AU5669886A

AU5669886A - Waveguide slot array antenna

Info

Publication number: AU5669886A
Application number: AU56698/86A
Authority: AU
Inventors: Joseph H. Acoraci
Original assignee: Allied Corp
Current assignee: Allied Corp
Priority date: 1985-10-03
Filing date: 1986-03-25
Publication date: 1987-04-24
Also published as: WO1987002192A1

Description

WAVEGUIDE SLOT ARRAY ANTENNA

Technical Field of the Invention

This invention relates to antennas and more particularly to a waveguide slot array antenna. Background of the Invention

A waveguide slot array antenna may comprise a number of waveguide elements which are fed with R.F. energy from one end and terminated at the other end of the waveguide. Slots are formed in the waveguide to radiate or leak R.F. energy. The amount of energy coupled out of a slot in a waveguide is determined by the slot size and angle. The antenna pattern from the antenna aperture of the slot array antenna is determined by the amplitude and phase distribution of the R.F. energy radiating from the slots in the waveguides. The angle, orientation and location of the slots along the waveguides provides the amplitude and phase distribution from each slot.

Each waveguide element may be rectangular in cross section having a broadside and a narrow side or edge. For obtaining R.F. energy from a waveguide which is polarized in the direction of the longitudinal axis of the waveguide, slots are formed in the narrow side or edge of the waveguide element. The antenna pattern from an array antenna may be electronically scanned for example ± 60°in azimuth by controlling the phase and amount of radio frequency energy coupled to each waveguide element. A power divider and phase shifters are normally used to control the phase and amplitude of the R.F. energy at the input of each waveguide element. Waveguide slot array antennas may form a pencil beam with low side lobes which radiates broadside to the slot array antenna. When the pencil beam is directed at 90° to the antenna aperture or waveguide longitudinal axis or at an acute angle with respect to the the waveguide longitudinal axis at the end fed by R.F. energy, a Single pencil beam with low side lobes may be generated. When the pencil beam is directed at an angle greater than 90° with respect to the antenna aperture and away from the waveguide end fed by R.F. energy, multiple beams are formed, for example, the desired pencil beam and a grating lobe. In many applications the presence of a grating lobe is unacceptable.

Presently for a particular application, a waveguide slot array antenna is positioned near the surface of the ground for radiating a pencil beam at ground level and extending up to an elevation of 20°. The waveguide slot array antenna is positioned ^vertical to the ground with the waveguide elements positioned parallel to one another and vertical to the ground with the R.F. energy fed at the upper end of each waveguide with the lower end of each waveguide terminated. The -6 db point of the underside of the pencil beam pattern is positioned at the horizon, thereby reducing undesirable ground reflections.

A general discussion of waveguide slot array antennas is provided in a book by H. Jasik entitled "Antenna Engineering Handbook" McGraw-Hill Book Company, Inc. 1961, pgs. 9-1 thru 9-18 which shows an edge slot in a rectangular wave guide in Fig. 9-11.

It is therefore desirable to provide a waveguide slot array antenna that may radiate a single pencil-beam from the antenna aperture at angles greater than 90° with respect to the waveguide longitudinal axis at the end fed by R.F. energy.

It is further desirable to provide a waveguide slot array antenna having additional slots? to provide R.F. energy to cancel a grating lobe. It is further desirable to provide a waveguide slot array antenna which may be positioned above the ground with a plurality of vertical waveguide elements having the end nearest the ground fed with R.F. energy and the upper end of each waveguide element terminated.

Summary of the Invention

An apparatus is described for radiating electromagnetic energy in substantially a single beam comprising a plurality of waveguides spaced apart from one another, each waveguide having a first wall facing in a first direction, each waveguide having a plurality of first slots in the first wall having a first spacing along the waveguide for generating a first beam and an undesired second beam, each waveguide having a plurality of second slots in the first wall having a second spacing along the waveguide less than the first spacing for generating a third beam to substantially cancel the second beam, and each waveguide adapted for coupling electromagnetic energy thereto having a predetermined amplitude and phase, respectively.

Description of the Drawing

Fig. 1 is a schematic and pictorial view of a waveguide array antenna of the prior art. Fig. 2 is a graph showing a typical radiation pattern from the antenna Fig. 1.

Fig. 3 is a schematic and pictorial view of one embodiment of the invention.

Fig. 4A is a front view of a portion of one waveguide element in Fig. 3.

Fig. 4B is an enlarged view of a portion of Fig. 4A.

Figs. 5 and 6 are front views of a portion of a waveguide element.

