Beam Width and Ground Clutter

While the strongest part of the radar beam is in the center of the beam, and the certified beam width (antenna size dependent) contains the most energy, additional beam energy remains beyond this point and may extend out several degrees beyond the certified beam width on both top and bottom. Any radar energy that hits a highly reflective surface will produce a return, and terrain and cities are both highly reflective and will produce ground clutter returns.

With a 10 inch antenna set to 0.00° tilt, you can expect to see ground returns (over flat land) at a distance that is equal to your current height above ground level divided by 1000. Figure 4 provides an example of how ground clutter would appear on a display with a 10 inch antenna system, and the tilt angle set at 0.00 while flying at 25,000 ft. AGL. Adjusting the tilt upwards would reduce the amount of ground clutter on the display.

Figure 5 shows the relationship of height above ground level to the location of the beginning of ground clutter, and the location of the bottom of the certified beam width, for the 10 inch antenna.

NOTE: As antenna size increases both the certified beam width and the width of the additional energy beyond the certified beam width will decrease, as will ground clutter from the additional beam energy.

KEY POINT: You should raise the tilt angle sufficiently to minimize ground clutter to enable the differentiation of precipitation returns.

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