US3241147A

US3241147A - Antenna utilizing intermediate cuspate reflector to couple energy from feed to main reflector

Info

Publication number: US3241147A
Application number: US330701A
Authority: US
Inventors: Samuel P Morgan
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1963-12-16
Filing date: 1963-12-16
Publication date: 1966-03-15
Anticipated expiration: 1983-03-15

Description

March 15, 1966 s. P. MORGAN ANTENNA UTILIZING INTERMEDIATE CUSPATE REFLEGTOR T0 COUPLE ENERGY FROM FEED TO MAIN REFLECTOR 196s Filed Dec. 16,

N GP* /A/l/EA/TOA S. MORGAN 7' TORA/EV March 15, 1966 s. P. MORGAN ANTENNA UTILIZING INTERMEDIATE CUSPATE REFLECTOR TO COUPLE ENERGY FROM FEED TO MAIN REFLECTOR 2 Sheets-Sheet 2 Filed Deo. 16, 1963 v .MSK

United States Patent O ANTENNA UTILIZING INTERMEDIATE CUSPATE REFLECTOR T COUPLE ENERGY FROM FEED TO MAIN REFLECTOR Samuel P. Morgan, Morristown, NJ., assignor to Bell Telephone Laboratories, Incorporated, New York, N.Y., a corporation of New York Filed Dec. 16, 1963, Ser. No. 330,701

18 Claims. (Cl. 343-781) This invention relates to antenna systems, and more particularly, to improvements in Cassegrainian and Gregorian antenna systems.

Recently antennas employing Cassegrainian .and Gregorian telescope principles have been found to be very useful in radio communication systems. yEach antenna, taking the name of the optical telescope after which it is patterned, has a main, concave paraboloidal reflector with a feed element at its vertex and a smaller, intermediate reflector located in front of the main reflector to couple radio waves between the main reflector and the feed element. The Cassegrainian .antenna employs an intermediate reflector having a convex surface, usually hyperboloidal, situated between the main reflector and its focus, and theGregorian antenna employs an intermediate reflector having a concave surface, usually ellipsoidal, so situated that the focus of the main reflector lies between the main and intermediate reflectors. Among the advantages of Cassegrainian and Gregorian an-tenna arrangements i-s the elimination of the transmission line found necessary in conventional feed-at-the-focus paraboloidal antennas to interconnect the terminal equipment to the feed element.

A problem in the design of Cassegrainian and Gregorian antennas (visualizing the antenna as transmitting) is reflection back into the feed element of energy that is radi-ated from the feed element and that is reflected from the intermediate reflector and intended for transmission to the main reflector. This phenomenon, hereafter called back reflection, has the effect of creating an impedance mismatch for energy coupled Ibetween the feed element and the reflectors when the antenna is receiving as well as when it is tr-ansmitting. Although an impedance mismatch can quite easily be elimina-ted for waves of a single frequency or a narrow band of frequencies, it is not possible to eliminate impedance mismatch over a broadband of frequencies. As a result the transmission characteristics between the feed element and the reflectors are frequency dependent outside of avery restricted band of frequencies.

Back reflection has been avoided in one prior art antenna .arrangement by mounting a small cone on the surface of the intermediate reflector to .scatter the energy that would otherwise be reflected back into the feed. A scattering cone has several adverse effects on the electrical characteristics of an antenna system. First, it may unduly increase the wide-angle side lobes of the antenna radiation pattern. Wide-angle `side lobes are particularly undesirable in receiving applications where it is important that the antenna be insensitive to noise coming from directions far removed from the direction of the main beam. Second, the scattered energy is not utilized and therefore the potential gain of the antenna system is not fully realized. This second effect is especially pronounced in antennas in which the feed element is a so-called near field feed, disclosed in a patent application of D. C. Hogg, Serial No. 143,078, filed October 5,1961, and .assigned to the assignee of this invention. In this case, the feed aperture is very large so the potential lback reflection is large and the cone scatters an inordinate amount of energy.

