US3623114A

US3623114A - Conical reflector antenna

Info

Publication number: US3623114A
Application number: US848776A
Authority: US
Inventors: Arthur F Seaton
Original assignee: National Aeronautics and Space Administration NASA
Current assignee: National Aeronautics and Space Administration NASA
Priority date: 1969-08-11
Filing date: 1969-08-11
Publication date: 1971-11-23
Anticipated expiration: 1988-11-23

Abstract

A conical reflector antenna is disclosed with a feed that is either a close approximation to a line source or a cylindrical structure which is electrically equivalent to a line source. Nearly complete control of its aperture illumination function is achieved by exciting various amounts of the line source or cylindrical feed. In addition to beamwidth control of a collimated beam that is linearly or circularly polarized, phasesensing monopulse operation is made possible by dividing a cylindrical feed structure into three equal sectors and incorporating conventional directional couplers for obtaining the difference between received signals from sectors on either side of a vertical axis for azimuth error, and the weighted difference between the sum of those signals and the signals from the third sector for an elevation error.

Description

United States Patent inventors T. 0. Paine Administrator of the National Aeronautics and Space Administration with respect to an invention of; Arthur F. Seaton, Palos Verdes Estates,

Calif. Appl. No. 848,776 Filed Aug. 11, 1969 Patented Nov. 23, 1971 CONlCAL REFLECTOR ANTENNA 13 Claims, 13 Drawing Figs.

References Cited UNITED STATES PATENTS 8/1965 Schell 3,308,468 3/1967 Hannan .i 343/779 FOREIGN PATENTS 801,886 9/1958 Great Britain 343/840 Primary Examiner-Eli Lieberman Attorneys-G. T. McCoy, .1. H. Warden and Monte F. Mott ABSTRACT: A conical reflector antenna is disclosed with a feed that is either a close approximation to a line source or a cylindrical structure which is electrically equivalent to a line source. Nearly complete control of its aperture illumination function is achieved by exciting various amounts of the line source or cylindrical feed. in addition to beamwidth control of a collimated beam that is linearly or circularly polarized, phase-sensing monopulse operation is made possible by dividing a cylindrical feed structure into three equal sectors and incorporating conventional directional couplers for obtaining the difference between received signals from sectors on either side of a vertical axis for azimuth error, and the weighted difference between the sum of those signals and the signals from the third sector for an elevation error PATENTEBNHV 2 MI I 3,623,114

SHEET 1 [IF 3 ARTHUR F. SEATON INVIZNTOR,

ATTORNEYS PATENTEDNUV 23 I971 SHEET 2 BF 3 F. SEATON INVIZN'I'OR.

% Wu/MN ATTORNEY ARTHUR PATENTEUuuv 23 1971 3,623.1 14

SHEET 3 [1F 3 3o 33 FIGT To P TOP V A H Zxy Exyz Av ARTHUR F. SEATON INVEN'IOR.

ATTORNEYS CONHCAL REFLECTOR ANTENNA ORIGIN OF THE INVENTION BACKGROUND OF THE INVENTION This invention relates to a reflector-type antenna and more particularly to the combination of a new reflector and means for feeding the reflector to control its aperture illumination function, and for phase-sensing monopulse operation, dividing the feed into three sectors.

In the past parabolic reflectors have been employed for antennas with considerable success, but proper focusing requires a point source feed so that control over the amplitude of the aperture illumination function is severely limited. It would be desirable to have a reflector-type antenna having all of the advantages of a parabolic reflector antenna with the additional advantage of control over its aperture illumination function so that design for maximum gain of low sidelobes may be accomplished as desired.

SUMMARY OF THE INVENTION In accordance with the present invention, a reflector-type antenna isprovided with a conical reflector and a feed which is either a close approximation to a line source or some structure which is electrically equivalent to a line source such that any desired distribution of the feed can be provided to reflect a beam out through the aperture of the cone in a predictable manner. Beamwidth is then easily changed by exciting various amounts of the line source feed, with a wide beamwidth generated when only a small portion of the feed near the apex of the cone is excited and progressively narrower beams generated as additional sections of the feed successively further from the apex are excited. The narrowest beam results when the full feed is excited. For phase-sensing monopulse operation in a tracking system, the feed is divided into three 120 sectors, two sectors A and B on opposite sides of a given axis and the third sector C centered about that axis. A directional coupler combines signals received by sectors A and B to provide an error signal about that given axis in a difference arm. The sum of the signals received by the sectors A and B are then combined in a second directional coupler to provide an error signal about a second axis perpendicular to the given axis, with proper attenuation of the sum (A+B) such that the difference [(A+B)C] is equal to zero when the target is on the second axis.

DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates schematically a conical reflector antenna with its reflector in section through the axis thereof.

FIG. 2 illustrates schematically a conical reflector antenna for a half-angle of 45 at the vertex thereof.

FIGS. 3a to 3d illustrates diagrammatically the illumination of a conical reflector aperture with a linearly polarized line source i'eed arrangement.

FIG. 4 illustrates schematically a front view of a conical reflector antenna with a linearly polarized cylindrical source feed arrangement.

FIG. 5 illustrates a side view of the cylindrical source feed arrangement of FIG. 4 with the entire circumference of the cylinder thereof projected into the plane of the paper.

FIG. 6 illustrates schematically the arrangement of sections of a cylindrical source feed in a conical reflector antenna, each section being adapted for selective excitation to switch beamwidth.

FIG. 7 is a diagram which illustrates the geometry of a cylindrical feed arrangement.

FIG. 8 illustrates in a perspective view a planar antenna array (with crossed slots for circular polarization) adapted to be used iii a cylindrical form for the feed arrangement of FIG. 7.

FIG. 9 illustrates a cylindrical source feed arrangement for phase-sensing monopulse operation.

FIG. 10 illustrates a network for the monopulse arrangement of FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS It has been discovered that a conical reflector will collimate a beam when excited by a properly constructed line source feed disposed along the axis of the reflector. By exciting more or less of the line source feed from the apex of the cone toward its aperture, the beamwidth may be readily changed for different uses, such as for communications in a deep space mission when it is desired to cover the earth between the 3 db. points at all ranges without losing too much signal strength at long ranges.

Other advantages will become apparent from the following detailed description, such as the inherently stronger structure of the conical reflecting surface if solid, and the ease with which it can be made of a lightweight material if flexible. It is especially applicable for large antennas and may be built with a collapsible frame for convenient storage when such a large antenna is not in use, particularly in a space craft.

Referring now to the drawings, a conical reflector 10 with a line source feed is shown schematically in FIG. I to illustrate that a conical reflector can be used as a directional antenna when the proper phase and polarization constraints in the feed are satisfied. For a cone with a vertex angle 2 1 and an axial line source feed, the phase constraint is satisfied and perfect collimation is achieved if rays emanate from the axis at the angle 0,, when There is a general class of line source feeds which may be employed with a conical reflector that will satisfy the constraints of equation (I), namely feeds working in the broadside mode. However, except for the special case of a conical reflector with a half-angle of approximately 45", a broadside feed will require the various radiating elements (illustrated in FIG. I by dots, such as a dot 12) to be excited with a progressive phase delay for a uniform phase front illustrated by a dotted line 13.

For the special case of a conical reflector with a half-angle of 45, no interelement progressive phase delay is required and a standing wave array can be used for the feed. Accordingly, in order to describe the present invention with the simplest feed system, a linear feed comprising radiative elements on the cone axis, such as

elements

16 and 17 and a reflector half-angle of 45 is assumed, as for a reflector 18 in FIG. 2. However, any half-angle and feed system which satisfies the restraints of equation (I) may be employed to practice the present invention if a uniform wave front is maintained by proper phase delay in the excitation of the progressive elements of the line feed.

The polarization restraint is more difficult to satisfy than the phase restraint. To introduce the problems in satisfying the polarization restraint, an ideal line source feed as schematically illustrated in FIGS. 1 and 2 will be assumed. The problem involves the creation around the feed of a field that has a proper magnitude and polarization so that waves reflected from a conical surface will be oriented in the same direction and will have the proper amplitude distribution. This of course assumes the phase restraint is satisfied by either a 45 halfcone angle or proper interelement phase delay.

