US 2840818 A
Description (OCR text may contain errors)
June 24, 1958 R. H. REED ET AL SLOTTED ANTENNA Filed April l5, 1954 il M/ M www fr/41Min tCalif., as n s to rghes Aircraft Company, Culver City, Cal i., a corporation of Delaware Application April l5, i954, Serial No. 423,400
7 Cla-tima (Cl. 343-770) rhis invention relates generally to antennas for electromagnetic waves and more particularly to a linear array of closely-spaced radiating slots placed broadside to each other in the side of a waveguide or coaxial transmission line.
As is well known in the prior art, an antenna comprising a linear array of individual radiating elements is often used to form a beam of radiation which has a desired directivity and side-lobe level in a plane containing the array. The distance between adjacent radiating elements, the amplitude of the radiation from each element, and the phase of the radiation between adjacent elements are the three factors which determine the radiation pattern for such an array. The mathematical derivation of the resulting radiation pattern is based upon the theory of interference from isotropic sources and is exhaustively treated in the literature, for example, Antennas, by Kraus, McGraw-Hill Book Co., lne., 1950. A broadside type of array can be constructed by equispaced radiating elements, fed by in-phase microwave energy-sources. lf the distance between radiating elements is large, it is found that in addition to a broadside beam there will berpresent other main lobes which malte angles with the broadside pattern. In other words, the number of main lobes is determined by the spacing between the radiating elements. lf the distance between adjacent radiating elements is equal to or less than half a wavelength of the radiation, only one main lobe exists. if the radiating elements are all spaced one-half wavelength apart and are excited in the same phase, the resul*- ing main beam for equal amplitudes from all radiating elements is as narrow as possible but contains sizeable side-lobes. lt has been found that by controlling the amplitudes of the radiations from the radiating elements, as adjusting the lengths of the elements, the side-lobes can be substantially reduced by sacrificing a small amount of the directivity of the main beam. A preferred method for doing this is referred to as the well-known Dolph- Tchebyscheli1 Optimum iistribution of the amplitudes of the radiating elements.
A further example of the prior art is the so-called end lire array. in such an array, the phase between adjacent radiating elements is retarded progressively by the same amount as the spacing between them. Of course a main beam at any direction is obtainable by some intermediate arrangement of spacing and phase between adjacent radiating elements. ln order that an end lire array have only one main beam, it may be necessary to have the spacing between adjacent radiating elements a quarter of a wavelength apart. lf the distance becomes larger, additional main beams may be present.
A major problem with a practical array design which has caused dil'liculty in the prior art is that of interaction due to mutual coupling between radiating elements, which are spaced close to one another. This mutual coupling destroys the isolation or independence of adjacent radiating elements and creates a dependence between the elements. The theory of the interference of eluted lune 24, 1.95
isotropic sources disclosed above is applicable only to independent sources. lt was found in the prior art that a spacing of a quarter Wavelength between adjacent elements gave rise to a mutual coupling which could produce strong adverse eects in the radiation pattern. "l" his condition was aggravated by the fact that the mutual coupling was dependent on the frequency of the radiation. Although the adverse effects due to mutual coupling could be minimized by a trial and error adjustment of slot dimensions and of phase relations between slots, slight changes of frequency would upset these adjustments completely.
it is, therefore, an object of this invention to provide an antenna having an improved linear array of radiating elements.
lt is another object of this invention to provide an antenna transmission line having a plurality of radiating slots to provide a substantially constant radiation pattern over a wide range of frequencies.
lt is still another object to provide a two-dimensional multiple slot array antenna in which there is no intercoupling between adjacent linear arrays.
The present invention discloses an antenna comprising a linear array having broadside slots as radiators which are spaced extremely close to one another. rhe spacing between adjacent slots is under all circumstances less than one-quarter wavelength, and preferably less than one-tenth wavelength. lt has been found that such an arrangement results in a very much improved radiation pattern in which the adverse effects due to mutual coupling are almost completely eliminated. Furthermore, such an array was found to be rather insensitive to changes in frequency. lt is theorized that the reason for these changes in the array characteristics can be explained on the basis of a continuous distribution of radiating elements. ln fact it was found that the radiation pattern from such an array gave a good approximation t0 a sin X distribution, the characteristic radiation pattern of a continuous distribution having a uniform illumination.
The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be betterunderstood from the following description considered in connection with the accompanying drawings in which several embodiments of the invention are illustrated by way of examples. it is to be expressly understood, however, that the drawings are for the purpose of illustration and description only, and are not intended as a definition of the limits of the invention. I Fig. l is a perspective View of a rectangular coaxial transmission line having closely spaced parallel slots, in accordance with this invention;
Fig. 2 is a curve illustrating certain characteristics of the device of Fig. l;
Fig. 3 is a perspective view of a two-dimensional array of the transmission line shown in Fig. l, for the purpose of describing an illustrative use of the device of this invention; and
Fig. 4 is a perspective view of a hollow rectangular waveguide having closely spaced parallel slots in accordance with this invention.
Referring to the drawings, which are made a part of this specication, like reference characters are used to indicate like parts throughout. Referring particularly lto Figs. l and 2, Fig. l shows a coaxial transmission `line adapted to propagate the TEM mode, and is shown by way of example as a rectangular transmission line i@ having a center conductor 11 and outer conductor 12. A series of transverse slots 14, which are situated parallel and very close to one another, are provided along one section of the outer conductor wall 12. As illustrated, slots 14 may be spaced along one broad wall 16 of outer conductor 12. The spacing between these slots is such that the number of them occupying a distance of one operating wavelength is under all circumstances greater than four and preferably greater than ten. Where uniform illumination is desired, a requirement for a manyelement traveling wave array is that the length of the slots be substantially smaller than resonant slot lengths to keep the excitation coefficients very small.
