US3364490A

US3364490A - Variable beamwidth antennas utilizing defocusing

Info

Publication number: US3364490A
Application number: US311849A
Authority: US
Inventors: Peter W Hannan
Original assignee: Hazeltine Research Inc
Current assignee: Hazeltine Research Inc
Priority date: 1963-09-26
Filing date: 1963-09-26
Publication date: 1968-01-16
Anticipated expiration: 1985-01-16

Description

Jan. 16, 1968 P. w. HANNAN 3,364,490

VARIABLE BEAMWIDTH ANTENNAS UTILIZING DEFOCUSING Filed Sept. 26, 1963 3 h ets-Sheet 1 F I I I5 2 POSITION I ADJUSTING 20 I EANS ADJUSTABLE PHASE SHIFT MEANS ILLUMINATION PATTERNS PHASE AMPLlTUDE AMPLITUDE PHASE DIFFERENCE FIG, 1b S FOCUSED Q I =O D D=S I=| PoslTIoN I ADJ. PHASE gfi' I 53 J SHIFT FIG 2 MEANS L A IS ILLUMINATION PATTERNS PHASE AMPLITUDE AMPLITUDE PHASE DIFFERENCE FIG, 2b FOCUSED Q I =o D=S 2=l FIG. 2C

DEFOCUSED Q I 90 Jan. 16, 1968 P. W. HANNAN VARIABLE BEAMWIDTH ANTENNAS UTILIZING DEFOCUSING 3 Sheets-Sheet 2 Filed Sept. 26, 1963 a 3 G F PATTERNS ILLUMINATION AMPLITUDE AMPLITUDE PHASE DIFFERENCE PHASE FOCUSED FIG. 3b

DEFOCUSED FIG. 50.

PATTER NS ILLUMINATION AMPLITUDE AMPLITUDE PHASE DIFFERENCE PHASE FOCUSED FIG. 5b

DEFOCUSED Jan. 16, 1968 P. w. HANNAN VARIABLE BEAMWIDTH ANTENNAS UTILIZING DEFOCUSING 3 Sheets-Sheet Filed Sept. 26, 1963 POSITION ADJUSTING MEANS FEED A D J. PHASE SHIFT M EANS (0or 90SHlFT) FIG. 4a

FOCUSED FIG. 4b

DEFOCUSED FIG. 40

TRANSMITTED POLARIZATION REFLECTED POLARIZATION FIG. 4e

FIG. 4d

United States Patent 3,364,490 VARIABLE BEAMWIDTH ANTENNAS UTILIZING DEFOCUSING Peter W. Harman, Northport, N.Y., assignor to Hazeltine Research, Inc., a corporation of Illinois Filed Sept. 26, 1963, Ser. No. 311,849 16 Claims. (Cl. 343-454) This invention relates to variable beamwidth antennas utilizing defocusing and, as a specific example, to variable beamwidth double-reflector monopulse antennas whose focus can be varied while maintaining the phase relations required in monopulse operation and also maintaining an approximately invariant ratio of sum and difference mode pattern beamwidths.

In a microwave antenna, it may be desired to vary the width of the radiated beam in a continuously variable manner or, alternatively, in a stepped manner. This may be desired for example in tracking and communicating with a space vehicle where it is necessary first to roughly locate the vehicle and acquire it in the antenna beam. This acquiring function is most easily performed with a monopulse antenna having wide beams. Then it is desired to narrow the beams to help obtain accurate tracking or efiicient communication information. Another example involves the problem of location of one or more targets within a region which is fixed in size but is moving toward or away from the antenna. At short range, wide beams with low gain are needed; at long range narrow beams with high gain are needed. In both these cases a continuous and simple method for changing the beamwidth of an antenna facilitates the system operation.

Widening the beamwidth of an antenna could be accomplished by reducing the diameter of the radiating aperture. Such a method, however, is usually inefiicient or complicated and is usually subject to various defects related to operation with a reduced aperture. If, instead, beamwidth is widened by defocusing the antenna, high efficiency is maintained as the beam is widened and the variable mechanism is relatively simple. Furthermore, since essentially the entire aperture radiates at all times, many of .the defects related to a reduced aperture are avoided.

One problem arising as a result of the defocusing of an antenna is the change in shape of the radiation patterns as the antenna departs from the focused condition; especially serious are those ripples in the pattern that are associated with Fresnel diffraction. However, in accordance with the invention, the aperture illumination preferably is smoothly tapered down at the edge with the result that the generation of these ripples is minimized and the pattern tends to retain a similar shape as the antenna is defocused.

