|Covid-19|Null (radio) Wikipedia
In radio electronics, a null is an area or vector in an antenna's radiation pattern where the signal 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 unintentionally prevent reception of a signal. Null fill in the vertical plane is used to prevent this.
Also, some directional antennas have very narrow, sharp nulls. These antennas when used for direction finding are aimed away from the signal of interest until the antenna's null is pointed at the signal, and the signal cannot be detected.