It is therefore the obje-ct of this invention to avoid back reflection in a Cassegrainian or Gregorian antenna system .3,24l,l47 Patented Mar. 15, 1966 ICC without adversely affecting the antennas electrical characteristics.

In accordance with the above object an antenna system is provided havin-g -a large, concave main reflector, a feed element located near the main reflector, and a small intermediate reflector having a cuspate reflecting surface, the intermediate reflector coupling waves Ibetween the feed element and the main reflector. As employed in this specification, the term cuspate surface is a surface that projects two curved lines meeting at an apex onto at least one plane intersecting the surface. The cuspate shape of the intermediate reflector permits coupling between it and the main reflector of a wave having -a hole in its wave front. The hole is so situated and is of such size that the inside boundary of the wave front irradiated from the cuspate reflector circumscribes the effective feed aperture when the Wave front is impinging upon the main reflector. The effective feed -aperture is taken to mean the area ofthe feed element over which waves impinging upon the feed element are received by it, regardless of whether the feed element -actually has a physical aperture through which waves are received. As a result, no back reflection into the feed element takes place and no measures to correct impedance mismatch are necessary. The shape of the surface of the main reflector is appropriately deformed from the convention-al parabolic shape so that antenna system accommodates plane Waves at the aperture of the main reflector.

The intermediate reflector can take the shape of the inside surface of a cusp having concave sides, the outside surface of a cusp having concave sides, `the inside surface of a cusp having convex sides, or the outside surface of a cusp having convex sides, depending upon the manner in which energy is coupled between the feed element and the main reflector.

These and other features of the .invention will become more apparent from consideration of the following detailed description taken in conjunction with the drawings in which FIGS. 1 through 4 show antenna arrangements according to the invention, each having a different type of cuspate intermediate reflector. For ease of visualization the antenna arrangements shown in lFIGS. 1 through 4 will be described in terms of their use as transmitting antennas. They function in a reciprocal fashion in reception of electromagnetic waves and are equally advantageous for this purpose.

FIG. l discloses a Cassegrainian antenna comprising a feed element 6, an intermediate reflector 10, and a main reflector 8 whose surfaces can be defined in terms of a cylindrical coordinate system having an axial coordinate designated z, a radial coordinate designated r, and an angle 0 that a plane passing through the z axis and the point to be defined makes with the plane of the drawing. It is assumed that the intermediate reflectors and the main reflectors of all the embodiments exhibit rotational symmetry around an axis, represented by the z axis, in FIG. l. Thus, the surfaces of reectors 8 and 10 are formed by rotating the curves representing reflectors 8 and 10 about the z axis and the coordinate angle 0 can be disregarded because the coordinate r and z values of a surface are the same for all. values of 0. Thus, the usual way of forming the reflectors of a directional antenna system results in a radiation pattern for the antenna system which is directional in any plane passing through the z axis. Feed element 6 is of the so-called near field type disclosed in the above-mentioned D. C. Hogg application. It is a conventional horn-reflector that accommodates waves having a plane wave front and that has an opening so designed that intermediate reflector 10 lies in its near eld. mission, electromagnetic waves emanating from terminal equipment block 2 are coupled to feed element 6 by' 3 means of a wave guide section 4. Feed element 6 directs the waves in a plane wave front toward inter mediate reflector 10.

