US2874382A - Dual beam antenna - Google Patents

Description

Feb. 17, 1959 G. STAVIS DUAL BEAM ANTENNA 2 Sheets-Sheet 1 Filed June 9. 1954 INVENTOR. 62/8 673 7106 Feb. 17, 1959 Filed June 9, 1954 2 Sheets-Sheet 2 V f 6/ I- T 76 2 MW .2 1 (6 L SOURCE j N 4, 77 i 7/ 72 m 7 Q3 PHASE lam/seams 3 74 M sw/nw I 5 INVENTOR.

GUS ST/QV/S United States Patent Of ce DUAL BEAM ANTENNA Gus Stavis, Ossining, N. Y., assignor to General Precision Laboratory Incorporated, a corporation of New York Application June 9, 1954, Serial No. 435,584 '9 Claims. (Cl. 343-771 This invention relates to an antenna for alternately radiating one or another pair of beams of energy.

The art of aerial navigation by means of airborne radio apparatus has received considerable attention in recent years. frequently desirable to propagate radio energy from the aircraft toward the ground in four discrete beams. These beams are paired, the first pair consisting of a beam directed forward and to the right and a beam directed rearward and to the left while the second pair consists of a beam directed forward and to the left and a beam directed rearward and to the right. The complete system normally includes means for generating the pairs of beams alternately. The individual beams may be formed in a number of ways, but linear arrays, having the in- 7 linear array antenna capable of. radiating simultaneously dividual radiators aligned substantially parallel to the direction of travel of the aircraft, are favored because the shape of the portion of the ground illuminated by such an array is appropriate for navigational purposes. A linear array inherently generates a beam in the form of a hollow cone the axis of which coincides with the axis of the array and when such a cone is extended to 2,874,382 Patented Feb. 17, 1959 the individual arrays combine to form two beams, one

In connection with such apparatus, it is directed forward and to the right and the other directed rearward and to the left. The transverse angle of the beams can be varied by adjusting the transverse spacing between adjoining arrays and the other two beams may be formed by reversing the phase of the excitation of alternate arrays.

The foregoing antenna arrangement is more fully described in the copending application of George R. Gamertsfelder, Serial No. 334,914, filed February 3, 1953, for Planar Microwave Antenna Array, now Patent No. 2,854,666. It has been found to be fully satisfactory but is subject to the limitation that it cannot be designed-to produce beams making angles greater than 70 /z with the array axis without also producing anintersect a plane parallel to its axis the intersection is a hyperbola. In physically realizable structures the conical shell of radiation has an appreciable thickness so that the spot illuminated on the ground is bounded by two hyperbolas having a common transverse axis which is parallel to and lies directly beneath the axis of the array.

One antenna system for obtaining two pairs of beams as above described comprises four linear arraysone for each beam. Two of the arraysare arranged so that the conical shell of radiation is directed forward while the other two are arranged to direct the radiation toward the rear. Each array is provided with reflectors to confine the radiation to the downward portion on one or the other side. In this fashion, the fourbe'a'ms are produced. However, this arrangement has several disadvantages. First, the reflectors necessary to confine the beam sufficiently are quite bulky and bulk is always an important consideration in airborne equipment. Second, the forward and rearward beams are produced by separate arrays. Now it is desirable that the forward and rearward angles be identical and that this identity be independent of the frequency of transmission. If the individual radiators are fed by a waveguide, the angle depends, among other things,- upon the ratio of the free space wavelength to the guide wavelength. This means that the guide wavelengths, and therefore the guide dimensions, in the. individual arrays must be iden tical, which, in turn, imposes severe manufacturing tolerances on'the waveguides; Third, only one-half of the individual radiators are in use at any instant of time.

Another antenna system for producing the'four beams opment of airborne navigation equipment hasrecently made it' desirable to obtain beam angles in the range from to It is an object of this invention to provide a compact two beams making angles with the array axis in the range of -70 to 90, 5 a 7 Another object of the invention is to provide an antenna system for alternately radiating one or another pair of narrow beams.