When the antenna is a monopulse type, it operates in a sum mode and one or two difference modes. In the case of a prior art amplitude-comparison monopulse antenna comprising a focusing lens or reflector illuminated by an ordinary four-horn feed, usually the sum illumination is well tapered but the difference illuminations are not. This lack of tapering, which degrades the performance in the focused condition, would generate intolerable ripples in the difference patterns in the defocused condition. However, by means of special techniques, it is possible to properly taper the difference illuminations while retaining the optimum taper in the sum illumination. These techniques are described in the applicants patent application entitled, Antenna Systems Providing Independent Control in a Plurality of Modes of Operation, Ser. No. 111,542, filed May 22, 1961, now Patent No. 3,308,468. By utilizing such techniques, together with other concepts explained below, the invention of the present application permits a variable beamwidth monopulse 3,364,490 Patented Jan. 16, 1968 antenna to be achieved by defocusing, while retaining good pattern shapes and high efficiency in all modes for all beamwidths.

In order that a monopulse antenna system provide the proper information for accurately determining the direction of a distant target, it is necessary to preserve the phase relationship between signals in the sum and difference modes. The fundamental problem to be overcome in providing variable beamwidth monopulse antennas utilizing defocusing is the requirement to maintain these phase relations in spite of defocusing. In an ordinary focused monopulse antenna system, maintenance of these phase relations requires careful design, control, and measurement of both the antenna and the circuits. In the case of the antenna itself, any defect of focusing, such as astigmatism or chromatism, causes a change of the phase relations which must be accounted for. In a monopulse antenna which is varied continuously from the focused condition to an extremely defocused condition for purposes of beam widening, the phase relations vary continuously and reach an ultimate value of about Such a change would ordinarily destroy the capability of the antenna system to accurately determine the target direction. In accordance with the present invention, the focus of a monopulse antenna can be varied while maintaining the phase relations relied upon in monopulse operation.

Although not essential, it is very desirable in some applications that the error pattern, defined as the ratio of the difference mode pattern to the sum mode pattern, have an invariant shape as it Widens with defocusing. One way to achieve this requires that, in addition to the pattern shapes remaining invariant with defocusing, the sum and difference pattern beamwidths should maintain the same ratio. This concept, together with methods for accomplishing this desired result, is explained in greater detail below.

In array type antennas, defocusing would be obtained by varying the phase of excitation of each radiating element so as to produce a curved wavefront across the antenna aperture. For antennas consisting of a feed together with a lens or a reflector, defocusing may be accomplished simply by moving the feed or the focusing element. There is some advantage, however, in employing the Cassegrain (or double-reflector) type of antenna for this purpose. In addition to the usual benefits, such as high efficiency and convenient location of the feed and associated waveguides, etc., the Cassegrain type antenna permits the accomplishment of defocusing by motion of the subreflector. This is usually easier than moving either the large main reflector or the feed and its associated waveguides, etc. Defocusing can also be accomplished by varying the power of the focusing element; for example, in a reflector type antenna the curvature of the reflector may be varied. Also, if it is required only to change from a narrow beam conditionto a wide beam condition without a continuous variation inbetween, some special simple techniques are available. For example, in the Cassegrain antenna the subreflector may be removed, or rendered transparent, to permit direct radiation by the feed; this corresponds to a fiat reflector and hence a greatly defocused condition which yields greatly widened beams.

It is an object of this invention therefore to provide new and improved variable beamwidth antennas, for use in systems which derive information on the basis of phase relations existing between different portions of a signal, wherein the beamwidth is changed by a varying focus of the antenna while maintaining these phase relations.

An additional object of this invention is to provide new and improved variable beamwidth monopulse antennas whose focus can be varied while maintaining desired phase relations of signals and one or more of the following: essentially invariant mode pattern shapes; high efficiency in all modes; an essentially invariant ratio of mode pattern beamwidths; and an error pattern of essentially invariant shape.

In accordance with the invention, a variable beam width antenna for use in a system which derives information dependent on phase relations existing between different portions of a received signal comprises an antenna, first means for changing the focusing effect provided by the antenna and second means coupled to the first means for providing phase shift compensation related to the change in focusing effect produced by the first means; whereby the focus of the antenna can be varied while maintaining the phase relations substantially unchanged.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description, taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

In the drawings:

FIG. 1a shows a single-reflector, variable beamwidth antenna utilizing the invention;

FIGS. 1b and 10 comprise diagrams useful in describing the operation of the FIG. 1a antenna;

FIG. 2a shows a double-reflector, variable beamwidth antenna utilizing the invention;

FIGS. 2b and 20 comprise diagrams useful in describing the operation of the FIG. 2a antenna; and

FIGS. 3a, 3b and 3c, FIGS. 4a, 4b, 4c, 4d and 4e and FIGS. 5a, 5b and 50 show alternative constructions for portions of the FIG. 2a antenna, together with diagrams useful in describing operation with such alternative constructions.