The path of electromagnetic energy between feed element 6 and the far eld of the antenna is illustrated by means of a ray diagram. For the purpose of the following qualitative explanation of the mode of operation and the later quantitative analysis, it is assumed that the behavior of electromagnetic waves can be explained in terms of geometric optics, because the effects of diffraction on the antenna characteristics the invention seeks to improve are small. The center of the wave emanating from feed element 6, represented by ray A, after impinging upon intermediate reflector 10, divides into segments B and B', which impinge upon main reflector 8 at the inside extremity of its geometrically illuminated surface. After reflection from main reflector 8, resulting rays C and C' are radiated into the antenna far field. Rays D andY D', showing the boundaries of the wave radiated from feed element 6, impinge upon the geometrically illuminated extremity of intermediate reflector 10 and are reflected therefrom as rays E and E'. Rays E and E illuminate main reflector 8 at the outside extremity of its geometrically illuminated surface and are reflected therefrom into the far field of the antenna as rays F and F. Translation of the ray diagram into three dimensional terms indicates that intermediate reflector 10, having the shape of the outside surface of a convex cusp, transforms the plane wave front that emanates from feed element 6, having a circular surface, into a curved wave front, having a ring-shaped surface the inside and outside circumferences of which expand as the wave `front progresses from intermediate reflector 10 to main reflector 8. By the time that the curved wave front has reached main reflector 8, the inside circumference of its surface has expanded to such a size that it circumscribes the effective aperture of feed element 6. As a result, substantially no energy emanating from feed element 6 is reflected back into it. All the energy is used to illuminate main reflector 8, and thus contributes to the antenna system gain. Main reflector 8 is correspondingly deformed from the conventional paraboloidal shape to effect transformation of the curved wave front impinging upon it into an emergent plane wave front.

The remaining embodiments of the invention, shown in FIGS. 2 through 4, differ from the arrangement in FIG. 1 only in the shape of the intermediate and main reflectors. In considering the modes of operation of these arrangements, the same letters are used to identify corresponding rays in the ray diagrams in order to facilitate comparis-on of the ray paths of the various embodiments.

FIG. 2 discloses a Gregorian antenna comprising a main reflector 12 and an intermediate reflector 14 having the shape of the inside surface of a convex cusp. In this arrangement, unlike the Cassegrainian antenna of FIG. 1, rays emanating from feed element 6 are reflected from one side of intermediate reflector 14 to the opposite side of main reflector 12. Thus, rays D and D' reflect lfrom the geometrically illuminated extremity of intermediate reflector 14 as rays E and E', which cross each other. As explained in connection with FIG. 1, cuspate intermediate reflector 14 transforms the plane wave `front having a circular surface that emanates from feed element 6 into a curved wave front having a ring-shaped surface whose inside circumference circumscribes the effective aperture of feed element 6.

In FIG. 3 an antenna arrangement is shown comprising a main reflector 16 and an intermediate reflector 18 having the shape of the inside surface of a concave cusp. This arrangement can be considered to be a crossed Cassegrainian antenna because the rays impinging upon one side of the surface of intermediate reflector 18 from feed element 6 impinge upon the other side of the surface of main reflector 16. Also, rays E and E', reflected yfrom the geometrically illuminated extremity of intermediate reflector 18, impinge upon the geometrically illuminated inside edge of main reflector 16. If the two reflectors are chosen to have practical proportions, it may easily occur that intermediate reflector 18 blocks some energy impinging upon it for transmission to main reflector 16. This drawback can be seen in FIG. 3 by tracing the path of ray A, which ought to reflect from intermediate reflector 18 as shown by dashed lines B, C, B', and- C', but which is prevented from so doing by the shape of intermediate reflector 18. As a result, illumination of main reflector 16 may be somewhat inefficient and the gain of the antenna system may thus be smaller than in the other arrangements.

In FIG. 4 an antenna arrangement is shown compr1s ing a main reflector 2f) and an intermediate reflector 22 having the shape of the outside surface of a concave cusp. This arrangement can be considered to be an uncrossed Gregorian antenna because the rays impmgmg upon one side of the surface of intermediate reflector 22 from feed element 6 impinge upon the same side of the surface of main reflector 20. Rays E and E', reflected from the geometrically illuminated extremity of intermediate reflector 22, impinge upon the geometrically 1lluminated inside edge of main reflector 20, and rays B and B', reflected from the apex of intermediate reflector 22, impinge upon the geometrically illuminated outside edge of main reflector 20. l