In accordance with the invention, a plurality of linearly arranged individual radiators are excited from a waveguide containing radio energy. The current distribution is sinusoidal so as to produce two conical beams making angles equal in magnitude but opposite in sign with the axis of the array. The two conical beams can be restricted to two narrow beams byarranging several such linear arrays side by side and suitably selecting the relative positions of the sinusoidal current distribution of each array. g 3

For a clearer understanding of the invention, reference may be made to the accompanying drawing, in which:

Figure l is a diagram useful in explaining how two conical beams can be produced from a linear array antenna;

Figure 2 is a diagram of the radiation from a linear array antenna; 7

Figure 3 is a graph showing the angles which conical beams make with the array axis under various circumstances;

Figure 4 is a plan view of a single linear array antenna in accordance with the invention together with a curve further describing its construction;

Figure 5 is a diagram showing the direction of the beams produced by the planar array of the invention;

Figures 6 and 7 are diagramsuseful in explaining the operation of a planar array antenna; and

Figure 8 is a schematic showing of a planar array antenna according to the invention.

Referring now to Fig. 1, let us consider a number of isotropic radiators, represented by the dots 11, 12, 13 etc. arranged along a line such as AA, cophasally excited, and having a sinusoidal .current distribution as indicatedby the curve 15. It can be shown that such 3 an array of radiators will propagate two beams of energy in such directions that ls1n(:l;0) +n (1) where 'j =the angle between the normal, N, to the array and the beams;

A=free space wavelength;

L=length of one cycle of the current distribution s=distance between radiators, and

n=a positive or negative integer, or zero.

A convenient way of obtaining the distribution illustrated in Fig. 1 is to feed the individual radiators from a waveguide and to make the spacing between radiators a half guide wavelength. The current in the individual radiators can be controlled by well-known techniques, the method selected depending upon the type of radiators used. For example, if slots on the broad face of the guide parallel to the axis are used, variation in the displacement of the individual slots from the center line of the guide will vary the current in the'individual slots. The possibilities and limitations of such an antenna may best be understood by considering an example; In Fig. 3 there is plotted the variation in beam angles (0) of the various order beams as function of radiator spacing in terms of free space wavelength (s/X) for the case where )\==l.34" and 'y =7O (0=20). Variations in s/)\ are assumed to be made by varying the size of the waveguide.

As can be seen from Fig. 3, two zero order beams (n==0) are produced at angles of 120, regardless of radiator spacing. If radiation'is to be confined to two major beams, the spacing s/X must be kept below the critical value .745 as indicated by theordinate 16. The spacing s/a must also be held above a minimum value to preclude the propagation of the TE mode in the waveguide and this minimum value is .577, as indicated by the ordinate 17. It is thus seen that for the case illustrated in Fig. 3 where 0 =:20, there is a range of possible values for s/k which means that there 'is a range of guide sizes which can be used. Let us now consider the values of'a which can be selected.

The lower limit of s/A occurs when the broad, or a," dimension of the guide approaches A.

sin (:59) =cos -l-n g- The lower limit of s/X is thus seen to be independent of both A and the value of 0 selected and is .577 for all cases. The upper limit varies with (hand is determined by the threshold at which the n=-l lobe appears. The n=-1 lobe first appears at 90, so the limiting value of SA occurs when, in Equation 2, sin (:0) equals l, when n=-1.

From Equation 3 it can be seen that for 0 =20 :70) the limiting value of s/A is .745, as previously stated. When 0 =0, the limiting value is 1. There is therefore no lower limit to 0 imposed by the necessity for suppressing higher order lobes, since, as 0 is chosen smaller and smaller, the range of values of s/a increases.

There is, however, a practical lower limit to 0 From Equation 1 it can be seen that Bill 90 so that when 0 approaches zero, L becomes infinite. As 0 is selected smaller and smaller it is necessary to make the array longer and longer in order to obtain a full wavelength of the current distribution.

As fl is made larger, the range of possible values of s/X decreases until at some point the lower and upper limits of s/a coincide. The upper limit of a, is found by setting s/ \=.577 in Equation 3 and solving for from which it is seen that this design is suitable for producing two conical beams at any angle ('7) to the array from 43 to 90".

