US2415352A

US2415352A - Lens for radio-frequency waves

Info

Publication number: US2415352A
Application number: US532381A
Authority: US
Inventors: Harley A Iams
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1944-04-22
Filing date: 1944-04-22
Publication date: 1947-02-04
Anticipated expiration: 1964-02-04
Also published as: BE474864A; GB608949A

Description

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LENS Fox RADIO FnEQUENcY'wAvEs frequency lens disposed between surfaces;` and Fig. 10 is a sectional view of Patented Feb. 4, 1947 STATES i 2,415,352 Y Lens Foa RADIO-FREQUENCY WAVES Harley A. Iams, Princeton,

N. J., assigner to Radio Corporation of America, a corporation of Delaware 16 Claims.

'I'his invention relates to lenses for radio frequency waves and more particularly to such lenses in which the wave reflections from the surfaces are reduced.

It is known that radio frequency waves may be reflected and refracted. Reflections or refractions of radio waves occur when a wave travelling through a medium'reaches a, discontinuity in the medium or goes from one medium into another in which its velocity is different. If the surface or object in the path of the wave is composed of non-conductive material, both reflection and transmission with refraction may occur depending upon the angle between the direction of propagation and the reflecting surface, and upon other factors which do not require discussion here.

Lenses for focusing radio frequency waves have been made of materials such as pitch, paran-ln, and other insulators. The abrupt change from the dielectric constant of the medium of propagation to the dielectric constant of the lens produces an undesired reflection of the waves. The instant invention relates to the reduction of such undesired reflections.

One of the objects of the invention is to provide means for reducing the undesired reflection from the surface of a radio frequency lens. Another object is to provide an improved lens for radio waves of ultra high frequency. Another object is to provide a lens including reflection reducing surfaces whereby radio waves will be transmitted with less loss. An additional object is to provide an improved lens with a skeletonized surface for minimizing the reflection of radio frequency waves. l

The invention will be described by referring to the accompanying drawing in which Figs. 1 and 2 y are plan and section views, respectively, of one embodiment of theinvention; Figs. 3 and 4 are plan and section views, respectively, of a radio conductive members; Figs. 5 and 6 are plan and section views, respectively, of a biconvex lens including reflection reducing coatings; Figs. 7, 8 and 9 are plan, section views and a detailed section view, respectively, of a biconvex lens including a skeletonized a modification of the invention.

Referring to Figs. 1 and-2, a lens including a flat core I of wave refracting material such as Micalex is constructed with one curved surface 3 and one plane surface 5. By Way of example, the dimensions are shown for a lens having a focal length of 1'71/2 inches (measured from the straight edge) which was used to form a line image of a scene illuminated with radiation having a frequency of 24,000 megacycles. The core material has a dielectric constant of 6.1. Coatings '7 and 9, consisting of Lucite or polystyrene having a thickness 0f approximately one quarter wave length in the coating material, are cemented or otherwise secured to the effective surfaces 3, 5 of the lens. The dielectric constant of the coating material should be the square root of that of the core material; since the dielectric constant of polystyrene is 2.5, it is well suited for coating MicaleX. .In the case of the described lens, a substantial increase in transmission and a substantial reduction in undesired reflections were observed when radio waves of a frequency of 24,000 megacycles were applied. The image appears in the focal line represented by the dash line I I.

The lens described in the foregoing paragraph was inserted between a pair of conductive sheets I3, I5, preferably spaced by 1/2 wavelength or less. The sheets may be curved to form an aperture Il which may be disposed otherwise than in geometric alignmentwith the effective surface 'I of the lens. The conductive sheets guide the radio waves toward the lens and from the lens to the focal line II. The lens may be used for transmission by applying the wave energy at the open end of the sheets which may be shaped or bent to conform to the focal line.

In place of the section of plano-convex lens of Figs. 1 and 2, the invention may be applied to a biconvex lens as illustrated in Figs. 5 and 6 or '7 and 8. By way of example, a lens I9 was formed of Micalex. The following dimensions are given by way of example rather than limitation: diameter 14 inches, radius of each face 40 inches, and focal length 13 inches. The coatings 2| and 23 should be made of an insulator having a dielectric constant of the order of 2.5 and a thickness substantially equal to a quarter wave length of the applied radio frequency waves as measured in the insulator. Since this thickness of the reflection reducing coatings is based upon normal incidence, it follows that the coating will not minimize rreflections at all angles. For incidence other than normal, the ideal thickness is somewhat greater, being cos 0 where E is the dielectric constant of the material. In the case of Micalex, E=6.1; therefore the material to be removed equals 29% of the total surface layer. In the case of polystyrene, E=2.5 and the material to be removed equals 39% of the surface layer.

Another method of reducing the reection is to avoid abrupt changes in dielectric constants.