FIG. 1 ANTENNA Referring now to FIG. 1a, there is shown a variable beamwidth antenna, constructed in accordance with the invention, for use in a system which derives information dependent on phase relations existin between different portions of a signal. More specifically, FIG. la shows a single-reflector monopulse antenna for use with an amplitude-sensing monopulse system. The FIG. 1a antenna includes feed means, shown as

horns

10, 11, 12 and 13 arranged in a four-horn cluster well known in the prior art. The antenna also includes focusing means, shown as reflector 14, cooperating with the feed means. Also included is first means, shown as position adjusting means 15, for changing the focusing effect provided by reflector 14. Position adjusting means 15 may, for example, be a mechanical arrangement for moving the reflector 14 as indicated by the dashed reflector contour 14' and the twoheaded arrow. The antenna also includes a comparator arrangement made up of

hybrid junctions

16, 17, 18 and 19 utilized in well-known fashion to produce sum, elevation difference and azimuth difference mode signals in the lines labeled S, E and A, respectively, It will be understood that the lines interconnecting the horns 1043 and hybrid junctions 16-19 may be waveguides, coaxial transmission lines or other desired type of electromagnetic wave transmission means. The antenna of FIG. la finally includes second means, shown as adjustable phase shift means 20, coupled to the feed means (horns -13) for providing a phase shift correction related to the change in focusing effect produced by the action of the position adjusting means 15. In a complete radar system the lines S, E and A will be coupled to a transmitter, receivers, etc., in known manner.

Ignoring for the moment the action of means and 20, the antenna of FIG. 1a operates in the manner of prior art monopulse systems so that no detailed description of operation is necessary. Such an antenna may, for example, be used with a monopulse radar system which derives information as to the azimuth and elevation of a target aircraft on the basis of the amplitude and phase relations existing between the different portions of a radar signal which exists in the lines S, E and A. The solid line labeled 4t 14 can be considered to be a cross-sectional profile of a reflector in the position it would normally be found in a prior art antenna, i.e., the position for providing the optimum focusing effect.

FIG. lb illustrates certain relationships which exist in the focused condition, that is with the reflector in the position shown at 14 and no compensating phase shift introduced by means 20. It will be understood that the important monopulse relations are usually concerned with reception and not with transmission, however, reciprocity applies and the following discussion will be largely in terms related to transmission as is customary in discussing antenna operation. In FIG. lb, the straight line labeled Illumination-Phase indicates that for the focused condition the antenna aperture is illuminated in a uniform phase relation across the aperture. That is to say, there is substantially no phase variation across the aperture clue to the focusing system. In the present discussion, antenna aperture illumination refers to the field distribution across the reflector. The drawing 1abeled "Illumination-Amplitude shows the relation of the sum mode amplitude to the difference mode amplitude at the antenna aperture. The difference mode just referred to may be either the elevation difference mode or the azimuth difference mode, since both are similar except for the difference in coordinates. The basic amplitude contours of the patterns produced are labeled Patterns-Amplitude and it will be seen that the ratio of the widths of the difference and sum patterns is effectively one-toone as shown.

It will be understood that in normal amplitude-sensing monopulse operation the two portions of the difference mode signal will be 180 out of phase with each other. Also, one portion of the difference mode signal will be in phase with the sum mode signal. This set of relations will be considered a zero degree phase difference condition as indicated at Patterns-Phase Difference. If the portion of the difference mode signal which should normally be in phase with the sum mode signal were in fact out of phase with the sum mode signal, by say 30, this would be termed a 30 phase difference. It will be noticed that the other portion of the difference mode signal is normally 180 out of phase with the sum mode signal, but this is not the phase difference referred to. With respect to this other portion of the difference mode signal, the phase difference would be measured using 180 as the reference and measuring deviations from this normal 180' relation.

