fig. 4 is an elevation of the lens shown in Fig. 2 mounted on an electromagnetic horn, and showing the lens and its housing partially broken away; and

Fig. 5 is a section on line 5-5 of Fig. 4.

The lens geometry is established under the assumption that R. F. energy radiations emitting from a radiation source obey the principles of ordinary ray optics. Referring to Fig. 1, there is shown a radiation source S, which may be the waveguide inlet to an electromagnetic horn, a dipole, or a waveguide aperture; a lens L, the geometry of which is unknown at this stage, having a base B confronting the source S, and a focus point P at which it is desired to converge the radiations emitting from the source S. An equation relating the times of travel of radiations in the several directions from the source S can be derived using the symbols defined as follows with reference to Fig. 1:

a=the distance from the source S to the base B of the lens L;

f=the distance from the base B to the focus point P;

y=the radius of the base B;

x=a variable distance from the base B along the axis of the lens L and constitutes the abscissa of the lens profile;

y=the radial dimension of the lens L at the distance x from the base B and constitutes the ordinate of the lens profile.

The paths of the individual radiations through lens L are considered to be parallel to the axis of said lens in the interests of simplicity This assumption is reasonable if the distance a from the source S to the base B is large compared with the diameter of said base, inasmuch as the individual radiations will then enter the lens L substantially normal to the base thereof and little refraction will occur.

The complete distance of travel of each radiation from the source S to the focus point P is represented by the summation of the following distances which may be expressed in terms of the variables x and y by the application of the Pythagoran principle.

Where z=distance of path from the source S to the base B;

l=distance of path through the lens L;

d=distance of path from the lens L to the focus point P; and

n=the index of refraction of the lens material, then dam-ma 3 The time of travel of each ray from the source S to the focus point F, therefore, is

where c=speed of the rays which is constant.

' If the radiations converge on the focus point P, then the time of travel of every ray from the source S to the focus point F is the same. Thus, imposing a time cou The time constant K may be determined from the Equation 1 by inserting therein values corresponding to a radiation passing through the outermost peripheral portion of the lens L.

In order to calculate the lens geometry, it is necessary to specify the particular circumstances'under which the lens is intended to operate. More specifically, the values of a, f, y and It must be fixed. The value of a and y may be limited by the structure of the radiation source, i. e. if the source comprises an electromagnetic horn the value of a is the axial length of said horn, and y is the radius of the outlet of said horn. Also, the values of a and y may be determined as a matter of convenience when there are no structural limitations. The value of f, of course, depends upon the focal length desired, and the value of n is governed by the chosen lens material. A substitution of the specified values into Equation 1 provides a mathematical expression representative of a parabola which defines the lens profile.

In constructing a lens in accordance with this invention, the parabolic profile is approximated. As shown in Figs. 2, 3, 4 and 5, a lens L is constructed of a series of disks 11 of varying diameters held in stacked relation to form a generally stepped paraboloidal solid 12 having a base 13 and an apex 14. The disks 11, from the base 13 to the apex 14, are of successively decreasing diameters so that an imaginary plane passing through the axis of the solid 12 intersects the peripheries of the rear faces of each of said disks to form a series of points the locii of which describe a parabola corresponding to the calculated lens profile. A hole 15 is concentrically formed in each of the disks 11 and a dowel 16 extends halfway into the holes of adjacent disks to retain said disks in position. This type of construction permits certain of the disks 11 to be interchanged with other disks in order to vary the focal length of the lens L or to replace damaged disks.

In mounting the lens described above on an electromagnetic energy radiating horn, reference is made to Figs. 3, 4 and 5. A horn 17 is shown in these figures as comprising a conically shaped portion 18 formed with a waveguide terminal 19 at its convergent end, and a circular mouth 21 and a flange 22 at its divergent end. The flange 22 is provided with a series of equally spaced holes 23 therein and is adapted to mount a lens assembly including a dielectric lens L and a housing 24, in a manner to be described hereinafter.

