US 3045238 A
Description (OCR text may contain errors)
July 17, 1962 Y ATTORNEYS July 17, 1962 T. c. cHEsToN 3,045,238
FIVE APERTURE DIRECTION FINDING ANTENNA Filed June 2, 1960 4 Sheets-Sheet 4 A c 2 E E Z- F s o 4 E-PLANE DIFFERENCE 7 HORIZONTAL POLARIZATION s -EI a IA+B)-(c+D) db I2 24 I I I I I I l I l I I I I I I I I 90 8O 70 60 50 40 50 2O IO O I0 2O 30 40 50 60 70 8O 90 DEGREES TIT 2 931- J3- A F I D i C 4 H-PLANE DIFFERENCE VERTICAL POLARIZATION e (I E) s (A+C)(B+D) db I2 24 I I I I I I I I I I I I I I I INVENTOR THEODORE C. CHESTON ATTORNEYS Patented July 17, 1962 3,345,238 FIVE APERTURE BIRECTIGN FINDING ANTENNA Theodore C. Cheston, Silver Spring, Md., .assigner to the United States of America as represented by the Secretary of the Navy Filed .lune 2, 1960, Ser. No. 33,601 7 Claims. (Cl. 343-776) This invention relates generally to electromagneticwave energy radiating and radiation-receiving devices; more specifically, it relates to an improved multi-lobe antenna for use insdirection nding devices such as radar.
In order to obtain greater precision in certain direction iinding devices, or radars, it has been found desirable to utilize what is known as the monopulse method. In a monopulse system the sum and dilerence quantities of the outputs from a plurality of antennas, usually four, are employed to derive the location of a target.
, The design of suitable antennas for a monopulse system has not been too ditlicult where limitations on size are not severe. Parabolic reflector and lens type antennas having four separate feeds are commonly employed. However, in certain applications, such as guided missiles, an antenna of small dimensions is required. The parabolic reiector type antenna is unsuitable where the overall size of the antenna must be relatively small, as the size of the four antenna feeds with respect to the size of the reilector results in a blocking of the antennas total aperture by the feeds. The lens type antenna normally requires a much larger overall depth than is desirable in a small antenna. Accordingly, a need has existed for a monopulse antenna that is small in size and that possesses desirable radiation patterns.
It has been known that an antenna composed of four adjacent pyramidal horns would provide an antenna of suiliciently small size. However, the radiation patterns of such an antenna are far from acceptable, and such an arrangement has accordingly received little serious consideration as a practical antenna for a monopulse system.
According to the present invention, an antenna possessing the desired physical size and radiation patterns for use in a monopulse system can be constructed by employing five pyramidal horns arranged in a certain manner.
It is an object of this invention to provide a five-aperture direction finding antenna capable of giving a plurality of well shaped radiation patterns of narrow beamwidth and with relatively small side lobes.
Another object of the invention is to provide a fiveaperture direction nding antenna that is so constructed as to be small in physical size and non-critical in operation.
It is also an object of the invention to provide a liveaperture direction inding antenna for use in a monopulse system that is so constructed that the diilerence and sum radiation patterns thereof may be separately adjusted.
Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:
FIG. l is a diagrammatic view of a typical monopulse antenna array;
FIG. 2 is a graphical representation of the desired sum radiation pattern for a monopulse antenna;
FIG. 3 is a graphical representation of the desired difference radiation pattern for a monopulse antenna;
FIG. 4 is a graphical representation of the desired amplitude distribution across the face of an antenna to produce the radiation pattern of FIG. 2;
FIG. 5 is a graphical representation of the desired amplitude distribution across the face of an antenna to produce the radiation pattern of FIG. 3;
FIG. 6 is a perspective view of an antenna constructed according to the present invention;
FIG. 7 is a View in elevation of an antenna constructed according to the present invention, showing the ve pyramidal horns of the antenna and their relative dimensions;
FIG. 8 is a graphical representation of the amplitude distributoin for a difference radiation pattern in the E plane across the faces of a pair of adjacent, rectangular antenna horns;
FIG. 9 is a graphical representation of the amplitude distribution for a difference radiation pattern in the E plane across the faces of a pair of spaced rectangular horns;
FIG. l() is a graphic representation of the sum radiation pattern in the E plane of the antenna of FIG. 7;
FIG. ll is a graphic representation of the sum radiation pattern in the H plane of the antenna of FIG. 7;
FIG. 12 is a graphic representation of the difference radiation pattern in the E plane of the antenna of FIG. 7; and
FIG. 13 is a graphic representation of the difference radiation pattern in the H plane of the antenna of FIG. 7.
Radars of the phase comparison, monopulse type require an antenna capable of presenting `at least two lobes in each of the vertical and horizontal planes. A signal from a target is simultaneously received on all channels ofthe antenna, the sum and difference quantities of the outputs being employed to derive the location of the target in space.
