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:
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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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