US2629052A - Scanning antenna - Google Patents

Description

Feb. 17, 1953 H. I'AMS 2,629,052

SCANNING ANTENNA F'iled Dec. 12, 194'? C f@ Jpg. a 12- n u 'o ,4m-H2450 ciu/05 S3: g PoLYsry/mvf 15) 6 f/L50 @w05 x u, 3f- 4 53 uns, 2' o I! b2 lbs fefquf/vcrf Mawr-off' mman/CW lwnvr:

Harleylams Ai'tgzgney Patented Feb. 17, 1953 OFFICE SCANNING ANTENNA Harley Iams, Venice, Calif., assignor to Radio Corporation of America, a corporation of Bela Ware Application December 12, 1947, Serial No. '791,178

More specifically, it is one of the objects of the Y present invention to improve the deflection sensitivity of frequency-responsive scanning antennas of the leaky waveguide type by lling the waveguide with dielectric material.

Another specific object is to provide deection increasers for frequency responsive scanning antennas, comprising arrays of waveguides of different lengths arranged in such manner as to accentuate the displacement of phase front with variation in frequency.

The invention will be described with reference to the accompanying drawing, wherein:

Figure 1 is a pictorial view of a portion of a scanning antenna embodying the present invention,

Figure 2 is a graph showing the performance of the device of Fig. 1,

Figure 3 is a pictorial view of a structure like that of Fig. 1 in combination with means for increasing the beam deection provided thereby, and

Figure 4 is a pictorial view of a modification of the structure of Fig. 3.

It is well known in the radio art that a waveguide having a series of openings along one of its walls will act as a beam forming antenna. Such antennas are often called leaky waveguide antennas. Some of the applied energy leaks out each of the openings and is radiated, so that the device behaves as a linear array of radiators. The direction of the beam depends somewhat upon the frequency of the energy applied to the waveguide.

According to the present invention, the effect of frequency upon beam direction is utilized for scanning. Ordinarily, the variation in beam direction with small variations in frequency is too slight to be of substantial utility. I have found that by making the waveguide approximately one-half wavelength wide in the direc- (Cl. Z50- $3.63)

tion perpendicular to the electric vector, and spacing the openings slightly more than onehalf wavelength apart, the desired variation of beam direction is substantially at a maximum. The foregoing dimensions are in terms of the wavelength in free space of the energy with which the antenna is to operate.

The deection sensitivity (i. e. the change in beam direction for a given change in frequency) may be increased further by filling the waveguide with insulating material having a dielectric constant substantially greater than unity. Polystyrene is an example of such a material, having a dielectric constant 6:2563.

Figure l shows a portion of a dielectric filled leaky waveguide antenna. A hollow waveguide l is provided with radiator openings 3 in one of its walls 5, spaced along the length of the guide. The wall 5 is parallel to the electric vector of energy propagating along the guide. The interior is filled with dielectric material l. The width w of the guide I, from the wall 5 to the opposite wall, is approximately where A is the mean wavelength of the energy with which the antenna is to operate, and e is the dielectric constant of the material 1.

Energy is supplied to one end of the guide l, as indicated by the arrow S, and ows toward the other end, a portion of it escaping through each of the apertures 3. Suppose the frequency to be such that' the above relationship between wavelength and guide width is fulfilled exactly. Then the phase velocity in the guide is substantially infinite, and all of the apertures 3 will radiate in phase, producing a beam perpendicular to the wall 5. Now suppose the frequency to be increased slightly. The phase velocity in the guide is reduced, and the radiation from apertures nearer the end of the guide where the energy is applied will lead (in phase) that from apertures further from said end. This produces a linear phase front at some angle to the wall 5, and the resulting beam is tilted toward the direction of propagation through the waveguide.

Refer to Fig. 2. The curve Il shows the beam deviation vs. frequency characteristic of a dielectric filled guide like that of Fig. l. It is evident that a one percent change in frequency causes a change of about 14 degrees in the direction of the beam. The curve I3 shows that an air-filled 1 guide of optimum dimensions requires a frequency variation of approximately 21/2 times as much to cause the same beam deflection.

Referring to Fig. 3 an assembly of waveguides I5 is shown, the individual guides being bent or curved so that their respective ends are in line. Each of the waveguides in the bundle differs in length from its neighbors by -a predetermined amount, and the guides are arranged in order of their lengths. For convenience in explanation, it is assumed that each of the waveguides has an inside width whi-ch is just equal to \o/2, where ku is the free-space wavelength. The wavelength in the guides is then infinite, and power which enters adjacent waveguides in the same phase emerges still in phase. N ow let the frequency be increased by 1 percent. The wavelength in the guides may be computed and found to be 7.07m. The center to center spacing of the guides may be made some convenient distance, such as 0.55m. The difference in path length in adjacent guides period delay between adjacent guides. The

emerging wave front is then tilted by sin*1 gigg=about 26 Omo This is the deiection which is caused by the waveguide assembly, independently of any deflection produced by the leaky waveguide I'I.

It will be apparent to those skilled in the art that the waveguides in the assembly I5 need not be bent through an angle of 180 degrees, as shown in Fig. 3, but may extend through a relatively small sector or, if greater deflection sensitivity is desired, may be formed in a helix with several turns as shown in Fig. 4. The difference in length between adjacent guides need not be exactly one wavelength, but may be any constant amount. In the waveguide yassemblies either of Fig. 3 or Fig. 4, the lengths of the curved guides are Aproportional to their respective radii of curvature, each waveguide of an assembly being curved I through the same angle as that of the others of the same assembly. The arrangement illustrated may `be described as an assembly of waveguides each having a wall in common with an ,-f

adjacent waveguide and each waveguide being curved.

I claim as my invention:

1. A radio antenna for providing a beam whose direction varies with variation in Ifrequency of fi energy applied thereto, including a hollow waveguide filled with insulating material having a dielectric constant substantially greater than unity,

a. series of radiator apertures in the wall of said waveguide, spaced apart from each other in the direction of propagation of energy along said guide, and an array vof further waveguides, one end of each of said further waveguides being positioned with respect to the corresponding ends of the others of said further waveguides so that said ends lie in a line parallel and adjacent to the line of said radiator apertures, and the other ends of said further waveguides being similarly positioned with respect to each other to dene a second line, the length of each of said further waveguides differing from those of its neighbors by a predetermined amount.

2. In a scanning antenna system which provides a beam having a substantially linear phase front and varies the direction of sai-d phase front to deect said beam, means for increasing the delection of said beam comprising an array of waveguides of diierent lengths each curved to a substantially semicircular shape, each differing in length from at least one of the others by a predetermined amount, each end of each of said waveguides being positioned with respect to the corresponding ends of the others of said waveguides so that said ends dene substantially straight lines; sai-d waveguidesl being arranged in the order of their respective lengths.

3. In la scanning antenna system which provides a beam having a substantially linear phase front and varies the direction of said phase front to deflect said beam, means for increasing the deflection of said beam comprising an array of waveguides of Idifferent lengths each curved to a substantially helical shape, each diiering in length from at least one of the others by a predetermined amount, each end of each of said waveguides being positioned with respect to the corresponding ends of the others of said waveguides so that said ends define substantially straight lines; said waveguides being arranged in the order of their respective lengths.

HARLEY IAMS.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,369,808 Southworth Feb. 20, 1945 2,408,435 Mason Oct. l, 1946 2,411,872 Feld-man Dec. 3, 1946 2,436,408 Tawney Feb. 24, 1948 2,442,951 Iams June 8, 1948 2,447,768 Mueller Aug. 24, 1948 2,461,005 Southworth Feb. 8, 1949

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