In a lens such as illustrated in Figs. 1, 2, 3 and 4.

the reflection may be reduced by tapering the effective surface as shown in Fig. 10. The ideal length t1 of the taper may -be determined from the equation where A2=wave length in air, 7q=wave length in material of lens, p.=index of refraction of material, and a=any integer. For polystyrene and radio Waves having a length of approximately 1/2 inch in air, the reiiection was found to fall steadily as d was increased from zero to about o inch, and to be negligible for any length over W inch.

In a lens as illustrated in Figs. 7 and 8, reection may also be reduced by the use of tapered grooves, instead of the holes as shown. In this case, there should preferably be two or more grooves per wave length, and the depth should be as indicated in the preceding paragraph.

Thus the invention has been described as an` improved lens for radio frequency waves. The refiections are reduced by applying a coating to the lens surface, or by skeletonizing the surface layer. The undesired reflections may also be reduced by tapering the eifective surfaces of the lens, i. e., by tapering the surfaces which reiiect the radio wavesinto the medium from which the waves have come.

I claim as my invention:

l. An ultra high frequency device consisting of an electric lens of a wave refracting material and l a reflection reducing surface disposed on at least one effective surface of said lens.

2. An ultra high frequency device consisting of an electric lens of a wave refracting material having substantially parallel sides and a reneetion reducing surface disposed on at least one effective surface of said lens.

3. An ultra high frequency device consisting of an electric lens of a Wave refracting material and a reiiection reducing coating disposed on at least one eiective surface of said lens.

4. An ultra high frequency device consisting of an electric lens of a wave refracting material having substantially parallel sides and a reliection reducing coating disposed on at least one effective surface of said lens.

5. An ultra high frequency device consisting of an electric lens of a wave refracting material and a surface having an effective thickness of an odd number of quarter wave lengths of said waves. disposed on at least one wave impinging surface of said lens whereby the reflection of said waves from said surfaces is reduced.

6. An ultra high frequency device consisting of an electric lens of a wave refracting material and a coating having an effective thickness of an odd number of quarter wave lengths of said waves in said coating, disposed on at least one wave impinging surface of s aid lens whereby the reection of said waves from said surfaces is reduced.

7. An ultra high frequency device consisting of an electric lens of a wave refracting material having at least one curved surface and a skeletonized layer disposed on said curved surface and having an eifective thickness of an odd integral number of quarter wave lengths of the applied electric waves whereby the reection of said waves from said surface is reduced.

8. An ultra high frequency wave focusing device consisting of an electric lens of a wave refracting material having substantially parallel sides and at least one curved surface and a pair of conductive elements disposed parallel to each other, parallel to said surfaces and extending beyond said lens and away from said curved surface..

9. A device of the character of claim 8 in which the pairs of conductive elements are curved in the region extending away from said curved surface.

10. A device of the character of claim 8 in which a reflection reducing coating is disposed on said curved surface.

11. An ultra high frequency device consisting of an electric lens of a wave refracting material having at least one curved surface and having parallel sides and means for reducing the index of refraction of the material on said curved surface and having an effective thickness of an odd number of quarter wave lengths of the applied electric waves whereby the reflection of said Waves from said surface is reduced.

12. An ultra high frequency wave focusing device consisting of an electric lens of a wave refracting material having substantially parallel sides and at least one curved surface and a pair of conductive elements disposed-parallel to each other, parallel to said surfaces and extending beyond said lens in at least one direction.

13. An ultra high frequency wave refracting device consisting of wave refracting material having substantially parallel sides ve or more wave lengths long perpendicular to the direction of motion of the wavefront placed between parallel conducting sheets separated by 7i/2 or less.

14. An ultra high frequency device consisting of on electric lens of a wave refracting material and a reiiection reducing surface, saidsurface including at least one projecting portion tapering along the axis of the lens.

15. An ultra high frequency device consisting of an electric lens of a wave refracting material and a reiiection reducing surface, said surface including at least one projecting portion -tapering along the axis of the lens in accordance with the equation in which d=the length of' the taper, 7i1=wave length in material of lens, M=wave length in air,

0f refl'aCiOn Of material, and G=ny REFERENCES CITED 16. radio wave device consisting of a mate- The following references are of record in the rial having a. characteristic dened by the equame 0f this Patent: tion n=\/e' where n=ndex of refraction and 5 UNITED STATES PATENTS e=delectric constantl and a surface layer having an eiective depth substantially equal to an odd Ilggeos Zwolgyag Jungjzaelgm integral number of quarter wave lengths of the 2273447 om Feb 17' 1942 applied Waves and in which e'=n" less than 4 10 2'361'589 Berufen-tee-{5-:2::0:11:31'1944` at the frequency of the applied waves. 2:3311716 Nadeauet al.' oct. 12: 1944 HARLEY A, IAMs. 2,240,157 Gagnon, et a1. Apr. 2,9, 1941