US 2547414 A
Abstract available in
Claims available in
Description (OCR text may contain errors)
W. SICHAK pri! 3, 1951 ANTENNA Filed Aug. 8, 1945 FIG.I
1N VEN TOR.
. WILLIAM SICHAK A TTORNEY Patented Apr. 3, 1951 ANTENNA William Sichak, Cambridge, Mass., assignor, by mesne assignments, to the United States of America as represented by the Secretary of War Application August 8, 1945, Serial No. 609,652
This invention relates to antennas for communication systems and is particularly directed. to an antenna having means for shaping the primary radiation pattern of a beam of electromagnetic energy emitted by a radiating element, such as a horn-type feed.
Such a radiating element has wide application. It can be used to match a wave guide transmission line to free space and thus may be used for radiating waves of electromagnetic energy directly to free space. However, its principal a plication is in feeding energy to, and illuminating, a refiector adapted to direct the radiant energy into free space in any desired beam pattern.
The gain, half power beam width and side lobes of the radiation pattern of an antenna having a paraboloidal reflector depend upon the phase and amplitude distribution of the radiation pattern of the reflector feed. The latter may be referred to as the primary pattern of the feed. For a circular dish-like reflector the problem of feed is relatively simple. For a truncated paraboloidal reflector it is necessary to use a feed adapted to produce a beam suitably shaped for efficiently illuminating the reflector.
One objective in designing a paraboloid antenna is to realize the maximum possible gain consistent with side lobe requirements. In order to get the highest gain, it is required to have a relatively uniform illumination across the aperture of the paraboloid and low spill over the edge. To achieve this, one can either shape the primary beam of a feed to fit a given reflector or shape the reflector, such as to form a truncated par-aboloid, to fit a given feed, or both.
The gain of a paraboloid antenna with a given feed is a maximum if the amplitude at the edge is half the average amplitude of the illumination over the aperture of the paraboloid. For a given feed, this provides a simple criterion for determining the siZe of the paraboloid. In general, the gain factor does not drop sharply with the increase of the aperture of the dish. Thus, a slightly bigger aperture can always be used to reduce the side lobes of the radiation pattern of the antenna. In most cases, however, the converse problem is encountered; namely, for a given paraboloid how to shape the primary pattern of a feed to achieve the best results.
In order to design an antenna of high gain factor, it is necessary to shape a primary pattern of some special type to provide more uniform i1lu-- mination with little energy shooting outside the reflector. Thus it is frequently necessary that the radiation from a horn or a wave guide type feed meets certain requirements as to intensity near the edge of the beam. For example, it may be desired that the intensity patterns in the H-plane and in the E-plane be similar. Another possible requirement would be that the radiation angle be about between the 10 decibel points (points A and A in Fig. 1 with point B used as a basis for comparison), as would be desired for illumination of a paraboloid for which the focus is located in the plane of the periphery. Ordinarily the radiation pattern for an open wave guide type feed is too broad in the E-plane and too narrow in the H-plane to achieve the desired illumination. By known means, the beam may be narrowed in the E-plane and adjusted so that the desired attenuation of the primary pattern is attained at the periphery of the'paraboloid or other suitably shaped reflector.
Accordingly, it is the principal object of the present invention to provide means for broadening the primary radiation pattern of an open wave guide or horn type feed in the H-plane so that the attenuation in the H-plane may be adjusted to be substantially the same as that in the E- plane, or vice versa.
Another object of the invention is to provide means for shaping the primary pattern of a radiating feed element to fit specified reflectors.
Still another object of the invention is to provide a radiating feed element for an antenna which is adapted to produce a desired energy beam pattern.
Other objects, advantages and novel features of the invention will become apparent when taken into consideration with the accompanying drawings, in which:
Fig. 1 is a diagrammatic view illustrating the radiation pattern ordinarily produced by an open wave guide or horn type feed and the pattern desired according to this invention;
Fig. 2 is a top plan view of an open wave guide with a shaped end according to one embodiment of the invention; and
Fig. 3 is a perspective view of a horn type radiating element according to a modification of the invention.
Referring now to Fig. 2, l0 designates an open wave guide of rectangular cross-section and having a broader dimension a. Wave guide I 0 is adapted when properly excited, to radiate energy from'its open end or'aperturel l. 'When'iprop erly positioned relative to a reflector of any suitable shape, wave guide I0 is adapted to feed energy to and illuminate the reflector. As indicated hereinbefore, it is frequently desirable that the primary radiation pattern from a wave guide or horn be shaped to efliciently illuminate a specified reflector. According to this invention, it is desired to broaden the primary pattern in the H-plane (the plane parallel to the magnetic vector H and perpendicular to the electric vector E). This is accomplished by Cutting ofi the cornersof the broad side walls of'wave guide iii as at l2 and 13 so that the broad side walls appear to be tapered inwardly and towards the free en of Wave guide In. Thus in efiect the Wave guide aperture I l is increased in one plane, or as illustrated in the H-plane.
