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Because of the "diffraction limit", larger dishes produce narrower (smaller angle) beams than small dishes: ----

Because of the "diffraction limit", larger dishes produce narrower (smaller angle) beams than small dishes (with larger angle beams). The upper plot shows the dish changing size, the lower plot shows the size of a ground spot far below a larger or smaller orbiting dish:
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Large dishes make smaller spots, but they are heavy. On the ground, they are distorted by weight and wind, and lose their shape. A parabolic dish is like a telescope mirror, it should keep its shape within a fraction of a wavelength or the distortions will spread the beam, making it fuzzy.

Also, big dishes take time to move around. Old style rotating dishes, and steered dishes, took many seconds to move from target to target. This was troublesome when detecting and tracking single airplanes; for ballistic missile early warning, a second is too long. As electronics became cheaper and budgets bigger, radars moved to a new system. phased array antennas.

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=== Phased Arrays ===
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Grating Lobes

The first precision microwave radars used parabolic dishes to focus the beam. The waves from the transmitter source bounce off the parabolic dish reflector and reflect back into a narrow beam with a horizontal wavefront:

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Because of the "diffraction limit", larger dishes produce narrower (smaller angle) beams than small dishes (with larger angle beams). The upper plot shows the dish changing size, the lower plot shows the size of a ground spot far below a larger or smaller orbiting dish:

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Large dishes make smaller spots, but they are heavy. On the ground, they are distorted by weight and wind, and lose their shape. A parabolic dish is like a telescope mirror, it should keep its shape within a fraction of a wavelength or the distortions will spread the beam, making it fuzzy.

Also, big dishes take time to move around. Old style rotating dishes, and steered dishes, took many seconds to move from target to target. This was troublesome when detecting and tracking single airplanes; for ballistic missile early warning, a second is too long. As electronics became cheaper and budgets bigger, radars moved to a new system. phased array antennas.


Phased Arrays

Traditional phased arrays are two dimensional uniformly spaced grids of antennas in a flat plane. By changing the delays of the RF signal to from the antennas, the sum of the wavefronts from the antennas can be steered in different directions, electronically, very rapidly, while the planar antenna stays fixed.

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GratingLobes (last edited 2021-06-08 17:49:40 by KeithLofstrom)