shifters. These requirements will insure a light weight-high efficiency array. In layout No. 1, the tubes are arranged on a square grid to yield a square subarray that utilizes approximately 100 percent of the available aperture. A square grid, however, does not allow a tube to be located at the input of each slotted waveguide and, hence, additional transmission lines are required. Transmission lines are also required to reverse the direction of rf power flow at the subarray edges. This approach is undesirable because of the complexity and greatly increased weight. Layout No. 2 shows an approach whereby rf power flows continuously across a large portion of the array. This approach yields a very simple rf power distribution system but greatly complicates the subarray phasing approach. Since the rf flows across several subarrays, it will accumulate the phase error of each subarray. Consequently, the phase of each row must be compensated as it enters into each subarray. This requires a large number 150/subarray) of high power lossy () phase shifters. These phase shifters would greatly increase the weight, losses, and cost of the array. Layout No. 3 does not have the disadvantages of the previous layouts (i. e. , large number of transmission lines or high power phase shifters). A small corporate feed structure is required to start each row. Staggering the tubes allows a tube to be located at the input to each slotted waveguide. At the input (i. e. , left) side of the subarray, the tube staggering yields a staggered aperture edge and, hence, some of the available rectangular subarray aperture is not used. If the right edge of the subarray was also staggered, the subarrays could be interlaced with approximately 100 percent aperture utilization. The staggered left edge yields a loss in aperture. There are two approaches for the right edge that must be compared. Figure 6-25 demonstrates the two approaches. In the first approach, all rows are identical and the right edge is staggered similar to the left edge. In the second approach, the end waveguide of each row is extended to yield a straight edge. The power density across the end waveguides are not identical and the subarray aperture is no longer uniformly illuminated. The efficiency and sidelobes associated with these two right edge approaches are compared below. The power coupled out of the end of a waveguide is used as input to the next tube in the row. The last waveguide in a row does not excite another
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