To illustrate the foregoing principles, suppose it is required to design 'a waveguide fed slot antenna for prod'ucing two beams at angles of to the array axis, for operation at A=1.34. The antenna will comprise a waveguide fed from one end and terminated in a short circuiting plate to produce a standing wave within the guide and the slots will be located a half guide wavelength apart at voltage maxima. The first requirement is that the guide must be small enough to preclude propagation of the TE mode. The a dimension therefore cannot exceed 1.34", and the largest standard guide which can be used has an a dimension of 1.122". The guide wavelength is The spacing is From Equation 1, the length of the current distribution is A 1.34 sin 0 sin 10 Referring to Fig. 1 let us take as a reference the slot at the center of the array and call this the 11:0 slot. The current in the nth slot will be from which the current in each-slot may be readily computed. Slot currents are, of course, determined by the lateral spacing from the centerline of the guide.

Fig. 4 shows an'autenna designed as above described comprising a rectangular waveguide 21 having a plurality of slots such as 22,231 and 24 formed in the broad side of the guide. The antenna is fed from the left, as shown, in Fig. 4, and the other end is closed by means of a conductive plate 25, located M74 from the center of the last slot 24. The slots are spaced *apart'a distance s=7\ /2 and the current distribution followsthe curve 26.

Referring back to Fig. 3, there is another possible mode of operation. It may be seen from the figure that there is a value of s/)\ for which the 11:0 and the n=1 lobes coincide. This value of s/)\ is is indicated in Fig. 3 by the ordinate 18 and may be determined by solving Equation 1 for n=0 and n=1 and equating the results.

That is, the length of the current distribution is equal to the guide wavelength. This is the mode of operation of the antenna described in the aforementioned Gamertsfelder application Serial No. 334,914.

The angular limits of operation may be determined as before. The spacing s/A must be large enough to preclude propagation of the TEgo mode in the guide and this value has previously been found to be s/ \=.577. From Equation 1 The upper limit of 0 is thus seen to be 60, which corresponds to a lower limit of :30". As 70 is increased, a point is reached at which the n=+1 lobes appear. from Equation 2, by setting 6:90", n=+l, and remembering that, for the special case under consideration,

als= g=2 cos yo sin 90=cos rtu 1=cos 'y +2 cos 7 7 6 i It is thus seen that this special case is limited to values of '7 between 30 and 70% or, in other words, to values of 0 between 19% and 60.

The linear arrays so far discussed produce two conical beams in which the axis of the cones coincides with the axis of the array. In Fig. 5 there is shown such an array 31 having a longitudinal axis 32. The two conical beams when extended to intersect a plane parallel to the array will illuminate'two surfaces, the first boundedby hyperbolas 33 and 34 and the second bounded by hyperbolas 35 and 36.

It will be understood that a linear array of isotropic radiators would normally produce two complete cones of radiation, as illustrated in Fig. 2. However when the radiators are fed by a waveguide, and the array is mounted on an aircraft, the portion of the conical radiations above the array axis 32 is effectively suppressed. For aerial navigation purposes, it is desirable to further confine the illuminated portion to two small areas, such as the areas 37 and 38, lying to one side of the projection 39 of the axis 32 on the horizontal surface. This may be accomplished by arranging several linear arrays side by side.

In Fig. 6, the lines 41, 42, 43, 44 and 45 represent linear arrays, similar to that shown in Fig. 4, having sinusoidal current distributions represented by curves 46, 47, 48, 49 and 50 respectively. The lines 52, 53 and 54 represent the equipotential lines of the standing wave pattern created by the arrays making an angle A with the array axis. It can be shown that the dual beams will have their maxima in the plane which is normal to the array and to the'equipotential lines of the field of the array. Thus, in Fig. 6, the directions of the two maxima are indicated by the arrows 55 and 56 making an angle a with the array axis. It can be seen that the angle on can be varied by varying the relative space phase of the current distribution in the various arrays. Thus, if the distribution were identical in all arrays, a would be zero. If the distribution is shifted uniformly between arrays, u will be different from zero and will depend upon the amount of the shift and upon the spacing, s between arrays. In the instant case, it is desirable to be able to produce pairs of beams alternately, the first pair making an angle a and the second pair making an angle -a with the array axis. This can be accomplished conveniently by displacing the distribution between adjoining arrays by 90. If the microwave excitation to alternate arrays be reversed in phase, the current distribution This limiting value of 7 may be determined of these arrays will be shifted by 180. As shown in Fig. 7, the equipotential lines are angularly shifted so that the beams now make an angle of u with the array axis as shown by the arrows 58 and 59.