Referring now to FIG. 1c, there is shown how the FIG. 1b relationships change when the reflector is moved to the dashed position 14. Thus, the antenna is now defocused and a phase variation exists across the aperture as shown by the dashed line labeled Illumination- Phase. (The phase contour is shown clashed to correspond to the dashed position of the reflector.) As shown, the Illumination-Amplitude relations remain substantially unchanged. Also, as shown at Patterns-Amplitude, both the sum and difference mode patterns will be wider, however, now the ratio of the width of the difference and sum patterns is approximately two-to-one (instead of the original one-to-one ratio) so that a change in the error pattern has been introduced. Furthermore, the non-uniform phase illumination of the aperture results in a phase difference between the sum and difference mode signals as noted in FIG. 10. Examining these results, it will be seen that the beamwidth of the antenna has been changed as desired by the action of positioning means 15 in moving the subreflector 14. However, the required phase relations utilized in monopulse operations have been destroyed as indicated by the 90 phase difference produced. In accordance with the invention, the proper phase relations are restored by adjusting adjustable phase shift means 20 to provide a compensating phase shift. This is the essential feature which permits the defocused antenna to provide useful monopulse signals. As noted, the error pattern has been changer, which may detract from the usefulness of the signals produced. Also, in order to have tapered illumination in the difference mode in the defocused condition, the sum mode illumination must be made much narrower than the antenna aperture in the focused condition, yielding low antenna gain in the sum mode focused pattern. These two deficiencies will be improved in the subsequent forms of this invention.

It will be appreciated that although only the two limits of adjustment of the reflector 14 have been discussed above, actually the position adjusting means can be utilized to provide any intermediate degree of defocus ing by moving the reflector to a position intermediate to positions labeled 14 and 14'. The adjustable phase shift means would then be adjusted to provide the required compensating phase shift to restore the normal phase relations. The adjustment of means 20 will be related to the change in focusing effect produced by means 15, and if desired the

means

15 and 20 can be intercoupled In chanically or electrically so that both vary simultaneously in proper relation. Adjustable phase shift devices are well known, as are mechanical means usable for adjusting position of an object such as a subreflector, so that no detailed description of

means

15 or 20 is re' quired.

FIG. 2 ANTENNA Referring now to FIG. 2a, there is shown a variable beamwidth, double-reflector monopulse antenna, of the Cassegrain type, constructed in accordance with the invention. This antenna utilizes the principles of independent control described and claimed in the applicants above-referenced patent application. Ignoring for the moment the position adjusting means 15' and the adjustable phase shift means 20 (which correspond substantially to means 15 and 2-0, respectively, in FIG. 1a), the antenna of FIG. 2a operates in the manner of a prior art monopulse antenna except for the addition of the independent control function. Independent control permits efficient utilization of the antenna aperture with respect to each mode independently of the other modes involved. This can be seen by comparing the Illumination-Amplitude relation of FIG. 2b with that of FIG. 1b. Thus, the FIG. 2a antenna utilizes the aperture efficiently in both difference and sum modes as shown in FIG. 211, while in the FIG. 1a antenna the sum mode illumination is half as wide as the difference mode illumination at the aperture as shown in FIG. lb. This result is achieved in the FIG. 2a antenna by utilizing only horns 3033 for the sum mode, only horns 3tl-37 for the azimuth difference mode, and only horns 26-33 for the elevation difference mode. This horn utilization is achieved by the inter coupling of the hybrid junctions 4051 as shown in FIG. 2a. The result is that an effectively larger feed is used for each difference mode, as compared to the sum mode, so as to provide independent optimum illumination in each mode, yielding high antenna gain in the sum mode focused pattern.

Shown in FIG. 2b are the important relations for the focused condition of the FIG. 2a antenna, that is with the subreflector 52 in its solid line position and zero degrees phase shift compensation introduced by means 20'. The relations shown in FIG. 2b are similar to those shown in FIG. 1b except for the different Illumination- Amplitude relation as just discussed and except for a different Patterns-Amplitude relation. As shown, the ratio of the width of the difference mode and sum mode patterns will be effectively two-to-one in the focused condition of the FIG. 2a antenna. Also as in FIG. 1b, the phase illumination will be uniform across the aperture and the phase difference will be zero degrees, as shown.

Referring now to FIG. 20, there is shown the effect of activating position adjusting means 15 to defocus the antenna by moving the subrefiector to the dashed position 6 52'. As with the FIG. la antenna, a phase shift will be produced and will be compensated by adjustment of the adjustable phase shift means 20', which may be gauged to means 15' as noted with regard to FIG. In. One important change to be noted is the change in the ratio of widths of the difference and sum patterns to one-to-one, in the defocused condition. Again this causes an undesired change in the error pattern but this change will not destroy the usefulness of the signals received in the defocused condition. Only two positions of the subreflector 52 have been discussed, but varying degrees of defocusing can be produced by different adjustments of the subrefiector position.