A clamping band 25 having its end portions joined by a bolt and nut 26 fits around the base portion of the lens L so that by tightening said nut on said bolt the lens base portion is gripped by said band. A plurality of brackets 27, corresponding in number to the number of holes 23 in the flange 22 of the horn, are mounted in spaced relation on the periphery of the band 25. Each of the brackets 27 is constructed with a base leg 28 having an arcuate slot 29 therein and a perpendicularly extending leg 31 for connection to the band 25, as by rivets 32. The base leg 28 of each bracket is adapted to be secured to the flange 22 of the horn 17, as will appear hereinafter.

The housing 24 comprises a cylindrical shell 33 having an inwardly directed flange 34 on one end and a cylindrical can 35- slidably fitting over the other end. The flange 34 is formed with a series of holes 36 corresponding to those in the flange 34 of the horn 17, for mounting purposes as will be described hereinafter. The lens L, with the clamping band 25 gripping its base portion, is contained within the shell 33 and the base legs 28 of the brackets 27 abut the inner face of the flange 34 of the shell 33.

The can 35 is provided with an inwardly directed flange 37 at its outer end. A circular plate 38 of dielectric material fits within the can 35 and abuts the inside face of the flange 37 and closes the outer end of said can. Nuts 39 cooperate with bolts 41, passing through suitable holes 42 formed in the flange 37 and the plate 38, to hold said plate to said flange. Four equally spaced longitudinal slots 43 are formed in the inner end portion of the can 35 and receive four bolts 44 mounted on the shell 33. Wing nuts 45 cooperate with thev bolts 44 to adjustably fasten the shell and the can together. In fitting the can 35 over the shell 33 the circular plate 38 is caused to abut the apex 14 of the lens L to provide further support therefor.

A plurality of bolts 46 pass through the holes 23 in the flange 22 of the horn 17, the corresponding holes 36 in the flange 34 on the shell 33, and are received by corresponding slots 29 in the base plates 28 of the brackets 27. A washer- 47 is placed on each of the bolts 46 and a nut 48 is drawn up on each of said bolts to secure the housing24 and thelens L to the horn 17.

The lens L without the housing 24 provides a definite focusing of R. F. waves in a restricted region. However, a slight phase error occurs at the outer edge of the lens L. The'primary purpose of the housing is to protect the lens from mechanical damage. However, the flange 37 of the can 35 of said housing functions as an inductive window which can be suitably located with respect to the lens to compensate for the slight phase errors encountered. around the outer edge of the lens L.

The lens L with its housing 24 in place is further characterized by a small focal depth. Even in spite of a rapidly decreasing range, which normally operates to increase the transmitted power inversely to the square of the range, the. transmitted power actually rapidly decreases. For an increasing range, the transmitted power similarly decreases rapidly.

The characteristics of the lens of the present invention are more fully understood with reference to a lens of particular dimensions. A lens profile is derived for the following conditions:

a=1.625 feet y'=0.5 feet i=4 feet 71:1.1

The lens material chosen is a substance commonly known as Celltite and is a foam material produced by the Sponge Rubber Company of Shelton, Connecticut. Celltite has a refractive index of approximately 1.1 and a low loss tangent of approximately 0.0017, thus assuring a reasonable volume. The combination of the moderate refractive index and the low loss tangent of this substance results in a low attenuation of energy passing through the lens. In addition, the moderate refractive index permits the simplifying approximation of the lens profile.

A lens constructed in accordance with the limitations set forth above, when mounted on an electromagnetic energy radiating horn, provides an energy distribution in the focal plane (four feet from the lens base) having half-power points less than four inches apart. This separation of half-power points corresponds to a subtended angle of approximately five degrees which is similar in order of magnitude to the angle subtended by the halfpower points of the horn without the lens in the distant zone. Thus, the lens may be considered as modifying the wavefront of waves emitting from the horn so that distant zone patterns of energy distribution are obtained even in the near Zone.