Figs. 7A and 7B are graphs of the radiation pattern from an array of waveguide elements where one is shown in Fig. 5.

Figs. 8A and 8B are graphs of the radiation pattern from an array of waveguide elements where one is shown in Fig. 6.

Fig. 9A and 9B are graphs of the radiation pattern from an array of waveguide elements shown in Fig. 3. Detailed Description

Referring to Fig. 1, a waveguide slot array antenna 1 ^• of the prior ar is shown. A plurality of waveguid elements 11-19 are positioned side-by-side and spaced apar 5 from one another. The lower end of waveguide elements 11-1 are terminated by resistors 20-28. The upper end o waveguide elements 11-19 are coupled through phase shifter 29-37, respectively, to a respective input of power divide 39. Waveguide elements 11-19 have one edge wall which ma 0 be co-planar to one another with slots 42 formed therein t provide a radiating aperture 43. Radio frequency ^"(R.F.) power or microwave power is coupled to power divider 39 ove line 40. Below waveguide elements 11-19 is shown th earth's surface or ground 41. 5 Fig. 2 is a graph showing a typical radiation patter from waveguide slot array antenna 10 of Fig. 1. In Fig. the ^' radius P represents amplitude and the polar angle 5 represents elevation angle with 90° at the horizon. A shown in Fig. 2 a single lobe or beam is radiated extendin o from the horizon upwards in elevation to an angle of 20 shown by arrow 45. Curve 46 shows the elevation directio and amplitude of the main beam. Curve 47 shows th amplitude and direction of the upper side lobe adjacent th main beam 46 and curve 48 shows the amplitude and directio 5 of the lower side lobe adjacent the main beam 46.

Fig. 3 is a schematic and pictorial view of a waveguid slot array antenna 50 for radiating electromagnetic energ in substantially a single beam. Waveguides 51-59 are show spaced apart from one another. Waveguides 51-59 have a wal 0 60-68,; respectively, which may have a surface facing in th same direction. Walls 60-68 may be, for example, co-planar. Waveguides 51-59 are terminated by resistors 69-77, respectively, to ground potential, ' for example, b line 78. The lower end of waveguides 51-59 are coupled over 5 lines 79-87, respectively, through phase shifters 88-96, respectively, and over lines 97-105/ respectively, t respective inputs of power divider 106. Power divider 106 receives electromagnetic energy such as in the form of radio frequency or microwave energy or power over line 107. The microwave power received on lead 107 is divided by power divider 106 and distributed over lines 97-105.

Walls 60-68 have a plurality of first slots 110 cut therein for radiating R.F. power in a first beam. In addition to slots 110, slots 112 are cut in walls 60-68 to radiate R.F. power to cancel out a grating lobe which is also formed at the time the first beam of R.F. power is formed. For example, the desired beam of R.F. power may radiate at an angle of θ measured from the normal to the respective wall of the waveguide and away from the input end of waveguides 51-59. Reference line 114 is normal to wall 68 and to reference line 115 which is then lying on the surface of wall 68 and parallel to the longitudinal axis 113 of waveguide 59. Angle θ as shown by arrow 116 is formed between reference line 114 and reference line 117 and is in the plane formed by reference lines 114 and 115. Angle θ which may be, for example, 10° may be the peak of a beam radiated from aperture 118 formed by slots 110 in walls 60-68. Angle 0 which may be, for example, -57.11° is formed between reference line 114 and reference line 120 as shown by arrow 121. Reference line 120 is in the plane formed by reference lines 114 and 115. Angle Sf may be the peak amplitude in a grating lobe or beam radiated from slots 110 formed in walls 60-68.

Slots 112 formed in walls 60-68 may also form a beam having a peak at angle £f and having a phase opposite to the grating lobe radiated by slots 110. Slots 110 and 112 form radiating aperture 118.