Equations will now be developed which define the in-j termedate reflector and main reflector shapes requiredV to satisfy the ray diagrams shown in the drawings. For the purpose of this analysis, it is assumed -that geometnc optics describes the electromagnetic field established between feed element 6 and the aperture of the main reflector and that a wave having a plane wave front across the aperture of the main reflector is desired. In this analysis terms are defined as follows:

r1 is a variable defining the r coordinate of the intermedi! ate reflector; n

Z1 is a variable defining the z coordinate of the intermediate reflector;

r2 is a variable defining the r coordinate of the main reflector;

z2 is a variable defining the z coordinate of the main reflector;

c1 is a constant representing the z coordinate of the apex of the intermediate reflector;

b1 is a constant representing the r coordinate of the geometrically illuminated extremity of the intermediate reflector surface also coinciding with the r coordinate of the edge of the aperture of feed element 6;

dl is a constant representing the z coordinate of the geometrically illuminated extremity of the intermediate reflector surface;

a2 is a constant representing the r coordinate of the geo-r metrically illuminated inside edge of the main reflector surface;

b2 is a constant representing the r coordinate of the geo# metrically illuminated outside edge of the main reflector surface; and

d2 is a constant representing the z coordinate of the geometrically illuminated outside edge of the main reflector surface.

Reference is now made to FIG. 1, in order to formulate the equations defining the reflectors shown therein. Because every ray path from feed element 6 to an arbitrarily chosen plane lying perpendicular td the Z axis must be the same in order to produce a plane, Wave front across the aperture of the main reflector,

C=C1+(fl22+C12)`/ (2) The power flow per unit area at the intermediate reflector, S1(r1), depends upon the radiation pattern of feed element 6 and the power flow per unit area at the main reflector, S2(r2), is chosen to produce the radiation pattern specified for the antenna system. The physical law of conservation of power makes possible formulation of the relationship between r1 and r2.

where S1 and S2 have been normalized relative to one another so that Inspection of the ray diagram of FIG. 1 indicates that tan 2 =tr ZF Z2 (5) where p is the angle of incidence andreection of a typical ray at each reflector (go is measured relative to a line normal to the reliector in question). By solving Equation 1 for zf-z2, substituting the expression for z1iz2 into Equation 5, and employing the trigonometric identity between the tangent of an angle and the tangent of its double angle, one obtains where dl'g Integrating Equations 7 results in 1 r 21(7'1) :C1-2li@ @L 1T2 (P1) dpi 2.. 2 r2 T2 2O@ -f-l/.z cada 8 where the relationship between r1 and r2 is given by Equation 3 and C is defined by Equation 2. Thus, given the antenna dimensions c1, b1, a2, and b2, the power flow at the intermediate reflector 81(11), and the power flow at the main reliector S2(r2), Equations 2, 3, and 8 permit computations of the surfaces of the intermediate reflector and main reflector 8. If a2 is made equal to zero and if S1 and S2 are made constant independent of position, Equations 8 reduce to the equations defining the conventional near-lield Cassegrainian antenna, Le., a paraboloidal main rellector confocal with a paraboloidal intermediate reflector. It is the specilication of a2 to be nonzero that accounts for the development of the cusp in the intermediate reflector and it is the specification of. a2 to be at least as large as the radius of the effective aperture of the feed element that accounts for elimination of back reection altogether.

Frequently applications call for a constant power flow at the reflector surfaces. In this case,

According to Equations l0 the intermediate' reflector ought to exhibit a minute area of concavity very near the apex of the cusp. This inflection in the equations can be ignored in the construction of physical embodiments of intermediate reflector 10 because the concavity only affects the geometrically determined power ow after reflection from intermediate reliector 10 at the inside boundary of the wave front, where the elfects of diffraction predominate in any event.