Referring to the drawing, FIG. 1 illustrates diagrammatically a known antenna arrangement for a phase comparison monopulse system. The antenna `array consists of four separate antennas, I, II, III and IV, arranged Ito form a square. All of the separate antennas have similar radiation patterns. When the sum and difference method of monopulse is employed, signals received in the four antennas are combined in the following manner:
Sum channel I-j-II-l-III-l-IV.
Difference channel (l) I-l-II- (IIl-l-IV) (elevation) Diiference channel (2) I-j-III-(II-f-IV) (azimuth) The desired radiation patterns and amplitude distributions for the antenna array shown are illustrated schematically in FIGS. 2, 3, 4 and 5. FIG. 2 shows the general form of the ldesired sum radiation pattern in both the elevation and azimuth planes, the beamwidth being narrow and the side lobes, if any, being at a minimum. The form of the desired difference channel radiation pattern-for either the elevation or azimuth plane is shown generally in FIG. 3, the beamwidth again being narrow and the side lobes at a minimum.
FIG. 4 illustrates schematically the amplitude distribu- `tion `across the face of the horns in both the E and the H planes required to yielda pattern of the form of FIG. 2. rDhe tapered distribution -across the antenna aperture tends to substanti-ally reduce the undesired side lobes. Similarly, FIG. 5 illustrates schematically the amplitude distribution across the antenna aperture for the dilerence channels in both the F. and the H planes required to produce an `antenna pattern of the form of FIG. 3, wherein, again, =a tapered distribution is desired to reduce the side lobes.
In applications' where the physical size of the antenna structure must be limited, it has been found desirable to construct the antenna by using pyramidal horns. However, when four pyramidal horns are arranged as in FIG. l, the resultant radiation patterns are far from those desired. Large, undesirable side lobes are present, particularly in -theE plane difference channel pattern and the H. plane `sum channel pattern. The present invention contemplates a five-horn antenna arrangement, which has been found to possess radiation patterns closely approximating those desired.
A five-aperture antenna constructed according to the inventionis shown in perspective at 1 in FIG. 6. The antenna comprises four generally similar, shaped, pyramidal-type microwave horns A, B, C and D, and a fifth, smaller, pyramidal-type born F. The horns are positioned so that the apertures of all tive horns lie in a commonplane andform the total antenna -aperture. The microwave horns are each constructed in `the usu-al manner from a suitable material, such as brass. The antenna 1 is shown in front elevation in FIG. 7, the direction of the electrical field being indicated at 2.
Asis best shown in F-IG. 7, the separate horns A, B, C `and D comprising `the antenna 1 are arranged to form an upper pair of horns 3 and a lower pair of horns 4, the pairs being separated in the E plane, rand the fifth horn F is positioned inthe space between the two pairs. The horns A and B of the upper pair, and the horns C and D of the lower pair, are placed as closely adjacent in the H plane as is possible.
'I'he reason `for spacing the pairs of horns in the E plane may be best understood by a comparison of FIGS. 8 and 9 with FIG. 5. FIG. 8 shows the idealized form of the difference channel amplitude distribution across the apertures of a pair of rectangular-shaped horns placed immediately adjacent in the E plane. FIG. 9 shows the idealized form of the difference channel for a pair of similar horns spacedin the E plane. As is evident from an examination of these views, the discontinuous characteristic .of FIG. 9 is a better yapproximation to -the desired distribution of FIG. than is the characteristic of FIG. 8, resulting in a substantial reduction of sidelobes in Athe radiation pat-tern of the horns of FIG. 9 over the pattern of the horns of FIG. 8.
Referring again t0 FIG. 7, each of `the horns A, B, CandD is shown to have a tapered corner yat 5, 6, 7 and 8, respectively; 'Ilhe purpose of the tapered corner is torreduce side-lobes in both the sum channel and the two difference channels of the antenna by effectively reducing theftotal amount of power radiated at the outside edges of the horns.
The amplitude distribution across -the apertures on the difference channel of a pair of adjacent horns, say A and B, in the ,H plane is by nature sinusoidal, and as a result gives rise to radiation patterns close to that desired. Shaping the horns by tapering their corners in the manner shown results in a reduction of the side lobes in said patterns.
The sum channel distribution resulting from the four horns A, B, C and D, as arranged in the spaced relationship of the invention, is different from that desired because-the amplitude at the center of the total aperture ofthe antenna 1 is zero and gives rise to very large side lobes.. To correctthis, a fifth horn, F, is added to the antenna. This fifth hornV is coupled directly to the sum channel :olf-"the antenna and thereby does not affect the difference channels. The horn F provides the high amplitudefin the center ofthe sum channel of the antenna required to obtain a good sum radiation pattern with small side-lobes.
The antenna of the present invention is so constructed that thesum. and difference channels may be individually adjusted. The ratio of overall aperture size in the E and` H planes are chosen for the desired beamwidths in `those. planes and may be readily arranged to give equal bearnwidth for pencil beams.