Referring to Figs. 4A and 4B, wall 60 of waveguide 51 is shown. Slots cut in wall 60 are spaced apart at their centers by the dimension d/3 which may be, for example, .8356 cm (.329"). Slot 123 may be cut at an angle 4, as shown by arrow 124 with respect to reference line 125. Reference line 125 is on the surface of .wall 60 and is orthogonal to the longitudinal axis 128 of waveguide 51. Slot 124 may likewise be cut at an angle _fif₂ with respect to reference line 126 shown by arrow 127. Reference line 126 is on the surface of wall 60 and is orthogonal to the longitudinal axis 128 of waveguide 51. Waveguide 51 may be, for example, rectangular in cross section with respect to the longitudinal axis 128. The position of the center of a slot: sirawjϊv for example, by reference point 130 on slot 123 and the. αc±entation of the slot with respect to reference line 1255,, determines the phase of the R.F. energy coupled out. Fox- positive angles of #, the R.F. energy coupled out of slot 123 will be at one phase angle and for negative angles of jøf, the R.F. energy coupled out of slot 123 will be shifted by 180°. The angle & of the slot determines the amount of R.F. energy coupled out of the slot. For example, when £f, is 0° no energy is coupled out of slot 123. When SS, is 90° the maximum energy is coupled out of slot 123. The polarization of the R.F. energy coupled out of slots 110 and 112 is aligned with reference axis 128. Fig. 5 shows waveguide 51* with slots 110' cut in wall 60*. Slots 110' are spaced at distance d which corresponds to 2.507 cm (.987"). Slots 110' are selected to provide the desired main beam at an angle θ having a peak amplitude along reference line 117 shown in Fig. 3. An undesired grating lobe may also be formed.

Fig. 6 shows a waveguide 51" having slots 112" formed in wall 60". Slots 112" are spaced apart by distance d/3 which corresponds to .836 cm (.329"). Slots 112" are selected to provide a single beam at an angle % having a peak amplitude along reference line 120 in Fig. 3 and shifted 180° in phase to cancel out a grating lobe formed by slots 110* shown in Fig. 5.

Fig. 4A is a combination or summation of slots 110* and 112" shown in Figs. 5 and 6, respectively, to provide a single main beam having a peak amplitude along reference line 117 shown in Fig. 3 with the undesired grating lobe cancelled out. The angle θ of the peak radiation from a beam of R.F. energy radiated from a waveguide array having equally spaced anti-phased slots is given by equation (1) .

sin θ = - -°- + - ≥ (n - 1/2) (1)

In equation (1) , θ is the angle at which the beam is formed, λ « is the free space wavelength, is the waveguide wavelength, d is the slot spacing, and n is an integer value of zero, —1, —2, .... The value selected for d, λ _Q and \ determine what interger values n can have for real values of θ, that is, for sin θ less than or equal to one. If in equation (1) , angle θ is real only for the value of n equals zero, then only a single beam (main beam) is formed. If angle θ is real for more than one value of n, then the main beam plus other beams called grating lobes are formed. One grating lobe is formed for each non-zero value of n. Using the values of 0.68 forχ_Q/ , 0.987 for d/λ_Q and with n equal to zero in equation (1) yields θ equal to +10°, which corresponds to a single main beam. Using the same values except for n equal to minus one in equation (1) yields θ equal to -57.11°, which is a grating lobe. The above values would be generated from a waveguide slot antenna fabricated with waveguides 51' shown in Fig. 5.

Fig. 7A shows a graph of the radiation pattern from a waveguide slot array antenna fabricated with waveguides 51* shown in Fig. 5. In Fig. 7A the ordinate represents power in decibels and the abscissa represents elevation angle in degrees. Curve 130 shows the main beam and curve 131 shows the grating lobe. Fig. 7B shows in polar coordinates the information shown in Fig. 7A. In Fig. 7B the radius represents power in decibels and the angle represents elevation angle in degrees. An angle of 90° as shown in Figs. 7A and 7B corresponds to the broadside direction from aperture 118 shown, for example, by reference line 114. Using the value of 0.329 for d/ λ_Q and the value of 0.68 for λ ₀/ λ in equation (1) with n equal to zero yields θ equal to -57.11°. Further, the beam at -57.11° is the only beam formed. A waveguide slot array antenna using waveguides 51", as shown in Fig. 6, would provide a single beam at -57.11°. The radiation pattern is shown in Figs. 8A and 8B. In Fig. 8A the ordinate represents power in decibels and the abscissa represents elevation angle in degrees. Curve 134 shows a single beam and curve portion 135 shows the radiation of side lobes. Referring to Fig. 8B the radius represents power in decibels and the polar angle represents elevation angle in degrees. Curves 134* and 135' in Fig. 8B correspond to the information shown in Fig. 8A. In Figs. 7A and 8A, the amplitude distributions of curves 131 and 134, respectively, are substantially the same. The amplitude distributions shown in Figs. 7A and 8A can be weighted appropriately and combined out of phase to yield an amplitude distribution having the desired main beam and no grating lobe as shown in Figs. 9A and 9B. In Fig. 9A the ordinate represents power in decibels and the abscissa represents elevation angle in degrees. Curve portions 137 and 138 represent low side lobes, while curve portion 139 represents the single main beam. In Fig. 9B the radius represents power in decibels and the polar angle represents the elevation angle in degrees. Curve 139' corresponds to curve 139 in Fig. 9A and curves 137' and 138* correspond to curves 137 and 138 in Fig. 9A.

