Null (radio)
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In radio electronics, a null is an area or vector at which the signal from two or more of the antenna elements in an antenna system cancels out almost entirely.
This can be an advantage, as nulls in the horizontal plane can be used to protect other transmitters from interference. If not carefully planned however, nulls can affect the receivability of a signal in any given place. Null fill in the vertical plane is used to prevent this.