A planar slotted waveguide antenna array having a front, radiating surface and a back-plane, a length dimension L and a width dimension W, comprising a plurality of radiating waveguides parallel to the width dimension; a plurality of co-planar radiating apertures in each of said plurality of radiating waveguides constituting said radiating surface; a feeder waveguide along at least part of the length dimension contiguous a predetermined edge of the array; and a plurality of coupling apertures for coupling microwave energy between said feeder waveguide and each of said plurality of radiating waveguides.

CROSS-REFERENCE TO RELATED APPLICATION 
This application is related to concurrently filed, commonly assigned 
application by the same inventor entitled COMPOSITE WAVEGUIDE COUPLING 
APERTURE HAVING A THICKNESS DIMENSION which is incorporated herein by 
reference. 
FIELD OF THE INVENTION 
The present invention relates to antennas in general and in particular to 
planar slotted-waveguide array antennas. More particularly still, it 
relates to planar waveguide-fed slot-antenna arrays suitable for 
terrain-mapping side-looking airborne radar (SLAR) antennas. 
BACKGROUND OF THE INVENTION 
Using SLAR is an efficient, low-cost method of viewing and mapping terrains 
over a wide swath of territory on either side of the flight path of the 
carrier aircraft. Two SLAR antennas on either side of the aircraft 
illuminate a long, preferably narrow strip of the terrain with a 
high-powered short radar pulse, normally in the X-band of the microwave 
spectrum. As the radiated impulse power is reflected by the illuminated 
terrain and received by the now receiving SLAR antenna, the intensity and 
times of arrival of the reflections are processed electronically to 
produce an instantaneous terrain map. As the aircraft proceeds along its 
path the terrain map is updated. As an example a suitable radar pulse 
repetition frequency of 800 Hz could be used, with a pulse duration of 
approximately 250 nanoseconds. The quality of the terrain map depends 
strongly from the precision of the radiated illumination pattern. It is 
known in the art that a narrow beam in the horizontal plane (a so-called 
pencil beam in the azimuth plane) having its peak intensity along an axis 
perpendicular to the flight path and slightly inclined with respect to the 
horizontal plane, and illuminating the terrain with gradually declining 
intensity reaching a null underneath the flight path is required. 
Accordingly, the terrain is approximately uniformly illuminated 
irrespective of the distance from the antenna. A narrow beam in the 
horizontal plane is necessary in order to provide good azimuth resolution 
of the terrain of the strip just under the antenna as an illuminating 
radar pulse is emitted. Therefore, the far-field azimuth angle of the beam 
should be as small as possible, and the illumination intensity should 
decline from its peak at the near horizontal to the near vertical 
(downward from the aircraft) as uniformly as possible. These 
characteristics are, of course, desirable in any planar antenna array, and 
imply minimal side-lobe illumination. 
PRIOR ART OF THE INVENTION 
As may be seen from the above description, the antenna arrays used in SLAR 
applications are among those that are required to meet the strictest 
standards in manufacturing and performance. It is therefore not surprising 
that the closest prior art to the present invention is a SLAR antenna. 
Indeed, as will be seen later when describing the preferred embodiment, 
the latter was realized to physically fit into the same antenna radome. 
The existing SLAR antenna comprises sixteen horizontal waveguides, in a 
single plane each of which is approximately seventeen feet long. The 
planar front surface of the waveguide array shows the slotted narrow side 
of the waveguides. The slots are what is known in the art as "edge-wall" 
slots. The array's waveguides are fed by a tree of T-splitters. As will be 
appreciated, it is difficult to maintain the waveguide width to within the 
required extremely narrow tolerance due to the extreme length of the 
waveguide, particularly because there are sixteen waveguides which could 
deviate from the nominal and important broad-face width at random. In 
addition, a substantial support structure is necessary, which, in any 
event can not provide the uniformity required for a well-shaped beam. But 
even the support structure would not mitigate non-uniformities inherent in 
machining a seventeen foot waveguide. Note that the radiating slots in the 
waveguides are placed approximately half-wave length apart (at X-band 
about 1.5 cm) and any deviations from their ideal planar position causes 
beam distortions, which directly affect range and azimuth resolutions. 