By a parallel analysis, the surfaces of main reflector 12 and intermediate reliector 14 of the Gregorian antenna shown in FIG. 2 can be expressed as where Equation 3 gives the relationship between r1 and r2 and C is defined by Equation 2. In the special case of uniform power flow at the reliector surfaces, Equations 1l reduce to (l2) l Z2=l fz2ll22) +S21/272 7`22'*a22)1/2 S21/2,122 cosh-1 112 Using a parallel analysis for the so-called crossed Cassegraiuian arrangement of FIG. 3 the shapes of main reflector 16 and intermediate reflector 18 are determined to be C=d1+[(b1+2)2+d12]/ (14) and the relationship between r1 and r2 is given by L Smpapldwfm :smaad/12 15 In the special case of uniform power liow at the reflectors, Equations 13 reduce to which delines the shapes of main reflector 20 and intermediate reflector 22 where and Equation defines the relationship between r1 and r2. Uniform power flow at the reflector surfaces of this arrangement causes Equations 17 to reduce to Both the main reflector and the intermediate reflector shown in FIGS. 1, 2, and 4 are extended in area beyond the geometrically illuminated surface to reduce spillover, which in practice occurs due to diffraction. Most conveniently the shapes of the reflectors shown in FIGS. 1 and 2 can be extended in accordance with the equations defining their shapes within the geometrically illuminated region. As the equations defining the shapes of the reflectors of FIG. 4 do not produce real solutions outside of the geometrically illuminated region the shapes of these reflectors can conveniently be extended by extrapolation. The shape of the main reflector between the geometrically illuminated inside edge, a2, and the opening of feed element 6 can be any convenient shape.

It is possible to practice the invention using intermediate and main reflectors which have other than rotationally symmetric shapes, depending upon the type of antenna radiation pattern that is desired. For example, main and intermediate reflectors having a cylindrical shape might be employed if a radiation pattern having a directional characteristic in only one plane is desired. Such a reflector is formed by translating lcurves similar to those representing the reflectors in the drawings along an axis perpendicular to the plane `of the drawings. The precise reflector shapes are found by an Ianalysis parallel to the analysis employed for reflectors having rotational symmetry.

Although the invention provides a large improvement whn used with a near field feed, as described, the conventional horn feed element that accommodates waves having spherical wave fronts can also be employed. In this case, the intermediate reflectors remain cuspate, but the new equations exactly dening the intermediate and main reflector surfaces must be derived by means of an analysis parallel to that employed above in the cases of a feed element accommodating waves having plane wave fronts.

What is claimed is:

1, An antenna system accommodating electromagnetic waves of predetermined wavelength and having a plane wave front and comprising a concave reflector and cuspate reflector situated on a common axis, said cupsate reflector facing the concave surface of said concave reflector, and -a feed element situated near the intersection of said axis and said concave reflector, said feed element having an aperture facing toward said cuspate reflector and accommodating electromagnetic waves having a plane wave front, said aperture having a diameter of at least several `of said wavelengths, and said cupsate reflector having a diameter at least as large as the aperture of said feed element, said cuspate reflector thereby serving to couple electromagnetic waves `between said feed element and said concave reflector.

2. The antenna system of claim l in which said cuspate reflector has the shape of the outside surface of a convex cusp.

3. The antenna system of claim 1 in which said cuspate reflector has the shape of the inside surface of a convex cusp.

4. The antenna system of claim 1 in which said Icuspate reflector has the shape of the inside surface of a `concave cusp.

5. The antenna system of claim 1 in which said cuspate reflector has the shape of the outside surface of a concave cusp.

6. An antenna system accommodating electromagnetic waves of predetermined wavelength and having a plane wave front and comprising a main reflector situated on an axis, a feed element situated near the intersection of said axis and said main reflector, said feed element having an aperture facing away from said main reflector along said axis and accommodating electromagnetic waves having a plane wave front, said aperture having a diameter of at least several of said wavelengths, and an intermediate reflector situated on said axis and facing the reflective surface of said main reflector, said intermediate reflector having a cuspate surface with a diameter at least as large as the diameter of the aperture of said feed element such that when the antenna system is transmitting the inside circumference of the area of the wave front traveling from said intermediate reflector to said main reflector circumscribes the effective aperture of said feed element when said wave front illuminates said main reflector.

7. The antenna system of claim 6 in which said intermediate reflector has the shape of the outside surface of a convex cusp.

8. The antenna system of claim 6 in which said intermediate reflector has the shape of the inside surface of a convex cusp.