In Fig. 2, a curve 18 shows the relationship between the side-lobe level and the number of slots per wavelength for the transmission line of Fig. l. 18 has the form shown when transmission line 1) has uniform illumination and uniform phase shift between successive slots. These results were obtained by systematically increasing the number of slots per wavelength and adjusting the length of the individual slots, such that the amplitude of radiation contributed by each slot is substantially the same. It follows, therefore, that the length of the slots must be substantially smaller than the resonant slot lengths as mentioned above. The lengths of the slots can be adjusted by any suitable means to obtain points for curve 16. It should be noted that the sidelobe level is reduced as the number of slots per wavelength is increased, and that the theoretical side-lobe level of a continuous distribution having uniform illumination is approached asymptotically. Fig. 2 shows that for all practical purposes the theoretical side-lobe level is obtained when the number of slots per wavelength is approximately 16 or larger. This is an indication that the array characteristics have changed from one having discrete radiating elements to one which has continuous distribution. The resulting radiation pattern with the device of Fig. 1 having 16 or more slots per wavelength, has the distribution, which is characteristic of a continuous distribution, having uniform illumination, and where X is a variable signifying one-half of the phase difference in radians between the contributions from opposite ends of a continuous distribution of width equal to the effective aperture of the array.
This type of array has a substantially constant radiation pattern and input impedance as a function of frequency. The radiation pattern remains constant because the phase of the mutual coupling between slots is a slowly varying function of frequency due to the close inter-element spacing and, therefore, the slot coupling coecient is less frequency-sensitive than for larger slot spacings. The input impedance remains low and constant because each slot represents only a very small discontinuity to the energy fed to transmission line in other words, the slots are now non-resonant. The various slot discontinuities add to substantially cancel at the input to the array.
The application of the principle of approaching a continuous distribution by the use of large numbers of radiating elements per wavelength and controlling the amplitude of radiated energy by means of metallic tape is not restricted to coaxial transmission lines but has been successfully employed with waveguides. Thus, a hollow waveguide 2t), as shown in Fig. 4, may be utilized to provide a slotted antenna having essentially the same characteristics as the coaxial transmission line of Fig. l. Radiating elements 14 are included in the broad wall 16 having the same proximity and dimensions as those described above in connection with Fig. l. Further, as will be evident, the same results will be achieved regard- The curve CII less of the configuration of the transmission line or waveguide.
Fig. 3 illustrates a two=dimensional array wherein a transmission line 10 ofthe same construction shown in Fig. 1, is placed parallel to transmission line 10 for the purpose of narrowing the radiation pattern in the direction perpendicular to the individual array. The parts of transmission line 10 corresponding to those of transmission line 10 are indicated by primed numbers. Since the individual slots in a linear array are broadsided and the distance between slots in adjacent coaxial lines is comparatively large, there is substantially no interaction between adjacent arrays. Therefore, the radiation pattern due to an individual array is substantially undisturbed, and such pattern retains its original shape in the plane which is parallel to the array and perpendicular to the surface containing the radiating elements. Another consequence of this lack of mutual coupling makes it possible to treat the individual arrays as independent isotropic sources, the interference pattern of which can easily be calculated.
From the foregoing, it is apparent that there has been described an improved antenna comprising electromagnetic translator having a plurality of closely spaced radiating elements or slots, wherein the number of slots per wavelength is sutiiciently great to insure a substantially constant radiation pattern over a wider range of operating frequencies than has previously been possible.
What is claimed is:
1. A radiator of electromagnetic energy comprising an elongated electromagnetic translator; said electromagnetic translator being adapted to propagate electromagnetic energy over a predetermined frequency range, said translator having a plurality of spaced elongated parallel non-resonant slots, and where the ratio of the elongation of said slots to the distance of separation of said slots is substantially greater than two.
2. The combination defined in claim 1, wherein said electromagnetic translator is a coaxial waveguide.
3. The combination defined in claim 1, wherein said electromagnetic translator is a hollow waveguide.
4. A microwave antenna comprising a coaxial waveguide; means for feeding microwave energy into one end of said waveguide; a portion of said waveguide being perforated with a plurality of parallel identical slots having an elongation of less than one-half of the working wavelength, each of said slots having said elongation perpendicular to the center line of said waveguide and being spaced apart a distance which is smaller than onehalf of said elongation.
5. A microwave antenna comprising a hollow conductor, said hollow conductor being adapted to be excited in a predetermined mode by electromagnetic energy having a predetermined operating Wavelength, said hollow conductor being perforated with a plurality of parallel elongated identical non-resonant slots, the longitudinal dimensions of said slots being substantially perpendicular to the center line of said hollow conductor, adjacent pairs of said slots being spaced apart less than one-half of the length of said slots.
6. An antenna having a predetermined working wavelength comprising a section of a transmission line, said transmission line having an inner conductor and an outer conductor, a plurality of identical openings in said outer conductor, said openings having a length of less than onehalf of the working wavelength, said openings being parallel to one another and perpendicular to the axis of said transmission line, and adjacent pairs of said openings being spaced apart less than one-half of the length of said slots.
7. An antenna having a predetermined working wavelength for microwaves comprising a section of a waveguide, said waveguide having a plurality of elongated non-resonant slots in one portion thereof, the longitudinal dimensions of said slots being perpendicular to the axis 5 of said waveguide, said slots being spaced apart less than one-sixteenth of said working wavelength, said waveguide being adapted to be excited in a predetermined mode by microwaves of said working wavelength, said slots serving as radiating elements upon said waveguide being excited in said predetermined mode.
References Cited in the le of this patent UNITED STATES PATENTS