Ideally, each slot must radiate from its appointed relative position 
within the array the correct amount of power in the correct phase, in 
order to produce the desired far field illumination pattern. 
SUMMARY OF THE INVENTION 
It is, therefore, the object of the present invention to provide an 
improved planar antenna array suitable for satisfying the strict 
requirements of SLAR applications. 
In order to achieve this object, it was realized that the array itself must 
be its own supporting structure, and, as a consequence, that it must be 
machined from a single piece of metal as far as the radiating waveguides, 
which comprise the most important group of components, are concerned. But 
to have a milling machine, no matter how accurate, mill sixteen (or more) 
parallel seventeen-feet long waveguides in that piece of metal might avoid 
the neccessity for an external support structure but is likely to 
introduce the same or more non-uniformities that would be more difficult 
to correct or mitigate. 
Accordingly, it is a feature of the present invention that the main 
component group is machined in a single slab of metal. However, instead of 
a small number of radiating waveguides running along the array-length, a 
large number of relatively short waveguides run parallel to the array 
width. 
The machined piece of metal does not only integrally incorporate the 
radiating waveguides, but also has its edge serving as the key 
coupling-(broad side)-wall of a series-fed waveguide. 
Accordingly, it is another feature of the present invention that a single 
feeder waveguide has a coupling wall integral with, and machined in, the 
main slab of metal which incorporates the radiating waveguides. 
It will be appreciated by those skilled in the art, that to have all 
critical components of the antenna array integrally machined from a single 
slab of metal is advantageous. 
According to the present invention there is provided a planar slotted 
waveguide antenna array having a front, radiating, surface and a 
back-plane, a length dimension L and a width dimension W, comprising: 
(a) a plurality of radiating waveguides parallel to the width dimension; 
(b) a plurality of co-planar radiating apertures in each of said plurality 
of radiating waveguides constituting said radiating surface; 
(c) a feeder waveguide along at least part of the length dimension 
contiguous with a predetermined edge of the array; and 
(d) a plurality of coupling apertures for coupling microwave energy between 
said feeder waveguide and each of said plurality of radiating waveguides. 
According to a narrower aspect of the present invention, the plurality of 
radiating waveguides and the pluarlity of coupling apertures are machined 
in a single piece of suitable metal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 of the drawings shows a portion of the SLAR antenna array of the 
prior art. The horizontal, parallel slotted waveguide 10a to 10p continue 
to the left of the Figure for a total length of approximately seventeen 
feet. At the right edge of the Figure sixteen feeder waveguides 11a to 11p 
are shown, which themselves are fed via a tree of T-splitters (not shown), 
which is why the array comprises sixteen radiating waveguides 10a to 10p. 
If power is not to be wasted in dummy loads, such array must have 2.sup.n 
radiating waveguides. 
The far-field azimuth angle .alpha. of a radar beam is defined as the 
off-axis angle at which the beam intensity is -3 dB relative to its peak. 
For SLAR applications a small azimuth angular width .alpha. of the beam is 
desired, in order to increase mapping resolution in the horizontal plane 
along the flight path of a SLAR aircraft. The angular width .alpha. for 
the antenna of the present preferred embodiment is approximately 
0.4.degree., which is capable of yielding an azimuth resolution of less 
than 8 meters/km. The side lobes of the main beam should be as low as 
possible and are -25 dB in the present case. 
In order to achieve the desired far-field azimuth pattern, a near-field 
pattern as shown in FIG. 2 by the thin solid line is required. It means 
that along the length of the radiating antenna, maximum power is to be 
radiated from its central axis. A suitably smoothly tapering function for 
such radiation pattern is given by 
EQU (2/3)+(1/3) cos x, -.pi.&lt;x&lt;.pi.. 
Thus minimum power would be radiated along the narrow (vertical) edges of 
the array. 
