The term Antenna Gain describes how much the energy leaving the antenna is focused into a particular direction.
Extracts from "Your Guide to Weather Radar" published by Rockwell Collins.
"The ability to adjust the gain control during weather detection modes provides some advantages in weather detection at the shorter ranges. Specifically, aircraft operating at high altitudes run a greater risk of encountering low reflectivity targets such as ice crystals. Adjusting the gain to provide maximum detection will aid in locating areas of low reflectivity targets.
By increasing the system gain, the pilot can effectively broaden the beam width of the transmitted signal (within the first 50 nm) to produce returns from targets that might have been otherwsie over-scanned. Once again, judicious use of the tilt and gain controls to discover the extent of nearby targets is recommended".
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