The width of the primarybeaminthe'H-mlane is a function of both I) and :It has-been-found that with a constant angle the beam becomes broader as b is increased, and .for a constant 5b, the beam becomes broader for an increasing 1;. A plot of the relationship between Wp/m (the db. power width chosen as the representative angle .or beam width of interest in reflector illumination) of the vcut .corner Wave :guide 1-D .and the angle of cut shows that Wp/lO increases as qb is increased with a general maximum .at =45. For a larger o, the beam'becomesmore and more flat with a practically constant "W 1o. Thus, theefiect of cutting the .corners as .at I2 and :13 is that more diffraction of the .beam is produced and the primary beam in the H-plane from that normally produced by an openwave guide as indicated by the solid line configuration in Fig. 1 is broadened-to that shown by the dotted line configuration.
From studies ,of the secondary patterns inside a reflector, particularly a truncated paraboloidal reflector, fed by a cut corner wave guide as I'll, there is shown a substantial advantage in .using this type of feed over the ordinary open wave With both the ,corners ,of wave guide In out off at equal angles of as at I2 and [3, it will be apparent that the beam will be :broadened symmetrically. I-n certaincases it may be desired to obtain an asymmetrical primary beam pattern. This may be achieved by cutting off only one of the corners of wave guide it, or by cutting off both corners at unequal angles of .41.
A modification of :the present invention is shown in Fig. 3 as applied to a horn-type feed r d atin e e In ge e al. a :horn comprise a flared portion attached ,or connected at its smallest dimensions to a hollow pipe wave guide. As shown, horn I5 has a wave guide portion I6 and a flared portion l1, both having a rectangular cross-section adapted to feed energy waves in the appropriate mode, the E-vectoror E-plane being indicated by the arrow at E. Flared portion if! comprises side walls l8 and 1,9 as extensions in the same plane of the respective side walls of wave guide portion 16, and upper and lower walls 20 and 2| formed at .an angle to the remaining parallel walls of wave guide portion [.6 at throat 22. The dimension between upper and lower Walls 20 and 2] is larger at mouth 23 than at throat 22. Thus, horn I5 is shown as being flared out in the E-plane and may be called an E-horn.
According to this modification, one .or both .of the corners of each of upper and lower walls 20 and 21 are cut off at an angle o to broaden the radiation pattern Produced by horn in t 4 H-plane asymmetrically or symmetrically respectively.
While there has been described what is at present considered the preferred embodiments of the invention, it will be obvious toithose'skilled in the art that various changes and modifications may be made therein without departing from the invention.
What is claimed is:
1. A radiating device for feeding a parabolic reflector comprising a rectangular hollow pipe .wave guide for propagating electromagnetic energy, said wave guide having a pair of walls parallel to the magnetic field vector of said elec trcmagnetic energy, sa-id Wave guide being provided with an open end for radiating said electromagnetic energy, and each of said walls of said pair of walls parallel to said magnetic field vector having an extension beyond said open end in the direction of radiation, said extensions being substantially in the form of trapezoids.
2. A radiating device comprising a rectangularhollow pipe wave guide for propagating electromagnetic energy, said wave guide being provided with an open end for radiating said electromagnetic energy, and each of the wider walls of said wave guide having an extension beyond said open end in the direction of radiation, said extensions being substantially in the form of trapezoids.
3. A radiating device according to claim 1 'wherein the non-parallel sides of each of said trapezoidal extensions make an angle of 45 and respectively with the parallel sides thereof.
4. A radiating device for feeding a parabolic reflector comprising a rectangular hollow pipe 'wave guide for propagating electromagnetic energy, said wave guide being :provided with a flared open end for radiating said electromagnetic energy, said wave guide having at said open end a pair of walls parallelto the magnetic fleld vector of said electromagnetic energy, and each of ,said walls vof said'pairof 'wallsparallel to said magnetic field vector :having an extension beyond said open end in the :direction of radiation, said extensions being substantially in the form .oftrapezoids.
'5, A radiating device according to claim 4 in which the tnon-parallel sides of each of said trapezoidal extensions make an angle of 45 and 135 respectively with the parallel sides thereof.
.REFERENCES CITED The following references are of record in the file of this patent:
UNITED STATES PATENTS Number Name Date Re. 23,003 Barrow May 25, 1948 2,234,293 Usselman Mar. 11, 1941 2,235,506 Schelkunoff Mar. 18, 1941 2,297,202 Dallenbach Sept. 29, 1942 2,369,808 Southworth Feb. 20, 1945 2,423,073 Willoughby June 24, 1947 2,429,640 Mieher et a1. Oct. 28, 1947 FOREIGN PATENTS Number Country Date 878,830 France Feb. 4, 1943