The bold solid curve in FIG. 2 illustrates the power coupling coefficient 
from the feeder waveguide to the radiating waveguides along the length of 
the array of the present embodiment and will be discussed later in 
conjunction with FIG. 4 et seq. 
While FIG. 2 shows the azimuth plane pattern in the near-field, FIG. 3 
illustrates the desired intensity of illumination as a function of the 
elevation angle. In flight, the SLAR antenna hangs under the fuselage of 
the aircraft with its length parallel to the flight path and radiates to 
one side perpendicular to the path. As it is normally desired to 
illuminate and map, say, a 100 km swath, the intensity of illumination 
should be maximum at an elevation angle slightly more than the horizontal. 
The illumination should decline with increasing angle with the horizontal 
plane of the flight path and must be a null at 90.degree., i.e. under the 
aircraft, in order to prevent interference with the radiation from the 
antenna on the other side of the aircraft. The smoothness of the decline 
in radiation intensity in the elevation plane is important for the 
uniformity of reflection of the radiation off the terrain. 
We now turn to FIGS. 4 and 5, showing the structure of the SLAR antenna 
array. FIG. 4 is a plan view of the antenna as it hangs vertically either 
below the fuselage of an aircraft (not shown) or along the side thereof. 
FIG. 5 is a side elevation showing the back of the antenna with the cover 
plate removed and not shown, and which is simply a planar rectangular 
piece of aluminum coextensive with the outer dimensions of the radiating 
waveguides, and is when assembly is complete, screwed in place by means of 
6014 screws evenly spaced around the radiating waveguide cavities. The 
back wall thus serves as a broadside wall to the radiating waveguides and 
as such must be well secured thereto to ensure electrical integrity and 
prevent any power leakage. 
Referring to FIGS. 4 and 5, the antenna is constructed from a single piece 
of machined (by numerically controlled milling) aluminum member 20, a 
back-plane cover (not shown) with a flange along its long edge, a 
feeder-wave-guide forming U-shaped channel 21, and a flange 22 at the 
feeder end of the array. The aluminum member 20 has along its length on 
the side of the U-shaped channel 21 a raised flange 23 serving as a fourth 
wall together with the flange of the back-plane cover of the wave-guide 
forming U-shaped channel 21. Vertical radiating waveguide cavities W1 to 
W187 are milled into the member 20, which in its pristine form measured 
more than its machined length of approximately 206 inches and its machined 
width of approximately 15.25 inches. Into the front wall of each of the 
waveguide cavities W1 to W187 are milled radiating slots S1 to S16 (shown 
only in the cavity W1, as are all other details) which alternate on either 
side of the center line 24, lengthwise, of the wall. Each waveguide cavity 
has an identical load constructed of microwave-absorbing material at its 
end, and communicates at its opposite (feed) end by means of a plurality 
of composite coupling apertures A1 to A187, which alternate on either side 
of the centre line 26 of that part of the raised flange 23 which, along 
its length, forms the fourth wall of the feeder waveguide forming U-shaped 
channel 21. But the apertures A1 to A187 (only A1 and A187 are shown in 
FIG. 5) are not identical, neither in dimensions nor in position with 
respect to the centre line 24 of the radiating waveguide cavities W1 to 
W187. The feeder waveguide 21 is connected to the transmit/receive 
waveguide (not shown) through the flange 22 at an input/output end 27 and 
has a load constructed of microwave absorbing material 28 at its other end 
to absorb residual power and match the waveguide. Aligning dowells 28 and 
29 are press fitted into place and ensure integrity of the connections to 
prevent leakage or discontinuities in the path of the transmit power 
coupled via the input/output 27. For the same reasons, it is necessary to 
ensure good electrical connection between the flange 23 and the waveguide 
channel 21, which is bolted to the flange 23 through holes H1 to H189. 
In order to not clutter the drawings, details of machining instructions and 
other details that are considered known in the art were omitted. 