Optimum energy concentration is obtained with this lens at the focal plane, a distance of four feet from the base of the lens. The energy concentration progressively and rapidly decreases from the focal plane directly towards and away from the lens, thereby indicating an energy concentration of small depth at the focal plane.

A large straight-edged metal plate coincident with the focal plane but at a distance from the lens axis does not effect the transmitted energy until the leading edge of the plate is advanced to within three inches from said axis. As the plate is advanced further towards the lens axis the transmitted energy rapidly declines until a nearly zero value is reached when the leading edge of said plate extends approximately three inches beyond the lens axis. A concentration of energy is thus defined to be within a three inch radius about the lens axis in the focal plane.

Because of the sharp focusing of R. F. wave energy brought about through the use of lenses constructed in accordance with this invention, it is possible to investigate the effects of objects and phenomena in restricted areas. The confusing influences of reflections from adjacent structures and the ground are effectively eliminated, for all practical purposes, thus permitting accurate indoor investigations to be performed.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. it is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. An R. F. energy antenna for focusing R. F. energy radiations on a distant restricted area, comprising a source for emitting R. F. energy radiations and a dielectric lens having an axis passing through said source, said lens comprising a series of homogeneous disks in coaxial stacked relation, said disks being of different diameters so as to generally form a paraboloidal solid having a base confronting said source and an axial cross-section defined by a curve represented by the following equation wherein a is the distance from the source to the base of said solid, f is the distance from said base to the distant focal area, It is the refractive index of said disks, x is a distance from said base along the axis of said solid, y is the radial dimension of the solid at the distance x and K is represented by K=va r +vf r where y is the radius of the base of said solid, said radius y being substantially less than said distance a.

2. A dielectric lens for focusing R. F. energy radiations emitting from a substantially point source on a distant restricted area, comprising a series of homogeneous disks in stacked relation, said disks being of difierent p K? diameters so as to generally form a paraboloidal solid having a base and an axial cross-section defined by a curve represented by the following equation +y +x (f +y wherein a is the distance from the point source to the base of said solid, 1 is the distance from the base of said solid to the distant focal area, n is the refractive index of said disks, x is a distance from said base along the axis of the solid, 32 is the radial dimension of the solid at the distance x and K is represented by =w/ +(y') f +(y) where y is the radius of the base of said solid, said radius y being substantially less than said distance a.

3. A dielectric lens for focusing R. F. energy radiations emitting from a substantialy point source on a distant restricted area, comprising an adjustable cylindrical housing open at two ends, a dielectric plate closing one end of said housing, and a series of homogeneous dielectric disks in coaxial stacked relation mounted within said housing and closing the other end of said housing, said disks being of different dameters so as to generally form a paraboloidal solid having a base and an axial cross-section defined by a curve represented by the following equation =1/ +y +\/(f +y wherein a is the distance from the point source to the base of said solid, is the distance from the base of said solid to the distant focal area, n is the refractive index of said disks, x is a distance from said base along the axis of the solid, y is the radial distance of the solid at the distance x and K is represented by K= /a +(y) /f +(y) where y is the radius of the base of the solid, said radius y being substantially less than said distance a.

4. An arrangement as set forth in claim 3 wherein, the housing comprises a cylindrical shell, means for supporting the lens in one end of said shell, a cylindrical can slidable on said shell to adjust the axial length of said housing to accommodate different size lenses, a dielectric plate closing the outer end of said can and abutting against the outermost disk of said lens, and an inwardly directed flange on the outer end of said can for minimizing phase error.

References Cited in the file of this patent UNITED STATES PATENTS 625,823 Zickler May 30, 1899 2,202,380 Hollmann May 28, 1940 2,547,416 Skellett Apr. 3, 1951 2,577,619 Kock Dec. 4, 1951 OTHER REFERENCES Article by A. H. Lince, FM-TV, vol. 12, issue 3, March 1952, pages 22-24.

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