Assuming the 45 half-cone angle of FIG. 2, it can be shown that a feed line which radiates identically polarized waves in all directions will produce a cylindrical main beam that has a null on the axis of the cone. It can be further shown that the polarization of the beam makes one complete revolution in space as a test probe is carried around the axis of the cone one turn.

The ideal feed for a cone reflector will produce two figureeight patterns perpendicular to each other with polarizations as shown in FIG. 3a which represents schematically a section through a 45 half-angle conical reflector 20 normal to its axis. The feed comprises four linear arrays of dipoles a,a', b and b excited through coaxial cables 21 bunched in the center. Although this feed is linearly polarized to simplify the explanation which follows, it should be understood that the feed may be adapted for circular polarization as will be described hereinafter.

The dipoles a and a are excited 180 out of phase and oriented parallel to the feed, as indicated by the dot and cross adjacent to each, and the dipoles b and b are perpendicular to the feed or cone axis and a small distance from it.

The dipoles a and a illuminate the conical reflector with a figure-eight pattern represented by two circles A and A. That figure-eight pattern is polarized perpendicular to the paper with the E vector in the upper half pointing into the paper, as indicated by the cross near the dipole a, while the E vector in the lower half points out of the paper as indicated by the dot near the dipole :1. Upon reflection of the fields in this A-A pattern from the conical reflector 20 at a locus of points of reflection represented by a circle 22, the vectors labeled E, on the circle 22 will appear in the antenna aperture as shown.

The vectors E always point either directly toward or away from the feed axis, and their amplitude anywhere on the circle 22 is determined by the dimension of the A-A' pattern in that direction. It is evident that a considerable amount of cross-polarized energy is present in the E, field, and that the aperture is only partly illuminated.

The dipoles band b are also fed out-of-phase insofar as the axis of circular symmetry is concerned and illuminate the reflector with a figure-eight pattern represented by two circles B and B. The addition of the 8-H pattern fills the remainder of the aperture and reduces the cross-polarized field in the following manner. The polarization of the 8-8 pattern is in the plane of the paper and parallel to the locus of points of reflection represented by the circle 22. Upon reflection of the fields in this pattern from the conical surface, the vectors labeled E on the circle 22 will appear in the aperture as shown.

On the principal axes the E, and E vectors are parallel and of equal amplitude as indicated by the four vectors on the x and y axes. in the quadrants, components of both the E, and E vectors exist and are everywhere perpendicular to each other, The total signal, represented by the vector E, in those areas, is the vector sum of the vectors E and E,,.

An expression for the sum is most easily achieved by breaking both vectors 5,, and E into x and y components. Using the upper right hand quadrant vectors as an example, as shown in FIGS. 3b and 3c, the following equations may be written.

E =E cos i E =E 'sin i (2) and E E sin q E, =E cos l (3) It can be shown that these expressions hold for all quadrants. Assume the following conditions are imposed on the amplitudes of E, and E,, as a function of D.

where and are the maximum values reached by E, and E,, respectively.

The equations for the x and y components can then be written as:

A E =E sin 4 cos A A E =E cos s cos =En cos 4,

If the further condition is imposed that A A n h m a! then E =5 sin cos 4 E, =E, sin 41 (7) and E,,,=-E, cos 2 sin D E =E cos l (8) The total components in the two principal axes can now be summed: E ,=E., ,+E,, =E,, sin l cos I E cos l sin =O E,,=E,,;+E E (sin l -+cos l )=E, (10) Equation (10) shows that the fields assumed for the feed add to a constant value with the polarization parallel to the y axis regardless of the location in 9. Equation (9) shows that the cross-polarized component, E,, is everywhere reduced to zero, again completely independent of l Hence it can be stated that the conditions assumed and imposed on this feed produce the ideal feed for the conical reflector in which no energy is lost in cross-polarized lobes and the collimated energy is equally distributed on circles concentric with the axis.

The conditions for a linearly polarized beam are summarized for reference:

1. Two orthogonal figure-eight patterns must be generated by the feed. 2. One pattern, polarized parallel to the feed, must obey the amplitude function:

A E.,=E s1n (11) sin I 3. The other pattern, polarized perpendicular to the feed,

must obey the amplitude function.