FIG. 3 ANTENNA The FIG. 2a antenna with the dotted rectangle 55' of FIG. 3a substituted for the dotted rectangle 55 in FIG. 2a will be termed the FIG. 3a antenna. It will be seen that the FIG. 3a antenna as so constituted is similar to the FIG. 2a antenna except that

directional couplers

57 and 58 have been substituted in place of

hybrid junctions

50 and 51, respectively.

If the left-hand lines connecting to each of the

directional couplers

57 and 58 are traced in the FIG. 3a antenna it will be found that these lines are coupled to the outer horns of the feed. The function of the

directional couplers

57 and 58 is to modify the effect provided by the outer horns 26-29 and 3437. This is done by coupling less power between the left-hand lines and the respective E and A lines than between the right-hand lines and the respective E and A lines.

Referring to FIGS. 1 and 2 it will be seen that, as indicated in FIGS. 1b and 10, when the FIG. 1a antenna is changed from a focused to a defocuscd condition the pattern beamwidth ratio changes from D:S=1:1 to D:S=2:l. Also, as indicated in FIGS. 2b and 20, when the FIG. 2a antenna is changed from a focused to a defocused condition, the pattern beamwidth ratio changes from D:S=2:1 to D:S=l:l. As noted in the discussion of the FIGS. 1a and 2a antennas, these beamwidth ratio changes cause an undesired change in the error pattern (defined at the beginning of specification). The applicant has discovered that by providing a proper ratio of the beamwidths of the difference and sum mode patterns in the focused condition, for example, an antennas focus could then be varied while retaining the ratio of the beamwidths substantially constant. This constant ratio in turn allows a substantially constant error pattern to be achieved. In paricular, the applicant has found that if the difference and sum mode patterns are arrangedto have a ratio of beamwidths of approximately /2:l(D.-S= /2:l) then this ratio remains substantially constant as the focus of an antenna is varied.

In the FIG. 3a antenna this result is accomplished by the inclusion of the two similar

directional couplers

57 and 58. Each of

directional couplers

57 and 58 is designed for the proper ratio of power division between the righthand line and the left-hand line. In order to achieve the results shown in FIGS. 3b and 30 this ratio should be approximately four-to-one. That is to say, if unity power were applied to the E line, four-fifths of this power would emerge in the right-hand line and one-fifth of the power would emerge in the left-hand line. In this way, directional coupler 57 is utilized in providing an elevation difference mode pattern approximately 1.414 times as wide as the sum mode pattern. Directional coupler 58 is similarly designed and utilized in providing an azimuth difference mode pattern approximately 1.414 times as wide as the sum mode pattern.

FIGS. 3b and 3c indicate the results produced in operation of the FIG. 3a antenna. It will be seen that a 90 phase shift again results in the extreme position, as in the FIG. la and FIG. 2a antennas, and is compensated for by adjustable phase shift means 20'. The important point is that the FIG. 3a antenna has the important added advantage of an error pattern which remains substantially constant over the whole range of antenna defocusing regardless of whether the subrefiector 52 is in either extreme position or any intermediate position. There remains, however, a somewhat low gain in the sum mode focused pattern, because the sum mode illumination is somewhat narrower than optimum. This gain is higher than that of the FIG. 1 antenna.

FIG. 4 ANTENNA The FIG. 2a antenna with means replaced by position adjusting means 15", means replaced by adjustable phase shift means 20" and dotted rectangle replaced by rectangle 55" will be termed the FIG. 4a antenna. The antennas of FIGS. la, 2a and 3a were each constructed to permit continuous adjustment of subrefiector spacing so that any desired degree of defocusing could be provided. The FIG. 4a antenna is designed to provide two distinctly different degrees of focusing without provision for intermediate degrees of focusing. This is accomplished by providing position adjusting means for positioning the subrefiector in either the position for focused operation in a double-reflector antenna system, or, alternatively, in a substantially nonrefiecting position so that the antenna becomes substantially just an array of horns not utilizing any reflectors. The latter condition provides defocused operation of the antenna.

Referring more particularly to the specific ararngement shown in FIG. 4a, it will be seen that the position adjusting means 15" is shown as a mechanical arrangement able to position the subrefiector 52 either in the solid line focusing position or in the dashed line position 52'. In FIG. 4, the lines from hybrid junctions 48 and 49 (see FIG. 2a) connect to hybrid junction 60. The other two arms of hybrid junction connect to electrically independent, but mechanically interconnected, portions of multipole, double-throw switch 62. In the position shown, switch 62 acts to couple hybrid junction 61) directly to hybrid junction 50, to which the E line also connects. If the switch 62 is moved to its other position, hybrid junction 60 is connected to hybrid junction 56 via fixed

phaseshifters

64 and 65 which provide a negative 45 phase shift and a positive 45 phase shift, respectively. In a similar manner, the lines from hybrid junctions 46 and 47 (see FIG. 2a) connect to hybrid junction 61, which in turn connects to switch 63 (which is preferably ganged to switch 62). Switch 63 acts to couple hybrid junction 61 to hybrid junction 51 either directly or via fixed phase shifters 66 and 67 which provide a negative 45 phase shift and a positive 45 phase shift, respectively.

