Slotted antenna

June 24, 1958

2,840,818

R. H. REED ETAL

SLOTTED ANTENNA

Filed April 15, 1954

<img file="US2840818A_D0001.tif" />

<img file="US2840818A_D0002.tif" />

6'—-----------1--------------1_____________i______________i_ ^ΰ S /a /5 2a

Jlars ah Mmuww

-------------1---------------------------------------1 2$ 3a /mrus.

//. /?£££ /Vosπ 7 cZ Sna-EM, Sv /rra^E£/·

United States Patent Office

2,840,818

Patented Jtme 24, 1958

2,840,818

SLOTTED ANTENNA

Richard H· Reed sod Robert J. Stegen, Los Angeles, Calif., assignors .to Hughes Aircraft Company,'Culver City, Calif., a corporation of Delaware

ApplicatioB April 15, 1954, Serial No. 423,400

Claims. (CL 343—770)

This 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., Inc., 1950. A broadside type of array can be constructed by equispaced radiating elements, fed by in-phase microwave energy-sources. If the distance between radiating elements is large, it is found that in addition to a broadside beam there will be present other main lobes which make angles with the broadside pattern. In other words, the number of main lobes is determined by the spacing between the radiating elements. If 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 resulting main beam for equal amplitudes from all radiating elements is as narrow as possible but contains sizeable side-lobes. It 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 DolphTchebyschefi Optimum Distribution of the amplitudes of tire radiating elements.

A further example of the prior art is the so-called end fire 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. In order that an end fire array have only one main beam, it may be necessary to have the spacing between adjacent radiating elements a quarter of a wavelength apart. If the distance becomes larger, additional main beams may be present.

A major problem with a practical array design which has caused difficulty 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 isotropic sources disclosed above is applicable only to independent sources. It was found in the prior art that a spacing of a quarter wavelength between adjacent elements gave rise to a mutual coupling which could pro5 duce strong adverse effects in the radiation pattern. This 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 10 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 radiat15 ing elements.

It 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.

It 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 25 are spaced extremely close to one another. The spacing between adjacent slots is under all circumstances less than one-quarter wavelength, and preferably less than one-tenth wavelength. It has been found that such an arrangement results in a very much improved radia30 fion 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. It is theorized that the reason for these changes in the array characteristics can be ex35 plained on the basis of a continuous distribution of radiating elements. In fact it was found that the radiation pattern from such an array gave a good approximation to a sin X “IT 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 better understood 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.

Fig. 1 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. 1;

Fig. 3 is a perspective view of a two-dimensional array of the transmission line shown in Fig. 1, 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 specification, like reference characters are used to indicate like parts throughout. Referring particularly to Figs. 1 and 2, Fig. 1 shows a coaxial transmission line adapted to propagate the TEM mode, and is shown by way of example as a rectangular transmission line 10

2,840,818 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. 1. The curve 18 has the form shown when transmission line 10 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 sin X

X~ 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 coefficient 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 10; 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 20, 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. 1. Radiating elements 14 are included in the broad wall 16 having the same proximity and dimensions as those described above in connection with Fig. 1. Further, as will be evident, the same results will be achieved regard less of the configuration of the transmission line or waveguide.

Fig. 3 illustrates a two-dimensional array wherein a transmission line 10' of the 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 sufficiently great to insure a substantially constant radiation pattern over a wider range of operating frequencies than has previously been possible.