E =E cos 35 (12) cos I r. The peak amplitudes of E, and 13,, must be equal.

A E E b 1 3 Physical realization of this ideal feed line would be very difficult to achieve at any frequency due to the physical impossibility of placing all radiating elements on the axis of the cone. At low frequencies, an array of very short dipoles transverse to the axis of the cone would produce the 8-8 pattern substantially as shown in FIGS. 3a and 3d, but to produce the A-A' pattern, two additional dipoles would have to be mounted parallel to the axis with an extremely small distance in terms of the wavelength between them. In addition, the dipoles a,a' must be fed 1 out of phase.

FlG. 3d shows diagrammatically in a perspective view the manner in which the cables 21 extend from the apex of the conical reflector 20 along the axis thereof to feed dipoles a, a, b and b at the end, and similarly arranged dipoles at various levels between the apex and the dipoles a, a, b and b in accordance with the invention. The dipoles a and a are disposed parallel to each other at equal distances from the axis and parallel to the axis, while dipoles b and b are disposed parallel to each other at equal distances from the axis and perpendicular to the axis as shown in FIG. 3a,

Both the short dipoles b and b and the out-of-phase dipoles a and a would be extremely hard to feed efficiently. At microwave frequencies the use of short dipoles would be most difficult. Slotted waveguide arrays have proved to be much more practical. For example, a single waveguide having its longitudinal center line along the axis of the cone could be provided with four arrays of slots, two arrays of shunt slots on opposing broad walls. Such an arrangement would produce patterns which approximate the ideal patterns shown in FIG. 3a. However, the resulting collimated beam would suffer some loss of gain from cross-polarization components and phase errors. To approach the ideal more closely, the feed waveguide would have to be made to appear extremely small in terms of a wavelength, and loading of the waveguide with a material of large dielectric constant would be required. A compromise would be necessary between high performance and the difficulties of working with a heavily loaded waveguide.

Before proceeding with a description of embodiments for a line source feed which approaches the ideal, it should first be noted that circular polarization can be achieved by any line source feed simply by replacing the linear dipoles a, a, b and b of FIG. 3 with crossed dipoles. These dipoles would all be identical in that one of the crossed dipoles would be parallel to the axis of the cone and the other perpendicular thereto. Each pair of crossed dipoles would then be fed in phase rotation starting with 0 for one pair of crossed dipoles, and proceeding in one direction with a phase shift in the feed to the remaining three crossed dipoles at 90 intervals.

It should also be noted that reference has been made to only one set of dipoles for both linear and circular polarized feeds at one point along the axis of the cone reflector, but it should be understood that the pattern illustrated in FIG. 3a, or a similar one for a circularly polarized feed, would be repeated at regular intervals along the axis of the conical reflector as schematically illustrated in FIGS. 1 and 2 and in the diagrammatic perspective view of FIG. 3d. It should also be noted that the possibility exists of using waveguides with suitable slots for both the linear and circularly polarized feed instead of dipoles. Bearnwidth change could then be readily accomplished by switching different levels of the feed along the axis of the conical reflector in and out, either individually by level or in groups by sections. A smaller diameter of the cone near the apex would be illuminated for a wide beam by exciting only the first one or two levels; as narrower beams are desired, successively larger diameters of the cone would be illuminated by exciting additional levels of radiating elements along the axis of the conical reflector.

The line source feed arrangement described with reference to FIGS. 3a and 3d will approach the ideal for satisfactory results in a relatively small antenna with just a few levels of radiating elements. For a larger number of levels, a larger number of feed lines would be required, one set of feed lines for each level of radiating elements to be separately excited. Thus, even if coaxial lines are used, the multiplicity of feed lines disposed along the axis of the conical reflector would form such a large bundle that the radiating elements at each level would have to be spaced too far from the axis, with resulting phase errors and loss of energy in cross-polarized lobes.

To minimize phase errors and loss of energy for large antennas, where a conical reflector has its greatest advantage over prior art parabolic reflectors, a cylindrical feed may be employed, instead of a line source feed, in the form of a hollow cylinder which may consist of a cylindrical reflecting surface with dipole elements mounted outside in a manner to be described with reference to FIGS. 4 through 7, or waveguides curved into an annular form with radiating slots around the outside, a desired number of waveguides then being stacked to form the hollow cylinder.