ELECTRICAL DESIGN OF THE ANTENNA 
As mentioned hereinabove, the antenna of the preferred embodiment was 
constructed to fit in the existing housings of the prior art antenna shown 
in FIG. 1. This fact determined that at X-band (.lambda..apprxeq.3 cm) an 
antenna length of approximately 17 feet yields 187 radiating waveguides W1 
to W187 each of which has 16 radiating slots S1 to S16, sixteen being the 
number of parallel waveguides in the prior art antenna, dictated by the 
fact that eight would be too few and thirty-two too many. In the present 
design, however, there is no such restriction and the antenna array could 
have been designed to be wider but for the housing. 
A standard waveguide size for the X-band is 0.9.times.0.4 inches and such 
standard was chosen throughout for the cavities W1 to W187 as well as the 
feeder channel 21. The length of each cavity W1 to W187, given the 
permissible total antenna width, was chosen to be 
25.times.(.lambda./2)=14.66 inches. 
The design of the radiating-slot arrays S1 to S16, which are non-uniform 
travelling-wave arrays, follows known procedures, for example, as 
explained by H. Yee in Chapter 9 (Slot-Antenna Arrays) in the text 
"Antenna Engineering Handbook (Johson and Jasik, eds., second ed., 1984) 
published by McGraw-Hill. This Chapter is included herein in its entirety 
by reference. Reference is made particularly to Section 9-7, at p. 9-26 
titled "Travelling-Wave Slot-Array Design". The resultant slot length is 
0.614.+-.0.002 inch for all slots S1 to S16 in all cavities W1 to W187, 
while the width is 0.062 inch. The position of the slots S1 to S16 with 
reference to the centre line 24 and with reference to the feed-end of the 
cavities W1 to W187 is determinable following the known principles 
expounded in the above reference. 
The design of the coupling apertures A1 to A187 is not conventional. As may 
be seen from FIGS. 6 and 7, the apertures A1 to A187 constrict stepwise 
along their central axis. This composite coupling aperture construction 
became necessary due to, first, the wall thickness through which coupling 
was necessary and which was dictated by mechanical reasons to be 0.4 inch, 
and, second, by the large variation in the degree of coupling required as 
dictated by the bold solid curve shown in FIG. 2. For in order to produce 
the near-field pattern above mentioned, (and given that the feeder 
waveguide 21 begins to feed at one end of the array of radiating 
waveguides at W1 and ends feeding at W187), a variation in coupling as per 
the bold solid curve became necessary. Normally, such variation in the 
degree of coupling is accomplished by placing the conventional coupling 
slots closer to or farther away from the centre line (as with the slots S1 
to S16). But due to the mechanical constraints, among them that a hole 30 
has to be provided for the back-plane cover, the apertures A1 to A187 
cannot be moved too far away from their centre line to increase coupling. 
It was thus necessary to have a fixed spacing on either side of the centre 
line for all the coupling apertures A1 to A187 but make them variably 
shorter than the resonant length. That, however, introduces phase errors 
that would degrade the azimuth beam shape and increase the level of the 
side-lobes. In order to correct for phase errors, the apertures A1 to A187 
were variably positioned off the centre line 24 at the radiating 
waveguides W1 to W187, by the variable dimension C in FIG. 4. 
For the necessary variation in coupling, between --dB and -14 dB, in the 
preferred embodiment, the constant dimensions of the apertures A1 to A187 
as shown in FIGS. 6 and 7 are as follows: 
W1=0.188 inch .+-.0.005 
W2=0.100 inch .+-.0.005 
D1=0.140 inch (D1 should be as long as possible) 
D2=0.260 inch. 
The variable dimensions A, B (in FIG. 6) and C (in FIG. 4) for each of the 
apertures A1 to 187 are given in the table on the following pages. 
In order to compensate for deviation from the nominal broad-face width of 
the feeder waveguide 21, which would affect the propagation velocity in 
the guide, it is preferable to employ pairs of adjustable screws 
penetrating the broad face of the waveguide. Suitable special purpose 
screws are commercially available from a number of suppliers, one of these 
being Johanson. "Johanson screws" consist of an insert comprising a plated 
screw, threaded bushing, and locking device. 31 are needed along the 
outside broad wall thereof to compensate for such deviation from nominal 
waveguide velocity, which, of course, affects the phase. It is for this 
reason that the employment of a single 17 feet-long waveguide is 
advantageous. For it is very difficult to compensate in the prior SLAR 
antenna and attain uniformity among sixteen very long waveguides. 
