NULL

1. In an antenna radiation pattern , a zone in which the effective radiated power is at a minimum relative to the maximum effective radiated power of the main beam . Note 1: A null often has a narrow directivity angle compared to that of the main beam. Thus, the null is useful for several purposes, such as radio navigation and suppression of interfering signals in a given direction. Note 2: Because there is reciprocity between the transmitting and receiving characteristics of an antenna, there will be corresponding nulls for both the transmitting and receiving functions. 2. A dummy letter, letter symbol, or code group inserted in an encrypted message to delay or prevent its solution, or to complete encrypted groups for transmission or transmission security purposes. [ NIS ] 3. In database management systems, a special value assigned to a row or a column indicating either unknown values or inapplicable usage. 4. Synonym node (def. #4).