The cylinder may be made as large as necessary to accommodate the numerous transmission lines required to run down its center for the purpose of exciting the feed arrangement at various levels. The only requirement is that a sufficiently large number of radiating elements (dipoles or waveguide slots) be placed around the cylinder to keep the interelement spacing to approximately one-half wavelength or less so that an effectively continuous illumination of the conical reflector will result.

Referring now to FIG. 4, one level of linearly polarized feed is provided with a

cylinder

30 and 12 equidistant dipoles, such as dipoles 31- and 32, placed about the cylinder 30. In order that the linear polarization of the signal be kept properly oriented after reflection from the surface of a conical reflector 33, the dipoles are rotated with a progressive interelement rotation of 30 such that

dipoles

31 and 34 are parallel but l out-of-phase so that the vectorsof their radiating energy point along the axis of the conical reflector, one away from the vertex of the conical reflector, and the other toward the vertex as shown in FIG. 5 where the surface of the cylindrical reflector is shown in a flat plane in order that the relative positions of the dipoles may be shown. Thus, the dipoles 31. and 34 correspond directly to the dipoles a, a in the ideal line source feed arrangement illustrated in FIG. 3. Dipoles 35 and 36 are perpendicular to the axis of the conical reflector 33 and the

dipoles

31 and 34 to correspond to the dipoles b and b of the ideal line source feed of FIG. 3a. The pairs of dipoles provided between the orthogonal dipoles in the cylindrical feed arrangement of FIGS. 4 and 5, and not present in the ideal line source feed of FIG. 3a, effectively fill in the remainder of the aperture between the figure-eight patterns provided by the perpendicular dipoles. That fill-in minimizes phase errors and loss of energy which otherwise would result due to the significant space between diametrical pairs of the perpendicular dipoles caused by the need for a cylindrical column through which coaxial cables must run to each dipole at the various levels which are to be separately excited for beamwidth switching.

If each level of radiating elements is to be separately excited, the reflecting cylinder 30 will require a diameter suffciently large to accommodate a maximum number of transmission lines, and of course the greater the diameter of the reflecting cylinder 30 the larger the number of radiating elements required at each level to maintain a spacing between elements of approximately one-half wavelength or less.

To minimize the number of transmission lines running through the cylindrical reflector 30, and still provide for selective beam switching, the various levels may be grouped into sections W, X, Y and 2 as illustrated in FIG. 6, preferably with progressively more levels of radiating elements in each section such as l, 2, 4 and 8 levels in the respective sections W, X, Y and Z. With a 45 half-angle for the conical reflector 33 and a 12-foot aperture, the cylindrical reflector 30 may be 1 foot in diameter to accommodate 48 transmission lines. 12 for each of four sections having a cumulative total of elements. Each element is oriented for broadside radiation perpendicular to the axis of the conical reflector 33 since a 45 half-angle is selected. For any other half-angle the elements must be oriented to radiate into the conical reflector 33 in such a direction as to satisfy the conditions of equation (I). An appropriate phase delay would then be required between levels for a uniform phase front beyond the aperture of the conical reflector 33 as noted hereinbefore with reference to FIG. 1. Such a phase shift may be provided by a phasing network or properly designed feed line within each section.

Since the cylindrical reflector 30 has a sufiicient diameter to accommodate a conventional horn antenna for an extremely broad beam of approximately 40, it may be advantageous to provide one as illustrated by horn antenna 40 in FIG. 6. For a narrow beam, of approximately 20, only the section W would be excited. For yet a narrower beam such as 10, both sections W and X would be excited and for a 5 beam section W, X, Y and Z would be excited. For the narrowest beam of approximately 2.5 all sections W, X, Y and Z would be excited. For each of the narrower beams of 20, 10, 5 and 2.5, the collimated beam in the near field would have a null on the axis of the conical reflector 33 so that for each of the narrower beams, it may be desirable to also excite the conical horn 40 when any of the sections W, X, Y and Z are excited.