______________________________________ 
SLOT NO. "A" DIM "B" DIM "C" DIM 
______________________________________ 
1 0.480 0.558 +0.083 
2 0.480 0.558 +0.083 
3 0.481 0.559 +0.083 
4 0.481 0.559 +0.083 
5 0.481 0.559 +0.083 
6 0.482 0.560 +0.083 
7 0.482 0.560 +0.083 
8 0.483 0.561 +0.083 
9 0.483 0.561 +0.083 
10 0.484 0.562 +0.083 
11 0.085 0.563 +0.083 
12 0.486 0.564 +0.083 
13 0.487 0.565 +0.083 
14 0.488 0.566 +0.083 
15 0.489 0.567 +0.083 
16 0.490 0.568 +0.083 
17 0.491 0.569 +0.083 
18 0.493 0.571 +0.083 
19 0.494 0.572 +0.083 
20 0.496 0.574 +0.082 
21 0.497 0.575 +0.082 
22 0.499 0.577 +0.082 
23 0.501 0.579 +0.082 
24 0.502 0.580 +0.082 
25 0.504 0.582 +0.082 
26 0.506 0.584 +0.082 
27 0.508 0.586 +0.082 
28 0.510 0.588 +0.081 
29 0.512 0.590 +0.081 
30 0.514 0.592 +0.081 
31 0.516 0.594 +0.081 
32 0.517 0.595 +0.080 
33 0.519 0.597 +0.080 
34 0.521 0.599 +0.080 
35 0.523 0.601 +0.080 
36 0.525 0.603 +0.079 
37 0.527 0.605 +0.079 
38 0.528 0.606 +0.079 
39 0.530 0.608 +0.078 
40 0.531 0.609 +0.078 
41 0.533 0.611 +0.078 
42 0.534 0.612 +0.077 
43 0.535 0.613 +0.077 
44 0.535 0.613 +0.076 
45 0.536 0.614 +0.076 
46 0.536 0.614 +0.075 
47 0.537 0.615 +0.075 
48 0.538 0.616 +0.074 
49 0.539 0.617 +0.074 
50 0.541 0.619 +0.073 
51 0.542 0.620 +0.073 
52 0.543 0.621 +0.072 
53 0.544 0.622 +0.072 
54 0.545 0.623 +0.071 
55 0.546 0.624 +0.071 
56 0.547 0.625 +0.070 
57 0.548 0.626 +0.069 
58 0.549 0.627 +0.069 
59 0.550 0.628 +0.068 
60 0.551 0.629 +0.067 
61 0.551 0.630 +0.067 
62 0.552 0.630 +0.066 
63 0.552 0.630 +0.066 
64 0.552 0.630 +0.065 
65 0.552 0.630 +0.064 
66 0.552 0.630 +0.063 
67 0.552 0.630 +0.063 
68 0.553 0.631 +0.062 
69 0.554 0.632 +0.061 
70 0.554 0.632 +0.060 
71 0.555 0.633 +0.059 
72 0.555 0.633 +0.058 
73 0.556 0.634 +0.057 
74 0.556 0.634 +0.056 
75 0.557 0.635 +0.055 
76 0.557 0.635 +0.053 
77 0.557 0.635 +0.052 
78 0.558 0.636 +0.051 
79 0.558 0.636 +0.050 
80 0.559 0.637 +0.048 
81 0.559 0.637 +0.046 
82 0.560 0.638 +0.044 
83 0.560 0.638 +0.042 
84 0.561 0.639 +0.040 
85 0.561 0.639 +0.038 
86 0.562 0.640 +0.036 
87 0.562 0.640 +0.033 
88 0.563 0.641 +0.031 
89 0.563 0.641 +0.028 
90 0.564 0.642 +0.025 
91 0.564 0.642 +0.022 
92 0.565 0.643 +0.019 
93 0.565 0.643 +0.016 
94 0.566 0.644 +0.013 
95 0.566 0.644 +0.009 
96 0.567 0.645 +0.006 
97 0.567 0.645 +0.002 
98 0.568 0.646 -0.001 
99 0.568 0.646 -0.005 