By way of example, the physical dimensions for a typical X-band microwave antenna constructed according to this invention are shown in FIG. 7. The radiation patterns resulting `from such an antenna for-the E plane summation channel, H plane summation channel,
E plane difference channel, and H plane difference channel are shown in FIGS. l0, lil, l2 and 13, respectively. These views indicate that the, antenna radiation pattern has low side lobes, -good beamwidth, lgood beam separation for the difference channels, and good symmetry in the E and H planes; moreover, the physical size of the antenna is small. Thus, the present invention is seen to provide an antenna suitable for monopulse applications.
The antenna dimensions are, of course, dependent upon the particular antenna requirements, and they may be computed by known means `for `any specific antenna.
While the antenna has been described with reference to a monopulse radar, it is to be understood that it could be otherwise employed. For example, the individual outputs of horns A, B, C and D may be passed through variable phase Shifters, such as electronically-controlled ferrite phase shifters, and then all may be added to the output o-f horn F. Suitable sinusoidal phase modulation, where diagonally opposite horns are in modulation phase opposition and adjacent horns are in modulation phase quadrature, then produces a conically scanned radiation pattern. Such patterns vare frequently used for precision direction finding in `lieu o-f a monopulse system.
It would also lbe possible to employ lens-corrected electromagnetic horns, or other types of apertures, including end fire radiators such as dielectric rod antennas. In any case, the use of five apertures arranged according to the invention will provide an antenna having good radiation patterns and in which the sum and difference channels may be separately designed for optimum performance.y
Obviously, many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scopeof the appended claims the invention may be practiced otherwise than as specifically described.
What is claimed is:
1. A multi-lobe microwave antenna, comprising four separate antenna means arranged generally in the form of a rectangle, one of said antenna means being positioned at each corner of said rectangle, 'and a fifth antenna means disposed centrally of said four antenna means, said four antenna means being arranged to form an upper pair and a lower pair of antenna means, the two antenna means comprising each such pair being disposed immediately adjacent one another, and the two pairs of antenna means being spaced from one another vertically, said fifth antenna being disposed centrally within said space between said two pairs. v
2. A multi-lobe microwave antenna, comprising; four separate microwave antennas, said four antennas being arranged to form an upper pair of antennas and a lower pair of antennas, the antennas comprising each pair of antennas being disposed immediately adjacent in the magnetic plane thereof, and the upper pair and lower pair of antennasbeing spaced apart in the electrical plane of said antennas, and a fifth microwave antenna, disposed centrally of said four antennas in the space between said upper and said lower pair.
3. A multi-lobe microwave antenna, comprising; four pyramidal hornantennas, said four horns being arranged with `the electrical fields of all of said horns lying in a common plane and to form an upper pair of horns and aA lower pair of horns, the apertures of the horns comprising each pair of horns being disposed immediately adjacent in the magnetic plane thereof, and the apertures of the upper pair and lower pair of horns being spaced apart in the electrical plane of said horns, and a fifth pyramidal horn antenna disposed centrally of said four horns in the space between the apertures of said upper and said lower pair.
4. An antenna as claimed in claim 3, wherein each of said four hornantennas is tapered at its outermost corner, whereby the amplitude distribution in the electrical plane of each horn is effectively tapered across the face of the horn from a minimum a-t the edge of said horn that is furtherest removed from the horn adjacent to it in the magnetic plane to a maximum at the edge of said horn that is closest to said adjacent horn.
5. A multi-lobe, microwave antenna, comprising; a first antenna means, a second `antenna means, a third antenna means, and a fourth antenna means, each of said four antenna means being arranged so that their respective electrical fields lie in the same plane, the apertures of said first and said second antenna means being positioned immediately adjacent in the plane of the magnetic eld to form an upper pair of antenna means, the
apertures of said third and said fourth antenna means being positioned immediately adjacent in the plane of the magnetic field to form a lower pair of antenna means, the `apertures of said upper pair and said lower pair of antenna means being spaced apart in the plane of the electrical field, and ia 'ifth antenna means positioned centrally of said first four antenna mean-s in the space be tween the apertures ef said upper pair and said lower pair of antenna means and having its electrical field lying in the electrical iield plane of said first foinantenna means. v
6. An antenna as claimed in claim 5, wherein each of said rst four antenna means is a shaped pyramidal horn type antenna, the corner of each of said four horns that is furtherest removed from said fifth antenna means being tapered to thereby effectively shape the amplitude distribution in the electrical ield plane across each of said horns.
7. An antenna as claimed in claim 6, wherein, additionally, said iifth .antenna means is a pyramidal horn type ant-enna.
References Cited in the file of this patent UNITED STATES PATENTS Allen Apr. 27, 1954 Hageman Sept. 9, 1958