It should be noted that all the dipole elements illustrated in FIGS. 4 and are fed in phase to produce the desired illumination of the reflector. To illustrate that the desired illumination is produced, the vector representing the field radiated by each dipole may be divided into two components, one parallel to the axis of the cylindrical reflector 33 (parallel to dipoles 31 and 34) and one orthogonal thereto (parallel to a diameter of the cylinder 30). A study of the vector diagrams for each of the dipoles with reference to the axes just described indicates a sinusoidal-cosinusoidal variation in amplitude of the components along the axes as a function of location on the cylinder 30 when referenced to a particular point. By proper choice of the coordinate system, the amplitude variation function of either component may be expressed as a function of sin b as can be deduced from FIG. 7 where 1 is the angle between the position of the dipole 35 shown in FIGS. 4 and 5 to a particular location on the reflecting cylinder 30.

It is desired to obtain the far-field distribution at any farfield point P as a function of D. The problem is more readily formulated if instead of a discrete number of sources an infinite number of sources is allowed on the cylinder, each radiating uniformly into the half-space visible from its location on the outside of the cylinder. This assumption should be a very close approximation to the actual physical situation because the circumference is large in terms of a wavelength. Furthermore, in practice the elements would have about onehalf-wavelength spacing, which is sufiiciently close to be an approximation of a continuous distribution. The far-field point P then will receive energy only from the elements visible to it, and the limits of integration become D -n12 and l +1r/2. The phase of the signals reaching point P is determined by both 1 and 1 since the elements lie on the circumference of a circle. The path length difference in radians is then seen from FIG. 7 to be [21ra/A cos( l 1 ,,)1 between the limits of integration. It

, is now possible to write the expression for the total field at P as the integration of the contributions from those infinitesimal portions of the continuous distribution visible between the limits of integration as p a constant multiplier This expression is in the form assumed for the ideal feed in the equatiohs( l l to l 3). A similar solution for the other component of field gives E,(l ,,)=E,,p cos b, (l6) This expression is in the desired form and also indicates that the cylindrical feed, as stipulated and under the reasonable assumptions made in setting up the far-field expression, will completely suppress the cross-polarized components and provide axially symmetric illumination of the conical reflector.

If circular polarization is desired, the dipoles may be replaced by crossed dipoles or crossed slots. Then, because a phase shift is equivalent to rotation of a circularly polarized element, a choice is available between feeding the elements in phase with a progressive interelement rotation and feeding them with a progressive interelement phase shifi with no rotation. In the case of a waveguide feed the use of pve phase shift would allow more flexibility in the choice of slot pattern used to generate the circularly polarized wave and would generally be preferable. The total phase shift required in one trip around the feed is 360" (equal to the number of degrees of rotation required for the elements) or 30 per element for l2 elements. In general the interelement phase shift, 41,, will be tlt,,=360/n (17) where n is the number of elements at each station on the cylinder.

A practical antenna consisting of a conical reflector and a circularly polarized cylindrical feed may take the form shown in FIG. 6. As noted hereinbefore, five beamwidths can be generated at will by the excitation of different portions of the cylindrical feed. The 40 beam is generated by the circularly polarized conical horn 40 set in the open end of the cylindrical feed. The 20 beam uses the conical horn plus the small section of feed W. The horn is fed through a line length equal to the transit time of the wave reflected off the conical surface so that the two wave fronts will join at the top of the cylindrical feed and will always be in phase regardless of the wavelength. The 10 beam is obtained by excitation of the section X in addition to the section W and the horn. The 5 and 2.5 beams are obtained by successive excitation of the Y and Z sections so that the entire aperture is finally illuminated.

Each of section W, X, Y and Z may consist of a circularly polarized planar array rolled up into a cylinder. The wave in the radiating structure would be traveling around the feed from a feed point in a chosen direction as for a multilevel section 44 illustrated in FIG. 8. The various levels of the sections are fed in phase at points 45, 46,...for wave travel in the direction indicated by an arrow. The feed at each point may be by cables, as noted hereinbefore, using a corporate-feed system. A progressive interelement phase shift could be obtained by making the guide wavelength A, slightly longer than would normally be required for in-phase radiation. That is readily accomplished for a given free space wavelength A, by so providing the characteristics of the waveguide as to yield a 360ln interelement phase shift to satisfy the polarization requirement of the conical reflector antenna.