Referring again to Fig. 5, the relationship between various angles is shown. 7 is the angle between the array axis and the elements of the conical beam. at is the angle between the horizontal projections of the array axisand the conical element under consideration, and is identical to the angle cc of Fig. 6. B is the angle between the vertical line normal to the plane of the array 31 and the projection of the conical element under consideration on a vertical plane through the normal'perpendicular to the array axis 32. From the geometry of the figure,

tan cc=SiI1 ,8 tan 7 From Fig. 6, A=+ oz, t-an a=-cot A, whence -cot A=sin a tan 7.

To illustrate the above principles, suppose it is desired to construct a planar array for alternately producing two pairs of beams for which 'y=i80 fi=i15 and to be operated at a wavelength of 1.34". A linear array for producing beams for 'y=i80 has already been considered and is illustrated in Fig. 4. Several linear arrays similar to that of Fig. 4 may be arranged side by side to meet the present requirements and such an arrangement is shown in Fig. 8.

Referring now to Fig. 8, there are shown five linear array antennas, 61,62, 63, 64 and 65, arranged side by side to form a planar array. Arrays '61 and 65 may be identical to that shown in Fig. 4, while arrays 62, 63, and 64 differ only in the arrangement of the slots. The array 61 is provided with slots throughout the length of the broad face of the waveguide, arranged precisely as shown in Fig. 4. For the sake of clarity only a single slot, 67, is shown in Fig. 8. This slot corresponds to the slot 22 of Fig. 4 and represents the slot carrying the maximum current. Each of the antennas 62, 63, 64 and 65 is similarly provided with slots throughout their respective lengths of which only the slots carrying the maximum current are shown. These slots are, for antenna 62, slot 68; for antenna 63, slots 71 and 72; for antenna 64, slots 73 and 74; and for antenna 65, slot 75.

The longitudinal position of the maximum current slots are readily determined. Itwill be recalled in connection with the design of the antenna of Fig. 4 that the length L of the current distribution was determined to be L=7.719. From Fig. 6 it is seen that the position of the current distribution maximum in each array is displaced by 90, or L/4, from that of the preceding array. Thus slot 68 is shifted longitudinally by L/4=1.93" from slot 67. In a similar fashion, the positions of slots 71, 72, 73, 74 and 75 are determined.

Next, the spacing s, between arrays must be calculated. It will be recalled that cot A=sin ,8 tan 7 -cot A=sin 15 tan 80=1.47 A=.145.7

From Fig. 6, it is seen that In Fig. 8 there is also shown a source of microwave energy 76 connected directly to arrays 61, 63 and 65. Source 76 is also connected to a phase reversing switch 77 which in turn is connected to excite arrays 62 and 64. The switch 77 has two positions in the first of which the microwave energy is transmitted to excite arrays 62 and 64 in phase with the excitation of arrays 61, 63 and 65. When so excited, the complete planar array will produce dual beams at 6=+10, 13=+15 and at =-l0, /3=l5. In the other position of switch 77, arrays 62 and 64 are excited with microwave energy 180 out of phase with the excitation of arrays 61, 63 and 65. This effects the inversion of the current distribution of arrays 62 and 64, as shown in Fig. 7, and with this excitation the planar array will produce dual beams at 0=+10, ;8=15 and at 0=l0, 9=+15.

A planar array antenna according to the invention can be designed to produce dual beams making any angle 'y with the array axis from 43 to 90. No bulky reflectors are required to confine the beams to small spots. Since both beams are produced by the-same elements, equality of forward and rearward beam angles is secured without stringent tolerances on the dimensions of the waveguides.

A number of modifications may be made within the scope of the invention. For example, the sinusoidal current distribution may be modified by superimposing a suitable taper to reduce side lobes. vAs another example, the-individual radiators may comprise dipoles instead of slots. Many other modifications will. occur to those skilled in the art.

What is claimed is:

1. An antenna for radiating two beams of energy co1nprising, a plurality of linear arrays arranged side by side,

the lateral spacing between adjoining arrays being equal,

each of said linear-arrays comprising a row-of individual radiators, the maximum current in said radiators relative to each other varying sinusoidally throughout the length of the array,'the length of one cycle of said current distribution exceeding twice the spacing between adjacent radiators, and the longitudinal position of the maximum current in each array being longitudinally displaced from the corresponding position in adjacent .arrays.