9. The antenna system of claim 6 in which said intermediate reflector has the shape of the inside surface of a concave cusp.

10. The antenna system of claim 6 in which said intermediate reflector has the shape of the outside surface of a COIlCaVe CllSp.

lll. An antenna system comprising a concave reflector situated on an axis, a smaller cuspate reflector situated on said axis and facing the concave surface of said concave reflector, and a feed element situated near the intersection of said axis and said concave reflector, said feed element facing toward said cuspate reflector and said cuspate reflector serving to couple electromagnetic Iwaves between said feed element and said concave reflector, in which said reflectors are situated on mutually perpendicular r and z axes, said feed element accommodates electromagnetic waves having plane wave fronts, and said reflectors are shaped with rotational symmetry around the z axis such that they project on the plane of the r and z axes curves defined by Z1 is a variable defining the z coordinate of said cuspate reflector,

r1 is a variable defining the r coordinate of said cuspate reflector,

Z2 is a variable defining the z coordinate of said concave reflector,

r2 is a variable defining the r coordinate of said concave reflector,

c1 is a constant representing the z coordinate of the apex of said cuspate reflector,

a2 is a constant representing the r coordinate of the geometrically illuminated inside edge of said concave reflector,

S1(p1) is the function expressing power flow per unit area at said cuspate reflector, and

9 S2022) is the function expressing power flow per unit area at said concave reflector, a2 being larger than one-half of the diameter of the effective aperture of said feed element.

12. An antenna system comprising a concave reflector situated on an axis, a smaller .cuspate reflector situated on said axis and facing the concave surface of said concave reflector, and a feed element situated near the intersection of said axis and said concave reflector, said feed element facing toward said cuspate reflector and said cuspate reflector `serving to couple electromagnetic waves between said feed element and said concave reflector, in which `said reflectors are situated on mutually perpendicular r and z axes, said feed element accommodates electromagnetic waves having plane wave fronts, and said reflectors are shaped ywith rotational symmetry around the z axis such that they project on the plane of the r and z axes curves defined by Z1 is a variable defining the z coordinate of said cuspate reflector,

r1 is a variable defining the r coordinate of said cuspate reflector,

z2 is a variable defining the z coordinate of said concave reflector,

r2 is a variable defining the r coordinate of said concave reflector,

a2 is a constant representing the r coordinate of the geo metrically illuminated inside edge of said concave reflector,

S2022) is the function expressing flow per unit area at said concave reflector, a2 being larger than one-half the diameter of the effective aperture of said feed element.

13. An antenna system comprising a concave reflector situated on an axis, a smaller cuspate reflector situated on said axis and facing the concave surface of said concave reflector, and a feed element situated near the intersection of said axis and said concave reflector, said feed element facing toward said cuspate reflector and said cuspate reflector serving to couple electromagnetic waves between said feed element and said concave reflector, in which said reflectors yare situated on mutually perpendicular r and z axes, said feed element accommodates electromagnetic waves having plane wave fronts, and said reflectors are shaped with rotational symmetry around the z axis such that they project on the plane of the r and z axes curves defined by zl is a variable defining the z coordinate of said cuspate reflector,

r1 is a variable defining the r coordinate of said cuspate reflector,

z2 is a variable defining the z coordinate of said concave reflector,

r2 is a variable defining the r coordinate of said concave reflector,

b1 is a constant representing the r coordinate of the geometrically illuminated extremity of said cuspate rellector,

d1 is `a constant representing the z coordinate of the geometrically illuminated extremity of said cuspate reflector,

S2(p2) is the function expressing power flow per unit area at said concave reflector, a2 being larger than one-half the diameter of the effective aperture of said feed element.