A conical reflector antenna may be given phase-sensing monopulse capability for tracking by dividing a cylindrical feed structure into three equal sectors X, Y and 2 about horizontal and vertical axes H and V, as shown in FIG. 9, and the incorporation of conventional directional couplers SI and 52 as shown in FIG. 10 for obtaining appropriate sums and differences of separate signals x, y and 2 received by the respective sectors X, Y and Z from a conical reflector 50 (FIG. 9). Different feed lines connecting each of the sectors to the directional couplers are represented by the reference character x, y and z to correspond to the signal identification letter assigned to the sectors A, B and C of the cylindrical feed structure in FIG. 9.

The directional couplers 51 and 52 (FIG. 10) provide horizontal and vertical error signals AH and AV according to the following equations:

where E, is an input to the coupler 52 from a sum arm of the coupler 51, and k is a constant introduced by the difi'erent attenuation factors of 3 db. and 4.77 db. for the couplers 51 and 52, respectively, provided to compensate for the fact that twothirds of the full aperture is used to develop the signal 2,, and only one-third to develop the signal 2. The horizontal and vertical error signals may then be employed to redirect the antenna through a conventional servomechanism to reduce those error signals to zero, thereby tracking the target.

Phase shifters

53 and 54 are provided in the feed lines for the y and z signals to maintain proper phase of the signals being combined. For example, the phase shifter 53 may provide a phase shift while the phase shifter 54 provides a +90 phase shift.

By dividing a cylindrical feed into four equal sectors and incorporating a conventional sum and difference network, a conical antenna can provide even more sensitivity in phasesensing monopulse operation. However, that would involve additional complexity in feed line and beam switching networks for the four sectors.

It should be appreciated that the invention is in no sense dependent upon any particular fabrication technique and that modifications and variations will occur to those skilled in the art. Accordingly, it is not intended that the scope of the invention be determined by the disclosed exemplary embodiments, but rather should be determined by the breadth of the appended claims.

We claim:

1. In a directional antenna having a conical reflector and a feed disposed inside said reflector along the axis thereof, an improvement in the feed comprising a plurality of radiating elements disposed about the axis of said reflector at various levels from a level near the apex of said reflector to a level closer to the aperture of said reflector at the end thereof opposite said apex, said elements being oriented to direct rays of radiant energy toward the inside surface of said reflector at an angle with said axis of said conical reflector as measured from the'apex of said reflector, and said angle is substantially equal to I80 less twice the half-angle of said reflector as measured from said axis to the inside surface thereof, whereby collimation of said rays emanating from the aperture of said reflector is achieved, said elements at a given level being disposed about said axis in pairs, with elements of a given pair on opposite sides of said axis, said pairs being uniformly spaced about said axis and arranged to radiate with a progressive interelement rotation of 360/n, where n is the number of said elements at said given level, thereby substantially reducing cross-polarization.

2. Apparatus as defined in claim 1 wherein all of said elements at said given level are excited in phase.

3. Apparatus as defined in claim 2 wherein each element radiates linearly polarized energy.

4. Apparatus as defined in claim 1 wherein each element radiates circularly polarized energy, and elements of a given level are arranged to radiate with a progressive interelement rotation of 360ln by orienting each element in a like manner and feeding all elements of said given level in sequence around said axis with a progressive interelement phase shift of 360/n, where n is as before the number of said elements at said given level.

5. Apparatus as defined in claim 1 wherein said feed system for a given level comprises a waveguide curved into a cylindrical form having as its axis said axis of said reflector and having slots as elements disposed about its axis for radiating energy 6. Apparatus as defined in claim 5 wherein said slot elements are disposed about said axis of said cylindrical form in pairs, with slot elements of a given pair on opposite sides of said axis, and said slot elements are uniformly spaced.

7. Apparatus as defined in claim 6 wherein each of said slot elements radiates circularly polarized energy, and all of said slot elements are oriented in like manner and fed with a progressive interelement phase shift of 360ln, where n is the number of said elements at said given level.