By combining the slots 110' shown in Fig. 5 with slots 112" shown in Fig. 6 with slots 112" reoriented to provide an additional 180° phase shift, a waveguide 51 shown in Fig. 4Awill result. In Fig. 4Aevery third slot 112" overlaps slot 110* to provide a combined slot 110, 112 as shown in Fig..4A.

Table I provides the necessary data for a 63 slot waveguide for use in a waveguide slot array antenna to provide the radiation pattern shown in Figs. 9A and 9B. The synthesis technique used to determine the amplitude distribution for the main beam with a flat top pulse was to equate the coefficients of the antenna array factor given by equation (2) to the coefficients of the Fourier series expansion of a rectangular pulse.

J2d 2 s (θ) = a. + 22 m=l a m_ cos (m2 τr d/λ sinθ) (2)

In equation (2) , a is the amplitude of the center element and a equals a__m for a symmetric amplitude distribution, d represents the spacing between waveguide elements and θ represents the angle of radiation with respect to a reference line normal to the plane of the radiating surface of the waveguide elements.

Table I

d Ampli¬ Slot

Slot d Centi¬ d tude Phase Orien¬

# ^λo meters Inches (dB) (deg) tation

1 -10.199 0.000 ( 0.000) -27.034 0.000 / 0.

2 -9.870 1.948 ( 0.767) -20.869 -80.542 / 0.

3 -9.541 3.896 ( 1.534) -27.962 -161.084 / 0.

4 -9.212 5.845 ( 2.301) -30.716 -61.626 \ 180.

5 -8.883 7.793 ( 3.068) -31.296 -142.168 \ 180.

6 -8.554 9.741 ( 3.835) -46.189 -42.710 / 0.

7 -8.225 11.692 ( 4.603) -32.307 56.748 \ 180.

8 -7.896 13.640 ( 5.370) -21.510 -23.794 \ 180.

9 -7.567 15.588 ( 6.137) -25.088 -104.336 \ 180.

10 -7.238 17.536 ( 6.904) -23.987 -4.878 / 0.

11 -6.909 19.484 ( 7.671) -17.987 -85.420 / 0.

12 -6.580 21.433 ( 8.438) -25.300 -145.962 / 0.

13 -6.251 23.381 ( 9.205) -28.527 -66.504 \ 180.

14 -5.922 25.329 ( 9.972) -31.239 -147.047 \ 180.

15 -5.593 27.277 (10.739) -36.642 -47.589 / 0. 16 -5.264 29.225 (11.506) -26.995 51.869 \ 180.

17 -4.935 31.176 (12.274) -16.753 -28.673 \ 180.

18 -4.606 33.124 (13.041) -20.406 -109.215 \ 180.

19 -4.277 35.072 (13.808) -19.419 -9.757 / 0.

20 -.3,948 37.021 (14.575) -13.418 -90.299 / 0.

21 -3.619 38.969 (15.342) -20.786 -170.841 / 0.

22 -3.290 40.917 (16.109) -24.397 -71.303 \ 180.

23 -2.961 42.865 (16.876) -31.204 -151.925 \ 180.

2.4 -2.632 44.813 (17.643 -26.674 -52.467 / 0

25 -2.303 46.761 (18.410) -18.642 46.991 \ 180.

26 -1.974 48.710 (19.177) -8.233 -33.552 \ 180.

27 -1.645 50.660 (19.945) -11.330 -114.094 \ 180.

28 -1.316 52.608 (20.712) -9.251 -14.636 / 0.

29 -0.987 54.557 (21.479) -1.750 -95.178 / 0.

30 0.658 56.505 (22.246) -6.789 -175.720 / 0.

31 -0.329 58.453 (23.013) -6.217 -76.262- \ 180.

32 0.000 60.401 (23.780) 0.000 -156.804 \ 180.

33 0.329 62.349 (24.547) -6.217 122.654 \ 180.

34 0.658 64.298 (25.314) -6.789 -137.888 / 0.

35 0.987 66.246 (26.081) -1.750 141.570 / 0.

36 1.316 68.194 (26.848) -9.251 61.028 / 0.

37 1.645 70.142 (27.615) -11.330 160.486 \ 180.

38 1.974 72.093 (28.383) -8.233 79.944 \ 180.