2. An antenna for radiating two beams of energy comprising, a plurality of end fed microwave transmission lines arranged side by side, a plurality of radiators positioned along each of said transmission lines at selected spaced distances, means for coupling each of said radiators to the microwave energy existing in its associated transmission line, the coupling of the individual radiators as respects each other being selected to produce a sinusoidal current distribution in said radiators along the length of each of said transmission lines, the wavelength of said current distribution exceeding twice the spacing between adjacent radiators, the longitudinal position of the maximum of the current distributions along each transmission line being longitudinally displaced from the corresponding position in adjacent transmission lines.

3. An antenna for radiating two beams of energy comprising, means for impressing microwave energy on a plurality of Waveguides arranged side by side, each of said waveguides having a plurality of individual radiators coupled thereto along its length to form a linear array, the degree of coupling of said radiators to said waveguides varying so that the variation in the maximum current of the radiators produces a sinusoidal current distribution, the wavelength of said current distribution exceeding the wavelength of the microwave energy in said waveguides, and the longitudinal position of the maxima of said current distribution in each array being longitudinally displaced from the corresponding position in adjacent arrays.

4. An antenna for radiating alternatively either one of a pair of beams of energy, comprising, a plurality of waveguides arranged side by side, each of said waveguides having a plurality of individual radiators coupled thereto along its length to form a linear array, the degree of coupling of said radiators to said waveguide varying so that the variation in the maximum current of the radiators produces a sinusoidal current distribution, the wavelength of said current distribution exceeding the wavelength of the microwave energy in said waveguides, the longitudinal position of the maxima of said current distribution in each array being longitudinally displaced from the corresponding position in adjacent arrays, by a distance equal to one quarter of the wavelength of said current distribution, means for impressing microwave energy on all of said arrays, and means for reversing the phase of the microwave energy applied to alternate arrays.

5. An antenna for radiating alternatively either one of a pair of beams of energy, comprising, a plurality of Waveguides arranged side by side, the lateral spacing between adjoining arrays being equal, each of said waveguides having a plurality of individual radiators coupled thereto along its length to form a linear array, the amount of coupling of said radiators to said waveguides varying so that the variation in the maximum current of the radiators produces a sinusoidal current distribution along the length of each array, the wavelength of said current distribution exceeding the wavelength of the microwave energy in said Waveguides, the longitudinal position of the maxima of said current distribution in each array being longitudinally displaced from the corresponding position in adjacent arrays, by a distance equal to one quarter of the wavelength of said current distribution, a source ofmicrowave energy, means for exciting alternate arrays with energy of the same phase directly from said source and means for exciting the remaining arrays from said source through a phase reversing switch, whereby said remaining arrays are selectively excited with 9 microwave energy having the same or opposite phase as that of said alternate arrays.

6. An antenna comprising a plurality of individual radiators arranged in a line, and wave transmission means for coupling energy to said radiators, said radiators being sufiicient in number and having such varying degrees of coupling to said transmission means as to produce at least one cycle of a sinusoidal maximum current distribution in said radiators, the length of one cycle of said distribution being greater than twice the spacing between adjacent radiators.

7. An antenna comprising, a plurality of individual radiators arranged in a line, and wave transmission means for coupling energy to said radiators, said radiators havpling to said individual radiators being selected to produce a sinusoidal maximum current distribution from radiator to radiator along said waveguide, the length of one cycle of said current distribution being greater than twice the spacing between adjacent radiators, said antenna being long enough and having enough radiators to include at least one complete cycle of said current distribution.

9. A dual beam linear array antenna comprising, a waveguide, a conductive plate closing one end of said waveguide so that microwave energy entering said waveguide sets up a standing wave, and a plurality of individual radiators on said waveguide, one at each voltage maximum of said standing wave, said radiators having such various amounts of coupling to the energy within said waveguide that the maximum current varies sinusoidally from radiator to radiator along said waveguide, the length of one cycle of said current distribution being greater than the wavelength within said waveguide and less than the length of said antenna.

References Cited in the file of this patent UNITED STATES PATENTS 2,225,312 Mason Dec. 17, 1940 2,408,435 Mason Oct. 1, 1946 2,605,413 Alvarez July 29, 1952 2,607,008 Guarino Aug. 12, 1952 OTHER REFERENCES Antennas, by J. D. Kraus, McGraw-Hill Book Co., New York, 1950, pages 347-350.

Images