Antenna Beam Illumination - Garmin GTN 650 Series Pilot's Manual Upgrade Supplement

Airborne weather radar has other capabilities beyond weather detection. It
also has the ability to detect and provide distance to objects on the ground, such
as, cities, mountains, coastlines, rivers, lakes, and oceans.
5.1.2

Antenna Beam Illumination

It is important to understand the concept of the antenna beam illumination.
The radar beam is much like the beam of a spotlight. The farther the beam
travels, the wider it gets. The radar is only capable of "seeing" what is inside the
boundaries of the beam.
The vertical dimensions of the radar beam are shown in the figure above and
the same holds true for the horizontal dimensions. In other words, the beam will
be as wide as it is tall. Note that it is possible not to see areas of precipitation on
the radar display because of the antenna tilt setting. With the antenna tilt set to
zero in this illustration, the beam overshoots the precipitation at 15 NM. The
curvature of the earth can also be a factor, especially at range settings of 150 NM
or more.
Figure 5-2 Radar Beam in Relation to the Curvature of the Earth
GTN 650/750 Series SW V3.00 Upgrade Supplement 190- 01007-05 Rev A
Figure 5-1 Radar Beam from 12 inch Antenna
320 nm
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