39 2.303 74.041 (29.150) -18.642 -0.599 \ 180.

40 2.632 75.989 (29.917) -26.674 98.859 / 0.

41 '2.961 77.937 (30.684) -31.204 -161.683 \ 180.

42 3.290 79.886 (31.451) -24.397 117.775 \ 180.

43 3.619 81.834 (32.218) -20.786 -142.767 / 0.

44 3.948 83.782 (32.985) _τ13.418 136.691 / 0.

45 4.277 85.730 (33.752) -19.419 56.149 / 0.

46 4.606 87.678 (34.519) -20.486 155.607 \ 180.

47 4.935 89.626 (35.286) -16.753 75.065 \ 180.

48 5.264 91.577 (35.054) -26.995 -5.477 \ 180.

49 5,593 93.525 (36.821) -36.642 93.981 / 0.

50 5.922 95.474 (37.588) -31.239 -166.561 \ 180.

51 6.251 97.422 (38.355) -28.527 112.896 \ 180.

52 6.580 99.370 (39.122) -25.300 -147...646 / 0. 53 6.909 101.318 (39.889) -17.987 131.812 / 0.

54 7.238 103.266 (40.656) -23.987 51.270 / 0.

55 7.567 105.214 (41.423) -25.088 150.728 \ 180.

56 7.896 107.163 (42.190) -21.510 70.186' \ 180.

57 8.225 109.111 (42.957) -32.307 -10.356 \ 180.

58 8.554 111.059 (43.724) -46.189 89.102 / 0.

59 8.883 113.010 (44.492) -31.296 -171.440 \ 180.

6-01 9.212 114.958 (45.259) -30.716 118.018 \ 180.

61. 9.541 116.906 (46.026) -27.962 -152.524 / 0.

10 62 9.870 118.854 (46.793) -20.869 126.934 / 0.

63 10.199 120.802 (47.560) -27.034 46.392 / 0.

In Table I the waveguide element is WR159, the cente frequency is 5060.7 MHz, d/ λ _Q equals 3.29 and d/λ equals 0.223 with all slots equally spaced and formed on th

Ϊ5 narrow wall or edge of the waveguide. •

The invention describes an apparatus for radiatin electromagnetic energy in substantially a single bea comprising a plurality of waveguides spaced apart from on another and having a wall or surface facing in a firs

20 direction, each waveguide having a plurality of slots in th wall having a first spacing along the waveguide fo generating a main beam and a grating lobe, each waveguid having additional slots in the wall? of the waveguide havin a second spacing along the waveguide less 'than the firs

25 spacing for generating a third beam to substantially cance the second beam, and each waveguide adapted for couplin electromagnetic energy at one end having a predetermine amplitude and phase, respectively. The invention furthe provides utilizing slots at first and second spacings wit

30 overlapping slots being combined as a single slot on th waveguide.

Claims

We Claim:

1. Apparatus for radiating electromagnetic energy in substantially a single beam comprising: a plurality of waveguides (51-59) spaced apart from one another, each said waveguide (51-59) having a first wall (60-68) facing in a first direction (114) , each said waveguide (51-59) having a plurality of first slots (110) in said first wall (60-68) having a first spacing along said waveguide (51-59) for generating a first beam and an undesired second beam, each said waveguide (51-59) having a plurality of second slots (112) in said first wall (60-68) having a second spacing along said waveguide (51-59) less than said first spacing for generating a third beam to substantially cancel said second beam, and each said waveguide (51-59) adapted for coupling electromagnetic energy thereto having a predetermined amplitude and phase respectively.

2. The apparatus of claim 1 wherein said plurality of waveguides (51-59) are positioned transverse to and above the surface of the ground (41) .

3. The apparatus of claim 2 wherein each said waveguide is terminated by a resistive load (69-71) at an end furthest above said ground (41) .

4. The apparatus of claim 1 wherein each said waveguide (51-59) includes a rectangular waveguide having a straight longitudinal axis (113) and wherein said first wall (60-68) has a lesser width than a second wall.

5. The apparatus of claim 1 wherein each said waveguide (51-59) includes first means (79-87) for coupling electromagnetic energy to an end closest to said ground.

6. The apparatus of claim 1 wherein said first slots (110) are evenly spaced apart by a distance d.

7. The apparatus of claim 6 wherein said second slots (112) are evenly spaced apart by one third said distance d.

8. The apparatus of claim 6 wherein at least one of saa C first slots (110) and one of said second slots (112) are combined into one slot (110, 112) .