8. Apparatus as defined in claim 7 wherein said progressive interelement phase shift is achieved by making the guide wavelength of said waveguide longer than required for inphase radiation.

9. Apparatus as defined in claim 8 wherein said levels are grouped in sections for separate and selective excitation to provide a desired beam width of rays emanating from the aperture of said cone.

10. Apparatus as defined in claim 1 including means for combining said elements into groups of adjacent elements to provide signals proportional to radiant energy received by said elements in groups, and

means for deriving a first error signal proportional to the difierence between signals provided by first and second ones of said groups of elements in adjacent sectors, whereby tracking said target in one plane is provided upon directing said antenna to reduce said first error si al to zero. 11. pparatus as defined in claim 10 including means for deriving a second error signal proportional to the difference between signals provided by third and fourth ones of said groups of elements in adjacent sectors, whereby tracking said target in a second plane while tracking in said first plane is provided upon directing said antenna to reduce said second error to zero while directing it to reduce said first error signal to zero.

12. Apparatus as defined in claim ll wherein said first and second ones of said groups of elements receive radiant energy from equal sectors, said third one of said groups of elements consists of said first and second ones of said groups of elements combined, and said fourth one of said groups of elements consists of all remaining ones of said elements not included in said third one of said groups.

13. Apparatus as defined in claim 12 wherein all sectors are of equal size, and said first and second tracking planes are perpendicular.

Claims

1. In a directional antenna having a conical reflector and a feed disposed inside said reflector along the axis thereof, an improvement in the feed comprising a plurality of radiating elements disposed about the axis of said reflector at various levels from a level near the apex of said reflector to a level closer to the aperture of said reflector at the end thereof opposite said apex, said elements being oriented to direct rays of radiant energy toward the inside surface of said reflector at an angle phi with said axis of said conical reflector as measured from the apex of said reflector, and said angle is substantially equal to 180* less twice the half-angle of said reflector as measured from said axis to the inside surface thereof, whereby collimation of said rays emanating from the aperture of said reflector is achieved, said elements at a given level being disposed about said axis in pairs, with elements of a given pair on opposite sides of said axis, said pairs being uniformly spaced about said axis and arranged to radiate with a progressive interelement rotation of 360*/n, where n is the number of said elements at said given level, thereby substantially reducing cross-polarization.

4. Apparatus as defined in claim 1 wherein each element radiates circularly polarized energy, and elements of a given level are arranged to radiate with a progressive interelement rotation of 360*/n by orienting each element in a like manner and feeding all elements of said given level in sequence around said axis with a progressive interelement phase shift of 360*/n, where n is as before the number of said elements at said given level.

5. Apparatus as defined in claim 1 wherein said feed system for a given level comprises a waveguide curved into a cylindrical form having as its axis said axis of said reflector and having slots as elements disposed about its axis for radiating energy

6. Apparatus as defined in claim 5 wherein said slot elements are disposed about said axis of said cylindrical form in pairs, with slot elements of a given pair on opposite sides of said axis, and said slot elements are uniformly spaced.

7. Apparatus as defined in claim 6 wherein each of said slot elements radiates circularly polarized energy, and all of said slot elements are oriented in like manner and fed with a progressive interelement phase shift of 360*/n, where n is the number of said elements at said given level.

10. Apparatus as defined in claim 1 including means for combining said elements into groups of adjacent elements to provide signals proportional to radiant energy received by said elements in groups, and means for deriving a first error signal proportional to the difference between signals provided by first and second ones of said groups of elements in adjacent sectors, whereby tracking said target in one plane is provided upon directing said antenna to reduce said first error signal to zero.

11. Apparatus as defined in claim 10 including means for deriving a second error signal proportional to the difference between signals provided by third and fourth ones of said groups of elements in adjacent sectors, whereby tracking said target in a second plane while tracking in said first plane is provided upon directing said antenna to reduce said second error to zero while directing it to reduce said first error signal to zero.

12. Apparatus as defined in claim 11 wherein said first and second ones of said groups of elements receive radiant energy from equal sectors, said third one of said groups of elements consists of said first and second ones of said groups of elements combined, and said fourth one of said groups of elements consists of all remaining ones of said elements not included in said third one of said groups.