14. An antenna system comprising a concave reflector situated on an axis, a smaller cuspate reflector situated on said axis and facing the concave surface of said concave reflector, and a feed element situated near the intersection of said axis and said concave reflector, said feed element facing toward said cuspate reflector and said cuspate reflector serving to couple electromagnetic waves between said feed element and said concave reflector, in which said reflectors are situated on mutually perpendicular r and z axes, `said feed element accommodates electromagnetic waves having plane wave fronts, and said reflectors are shaped with rotational symmetry around the z axis such that they project on the plane of the r and z axes curves ldefined by Z1 is a variable defining the z coordinate of said cuspate reflector,

r1 is a variable defining the r coordinate of Said cuspate reflector,

r2 is a variable defining the z coordinate of said concave reflector,

b1 is a constant representing the r coordinate of the geometrically illuminated extremity of said cuspate reflector,

d1 is a constant representing the z coordinate of the geometrically illuminated extremity of said cuspate reflector,

a2 is a -constant representing the r coordinate of the geometrically illuminated inside edge of said concave reflector,

S2(p2) is the function expressing power flow per unit` area at said concave reflector, a2 being larger than one-half the diameter of the effective aperture of said feed element.

15. An antenna system comprising a main reflector situated on an axis, a feed element situated near the intersection of said axis and said main reflector, said feed element facing away from said main reflector and pointing along said axis, and an intermediate reflector situated on said axis and facing the rellectivesurface of said main reflector, said intermediate reflector having a cuspate surface such that when the antenna system is transmitting the inside circumference of the area of the wave front traveling from said intermediate reflector to said main reflector circumscribes the effective aperture of said feed element lwhen it illuminates said main reflector, in which said reflectors are situated on mutually perpendicular r and z axes, said feed element accommodates electromagnetic waves having plane wave fronts, and said reflectors are shaped with rotational symmetry around the z axis such that they project on the plane of the r and z axes curves defined by where Z1 is a variable defining the z coordinate of said intermediate reflector,

ri is a variable defining the r coordinate of said intermediate reflector,

z2 is a variable defining the z coordinate of said main reflector,

r2 is a variable defining the r coordinate of said main reflector,

c1 is a constant representing the z coordinate of the apex of said intermediate reflector,

b1 is a constant representing ther coordinate of the geometrically illuminated extremity of said intermediate reflector,

a2 is a constant representing the r coordinate of the geometrically illuminated inside edge of said main reflector, and

b2 is a constant representing the r coordinate of the geometrically illuminated edge of said main reflector, a2 being larger than one-half the diameter of the effective aperture of said feed element and the power flow per unit area at said intermediate reflector being constant.

16. An antenna system comprising a main reflector situated on an axis, a feed element situated near the intersection of said axis and said main reflector, said feed element facing away from said main reflector and pointing along said axis, and an intermediate reflector situated on said axis and facing the reflective surface of said main reflector, said intermediate reflector having a cuspate surface such that when the antenna system is transmitting the inside circumference of the area of the wave front traveling from said intermediate reflector to said main reflector circumscribes the effective aperture of said feed element when it illuminates said main reflector, in which said reflectors are situated on mutually perpendicular r and z axes, said feed element accommodates electromagnetic waves having plane wave fronts. and said reflectors are shaped with rotational symmetry around the three axes such that they project on the plane of the r and z axes curves defined by where 0:01 l (M24-C12) 1/2 blz Z1 is a variable defining the z coordinate of said intermediate reflector,

rl is a variable defining the r coordinate of said intermediate reflector,

z2 is a variable defining the z coordinate of said main reflector,

r2 is a variable defining the r coordinate of said main reflector,

b1 is a constant representing the r coordinate of the geometrically illuminated extremity of said intermediate reflector,

b2 is a constant representing the r coordinate of the geometrically illuminated outside edge 0f said main reflector, a2 being larger than one-half the diameter of the effective aperture of `said feed element and the power flow per unit area at said intermediate reflector being constant.