When the subrefiector is in the position 52, the antenna acts as a wellfocused Cassegrain type antenna. v /hen the subrefiector is in the dashed position 52 the feed (representing a side View of the l2-horn feed of FIG. 2a) acts substantially independent of any reflector and produces greatly widened patterns. When the subrefiector is in the dashed position 52 the swicthes 62 and 63, shown in rectangle 55", are placed in the position shown in the drawing. When the subrefiector is in the solid line position 52, the switches 62 and 63 are placed in the other position from that shown. In this position the combined effect of

hybrid junctions

50 and 60 and switch 62 would cause all power put in at the E line to come out of the arm of hybrid junction 60 which connects to hybrid junction 48. With the switch 62 in the other position, equal quantities of all power put in at the E line would come out of the arms coupled to hybrid junctions 48 and 49. Similarly, in the position shown all power entering the A line would come out of the arm of hybrid junction 61 which connects to hybrid junction 46 and with switch 63 in the other position equal quantities of power would come out of the arms coupled to

hybrid junctions

46 and 47.

The arrangement shown preserves the impedance match and the required phase relationships. The resulting rela tions are shown in FIGS. 4b and 40, where it will be seen that the same ratio of beamwidths of the difference and sum patterns are produced in each focusing condition. In addition, the sum mode focused gain will be high be cause the corresponding illumination is sufficiently wide. It should be explained that in FIG. 4a, the actual size shown for the illumination amplitudes is not intended to be significant, because there is no longer any physical aperture. The important factor is the ratio of sizes in the sum and difference modes.

Adjustable phase shift means 29" need only have a zero degree phase shift compensation position for the focused condition and a 90 phase shift compensation position for the unfocused condition; a continuously variable phase shift compensation is not required. Switches 62 and 63, means 15 and means 20" may readily be intercoupled for simultaneous or ganged operation. Such inter-coupling for ganged operation is especially easy in the FIG. 4a antenna because each element has only two discrete positions.

Referring now to FIGS. 41! and 42, there are shown two views of a particular form of subrefiector construction useful in the FIG. 4a antenna. In a subrefiector constructed in accordance with FIGS. 4d and 4e, grids of thin parallel conductors such as '75 are supported in a pair of comparatively high dielectric constant sheets 76 and sheets 76 are spaced uniformly from each other by a low dielectric constant material '77 (which may be a foam or honeycomb-type material). This type of construction has the property of reflecting a wave polarized parallel to the conductors and transmitting a wave polarized perpendicular to the conductors. Subrefiectors of this type are discussed in greater detail in applicants application entitled Double-Rer'lector, Double-Feed Antennas for Crossed Polarizations and Folarization Changing Devices Useful Therein, Ser. No. 173,501, filed Feb. 15, 1962. A subrefiector constructed in this manner can be substituted for the solid subrefiector 52. in the FIG. 4a antenna. Then the subrefiector can be rendered reflective or transparent merely by being rotated 90 by a position adjusting means designed to produce such rotary movement. Operation of the complete antenna and the resulting relations will be essential identical to those discussed with reference to the basic FIG. 4a antenna.

FIG. 5 ANTENNA The FIG. 2a antenna with dotted rectangle S5 replaced by rectangle 55'' will be termed the FIG. 5a antenna. If the subrefiector of an antenna such as the FIG. 2a antenna is moved incrementally between the focus position 52 and the dotted position 52, the error pattern will go through a continuous range of variation. The FIG. 3a arrangement using fixed directional couplers permits a substantially constant error pattern; however, the FIG. 3a antenna does not make optimum use of the antenna aperture in the focused condition because the sum mode illumination is too narrow, as is shown in the Illumination-Amplitude figure of FIG. 3b. The FIG. 4a arrangement provides identical error patterns for two positions, as well as optimum use of the antenna aperture in the focused condition, but would not provide these features if the subrefiector were moved to provide a continuous range of focus changes. The FIG. 5a antenna can provide a constant error pattern for all degrees of defocusing with optimum antenna aperture utilization for all modes.

