The cylindrical array must develop the same aperture area as the planar array in the direction of the ground antenna to maintain equivalent efficiency. Therefore it must have at least as much total area. It must also have as many dc-rf converters with some form of active heat transfer to radiating surfaces at the top and bottom of the cylinder, or it must locate the converters at the cylinder ends and suffer the power loss in distributing the microwave power to the cylinder walls. There also is the complexity of switching power on and off to the converters as the cylinder rotates. The weight and cost penalty for a rotary joint (see Section 8) is sufficiently small that the planar array is a clear choice. 6.3 SUBARRAY TYPES The options in subarray types are shown in Figure 6-20. In Figure 6-21 they are shown individually together with a listing of advantages and disadvant- ages. The indicated efficiencies are due to I R losses, spillover and aperture taper for the individual subarrays. Additional losses at the array level would be associated with the pyramidal horn and paraboloid types due to the higher array side lobes. Only the slotted waveguide and the helix configuration have adequately high aperture efficiency, and the helix has two drawbacks: (1) the beam would be circularly polarized which would necessitate a dual polarized receiving antenna at added cost and complexity, and (2) there would still be a need for microwave power distribution hardware, i. e. , waveguide, from the converters to the helices, and from converter to converter in the case of the amplitron. 6.4 SUBARRAY DIMENSIONS The size of a subarray is determined by the amount of power loss that is budgeted for the effects of mechanical distortion, misalignment and effects of attitude control errors. These effects are illustrated in Figure 6-22 where the high gain (narrow beam pattern) associated with the larger dimension exhibits greater power loss than the smaller dimension with relatively wide beam patterns. The trend to smaller subarrays must be traded off against (1) the added cost of the supporting electronics and control hardware that must be associated with subarrays of any size, and (2) the power lost due to poorer area utilization.
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