100 0.569 0.647 -0.009 
101 0.569 0.647 -0.012 
102 0.570 0.648 -0.013 
103 0.570 0.648 -0.015 
104 0.571 0.649 -0.017 
105 0.572 0.650 -0.019 
106 0.572 0.650 -0.020 
107 0.573 0.651 -0.022 
108 0.573 0.651 -0.023 
109 0.574 0.652 -0.024 
110 0.574 0.652 -0.026 
111 0.575 0.653 -0.027 
112 0.575 0.653 -0.028 
113 0.576 0.654 -0.029 
114 0.576 0.654 -0.030 
115 0.577 0.655 -0.031 
116 0.577 0.655 -0.031 
117 0.578 0.656 -0.032 
118 0.058 0.656 -0.032 
119 0.579 0.657 -0.033 
120 0.579 0.657 -0.033 
121 0.580 0.658 -0.034 
122 0.580 0.658 -0.934 
123 0.581 0.659 -0.034 
124 0.580 0.659 -0.035 
125 0.581 0.659 -0.035 
126 0.582 0.660 -0.035 
127 0.582 0.660 -0.035 
128 0.582 0.660 -0.035 
129 0.582 0.660 -0.036 
130 0.583 0.661 -0.036 
131 0.583 0.661 -0.036 
132 0.583 0.661 -0.037 
133 0.583 0.661 -0.037 
134 0.584 0.662 -0.037 
135 0.584 0.662 -0.037 
136 0.584 0.662 -0.037 
137 0.584 0.662 -0.937 
138 0.584 0.662 -0.037 
139 0.584 0.662 -0.037 
140 0.584 0.662 -0.037 
141 0.584 0.662 -0.037 
142 0.584 0.662 -0.038 
143 0.584 0.662 -0.038 
144 0.584 0.662 -0.038 
145 0.584 0.662 -0.037 
146 0.584 0.662 -0.037 
147 0.584 0.662 -0.037 
148 0.584 0.662 -0.037 
149 0.584 0.662 -0.037 
150 0.584 0.662 -0.037 
151 0.583 0.661 -0.037 
152 0.583 0.661 -0.036 
153 0.583 0.661 -0.036 
154 0.583 0.661 -0.036 
155 0.583 0.661 -0.036 
156 0.582 0.660 -0.035 
157 0.582 0.660 -0.035 
158 0.582 0.660 -0.035 
159 0.582 0.660 -0.035 
160 0.581 0.659 -0.035 
161 0.581 0.659 -0.035 
162 0.581 0.659 -0.035 
163 0.580 0.658 -0.034 
164 0.580 0.658 -0.034 
165 0.580 0.658 -0.034 
166 0.580 0.658 -0.034 
167 0.579 0.657 -0.034 
168 0.579 0.657 -0.034 
169 0.579 0.657 -0.033 
170 0.579 0.657 -0.033 
171 0.579 0.657 -0.033 
172 0.579 0.657 -0.033 
173 0.579 0.657 -0.033 
174 0.579 0.657 -0.033 
175 0.579 0.657 -0.033 
176 0.579 0.657 -0.034 
177 0.580 0.658 -0.034 
178 0.580 0.658 -0.034 
179 0.581 0.659 -0.035 
180 0.581 0.659 -0.035 
181 0.582 0.660 -0.035 
182 0.583 0.661 -0.036 
183 0.584 0.662 -0.037 
184 0.585 0.663 -0.038 
185 0.586 0.664 -0.039 
186 0.587 0.665 -0.040 
187 0.588 0.666 -0.040 
______________________________________ 
The composite coupling aperture (such as A1 to A187) and the method of its 
design are subject of concurrently filed patent application entitled 
"Composite Waveguide Coupling Aperture Having a Thickness Dimension" by 
the same inventor.