17. An antenna system comprising a main reflector situated on an axis, a feed element situated near the intersection of said axis and said main reflector, said feed element facing away from said main reflector and pointing along said axis, and an intermediate reflector situated on said axis and facing the reflective surface of said main reflector, said intermediate reflector having a cuspate surf-ace such that when the antenna system is transmitting the inside circumference of the area of the wave front traveling from said intermediate reflector to said main reflector circumscribes the effective aperture of said feed element when it illuminates said main reflector, in which said reflectors are situated on mutually perpendicular r and z axes, said feed element accommodates electromagnetic waves having plane wave fronts, and said reflectors are shaped with rotational symmetry around the z axis such that they project on the plane of the r and z axes curves defined by Z1 is a variable defining the z coordinate of said intermediate reflector, r1 is a variable defining the r coordinate of said intermediate reflector, z2 is a variable defining the z coordinate of said main reflector, r2 is a variable defining the r coordinate of said main reflector,

d1 is a constant representing the z coordinate of the geometrically illuminated extremity of said intermediate reflector,

a2 is a `constant representing the r coordinate of the geometrically illuminated inside edge of said main reflector, and

b2 is a constant representing the r coordinate of the geometrically illuminated outside edge of said main reflector, a2 being larger than one-half the diameter of the effective aperture of said feed element and the power flow per unit area at said intermediate reflector `being constant.

18. An antenna system comprising a main reector situated on an axis, a feed element situated near the intersection of said axis and said main reflector, said feed element facing away from said main reflector and pointing along saidaxis, and an intermediate reflector situated on said axis and facing the reflective surface of said main reflector, said intermediate reflector having a cuspate surface such that when the antenna system is transmitting the inside circumference of the area of the wave front traveling from said intermediate reflector to said main reflector circumscribes the effective aperture of said feed element when it illuminates said main reflector, in which said reflectors are situated on mutually perpendicular r and z axes, said feed element accommodates electromagnetic waves having plane wave fronts, and said reflectors are shaped with rotational symmetry around the z `axis such that they project on the plane of the r and z axes curves defined by zl is a variable defining the z coordinate of said intermediate reflector, 1'1 is a variable defining the r coordinate of said intermediate reflector, z2 is a variable defining the z coordinate of said main reflector, r2 is a Variable defining the r coordinate of said main reflector, b1 is a constant representing the r coordinate of the geometrically illuminated extremity of said intermediate reflector, d1 is a const-ant representing the z coordinate of the geometrically illuminated extremity of said intermediate reflector, a2 is a constant representing the r coordinate of the geometrically illuminated inside edge of said main reflector, and b2 is a constant representing the r coordinate of the geometrically illuminated outside edge of said main reflector, a2 being larger than one-half the diameter of the effective aperture of said feed element and the power flow per unit area at said intermediate reflector being constant.

References Cited by the Examiner UNITED STATES PATENTS 2,477,694 8/1949 Gutton 343--837 HERMAN KARL SAALBACH, Primary Examiner.

Claims

1. AN ANTENNA SYSTEM ACCOMMODATING ELECTROMAGNETIC WAVES OF PREDETERMINED WAVELENGTH AND HAVING A PLANE WAVE FRONT AND COMPRISING A CONCAVE REFLECTOR AND CUSPATE REFLECTOR SITUATED ON A COMMON AXIS, SAID CUPSATE REFLECTOR FACING THE CONCAVE SURFACE OF SAID CONCAVE REFLECTOR, AND A FEED ELEMENT SITUATED NEAR THE INTERSECTION OF SAID AXIS AND SAID CONCAVE REFLECTOR, SAID FEED ELEMENT HAVING AN APERTURE FACING TOWARD SAID CUSPATE REFLECTOR AND ACCOMMODATING ELECTROMAGNETIC WAVES HAVING A PLANE WAVE FRONT, SAID APERTURE HAVING A DIAMETER OF AT LEAST SEVERAL OF SAID WAVELENGTHS, AND SAID CUSPATE REFLECTOR HAVING A DIAMETER AT LEAST AS LARGE AS THE APERTURE OF SAID FEED ELEMENT, SAID CUSPATE REFLECTOR THEREBY SERVING TO COUPLE ELECTROMAGNETIC WAVES BETWEEN SAID FEED ELEMENT AND SAID CONCAVE REFLECTOR.