Referring more particularly to rectangle 55' of FIG. 5a, means are provided for continuously varying the effect provided by the outer horns 26-29 and 34-37. For the elevation difference channel, these means are shown as including hybrid junction 60 which connects to hybrid junctions 48 and 49 (see FIG. 2). The remaining arms of hybrid junction 60 connect to two variable phase shifters 80 and 81, which in turn connect to hybrid junction 50, to which the E line also connects. Similarly, for the azimuth difference channel hybrid junction 61 connects to hybrid junctions 46 and 47 (see FIG. 2) and to adjustable phase shift means 82 and 83, which in turn connect to hybrid junction 51, to which the A line also connects. Adjustable phase shift means 80 and 82 provide a phase shift continuously adjustable between and negative 45 and phase shift means 81 and 83 provide a phase shift continuously adjustable between 0 and positive 45. Phase shift means 80, 81, 82 and 83 are ganged together so that all can be adjusted simultaneously in a coordinated manner. Phase shift means 80-83 are ganged so that at one extreme all provide 0 phase shift and at the other extreme 8t) and 82 provide a negative 45 phase shift and 81 and 83 provide a positive 45 phase shift. For intermediate positions each phase shifter 80-83 provides a phase shift of the same magnitude and of appropriate polarity. The position adjusting means adjustable phase shift means and the means of rectangle 55" can be intercoupled and arranged so that the effects produced by these three means vary in proper relation so that optimum performance is achieved for all degrees of defocusing.

In view of the discussions of the operation of the antennas of FIGS. 20 and 4a, the fundamentals of the operation of the FIG. 5a antenna will be clear. It will be seen that the relations as shown in FIGS. 5b and 5c are identical to those shown in FIGS. 4b and 4c. The principal difference between the operation of the antennas of FIGS. 4a and 5a is that: whereas the FIG. 4a antenna provides maintenance of required phase relations, essentially invariant mode pattern shapes, high efficiency in all modes, an essentially invariant ratio of mode pattern beamwidths and an error pattern of essentially invariant shape for two different focusing conditions; the FIG. 5a antenna provides the same results over a continuous range of focusing conditions.

It will be understood that, while in the antennas shown the compensating phase shift is applied in the sum mode line S, the compensation could just as well be applied in the difference mode lines E and A. However, in such case two variable phase shift means are required instead of one. It will also be understood that the adjustment of feed excitation could just as well have been applied in the sum mode, and that other forms of such adjustment, such as control of mode ratios in a multimode feed, could have been described.

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

What is claimed is:

1. A variable beamwidth antenna for use in a system which derives information dependent on phase relations existing between different portions of a received signal,

focusing means cooperating with said feed for providing a focusing effect;

first means for changing the focusing effect provided by said focusing means; and second means coupled to said feed for providing phase shift compensation related to the change in focusing effect produced by said first means;

whereby the focus of said antenna can be varied While maintaining said phase relation substantially unchanged.

3. A variable beamwidth antenna for use in a system which derives information dependent on phase relations existing between different portions of a received signal, comprising:

a feed;

reflector means cooperating with said feed for providing a focusing effect;

first means for changing the focusing effect provided by said reflector means; and second means coupled to said feed for providing phase shift compensation related to the change in focusing effect produced by said first means;

whereby the focus of said antenna can be varied while maintaining said phase relations substantially un changed.

4. A variable beamwidth double-reflector antenna for use in a system which derives information dependent on phase relations existing between different portions of a received signal, comprising:

a feed;

a subreflector spaced from said feed;

a main reflector spaced from said subreflector;

first means coupled to said subreflector for changing the focusing effect provided by said reflectors; and second means coupled to said feed for providing phase shift compensation related to the change in focusing effect produced by said first means;

whereby the focus of said antenna can be varied while maintaining said phase relations substantially unchanged.

5. A variable beamwidth double-reflector antenna for use in a system which derives information dependent on phase relations existing between different portions of a received signal, comprising:

a feed;

a subreflector spaced from said feed;

a main reflector spaced from said subreflector;

first means coupled tosaid subreflector for physically changing the position of said subreflector so as to change the focusing effect provided by said reflectors;

and second means coupled to said feed for providing phase shift compensation related to the change in focusing effect produced by said first means;

6 A variable beamwidth monopulse antenna comprising:

feed means for processing a sum mode signal and at least one difference mode signal;

focusing means cooperating with said feed means for providing a focusing effect;

first means for changing the focusing effect provided by said focusing means;

and second means coupled to said feed means for providing phase shift compensation related to the change in focusing effect produced by said first means;

whereby the focus of said antenna can be varied while maintaining the phase relations necessary to monopulse operation substantially unchanged.

7. A variable beamwidth monopulse antenna comprising:

reflector means cooperating with said feed means for providing a focusing effect;

first means for changing the focusing effect provided by said reflector means;

8. An antenna essentially as described in claim 7,

wherein the feed means produces a difference mode pattern approximately /2 times as wide as the sum mode pattern, so that the sum and difference mode patterns retain an essentially invariant ratio of beamwidths for different focusing conditions.

9. A variable beamwidth double-reflector monopulse antenna comprising:

feed means for processing a sum mode signal and at least one difference mode signal; a subreflector spaced from said feed means; a main reflector spaced from said reflector; first means coupled to said subreflector for changing the focusing effect provided by said subreflector; and second means coupled to said feed means for providing phase shift compensation related to the change in focusing effect produced by said first means; whereby the focus of said antenna can be varied while maintaining the phase relations necessary to monopulse operation substantially unchanged. 10. An antenna essentially as described in claim 9, wherein the feed means produces a difference mode pattern approximately /2 times as wide as the sum mode pattern, so that the sum and difference mode patterns retain an essentially invariant ratio of beamwidths for different focusing conditions.

11. A variable beamwidth double-reflector monopulse antenna comprising:

a subreflector spaced from said feed means;

a main reflector spaced from said subreflector;

first means coupled to said subreflector for physically changing the position of said subreflector so as to change the focusing effect provided by said subreflector; and second means coupled to said feed means for providing phase shift compensation to said sum mode signal, which compensation is related to said change in focusing effect produced by said first means;

12. An antenna essentially as described in claim 11, wherein the feed means produces a difference mode pattern approximately /2 times as wide as the sum mode pattern, so that the sum and difference mode patterns retain an essentially invariant ratio of beamwidths for different focusing conditions.

13. A variable beamwidth monopulse antenna comprising:

first means for changing the focusing effect provided by said focusing means;

second means coupled to said feed means for producing a relative change in the effective feed aperture for sum and difference mode signals, which change is related to said change in focusing effect produced by said first means;

and third means coupled to said feed means for producing phase shift compensation related to said change in the focusing effect produced by said first means;

whereby the focus of said antenna can be varied while maintaining the phase relations necessary to monopulse operation substantially unchanged and while maintaining an approximately invariant ratio of sum and difference mode pattern beamwidths and high gain in the focused condition.

14. A variable beamwidth monopulse antenna comprising:

first means for changing the focusing effect provided by said reflector means;

15. A variable beamwidth double-reflector monopulse antenna comprising:

a subreflector spaced from said feed;

a main reflector spaced from said subreflector;

first means for changing the focusing effect produced by said reflectors;

16. A variable beamwidth double-reflector monopulse antenna comprising:

a subreflector spaced from said feed;

a main reflector spaced from said subreflector;

a first means for physically changing the position of said subreflector so as to change the focusing effect produced by said reflectors;

a second means coupled to said feed means for producing a relative change in the effective feed aperture for sum and difference mode signals, which change is related to said change in focusing effect produced by said first means;

and third means coupled to said feed means for producing phase shift compensation to said sum mode signal, which compensation is related to said change in the focusing effect produced by said first means;

3,364,490 13 14 whereby the focus of said antenna can be varied While References Cited maintaining the phase relations necessary to mono- U T STATES PATENTS pulse operation substantially constant and While 2 716 746 8/1955 Howery 343 d755 X maintaining an approximately invariant ratio of sum and difference mode pattern beamwidths and high 5 HERMAN KARL SAALBACH, Primary Examiner.

M. L. NUSSBAUM, Assistant Examiner.

gain in the focused condition.

Claims

1. A VARIABLE BEAMWIDTH ANTENNA FOR USE IN A SYSTEM WHICH DERIVES INFORMATION DEPENDENT ON PHASE RELATIONS EXISTING BETWEN DIFFERENT PORTIONS OF A RECEIVED SIGNAL, COMPRISING: AN ANTENNA; FIRST MEANS FOR CHANGING THE FOCUSING EFFECT PROVIDED BY SAID ANTENNA; AND SECOND MEANS COUPLED TO SAID FIRST MEANS FOR PROVIDING PHASE SHIFT COMPENSATION RELATED TO THE CHANGE IN FOCUSING EFFECT PRODUCED BY SAID FIRST MEANS; WHEREBY THE FOCUS OF SAID ANTENNA CAN BE VARIED WHILE MAINTAINING SAID PHASE RELATIONSHIPS SUBSTANTIALLY UNCHANGED.